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Developing a High Thermal Conductivity Nuclear Fuel with Silicon Carbide Additives

Permanent Link: http://ufdc.ufl.edu/UFE0021716/00001

Material Information

Title: Developing a High Thermal Conductivity Nuclear Fuel with Silicon Carbide Additives
Physical Description: 1 online resource (132 p.)
Language: english
Creator: Wang, Jiwei
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: carbide, chemical, conductivity, deposition, dioxide, low, polymer, preceramic, silicon, sintering, temperature, thermal, uranium, vapor, whiskers
Nuclear and Radiological Engineering -- Dissertations, Academic -- UF
Genre: Nuclear Engineering Sciences thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Uranium dioxide (UO2) is the most common fuel material utilized in commercial nuclear power reactors. The main disadvantage of UO2 is its low thermal conductivity. During a reactor?s operation, there is a large temperature gradient in the UO2 fuel pellet, causing a very high centerline temperature, and introducing thermal stresses, which lead to extensive fuel pellet cracking. These cracks will add to the release of fission product gases after high burnup. The high fuel operating temperature also increases the rate of fission gas release and the fuel pellet swelling caused by both fission gases bubbles and thermal expansion. The amount of fission gas release and fuel swelling limits the life time of UO2 fuel in reactor. The objective of this research is to increase the thermal conductivity of UO2 while not significantly affecting the neutronic property of UO2. The concept is to incorporate another high thermal conductivity material, silicon carbide (SiC), into the UO2 pellet. Silicon carbide is expected to form a conductive percolation pathway in the UO2 for heat to flow out of the fuel pellet, thus increasing the UO2 thermal conductivity. Three methods were studied to incorporate SiC into UO2. Firstly, chemical vapor deposition (CVD) process was used to coat UO2 particles with a SiC layer prior to the low temperature sintering process of UO2. Secondly, allylhydridopolycarbosilane (AHPCS), a pre-ceramic polymer, was used to generate a SiC coating on UO2 particles prior to the low temperature sintering process of UO2. Thirdly, silicon carbide whiskers were mixed with UO2 particles prior to the low temperature sintering method of UO2. Though pellets with high density were not achieved by the first and second methods, pellets of 95% TD were achieved by pressureless sintering of UO2 with 5 vol% SiC whiskers and hot press sintering of UO2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. The thermal conductivity of the pellets will be measured at Idaho National Laboratory. The thermal conductivity data will not be discussed in this dissertation.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Jiwei Wang.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Tulenko, James S.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021716:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021716/00001

Material Information

Title: Developing a High Thermal Conductivity Nuclear Fuel with Silicon Carbide Additives
Physical Description: 1 online resource (132 p.)
Language: english
Creator: Wang, Jiwei
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: carbide, chemical, conductivity, deposition, dioxide, low, polymer, preceramic, silicon, sintering, temperature, thermal, uranium, vapor, whiskers
Nuclear and Radiological Engineering -- Dissertations, Academic -- UF
Genre: Nuclear Engineering Sciences thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Uranium dioxide (UO2) is the most common fuel material utilized in commercial nuclear power reactors. The main disadvantage of UO2 is its low thermal conductivity. During a reactor?s operation, there is a large temperature gradient in the UO2 fuel pellet, causing a very high centerline temperature, and introducing thermal stresses, which lead to extensive fuel pellet cracking. These cracks will add to the release of fission product gases after high burnup. The high fuel operating temperature also increases the rate of fission gas release and the fuel pellet swelling caused by both fission gases bubbles and thermal expansion. The amount of fission gas release and fuel swelling limits the life time of UO2 fuel in reactor. The objective of this research is to increase the thermal conductivity of UO2 while not significantly affecting the neutronic property of UO2. The concept is to incorporate another high thermal conductivity material, silicon carbide (SiC), into the UO2 pellet. Silicon carbide is expected to form a conductive percolation pathway in the UO2 for heat to flow out of the fuel pellet, thus increasing the UO2 thermal conductivity. Three methods were studied to incorporate SiC into UO2. Firstly, chemical vapor deposition (CVD) process was used to coat UO2 particles with a SiC layer prior to the low temperature sintering process of UO2. Secondly, allylhydridopolycarbosilane (AHPCS), a pre-ceramic polymer, was used to generate a SiC coating on UO2 particles prior to the low temperature sintering process of UO2. Thirdly, silicon carbide whiskers were mixed with UO2 particles prior to the low temperature sintering method of UO2. Though pellets with high density were not achieved by the first and second methods, pellets of 95% TD were achieved by pressureless sintering of UO2 with 5 vol% SiC whiskers and hot press sintering of UO2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. The thermal conductivity of the pellets will be measured at Idaho National Laboratory. The thermal conductivity data will not be discussed in this dissertation.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Jiwei Wang.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Tulenko, James S.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021716:00001


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d220450558408a552ba4910ffefa13bcd8f814ad







DEVELOPING A HIGH THERMAL CONDUCTIVITY NUCLEAR FUEL
WITH SILICON CARBIDE ADDITIVES























By

JIWEl WANG


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2008
































O 2008 Jiwei Wang


































To my parents, Li Wang and Shaofen Liu, for their love and support.









ACKNOWLEDGMENTS

I would like to thank Professor James Tulenko, my advisor and the supervisory committee

chair, for his guidance and support. I would also like to thank Dr. Ronald Baney, my supervisory

committee co-chair, for his patience and instruction. Without their help, this work would not

have been possible.

I would like to thank Dr. Samim Anghaie for being on my supervisory committee and his

kindness for providing the equipment at the Innovative Nuclear Space Powder and Propulsion

Institute. I would also like to thank my supervisory committee member Dr. Edward Dugan for

his help on the computer codes.

I would like to thank Shirvan Daryoosh at Innovative Nuclear Space Powder and

Propulsion Institute for his help on the experiment equipment. I would also like to thank

Department of energy for the funding of this study. Finally, I would like to thank my family for

their consistent support through the many years of my education.












TABLE OF CONTENTS


page

ACKNOWLEDGMENT S ................. ...............4....___ ......


LI ST OF T ABLE S ............ ............ ...............7...


LIST OF FIGURES .............. ...............8.....


LIST OF ABBREVIATIONS ................. ...............13.............


AB S TRAC T ............._. .......... ..............._ 14...


CHAPTER


1 INTRODUCTION ................. ...............16.......... ......


2 LITERATURE REVIEW .............. ...............20....


Properties of Uranium Dioxide ................. ...............20................
Properties of Silicon Carbide ................. ...............22................

3 NEUTRONIC CALCULATION................. .............3


Introducti on ............. ...... ._ ...............30...
Methods and Results ............. ...... .__ ...............30..
Discussion ............. ...... ._ ............... 1...


4 REACTION BETWEEN URAINIUM OXIDE AND SILICON CARBIDE ......................50


Background .............. .... .._ ...............50....
Experiments and Results................. .......... ..... .......5
Oxygen to Uranium Ratio of Uranium Oxide Powder ......____ ........._ ...............50
Particle Size Distribution of Uranium Oxide Powder ................. ................. ...._.5 1
Sintering UO2 and SiC at 1300 oC and 1650 oC ....._._.__ .... .._.... ......._.._.....5
Discussion ........._._... ......... ...............52....


5 LOW TEMPERATURE SINTERING OF URANIUM DIOXIDE ................. ................. 60


Back ground ............... .... ...............60.......... .....
Experiments and Results............... ...............61
Discussion ................. ...............62.................


6 SILICON CARBIDE COATING BY CHEMICAL VAPOR DEPOSITION .......................68


Back ground ............. .. ... ...............68...
Experiment and Result............... ...............69.
Discussion ............. ..... ...............70...












7 SILICON CARBIDE COATINTG FROM PRECERAMIC POLYMER ............... .... ........._..75


Back ground ............... ......._ ...............75....
Experiments and Results............... ...............75
Discussion ............ ..... .._ ...............76...


8 SILICON CARBIDE WHISKERS URANIUM DIOXIDE COMPOSITE ......................81


Back ground ............... .... ...............8. 1..............
Experiments and Results........................ ..................8
Characterization of Silicon Carbide Whiskers .............. ...............82....
Mixing of SiC Whiskers and Uranium Oxide Powder ................. .. .......................83
Pressureless Sintering of SiC Whiskers and Uranium Oxide Powder ............................83
Hot Press Sintering of SiC Whiskers and Uranium Oxide Powder ............... .... ........._..83
Discussion ............ ..... .._ ...............85...


9 CONCLUSIONS AND FUTURE WORK ....__ ......_____ .......___ ...........11


Conclusion ............ _...... ._ ...............111...
Future W ork ............ ..... .._ ...............112...


APPENDIX


A CASMO INPUT FILES ................. ...............113...............


B HOT PRES S SINTERING TEMPERATURE CALCULATION ................. ................ .. 126


LIST OF REFERENCES ................. ...............128................


BIOGRAPHICAL SKETCH ................. ...............132......... ......










LIST OF TABLES


Table page

2-1 Neutronic cross sections (bamns). ............. ...............26.....

3-1 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel. .........................34

3-2 Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel. ............. ...............34.....

3-3 MTC (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC fuel. ................... ..3 5

3-4 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature). .............. ...............3 5....

3-5 Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel (200 oC less in fuel temperature). ............. ...............36.....

3-6 MTC (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC
less in fuel temperature) ................. ...............36................

3-7 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature) at boron let-down. ............. ...............37.....

3-8 K-infinity versus burnup of UO2 COre compared to UO2 with SiC core (200 oC less in
fuel temperature) at boron let-down. ............. ...............37.....

3-9 Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel (200 oC less in fuel temperature) at boron let-down ................. ................ ...._.38

3-10 Doppler coefficient (pcm/K) versus burnup of UO2 COre compared to UO2 with SiC
core (200 oC less in fuel temperature) at boron let-down. ................ ..................3

3-11 MTC (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC
less in fuel temperature) at boron let-down. ............. ...............39.....

3-12 MTC (pcm/K) versus burnup of UO2 COre compared to UO2 with SiC core (200 oC
less in fuel temperature) at boron let-down. ............. ...............39.....

3-13 Centerline temperature of UO2 fuel COmpared to UO2 with SiC.................. ................ .40

4-1 Particle size distribution of received uranium oxide powder. .........._.... .......__... ........53

8-1 Densities of the pellets of UO2 and SiC whiskers after hot press sintering. ........._............87










LIST OF FIGURES


Figure page

2-1 Thermal conductivity of UO2............... ...............27.

2-2 Thermal conductivity of hyperstoichiometric UO2. ................ .............................27

2-3 Thermal conductivity of (Uo.sPu0.2)Ox. ............. ...............28.....

2-4 Thermal conductivity of UO2 before and after irradiation..........._._... ......._.. ..........28

2-5 Thermal conductivity of single crystal SiC and polycrystalline SiC compared to UO2....29

2-6 Thermal conductivity of SiC before and after irradiation compared to UO2. ................... .29

3-1 Crystal River 15X15 assembly design.. ............. ...............41.....

3-2 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel. ................... .........41

3-3 Doppler coefficient versus burnup of UO2 fuel COmpared to UO2 with SiC fuel. .............42

3-4 Moderator temperature coefficient versus burnup of UO2 fuel COmpared to UO2 with
SiC fuel. ............. ...............42.....

3-5 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel. ................... .........43

3-6 Doppler coefficient versus burnup of UO2 fuel COmpared to UO2 with SiC fuel. .............43

3-7 Moderator temperature coefficient versus burnup of UO2 fuel COmpared to UO2 with
SiC fuel. ............. ...............44.....

3-8 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature). .............. ...............44....

3-9 Doppler coefficient versus burnup of UO2 fuel COmpared to UO2 with SiC fuel
(200 oC less in fuel temperature). ............. ...............45.....

3-10 Moderator temperature coefficient versus burnup of UO2 fuel COmpared to UO2 with
SiC fuel (200 oC less in fuel temperature). ............. ...............45.....

3-11 Boron concentration versus burnup at boron let-down ......... ................. ...............46

3-12 K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature) at boron let-down. ............. ...............46.....

3-13 K-infinity versus burnup of UO2 COre compared to UO2 with SiC core (200 oC less in
fuel temperature) at boron let-down. ............. ...............47.....










3-14 Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel (200 oC less in fuel temperature) at boron let-down ................. ................ ...._.47

3-15 Doppler coefficient (pcm/K) versus burnup of UO2 COre compared to UO2 with SiC
core (200 oC less in fuel temperature) at boron let-down. ............. .....................4

3-16 Moderator temperature coefficient (pcm/K) versus burnup of UO2 fuel COmpared to
UO2 with SiC fuel (200 oC less in fuel temperature) at boron let-down............................48

3-17 Moderator temperature coefficient (pcm/K) versus burnup of UO2 COre compared to
UO2 with SiC core (200 oC less in fuel temperature) at boron let-down. ..........................49

4-1 Uranium oxide powders with different O/U ratio. A) UO2.10. B) UO2.27. C) U30s. D)
U O 2. ............. ...............54.....

4-2 X-ray diffraction pattern of UO2.10 pOwder. ................ ...............54........... ..

4-3 X-ray diffraction pattern of UO2.27 pOwder. ................ ...............55........... ..

4-4 X-ray diffraction pattern of U30s powder. ............. ...............55.....

4-5 X-ray diffraction pattern of UO2.0 pOwder ................. ...............56........... ..

4-6 Sieve and shaker for analyzing particle size distribution ................. ................. ...._56

4-7 Particle size distribution of received uranium oxide powder ................. .....................57

4-8 X-ray diffraction pattern of 30Onm P-SiC from Alfa Aesar ................. ........___..........57

4-9 Uranium dioxide-silicon carbide pellet after sintering at 1300 oC .............. ..................58

4-10 X-ray diffraction pattern of UO2-SiC pellet after sintering at 1300 oC ................... .........58

4-11 Uranium dioxide-silicon caribide pellet after sintering at 1650 oC .............. ..................59

4-12 X-ray diffraction pattern of UO2-SiC pellet after sintering at 1650 oC ................... ..........59

5-1 Oxygen to uranium ratio of UO2 pOwder oxidized in air at 140 oC. ............. .... ............64

5-2 Uranium dioxide pellet sintered at 1650 oC. .............. ...............64....

5-3 Scanning electron microscopy image of UO2 pellet sintered at 1650 oC (5,000X). ..........65

5-4 Uranium dioxide pellet sintered at 1200 oC. .............. ...............65....

5-5 Scanning electron microscopy image of UO2 pellet sintered at 1200 oC (5,000X). ..........66

5-6 Uranium dioxide pellet sintered at 1200 oC with pressure. ............. .....................6











5-7 Scanning electron microscopy image of UO2 pellet sintered at 1200 oC with pressure
(5,000X ). .............. ...............67....

6-1 Lindberg high temperature furnace ................. ...............72........... ...

6-2 Temperature profile of the CVD process. ......___ ... ................. ..........7

6-3 Uranium oxide powder after CVD process ................. ...............73......__. ..

6-4 Fourier transform infrared spectroscopy result of the powder after the CVD process......73

6-5 X-ray diffraction result of the powder after CVD process. ................ ............ .........74

6-6 Fumnace tube after CVD process. .............. ...............74....

7-1 Allylhydridopolycarbosilane (AHPCS), the SiC pre-ceramic polymer. ..........................78

7-2 Lindberg/Blue Mini-Mite 1100 oC furnace. .........._._ ........... ........__.........7

7-3 The temperature profile of sintering process. ............. ...............79.....

7-4 Silicon carbide pellet made by SiC powder (1 C) and AHPCS. ............. ....................79

7-5 Scanning electron microscopy image of the SiC pellet made by SiC powder (1 CL) and
AHP CS ........... __..... ._ ...............80....

8-1 Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar
(500X ) ................ ...............88.......... ......

8-2 Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar
(2,000X) ................. ...............88.......... .....

8-3 X-ray diffraction pattern of SiC whiskers from Alfa Aesar .............. ....................8

8-4 Scanning electron microscopy image of SiC whiskers as received from Advanced
Composite Material s (500X)............... ...............89.

8-5 Scanning electron microscopy image of SiC whiskers as received from Advanced
Composite Material s (2,000X)............... ...............90

8-6 X-ray diffraction pattern of SiC whiskers from Advanced Composite Materials .............90

8-7 Scanning electron microscopy image of SiC whiskers from Alfa Aesar after
dispersion (2,000X)............... ...............91

8-8 Scanning electron microscopy image of SiC whiskers from Advanced Composite
Materials after dispersion (2,000X) .............. ...............91....

8-9 The pellet of UO2 with 5 vol% SiC whiskers after pressureless sintering. ................... .....92










8-10 Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after
pressureless sintering, (2,000X)............... ...............92

8-11 Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after
pressureless sintering, (5,000X)............... ...............93

8-12 The pellet of UO2 with 10 vol% SiC whiskers after pressureless sintering. ................... ...93

8-13 Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after
pressureless sintering, (2,000X)............... ...............94

8-14 Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after
pressureless sintering, (5,000X)............... ...............94

8-15 Alumina die for hot press sintering ................. ...............95........... ..

8-16 Alumina die, graphite tube and sample holder. ............. ...............95.....

8-17 Geometry of the alumina die and graphite tube ................. ...............96........... .

8-18 Heated graphite tube observed through the optical pyrometer. .............. ....................96

8-19 Hot press sintering apparatus. .............. ...............97....

8-20 The pellet of UO2 with 5 vol% SiC whiskers after hot press sintering. ................... ..........98

8-21 Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after hot
press sintering, (2,000X) ................. ...............98........... ....

8-22 Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after hot
press sintering, (5,000X) ................. ...............99........... ....

8-23 X-ray diffraction result of UO2 with 5 vol% SiC whiskers after hot press sintering. ........99

8-24 The pellet of UO2 with 10 vol% SiC whiskers after hot press sintering...............__.. ......100

8-25 Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after hot
press sintering, (2,000X) ................. ...............100................

8-26 Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after hot
press sintering, (5,000X) ................. ...............101................

8-27 X-ray diffraction result of UO2 with 10 vol% SiC whiskers after hot press sintering.....101

8-28 The pellet of UO2 with 15 vol% SiC whiskers after hot press sintering. ................... ......102

8-29 Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers after hot
press sintering, (2,000X) ................. ...............102................










8-30 Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers after hot
press sintering, (5,000X) ................. ...............103................

8-3 1 X-ray diffraction result of UO2 with 15 vol% SiC whiskers after hot press sintering.....103

8-32 The pellet of UO2 (<25 CL) with 15 vol% SiC whiskers after hot press sintering. ...........104

8-33 Scanning electron microscopy image of UO2 (<25 CL) with 15 vol% SiC whiskers
after hot press sintering, (2,000X). ............. ...............104....

8-34 Scanning electron microscopy image of UO2 (<25 CL) with 15 vol% SiC whiskers
after hot press sintering, (5,000X). ............. ...............105....

8-35 X-ray diffraction result of UO2 (<25 CL) with 15 vol% SiC whiskers after hot press
sintering ................. ...............106................

8-36 The pellet of UO2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press
sintering ................. ...............107................

8-37 Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers (from Alfa
Aesar) after hot press sintering, (2,000X) ................. ...............107..............

8-38 Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers (from Alfa
Aesar) after hot press sintering, (5,000X) ................. ...............108..............

8-39 X-ray diffraction result of UO2 with 15 vol% SiC whiskers (from Alfa Aesar) after
hot press sintering. ............. ...............109....

8-40 Scanning electron microscopy image of the cross section of UO2 with 30 vol% SiC
whiskers after hot press sintering, (2,000X). ................ ...............110..............

8-41 Scanning electron microscopy image of the cross section of UO2 with 30 vol% SiC
whiskers after hot press sintering, (5,000X). ................ ...............110..............









LIST OF ABBREVIATIONS


AC Alternating current

AHPCS Al lyl hy dri dop oly carb osilane

BOL Beginning of life

CVD Chemical vapor deposition

DSC Differential scanning calorimetry

EOC End of cycle

EOL End of life

FTIR Fourier transform infrared spectroscopy

LOCA Lost of coolant accident

MTC Moderator temperature Coefficient

MTS Methyltri chl oro silane

PACVD Photo assisted chemical vapor deposition

PECVD Plasma enhanced chemical vapor deposition

SCB Silacyclobutane

SEM Scanning electron microscopy

SiC Silicon carbide

TD Theoretical Density

TMS Trimethylsilane

XRD X-ray diffraction

UO2 Uranium dioxide









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

DEVELOPING A HIGH THERMAL CONDUCTIVITY NUCLEAR FUEL
WITH SILICON CARBIDE ADDITIVES

By

Jiwei Wang

August 2008

Chair: James Tulenko
Maj or: Nuclear Engineering Sciences

Uranium dioxide (UO2) is the most common fuel material utilized in commercial nuclear

power reactors. The main disadvantage of UO2 is its low thermal conductivity. During a reactor' s

operation, there is a large temperature gradient in the UO2 fuel pellet, causing a very high

centerline temperature, and introducing thermal stresses, which lead to extensive fuel pellet

cracking. These cracks will add to the release of Eission product gases after high burnup. The

high fuel operating temperature also increases the rate of Eission gas release and the fuel pellet

swelling caused by both Hission gases bubbles and thermal expansion. The amount of Eission gas

release and fuel swelling limits the life time of UO2 fuel in reactor.

The obj ective of this research is to increase the thermal conductivity of UO2 while not

significantly affecting the neutronic property of UO2. The concept is to incorporate another high

thermal conductivity material, silicon carbide (SiC), into the UO2 pellet. Silicon carbide is

expected to form a conductive percolation pathway in the UO2 for heat to flow out of the fuel

pellet, thus increasing the UO2 thermal conductivity.

Three methods were studied to incorporate SiC into UO2. Firstly, chemical vapor

deposition (CVD) process was used to coat UO2 particles with a SiC layer prior to the low

temperature sintering process of UO2. Secondly, allylhydridopolycarbosilane (AHPCS), a pre-









ceramic polymer, was used to generate a SiC coating on UO2 particles prior to the low

temperature sintering process of UO2. Thirdly, silicon carbide whiskers were mixed with UO2

particles prior to the low temperature sintering method of UO2.

Though pellets with high density were not achieved by the first and second methods,

pellets of 95% TD were achieved by pressureless sintering of UO2 with 5 vol% SiC whiskers and

hot press sintering of UO2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. The thermal

conductivity of the pellets will be measured at Idaho National Laboratory. The thermal

conductivity data will not be discussed in this dissertation.









CHAPTER 1
INTTRODUCTION

Uranium dioxide (UO2) is the most common fuel material in commercial nuclear power

reactors. UO2 has the advantages of a high melting point, good high-temperature stability, good

chemical compatibility with cladding and coolant and resistance to radiation. The main

disadvantage of UO2 is its low thermal conductivity. During a reactor' s operation, because the

thermal conductivity of UO2 is Very low, for example, about 2.8 W/m-K at 1000 oC [1], there is a

large temperature gradient in the UO2 fuel pellet, causing a very high centerline temperature and

introducing thermal stresses, which lead to extensive fuel pellet cracking. These cracks will add

to the release of fission product gases after high burnup. The high fuel operating temperature also

increases the rate of Eission gas release and the fuel pellet swelling caused by fission gases

bubbles. The amount of Eission gas release and fuel swelling limits the life time of UO2 fuCI in a

reactor.

The obj ective of this research is to increase the thermal conductivity of UO2 while not

significantly affecting the neutronic property of UO2. The concept is to incorporate another

material with high thermal conductivity into UO2. It has been reported that a 50% increase in the

thermal conductivity of UO2 has been achieved by adding 5 w% molybdenum (Mo) at 1000 oC

[2], however, Mo has a large thermal neutron absorption cross section, which negates its use in

thermal reactors. It has also been reported that a 25% increase in the thermal conductivity of UO2

has been achieved by adding 1.2 w% BeO [3], however, BeO is a very toxic material for

handling.

Silicon carbide (SiC) is chosen in this research because the thermal conductivity of single

crystal SiC is sixty times higher than that of UO2 at room temperature and thirty times higher at









800 oC [4]. Silicon carbide also has the advantage of a low thermal neutron absorption cross

section, high melting point, good chemical stability and good irradiation stability [5].

The neutronic properties of UO2 will be affected by the addition of the SiC whiskers. The

effect of SiC whiskers on the neutronic property of a UO2 pellet was simulated by CASMO-3

[6], a multi-group two-dimensional transport theory code. The CASMO-3 result set the limit of

the amount of SiC additives that could be added to UO2 without significantly affecting the

neutronic property of UO2.

Several studies have shown that SiC whiskers can increase the thermal conductivity of

matrix materials. Russell et al. [7] reported the thermal conductivity of 30 vol% VLS SiC

whisker-mullite composite was three times higher at room temperature than that of single phase

mullite in the perpendicular direction to the hot pressing direction, and two times higher in the

parallel direction to the hot pressing direction. Johnson et al. [8] reported that the thermal

conductivity of 30 vol% SiC whisker-osumilite glass composite was four times higher at room

temperature than that of single phase mullite in the perpendicular direction to the hot pressing

direction, and two times higher in the parallel direction to the hot pressing direction. Hesselman

et al. [9] showed the thermal conductivity of 30 vol% VLS SiC whisker-lithium aluminosilicate

glass composite was five times higher at room temperature than that of lithium aluminosilicate

glass in the perpendicular direction to the hot pressing direction, and three times higher in the

parallel direction to the hot pressing direction. Hesselman et al. [9] suggested that a SiC whisker

"percolation" pathway was formed, and heat was conducted through SiC whiskers, bypassing the

matrix.

Solomon et al [10, 11i] have tried to increase the thermal conductivity of UO2 by

impregnating a SiC pre-ceramic polymer into a high dense UO2 pellet with 10 to 12 vol% open









porosity and transferring the SiC pre-ceramic polymer to a crystalline form at 1300 oC. There

was no improvement in the thermal conductivity of UO2 because the maximum density of SiC

was 75% TD even after several impregnation, which due to the blockage of the open porosity by

the SiC during the impregnation and the low crystallization temperature.

Uranium dioxide pellets are usually produced by sintering the green pellets at about 1700

oC in hydrogen atmosphere. The high sintering temperature around 1700 oC is necessary to

achieve the required high density of 95% TD. However, based on the study by Allen et al. [12],

UO2 reacts with SiC at the temperature above 1377 oC, evolving CO and SiO, forming uranium

carbide, uranium silicide and U3 3Si2. A two-stage low temperature sintering method of UO2

was studied to avoid the reaction between UO2 and SiC. Fuhrman et al. [13] have reported that

UO2 pellets of 95% to 97% theoretical density (TD) were achieved by sintering at 1200 oC in

nitrogen for one hour followed by one hour reduction in hydrogen, using a uranium oxide

powder with extra oxygen (O/U ~ 2.37). Langrod [14] has also achieved UO2 pellets of above

95% TD using mixture of UO2 and U30s (O/U ~ 2.30) by sintering at 1300 oC in argon or

nitrogen atmosphere for two hours followed by reduction in hydrogen.

Three methods were studied to incorporate SiC into UO2. Firstly, a chemical vapor

deposition (CVD) process was used to coat UO2 particles with a SiC layer prior to the low

temperature sintering process. Secondly, a SiC pre-ceramic polymer,

allylhydridopolycarbosilane (AHPCS), was used to coat UO2 particles prior to the low

temperature sintering process. Thirdly, SiC whiskers were mixed with UO2 particles prior to the

low temperature sintering process.

Though pellets with high density were not achieved by the first and second methods,

pellets of 95% TD were achieved by pressureless sintering of UO2 with 5 vol% SiC whiskers and









hot press sintering of UO2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. The thermal

conductivity of the pellets will be measured at Idaho National Laboratory. The thermal

conductivity data will not be discussed in this dissertation.









CHAPTER 2
LITERATURE REVIEW

Properties of Uranium Dioxide

The properties of uranium dioxide (UO2) have been investigated for decades. UO2 has a

cubic fluorite (CaF2) type crystal structure with a lattice parameter of 0.5468nm [15]. The

theoretical density of UO2 is 10.96 g/cm3 [16]. The melting point of UO2 is about 2850 oC [17].

UO2 alSo has the advantages of good high temperature stability, good chemical compatibility

with cladding and coolant and resistance to radiation [18].

The thermal conductivity is one of the most important properties of UO2, because it

determines the fuel temperature, thus directly affect the behavior and performance of fuel pellet

in a reactor. Based on the experiment data, Fink [1] pointed out that the thermal conductive of

95% dense UO2 can be calculated by Equation 2-1, which is plotted in Figure 2-1.

100 6400 -16.35
k = + exp( )(2-1)
7.5408 + 17.692t + 3.6142t 2 t5/2

Where t = T (K)/1000 and k is the thermal conductivity of 95% dense UO2 in W/m-K.

Uranium dioxide is easily oxidized in air. The uranium oxide with O/U ratio greater than

2.0 is called hyperstoichiometric UO2; the uranium oxide with O/U ratio less than 2.0 is called

hypostoichiometric UO2. The oxidation of UO2 in air is a two-step reaction:

UO29U307/U4099U30s. The intermediate oxidation products, U409 and U307, are derivatives

of the fluorite structure in which clusters of interstitial oxygen atoms are centered on unoccupied

cubic sites in the lattice, with accompanying displacement of neighboring U atoms [19]. U30s

has an orthorhombic lattice structure [20]. The density of U30s is 8.38 g/cm3, which is 24% less

than UO2. The density decrease results in an undesirable volume increase in the fuel.









The thermal conductivity of hyperstoichiometric UO2 [21]is shown in Figure 2-2. The

excess oxygen atoms act as phonon scattering centers, thus reduce the thermal conductivity of

UO2. The thermal conductivity of (Uo~sPu0.2)Ox (x < 2.0) [22] is shown in Figure 2-3. The

defects in the UO2 CryStal lattice also act as phonon scattering centers, thus reduce the thermal

conductivity of UO2. In addition, hypostoichiometric UO2 COuld contain uranium metal which

could be highly reactive with other materials; hyperstoichiometric UO2 may have an oxygen

partial pressure sufficient to cause interaction with other materials [23]. The O/U ratio of an

unknown UO2.x pOwder can be determined by measuring the weight difference of UO2.x and

U30s oxidized by UO2.x, Or UO2.x and UO2 reduced by UO2.x (fOr hyperstoichiometric UO2).

The thermal conductivity of irradiated UO2 is affected by the changes that take place in the

fuel during irradiation. During irradiation, fission products accumulate in the UO2 matrix and

cause the fuel swelling. The fission products dissolved in the UO2 lattice serve as phonon

scattering centers, thus reduce the thermal conductivity of UO2 fuel; the precipitated fission

products have a much higher thermal conductivity than UO2 and have a positive contribution to

the thermal conductivity of UO2 fuel. The fission product gases initially form in irradiated fuel as

dispersed atoms within the UO2 lattice, then form small bubbles. The small bubbles within the

UO2 lattice also serve as phonon scattering centers, thus reduce the thermal conductivity [24]. At

temperature below 1000 oC, uranium dioxide retains essentially all the fission gases, but above

this temperature the gases are released, little remains in those region of the fuel where the

temperature exceeds 1800 oC [18]. Radiation damage from neutrons, a-decay and fission

products increase the number of lattice defects and consequently reduces the thermal

conductivity of UO2 fuel. The thermal conductivities of UO2 before and after irradiation are

shown in Figure 2-4 [24]. The radiation-induced decrease in the thermal conductivity of UO2 is









large at low temperature. Oxygen and uranium defects are known to anneal at around 500 K and

1000 K, respectively. This explains why most changes in the thermal conductivity of UO2 aef

seen below 1000 K [24].

Properties of Silicon Carbide

Silicon carbide (SiC) is a ceramic compound of silicon and carbon, which was discovered

by Edward Goodrich Acheson around 1893. Several polytypes of SiC exist, the most common

polytype is a-SiC (6H-SiC) and the cubic form is P-SiC (3C-SiC). Only P-SiC is considered in

this research because of its property of isotropic expansion when heated. The density of P-SiC is

3.21 g/cm^'3 [25]. The melting temperature of P-SiC is about 2700 oC [5]. Silicon carbide has

low thermal neutron absorption cross section. The neutron cross-section for silicon, carbon and

other relative nuclides are shown in Table 2-1 [26].

One of the most attractive properties of SiC is its high thermal conductivity. The thermal

conductivity of single crystal SiC measured by Slack at room temperature is 490 W/m-K [4],

which is higher than copper, 398 W/m-K. The thermal conductivity of polycrystalline P-SiC by

CVD process is lower, about 70 W/m-K at room temperature [27]. Figure 2-5 shows the thermal

conductivities of single crystal SiC and polycrystalline P-SiC versus temperature. The single

crystal SiC has higher purity and less defects than the polycrystalline P-SiC. The boundaries in

the polycrystalline P-SiC can also serve as phonon scattering centers. Figure 2-6 shows the

thermal conductivity of polycrystalline P-SiC before and after irradiation [28]. The thermal

conductivity of unirradiated SiC decreases with increasing temperature. The thermal

conductivity of SiC is mainly controlled by the lattice vibration waves (Phonons). The phonon-

phonon scatterings increase with increasing temperature, which decrease the phonon mean free

path and consequently decrease the thermal conductivity.









The thermal conductivity of SiC decreases by a factor between 3 and 9 at room

temperature when SiC was irradiated with fast neutrons fluence of 2.7-7.7x 1021 H/CM2 (E>0. 18

MeV) at 550 oC-1100 oC [28]. During fast neutron irradiation, point defects are introduced in the

SiC lattice structure. These defects act as the scattering centers for phonons, so the thermal

conductivity is sharply reduced as a consequence. The phonon mean free path is determined by

the mean free path of defects, so the thermal conductivity is almost independent of temperature

after irradiation.

Amorphous SiC was studied by Snead [29] using transmission electron microscopy (TEM)

after irradiation of P-SiC with fast neutron of 2.6 x 1025 H/m2 (E>0. 1 MeV) at 53 oC. The thermal

conductivity of amorphous SiC is only 5.4 W/m-K at 800 oC, only slightly higher than UO2. The

annealing effect was also observed by Snead. The SiC crystallites grow slowly in the ~800-1100

oC temperature range and that the crystallite growth rate was approximately linear with annealing

temperature; the SiC crystallites grow rapidly in the temperature range of 1100-1150 oC, with

both faster growth of the existing crystallites and rapid nucleation of new crystallites throughout

the amorphous material. No amorphous SiC is found after annealing at 1150 oC for 30 min [29].

Silicon carbide has good chemical stability because of the protective silicon oxide (SiO2)

layer formed on it. Silicon carbide is not attacked by any acids or alkalis or molten salts at room

temperature. Verral reported that the SiC lost 2% at 573 K in deoxygenated water of pH 10.3

after 90 days and less than 2% at 573K in deoxygenated water of pH 3 after 32 days [5].

Hirayama also reported the weight loss increased with increasing pH value. The weight loss in

the oxygenated solution was more than that in the deoxygenated solution [3 0]. Verral reported

that there was no significant interaction between SiC and Zircaloy-4 at 1273 K. At 1773 K there









was a diffusion-based reaction to form ZrC and free SiC, but the SiC Zircaloy cladding

interaction was no worse than the UO2-Zircaloy cladding interaction [5].

Silicon carbide has been used as a layer in tristructral isotropic (TRISO) fuel particles for

gas cooled nuclear reactors. There are four layers on the spherical UO2 particle of TRISO fuel: a

porous pyrolytic carbon buffer layer, a dense pyrolytic carbon layer, a SiC layer and another

pyrolytic carbon layer. The SiC layer acts as a diffusion barrier to fission products and a pressure

vessel of the particle [31i]. Silicon carbide has shown the capability to maintain its properties

under high irradiation and temperature conditions. There is a slightly expansion at the fluences

up to 5 x1026n/m2 and irradiation temperature below 1000 oC; at higher temperature, irradiation

creates voids that cause continuing expansion, but the structural integrity is not affected. The

irradiation has a negligible effect on the strength of SiC [27].

Silicon carbide has also been used as electronic devices operating under extreme

conductions of power and temperature because of its wide band gap, high breakdown electric

field strength, high saturated electron drift velocity and high thermal conductivity. Because of

high purity requirement of the semiconductor device, the SiC is usually produced by a chemical

vapor deposition process.

Silicon carbide is commonly manufactured by combining silica sand (Sio2) and carbon at

high temperature, between 1600 oC to 2500 oC. The purity of the SiC crystals produced is

relatively low compared to the more expensive chemical vapor deposition (CVD) process.

Pre-ceramic polymers can also used to produce crystalline silicon carbide.

Allylhydridopolycarbosilane (AHPCS) was successfully converted to crystalline P-SiC at 1600

oC by Zheng et al. [32]. Silicon carbide fibers were also produced from the pre-ceramic polymer

[33]. The fibers were used to improve the mechanical properties of the matrix materials.









Besides SiC fibers, SiC whiskers were also produced to improve the mechanical properties

of the matrix material. The two main methods to produce SiC whiskers are Rice Hull method and

vapor-liquid-solid (VLS) method. The SiC whiskers are single crystal. The strengths, Young's

module and thermal conductivity can be extremely high. The SiC whiskers were not only used to

increase the mechanical properties [34, 3 5], but also the thermal conductivity of the matrix

materials [7-9].










Table 2-1. Neutronic cross sections (barns) [26].
Gabs(th) as(th) as(epi)
C 0.0032 4.8 4.66
Si 0.00019 4.2 3.7
O 0.13 5 3.4
Zr 0.18 8 6.2
Be 0.01 7 6.11
Mo 2.5 7 6.0














8-













2 -

1
0 0 0 0 20 10 10 10 20 70 30
Teprtr K

Fiue 21 hra odciiyo O


1.5
400


600 800 1000 1200 1400
Temperature (K)


1600 1800 2000


Figure 2-2. Thermal conductivity of hyperstoichiometric UO2 [21].





































1000 1500 2000
Temperature (K)


2500


Figure 2-3. Thermal conductivity of (Uo~sPu0.2)Ox [22].


W
5


~T~q
t:

3
Q

E


800 1000 1200 1400 1600
Temperature (K)


1800


Figure 2-4. Thermal conductivity of UO2 before and after irradiation [24].











































I I I I I I


500



400-




300-



~j200-



100-



0
300


400 500 600 700
Temperature (K)


Figure 2-5. Thermal conductivity of single crystal SiC and polycrystalline SiC compared to UO2
[1], [4], [27].


I l


l I


l l


Unirradiated SiC
- -~ Irradiated SiC at
Irradiated SiC at
UO 2


11000C
5500C


60


50


30-


20-


10 -


~


I I


I I


0 100 200 300 400
Temperature (K)


500 600 700


Figure 2-6. Thermal conductivity of SiC before and after irradiation compared to UO2 [1], [27],
[28].









CHAPTER 3
NEUTRONIC CALCULATION

Introduction

The effect of silicon carbide (SiC) additives on the neutronic properties of uranium dioxide

(UO2) Should be studied before the experiments. CASMO-3 [6], a multi-group two-dimensional

transport theory code, was used to simulate the burnup of UO2 fuel and UO2 fuCI With SiC

additives. The simulation utilized a Framatome Mark-B 15X15 assembly design. The power of

the assembly is 14.37 MW. There are 208 fuel rods, 16 guide tubes and 1 instrument tube per

assembly. The cross section view of Mark-B assembly is shown in Figure 3-1. The enrichment of

the fuel is 4.66%.

Methods and Results

The K-infinity versus burnup of UO2 fuel COmpared to UO2 with 5 w% and 10 w% SiC is

shown in Table 3-1 and Figure 3-2. The K-infinity of UO2 fuel is less than that of UO2 with 5

w% and 10 w% SiC at the beginning of life (BOL) because SiC replaces uranium-23 8, which has

a large resonance absorption cross section. As the fuel burns up, the K-infinity of UO2 fuCI

decreases slower than that of UO2 with 5 w% and 10 w% SiC because the thermal utilization

factor in UO2 decreases slower than in UO2 with SiC. At the end of life (EOL), 60 GWD/MTU,

the K-infinity ofUO2 with 5 w% SiC and 10 w% SiC are about 7% and 14% less than the K-

infinity of UO2 fuel, TOSpectively. The Doppler coefficient and moderator temperature coefficient

(MTC) versus burnup of UO2 fuel COmpared to UO2 with 5 w% and 10 w% SiC are shown in

Table 3-2, Table 3-3, Figure 3-3 and Figure 3-4. The Doppler coefficient of UO2 fuel is less than

that of UO2 with 5 w% and 10 w% SiC at BOC, but larger than that of UO2 with SiC at the end

of cycle (EOC). The MTC of UO2 fuel is less than UO2 with SiC throughout the burnup. All the

Doppler coefficients and MTCs of UO2 fuel and UO2 with SiC fuel are negative.









Because volume percentage is always used when considering SiC whiskers-matrix

composite, the K-infinity versus burnup of UO2 fuel COmpared to UO2 with 5 vol%, 10 vol% and

15 vol% SiC is shown in Table 3-1 and Figure 3-5. The 5 vol% SiC is equal to 1.5 w% SiC, the

10 vol% SiC is equal to 3.2% SiC and 15 vol% SiC is equal to 4.9% SiC. At 60 GWD/MTU, the

K-infinity of UO2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2. 1%, 4.5% and 7.2% less

than the K-infinity of UO2 fuel. The Doppler coefficient and moderator temperature coefficient

(MTC) versus burnup of UO2 fuel COmpared to UO2 with 5 vol%, 10 vol% and 15 vol% SiC are

shown in Table 3-2, Table 3-3, Figure 3-6 and Figure 3-7.

Discussion

In this research, the limit of the amount of SiC additives was set to 5 w% to limit the

change of K-infinity at EOL to 7 %. Because the SiC additives will affect the thermal

conductivity of the fuel, thus affecting the fuel temperature, the effect of fuel temperature change

on the neutronic properties of the fuel has to be considered.

Assuming there was 50% increase in thermal conductivity of UO2 with SiC additives

compared to UO2 fuel, the centerline temperature of the fuel rod was calculated according to

Equation 3-1. The results are shown in Table 3-13. The centerline temperature of UO2 with SiC

was 275 oC lower than the centerline temperature of UO2 fuC1.


T; -, = [ r, +-+ c+ rf ] (3-1)
at "27~rf 2kf hg ke h, (rf + tc)

Where Tci is the fuel centerline temperature (K)

T,; is the moderator temperature (K)

q 'is the linear power density (W/cm)

rf is the radius of fuel pellet (cm)

kf is the average thermal conductivity of fuel (W/(cm*K))









h, is the gap heat transfer coefficient (0.5---1.1) (W/(cm^'2*K))

kc is the thermal conductivity of cladding (W/(cm*K)

h, is the coefficient of convective heat transfer (2.8---4.5) (W/(cm^`2*K)

to is the clad thickness (cm)

Based on the centerline temperature calculation, a 200 oC decrease in fuel temperature of

UO2 with SiC was assumed in the CASMO-3 calculation. (Different amount of SiC will change

the fuel temperature differently. In this research, the changes of fuel temperatures of UO2 with

different amount of SiC were assumed to be the same, 200 oC, for simplicity) The K-infinity

versus burnup of UO2 fuel COmpared to UO2 with 5 vol%, 10 vol% and 15 vol% SiC, which are

200 oC lower in fuel temperature, is shown in Table 3-4 and Figure 3-8. At 60 GWD/MTU, the

K-infinity ofUO2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2.4%, 4.9% and 7.6% less

than the K-infinity of UO2 fuel. The differences in K-infinity are larger than that of UO2 with

SiC without considering the temperature change. The Doppler coefficient of UO2 with SiC is less

than UO2, aS shown in Table 3 -5 and Figure 3-9. The MTC of UO2 with SiC is still larger than

UO2, aS shown in Table 3-6 and Figure 3-10. All the Doppler coefficients and MTC of UO2 fuCI

and UO2 with SiC fuel are negative.

The effect of SiC additives on the neutronic properties of UO2 fuel at boron let down

situation was also studied. Considering a core with three batches (equal amount of fuel), a batch

of fresh fuel, a batch of fuel burned once and a batch of fuel burned twice. The power

distribution of the three batches was 1.25:1:0.75. In steady state, the cycle burnup of batch one

was 25 GWD/MTU; the cycle burnup of batch two was 20 GWD/MTU; and the cycle burnup of

batch three was 15 GWD/MTU. The boron concentration is shown in Figure 3-11. During boron

let down, the boron concentration changed from 1400 ppm at 0 GWD/MTU of fuel assembly to









10 ppm at 25 GWD/MTU of fuel assembly, from 1400 ppm at 25.01 GWD/MTU of fuel

assembly to 10 ppm at 45 GWD/MTU of fuel assembly and from 1400 ppm at 45.01

GWD/MTU of fuel assembly to 10 ppm at 60 GWD/MTU of fuel assembly. The K-infinity

versus burnup of UO2 fuel COmpared to UO2 with SiC fuel is shown in Table 3-7 and Figure 3-

12. The K-infinity versus burnup of UO2 COre compared with UO2 with SiC core is shown in

Table 3-8 and Figure 3-13. At EOC, which is 25 GWD/MTU of the fresh fuel assembly, the K-

infinity of the core of UO2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2. 1%, 4.5% and

7.2% less than the K-infinity of the core of UO2 fuel. The Doppler coefficient of UO2 with SiC is

less than UO2, aS shown in Table 3-9 and Figure 3-14. The Doppler coefficient of the core of

UO2 with SiC is less than the core of UO2, aS shown in Table 3-10 and Figure 3-15. All the

Doppler coefficients are negative. The MTC of UO2 with SiC is still larger than UO2, aS shown

in Table 3-11 and Figure 3-16. The MTC of UO2 with 10 vol% and 15 vol% SiC are positive at

the 0.5 GWD/MTU of the fuel assembly. However, all the MTCs of the core are negative, as

shown in Table 3-12 and Figure 3-17.












Table 3-1. K-infinity versus burnue of UO2 fuel COmpared to UO2 With SiC fuel.


Equivalent
burnup
(GWD/MTU)
0
0.5
5
10
15
20
25
30
35
40
45
50
55
60


UO2 + 5w%
SiC
1.42005
1.36314
1.30575
1.24631
1.19317
1.14419
1.0977
1.05264
1.00885
0.96639
0.92576
0.88752
0.85269
0.82183


UO2 + 10 w%
SiC
1.42592
1.36451
1.30048
1.23368
1.17308
1.11601
1.06056
1.00586
0.95243
0.90124
0.85395
0.8124
0.7779
0.75079


UO2 + 5 vol%
SiC
1.41447
1.3611
1.30778
1.25296
1.20426
1.15981
1.11807
1.07824
1.03983
1.00261
0.96676
0.93259
0.90049
0.87075


UO2 + 10
vol% SiC
1.41739
1.36221
1.30685
1.2499
1.19899
1.15238
1.10839
1.06614
1.02516
0.98546
0.94723
0.91097
0.87735
0.84677


UO2 + 15
vol% SiC
1.4202
1.36321
1.30565
1.24615
1.19276
1.14363
1.09698
1.05181
1.00776
0.96512
0.92432
0.88604
0.85108
0.82025


UO2
1.41154
1.35996
1.30833
1.2556
1.20872
1.16617
1.12643
1.08863
1.05234
1.01733
0.98355
0.95128
0.9207
0.89211


Table 3 -2. Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel .


Equivalent
burnup
(GWD/MTU)
0
0.5
5
10
15
20
25
30
35
40
45
50
55
60


UO2 + 5 w%
SiC
-1.50
-1.54
-1.61
-1.76
-1.93
-2.09
-2.24
-2.37
-2.49
-2.59
-2.67
-2.74
-2.79
-2.84


UO2 + 10 w%
SiC
-1.47
-1.51
-1.59
-1.76
-1.95
-2.13
-2.28
-2.44
-2.56
-2.67
-2.75
-2.82
-2.86
-2.87


UO2 + 5 vol%
SiC
-1.53
-1.56
-1.62
-1.76
-1.93
-2.08
-2.22
-2.34
-2.45
-2.54
-2.62
-2.70
-2.75
-2.80


UO2 + 10
vol% SiC
-1.52
-1.55
-1.62
-1.76
-1.92
-2.08
-2.22
-2.35
-2.45
-2.56
-2.64
-2.71
-2.77
-2.81


UO2 + 15
vol% SiC
-1.50
-1.53
-1.60
-1.76
-1.93
-2.09
-2.24
-2.37
-2.49
-2.59
-2.67
-2.74
-2.79
-2.82


UO2
-1.56
-1.58
-1.65
-1.79
-1.94
-2.09
-2.23
-2.35
-2.46
-2.54
-2.62
-2.69
-2.74
-2.79












Table 3-3. MTC (pcm/K) versus burnue of UO2 fuel COmpared to UO2 With SiC fuel.


Equivalent
burnup
(GWD/MTU)
0
0.5
5
10
15
20
25
30
35
40
45
50
55
60


UO2 + 5w%
SiC
-31.02
-29.18
-34.10
-39.80
-44.66
-48.91
-52.68
-55.94
-58.66
-60.96
-62.66
-63.67
-64.21
-64.29


UO2 + 10 w%
SiC
-27.20
-25.58
-30.75
-36.48
-41.20
-45.00
-47.97
-50.20
-51.33
-51.43
-50.51
-48.75
-46.67
-44.81


UO2 + 5 vol%
SiC
-33.63
-31.63
-36.31
-41.84
-46.69
-50.98
-54.98
-58.53
-61.76
-64.67
-67.15
-69.20
-70.87
-72.24


UO2 + 10
vol% SiC
-32.49
-30.48
-35.14
-40.61
-45.41
-49.71
-53.66
-57.14
-60.29
-63.04
-65.44
-67.28
-68.66
-69.71


UO2 + 15
vol% SiC
-30.93
-29.10
-34.01
-39.71
-44.57
-48.86
-52.60
-55.83
-58.55
-60.76
-62.47
-63.50
-63.92
-64.02


UO2
-34.40
-32.32
-36.84
-42.27
-47.05
-51.29
-55.32
-58.90
-62.25
-65.37
-67.92
-70.38
-72.58
-74.45


Table 3-4. K-infinity versus i
fuel temperature).
Equivalent burnup
(GWD/MTU) UO2
0 1.41154
0.5 1.35996
5 1.30833
10 1.2556
15 1.20872
20 1.16617
25 1.12643
30 1.08863
35 1.05234
40 1.01733
45 0.98355
50 0.95128
55 0.9207
60 0.89211


burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in


UO2 + 5 vol% SiC
1.42088
1.36718
1.31363
1.25868
1.20967
1.16473
1.12235
1.08171
1.0424
1.00417
0.96728
0.93209
0.899
0.8684


UO2 + 10 vol% SiC
1.42376
1.36825
1.31263
1.25552
1.20427
1.15711
1.11237
1.06922
1.02721
0.98639
0.947
0.90964
0.87501
0.84362


UO2 + 15 vol% SiC
1.42653
1.36921
1.31139
1.2517
1.1979
1.14813
1.10064
1.05442
1.00921
0.96531
0.92324
0.88379
0.84784
0.8163












Table 3 -5. Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel (200 oC less in fuel temperature).
Equivalent burnup
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC
0 -1.56 -1.67 -1.65 -1.64
0.5 -1.58 -1.71 -1.69 -1.68
5 -1.65 -1.78 -1.76 -1.75
10 -1.79 -1.92 -1.92 -1.91
15 -1.94 -2.10 -2.10 -2.11
20 -2.09 -2.27 -2.27 -2.28
25 -2.23 -2.42 -2.42 -2.44
30 -2.35 -2.55 -2.56 -2.59
35 -2.46 -2.67 -2.68 -2.71
40 -2.54 -2.78 -2.79 -2.82
45 -2.62 -2.86 -2.89 -2.92
50 -2.69 -2.95 -2.97 -3.00
55 -2.74 -3.02 -3.04 -3.07
60 -2.79 -3.07 -3.09 -3.12




Table 3-6. MTC (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less


in fuel temperature).
Equivalent burnup
(GWD/MTU) UO2
0 -34.40
0.5 -32.32
5 -36.84
10 -42.27
15 -47.05
20 -51.29
25 -55.32
30 -58.90
35 -62.25
40 -65.37
45 -67.92
50 -70.38
55 -72.58
60 -74.45


UO2 + 5 vol% SiC
-32.74
-30.75
-35.33
-40.74
-45.46
-49.63
-53.59
-57.06
-60.22
-62.99
-65.33
-67.29
-68.77
-69.97


UO2 + 10 vol% SiC
-31.54
-29.62
-34.34
-39.81
-44.54
-48.77
-52.58
-55.93
-58.85
-61.33
-63.39
-64.76
-65.67
-66.18


UO2 + 15 vol% SiC
-30.15
-28.32
-33.15
-38.71
-43.44
-47.59
-51.26
-54.40
-56.98
-59.05
-60.52
-61.28
-61.42
-61.18











Table 3-7. K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature) at boron let-down.
Equivalent burnup
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC
0 1.27265 1.27525 1.27131 1.26663
0.5 1.23342 1.23469 1.22975 1.22414
5 1.2104 1.21074 1.20511 1.1987
10 1.18497 1.18436 1.17774 1.17014
15 1.16305 1.16153 1.15393 1.14514
20 1.1442 1.14159 1.13303 1.12307
25 1.12757 1.12383 1.11428 1.1031
25.01 1.02681 1.01685 1.00175 D.98436
25.4 1.02499 1.01488 0.99968 0.98215
29 1.01752 1.00657 D.99079 D.97252
33 1.00981 0.99784 D.98129 D.96208
37 1.00246 0.98941 0.97203 0.95173
41 0.9957 0.98157 0.96336 0.94212
45 0.98974 0.97463 0.95579 0.93381
45.01 0.89612 0.87443 0.84991 0.82166
45.3 0.8952 D.87343 D.84887 D.82061
48 0.89583 0.87401 0.84983 0.82219
51 0.89699 0.87504 0.85112 0.82397
54 0.89835 0.87629 0.85257 0.82588
57 0.90022 0.87812 0.85472 0.82861
60 0.90282 0.88088 0.858 0.83272


Table 3-8. K-infinity versus burnup of UO2 COre compared to UO2 with SiC
fuel temperature) at boron let-down.
Equivalent burnup of
fresh fuel assembly
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC


core (200 oC less in


1.0966
1.0794
1.0675
1.0546
1.0433
1.0337
1.0254


1.0889
1.0711
1.0585
1.0449
1.0328
1.0224
1.0134


1.0761 1.0613
1.0578 1.0426
1.0449 1.0292
1.0306 1.0142
1.0180 1.0009
1.0069 D.9891
0.9974 0.9791


UO2 + 15 vol% SiC












Table 3 -9. Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel (200 oC less in fuel temperature) at boron let-down.
Equivalent burnup
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC
0 -1.75 -1.88 -1.88 -1.88
0.5 -1.77 -1.91 -1.91 -1.90
5 -1.79 -1.94 -1.93 -1.94
10 -1.90 -2.05 -2.05 -2.06
15 -2.03 -2.19 -2.19 -2.21
20 -2.14 -2.32 -2.32 -2.34
25 -2.23 -2.42 -2.42 -2.44
25.01 -2.46 -2.67 -2.70 -2.74
25.4 -2.46 -2.67 -2.71 -2.75
29 -2.50 -2.73 -2.76 -2.80
33 -2.54 -2.77 -2.80 -2.85
37 -2.58 -2.81 -2.84 -2.88
41 -2.60 -2.84 -2.86 -2.90
45 -2.60 -2.85 -2.86 -2.91
45.01 -2.87 -3.17 -3.23 -3.30
45.3 -2.87 -3.17 -3.22 -3.30
48 -2.86 -3.15 -3.19 -3.26
51 -2.84 -3.13 -3.17 -3.23
54 -2.82 -3.11 -3.14 -3.20
57 -2.80 -3.08 -3.11 -3.14
60 -2.77 -3.04 -3.07 -3.09


Table 3-10. Doppler coefficient (pcm/K) versus burnup of UO2 COre compared to UO2 with SiC
core (200 oC less in fuel temperature) at boron let-down.
Equivalent burnup of
fresh fuel assembly
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC


-2.27
-2.27
-2.30
-2.35
-2.41
-2.46
-2.49


-2.47
-2.48
-2.51
-2.56
-2.63
-2.68
-2.72


-2.49 -2.52
-2.50 -2.54
-2.52 -2.56
-2.58 -2.62
-2.65 -2.68
-2.69 -2.73
-2.73 -2.76












Table 3-11i. MTC (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC
less in fuel temperature) at boron let-down.
Equivalent burnup
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC
0 -5.21 -2.63 -0.21 2.48
0.5 -3.48 -0.97 1.38 4.00
5 -12.86 -10.44 -8.36 -5.90
10 -23.56 -21.19 -19.29 -17.11
15 -33.90 -31.66 -29.99 -28.04
20 -44.25 -42.20 -40.89 -39.25
25 -55.11 -53.38 -52.40 -51.09
25.01 -20.51 -16.52 -13.03 -8.73
25.4 -21.27 -17.31 -13.79 -9.50
29 -28.91 -25.03 -21.67 -17.54
33 -37.87 -34.15 -31.04 -27.00
37 -47.29 -43.83 -40.93 -37.23
41 -57.26 -54.15 -51.71 -48.49
45 -67.81 -65.27 -63.46 -60.92
45.01 -25.70 -18.81 -12.33 -3.93
45.3 -26.41 -19.54 -13.12 -4.72
48 -34.17 -27.70 -21.65 -13.84
51 -43.33 -37.24 -31.73 -24.59
54 -53.09 -47.49 -42.58 -36.20
57 -63.45 -58.49 -54.32 -48.84
60 -74.53 -70.45 -67.16 -62.87


Table 3-12. MTC (pcm/K) versus burnup of UO2 COre compared to UO2 with SiC core (200 oC
less in fuel temperature) at boron let-down.
Equivalent burnup of
fresh fuel assembly
(GWD/MTU) UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% Sit


-15.43
-15.14
-23.54
-33.27
-43.16
-53.39
-64.20


-11.30
-11.06
-19.62
-29.52
-39.67
-50.25
-61.61


-7.52 -2.86
-7.30 -2.68
-16.12 -11.77
-26.32 -22.28
-36.78 -33.14
-47.85 -44.73
-59.78 -57.31


C(











Table 3-13. Centerline temperature of UO2 fuel COmpared to UO2 with SiC.
Tm q' rf kf 14 k, h, to Tcl (K) Tcl (oC)
UO, with SiC 600 270 0.47 0.041 0.5 0.15 2.8 0.06731 1383 1110


4.5 1272
0.15 2.8 0.06731 1648
4.5 1538


999
1375
1265


1.1
600 270 0.47 0.027 0.5
1.1










OOOOOOOOOOOOOOO
OOIO O O O O O O O O OOOO
OOOOOOOOOOOOOOO
OOOO O @ O O O OO O OOOO
OOOOOOOOOOOOOQOO
Q OOQO OOOO O ~~O O ~O OO
OOOOOOOOOOOOOOO
SO O QO O OOO 3O O OOOO Q
OOOOOOOOOOOOOOO
OOOO~ 3O O O O OO OO
OOOOOOOOOOOOOQOO
OOOOO~ 38O~ 3O O O GO OO
OOOOOOOOOOOOOOO
OOOOO~ 3QO~ 3O O O OOOO
OOOOOOOOOOOOOOO


Figure 3-1. Crystal River 15X15 assembly design.


OUOa FuelPins


OGuide Tubes


+ Inskumen Tube


1.5

1.4-

1.3-

1.2-


1. -


0.9-

0.8-

0.7-

0.6
0 5 10 15 20 25 30 35 40 45
Equivalent Burnup (GWD/MTU)


50 55 60


Figure 3-2. K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel.



































0 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/MTU)


Figure 3-3. Doppler coefficient versus burnup of UO2 fuel COmpared to UO2 with SiC fuel.


-40

-45


0 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/MTU)


Figure 3-4. Moderator temperature coefficient versus burnup of UO2 fuel COmpared to UO2 with
SiC fuel.


































0 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/MTU)


Figure 3-5. K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel.


O 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/MTU)


Figure 3-6. Doppler coefficient versus burnup of UO2 fuel COmpared to UO2 with SiC fuel.














-30-

-35 Uo2 + 5 vol% SiC
UO2 + 10 vol% SiC
-40 Uo2 + 15 vol% SiC

-45-




-55-

-60-

-65-

-70-

75 III III
0 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/1VITU)


Figure 3-7.


Moderator temperature coefficient versus burnup of UO2 fuel COmpared to UO2 with
SiC fuel.


O 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/1VTU)


Figure 3-8. K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature).















-1.6-



UO




S-2.2-

S-2.4-


-2.6-


-2.8-




-3.2
0 5 10 15 20 25 30 35 40 45
Equivalnet Burnup (GWD/MTU)


50 55 60


Figure 3-9. Doppler coefficient versus burnup of UO2 fuel COmpared to UO2 with SiC fuel
(200 oC less in fuel temperature).


-25

-30


-35 -\\ UO2 + 5 vol'
S UO2 +10 vo
-40 U02 + 15 vo

-45-




-55-

-60-

-65-

-70-

-75
0 5 10 15 20 25 30 35 40 45
Equivalent Burnup (GWD/MTU)


50 55 60


Figure 3-10. Moderator temperature coefficient versus burnup of UO2 fuel COmpared to UO2
with SiC fuel (200 oC less in fuel temperature).












1400


1200-


E`1000-






S600-


400-


200-



0 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/MTU)


Figure 3-11. Boron concentration versus burnup at boron let-down.




1.25-

1.2-
UO2 +5 vol% SiC
UO2 +10 vol% SiC
1.15 ~UO2 +15 vol%SiC

1.1-






0.95-

0.9-

0.85-

g g I II II II
0 5 10 15 20 25 30 35 40 45 50 55 60
Equivalent Burnup (GWD/MTU)


Figure 3-12. K-infinity versus burnup of UO2 fuel COmpared to UO2 with SiC fuel (200 oC less in
fuel temperature) at boron let-down.















1.08 O
UO, +5 volo a SC
UO, + 10 volo SC
1.06 -\UO + 15 voloo5C






M 1.02-





0.98-



0 5 10 15 20 25
Equivalent Burnup of Fresh Fuel Assembly (GWD/MTU)


Figure 3-13. K-infinity versus burnup of UO2 COre compared to UO2 with SiC core (200 oC less
in fuel temperature) at boron let-down.


O 5 10 15 20 25 30 35 40 45
Equivalent Burnup (GWD/MTU)


50 55 60


Figure 3-14. Doppler coefficient (pcm/K) versus burnup of UO2 fuel COmpared to UO2 with SiC
fuel (200 oC less in fuel temperature) at boron let-down.
















-2.3



-2.4







-2.6



-2.7



-2.8


5 10 15 20
Equivalent Burnup of Fresh Fuel Assembly (GWD/MTU)


Figure 3-15. Doppler coefficient (pcm/K) versus burnup of UO2 COre compared to UO2 with SiC
core (200 oC less in fuel temperature) at boron let-down.


-5


-25


S-15-





-45-


-55- U2
UO2 +5 vol% SiC
UO2 + 10 vol% SiC
-65 UO2 + 15 vol% SiC


-75
0 5 10 15 20 25 30 35 40 45
Equivalent Burnup (GWD/MTU)


50 55 60


Figure 3-16. Moderator temperature coefficient (pcm/K) versus burnup of UO2 fuel COmpared to
UO2 with SiC fuel (200 oC less in fuel temperature) at boron let-down.


















-20 o2 T Iu voV 170 ~
UO2 +15 vol% SiC











-60-






0 5 10 15 20 25
Equivalent Burnup of Fresh Fuel Assembly (GWD/MTU)


Figure 3-17. Moderator temperature coefficient (pcm/K) versus burnup of UO2 COre compared to
UO2 with SiC core (200 oC less in fuel temperature) at boron let-down.









CHAPTER 4
REACTION BETWEEN URAINIUM OXIDE AND SILICON CARBIDE

Background

There are very few papers about the reaction between uranium dioxide (UO2) and silicon

carbide (SiC). G. C Allen et al. [12] have reported that UO2 reacts with SiC at a measurable rate

above 1377 oC. The reactions were described as Equation 4-1 to Equation 4-5.

UO2 + 2SiC t USi2 + 2CO (4-1)

UO2 + 2SiC t UC2 + 2SiO (4-2)

USi2 t USil.67 + 0.33Si (4-3)

5Si + 3UC2 t U3 3Si2 + 3SiC (4-4)

UO2 + 3UC2 t 4UC + 2CO (4-5)

W. Lippmann et al. [36] have confirmed the reaction between UO2 and SiC at the

temperature above 1700 oC in a system where gaseous products were free to escape. However,

they also found that there was no reaction up to 1800 oC in a system where gaseous products

were sealed. Solomon et al. [10] also observed reactions between UO2 and SiC at 1400 oC after

fifteen hours.

Experiments and Results

UO2 and SiC powder were sintered together at 1300 oC and 1650 oC to verify the literature

result. Before the sintering process, the uranium oxide powder from Framatome/Areva was

characterized for its oxygen to uranium ratio and particle size distribution.

Oxygen to Uranium Ratio of Uranium Oxide Powder

The powder was oxidized to U30s and the weight difference was measured to determine

the O/U ratio of the uranium oxide powder from Framatome/Areva. The powder was oxidized in

air at 350 oC for 24 hours. The weight change indicated the O/U ratio of received uranium oxide










powder was 2.10. The generated U30s was reduced to UO2.0 in argOn and 5% hydrogen

environment at 900 oC for 4 hours. The received uranium oxide powder was oxidized in air at

140 oC for 24 hours. The O/U ratio calculated by weight difference was 2.27.

Figure 4-1 shows the uranium oxide powder with different O/U ratio. The received UO2.10

powder was dark brown; the UO2.27 WAS black; the U30s powder was dark green; and the UO2.0

powder was orange. The X-ray diffraction (XRD) results of the four uranium oxide powders are

shown in Figure 4-2 to Figure 4-5. The XRD peaks of UO2.10 and UO2.27 WeTO ClOse to the peaks

of UO2.0, the extra oxygen in UO2.10 and UO2.27 Slightly broadens and offsets the peaks of UO2~.0

The XRD peaks of U30s were totally different from the peaks of UO2.0 because the crystal

structures were different.

Particle Size Distribution of Uranium Oxide Powder

The particle size of received UO2.10 pOwder was characterized by sieve analysis. Ten gram

UO2.10 pOwder was sieved through a series of screens with standardized mesh size. The sieve was

shaken for half an hour on an Octagon digital sieve shaker. The sieve and shaker are shown in

Figure 4-6. The powder between two screens was weighed and recorded in Table 4-1. The

experiments were repeated for three times. The average value was plotted in Figure 4-7. There

was about a 4% powder loss in the sieving process.

Sintering UO2 and SiC at 1300 oC and 1650 oC

The received uranium oxide powder was ball milled with the P-SiC powders (30 nm in

particle size, from Alfa Aesar) for half an hour. The weight ratio of SiC to UO2 is one. Figure 4-8

shows the X-ray diffraction (XRD) pattern of the 30 nm P-SiC. The mixture powders were cold

pressed at 200MPa, then sintered at 1300 oC in argon atmosphere. The pellet after sintering is

shown in Figure 4-9. The X-ray diffraction result (Figure 4-10) showed no new peaks at 1300 oC.









When the sintering temperature was increased to 1650 oC, the X-ray diffraction result (Figure 4-

12) showed that USil~ss was formed. The pellet after sintering is shown in Figure 4-11.

Discussion

The experiment result was different from Allen's result [12] because the ratio of SiC to

UO2 USed by Allen et al. was 2.5. Uranium dioxide pellets are usually produced by sintering the

green pellets at about 1700 oC in hydrogen atmosphere. High temperature around 1700 oC is

necessary to achieve the required high density of over 95% of theoretical. The reaction between

UO2 and SiC at 1377 oC has to be avoided to successfully incorporate SiC into UO2-










Table 4-1. Particle size distribution of received uranium oxide powder.
Particle Size (Cm) Weight (g) Weight (g) Weight (g) Average (g)
<25 0.6767 0.5633 0.6769 0.6390
>25 and < 45 1.9094 1.9317 1.9308 1.9240
>45 and <53 1.0295 1.1097 1.0454 1.0615
>53 and <63 0.8636 0.8886 0.9009 0.8844
>63 and <90 2.4028 2.4171 2.3799 2.3999
>90 and <150 1.4126 1.4529 1.4846 1.4500
>150 and <250 0.4147 0.4447 0.4161 0.4252
>250 0.8482 0.8681 0.8153 0.8439






















(b)







'"t
rr


Figure 4-1. Uranium oxide powders with different O/U ratio. A) UO2. 10. B) UO2.27. C) U30s. D)
UO2.


1.2



1-



0.8-



S0.6-



0.4-



0.2-



0"
10 20 30 40 50 60 70 80
26


Figure 4-2. X-ray diffraction pattern of UO2.10 pOwder.












1.2



1-



0.8-



~0.6-



0.4-



0.2-



0
10 20 30 40 50 60
26


Figure 4-3. X-ray diffraction pattern of UO2.27 pOwder.


1.2



1-



0.8-



%0.6-



0.4-



0.2-



0
10 20 30 40 50 60
26


Figure 4-4. X-ray diffraction pattern of U30s powder.


70 80


70 80












1.2



1-



0.8-



~0.6-



0.4-



0.2-




10 20 30 40 50 60
26


Figure 4-5. X-ray diffraction pattern of UO2.0 pOwder.


70 80


Figure 4-6. Sieve and shaker for analyzing particle size distribution.












1.2



1-



0.8-



a0.6-



0.4-



0.2



0
0 25 50 75 100 125 150 175 200 225 250
Particle Size (micron)


Figure 4-7. Particle size distribution of received uranium oxide powder.


1.2



1-



0.8-



%0.6-



0.4-



0.2-



0
10 20 30 40 50 60 70 80
26


Figure 4-8. X-ray diffraction pattern of 30nm P-SiC from Alfa Aesar.

































































10 20 30 40 50 60 70 80
26


Figure 4-10. X-ray diffraction pattern of UO2-SiC pellet after sintering at 1300 oC.


Figure 4-9. Uranium dioxide-silicon carbide pellet after sintering at 1300 oC.


0.8-



0.6-


0.2 1-


rw~
I


V SiC
r O


V






































Figure 4-11. Uranium dioxide-silicon caribide pellet after sintering at 1650 oC.


1.2


V UO2
1 USi1.8 I V V










0.4 *I



0.2 11 111 1 11




10 20 30 40 50 60 70 80
26


Figure 4-12. X-ray diffraction pattern of UO2-SiC pellet after sintering at 1650 oC.









CHAPTER 5
LOW TEMPERATURE SINTERING OF URANIUM DIOXIDE

Background

Uranium dioxide (UO2) pellets are usually produced by sintering the green pellets at about

1700 oC in hydrogen atmosphere [13, 14, 37-40]. The high sintering temperature around 1700 oC

is necessary to achieve the required high density of over 95% of theoretical. However, sintering

at a temperature around 1700 oC is expensive due to the furnace cost and maintenance cost.

Several studies have shown that UO2 pellets with high density can be achieved at lower

sintering temperature. Fuhrman et al. [13] have reported that UO2 pellets of 95 to 97%

theoretical density (TD) were achieved by sintering at 1200 oC in nitrogen for 1 hour followed by

1 hour reduction in hydrogen, while using uranium oxide powder with extra oxygen (O/U~

2.37). They also mentioned that UO2 pellets of 95% TD were achieved at temperatures as low as

1000 oC using the same method, though the result was not consistent. Langrod [14] has reported

that UO2 pellets of above 95% TD was achieved by sintering at 1300 oC in nitrogen atmosphere

for 2 hours followed by reduction in hydrogen, using mixture of UO2 and U30s (O/U ~ 2.30).

Ayaz et al. [39] have sintered UO2 pellets of 95% TD at 1 150 oC in CO2 and water vapor

atmosphere for 4 hour, followed by reduction in Ar+8% H2 for hour, using uranium oxide

powder with extra oxygen (O/U= 2.15). Williams et al. [41] have sintered uranium oxide with

different O/U ratio in argon, nitrogen, carbon dioxide and vacuum and achieved UO2 pellets of

94% TD at temperatures lower than 1400 oC in various gases.

Excess oxygen was believed to be the key factor to decrease the sintering temperature.

Fuhrman et al. [13] indicated that a minimum O/U ratio of 2.25 to 2.28 was required to achieve a

density above 95% of theoretical. In the sintering process, the uranium oxide particles undergo

solid state diffusion. Based on the theory by Williams et al. [41], the rate of the diffusion of









uranium ions determines the rate of the sintering process. The diffusion of uranium ions in non-

stoichiometric uranium oxides is more rapid than in stoichiometric UO2 because the extra

oxygen in non-stoichiometric uranium oxides lowers the lattice binding energies.

The uranium oxide powder was oxidized in air to increase the oxygen to uranium ratio.

The rate and degree of oxidation of UO2 VeTSus temperature studied by Langrod [14] is shown in

Figure 5-1. Langrod [14] also found that the sintering behavior of UO2 pOwder blended with

U30s was identical with the air oxidized UO2 pOwder of the same O/U ratio.

The non-stoichiometric pellets must be further processed to bring the oxygen to uranium

ratio back to 2.0 by soaking the pellets in hydrogen environment. Fuhrman et al. [13] reported

that 1100 oC was required to remove the excess oxygen in a reasonable time.

The study by Fuhrman et al. [13] showed that the grain size of the UO2 pellet sintered at

low temperature is smaller than the grain size of the UO2 pellet sintered at high temperature. The

result was confirmed by Ayaz et al. [39].

Experiments and Results

A UO2 pellet was first sintered in hydrogen atmosphere at high temperature. The received

uranium oxide powder was cold pressed in a 13 mm die at 200 MPa. The green pellet was

sintered at 1650 oC for 4 hours in hydrogen (balanced with argon) atmosphere. The density of

sintered pellet was 96.03% TD, which was measured by the Archimedes method (Archimedes

principle). The pellet is shown in Figure 5-2. The grain size of the pellet is in the range of 5 to 20

micron, as shown in the scanning electron microscopy (SEM) image (Figure 5-3).

The received uranium oxide powder was oxidized in air at 140 oC for 24 hours to increase

the O/M ratio to UO2.27. The UO2.27 pOwder was then cold compacted in a 13mm die at 200 MPa

pressure. The green pellet was sintered at 1200 oC for 1 hour in argon atmosphere, followed by

the reduction in Ar+5% H2 at 1200 oC for one hour. The picture of the sintered pellet is shown in









Figure 5-4. The density of the pellet measured by the Archimedes method was 95.71%TD. The

SEM image of the pellet is shown in Figure 5-5. The grain size of the pellet is in the range of 5 to

10 micron.

Another UO2 pellet was sintered at low temperature with pressure. The UO2.27 pOwder was

cold compacted in a 13mm die at 200 MPa pressure. The green pellet was then sintered at 1200

oC with 10 MPa pressure for 1 hour in argon atmosphere, followed by the reduction in Ar+5%

H2 at 1200 oC for one hour. (The detailed experiment apparatus of sintering process with pressure

will be explained in chapter 8.) The picture of the sintered pellet is shown in Figure 5-6. The

density of the pellet measured by the Archimedes method was 97.99%TD. The SEM image of

the pellet is shown in Figure 5-7. The grain size of the pellet is in the range of 5 to 10 micron, but

larger than the grain size of the UO2 pellet sintered at low temperature without pressure.

Discussion

Uranium dioxide fuel pellets with high density (>95%TD) can be achieved at 1200 oC,

which is lower than the temperature at which the reaction between UO2 and SiC occurs. So the

low temperature sintering method can be used to avoid the reaction between UO2 and Sic.

The density of the UO2 pellet sintered at 1200 oC is less than the density of the UO2 pellet

sintered at 1650 oC; the density of the UO2 pellet sintered at 1200 oC with pressure is larger than

the density of the UO2 pellet sintered at 1650 oC. The grain size of the UO2 pellet sintered at low

temperature is smaller than the grain size of the UO2 pellet sintered at 1650 oC. The pressure

applied during the sintering process increased the grain size. The grain size was still smaller than

the grain size of the UO2 pellet sintered at 1650 oC. A larger grain size is desired to lower the

amount of fission gas release because the fission gas has to diffuse longer distance to the grain

boundary.










The rate of fission gas release is related to the fuel temperature. The SiC additives are

expected to increase the thermal conductivity and lower the fuel temperature, thus lower the

fission gas release and offset the negative effect of smaller grain of UO2 pellet sintered at low

temperature.


























2 II



0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Figure 5-1. Oxygen to uranium ratio of UO2 pOwder oxidized in air at 140 oC [14].


Figure 5-2. Uranium dioxide pellet sintered at 1650 oC.
































Figure 5-3. Scanning electron microscopy image of UO2 pellet sintered at 1650 oC (5,000X).


Figure 5-4. Uranium dioxide pellet sintered at 1200 oC.
































Figure 5-5. Scanning electron microscopy image of UO2 pellet sintered at 1200 oC (5,000X).


Figure 5-6. Uranium dioxide pellet sintered at 1200 oC with pressure.
































Figure 5-7. Scanning electron microscopy image of UO2 pellet sintered at 1200 oC with pressure
(5,000X).









CHAPTER 6
SILICON CARBIDE COATINTG BY CHEMICAL VAPOR DEPOSITION

Background

The chemical vapor deposition (CVD) is a chemical process to produce high purity solid

materials by dissociation and /or chemical reactions of gaseous reactants (precursors) in an active

(heat, light, plasma etc) environment. The CVD process has the advantage of producing highly

dense and pure materials at relative low processing temperature. For example, SiC can be

produced below 1400 oC [42-47], much lower than the melting point. There are several variants

of CVD methods, which are initiated using different energy sources, such as plasma enhanced

CVD (PECVD) [48], photo-assisted CVD (PACVD) [49] and microwave CVD [50]. Though the

use of variant energy sources may requires sophisticated system and increase the cost of the

process, the cost for conventional CVD technique is reasonable [51].

The CVD process has been used to deposit a SiC layer in tristructral isotropic (TRISO)

fuel particles in a fluidized bed. In general, argon-hydrogen carrier gas is bubbled through a

liquid precursor, and then passed through a fluidized bed of carbon coated fuel particles

maintained at 1000 to 1800 oC. [52]. Methyltrichlorosilane (CH4SiCl3 Or MTS) was usually used

as the precursor because it contains the same number of silicon and carbon. The deposition rate

and microstructure of SiC were determined by the deposition temperature, MTS flow rate and

argon to hydrogen ratio.

Based on the study by Gulden [52], the deposition rate of SiC is independent of

temperature and only depends on the flow rate of MTS. The microstructure of SiC is strongly

dependent on temperature. A relative low dense laminar structure of SiC was formed at the

temperature between 1200 oC to 1400 oC and almost full dense columnar structure of SiC was

formed at the temperature above 1400 oC. However, by adjustment of argon to hydrogen ratio,









theoretical dense SiC can be deposited at the temperature between 1200 to 1400 oC [53]. The

crystalline size of SiC increases with increasing temperature and decreases with increasing argon

to hydrogen ratio. The study by Ford et al. [53] showed that the crystalline size of SiC was 0.25

micron at 1700 oC, using only hydrogen and 0.03 micron at 1300 oC, using an argon-hydrogen

mixture.

The CVD process has also been used to produce SiC electronic devices in semiconductor

industry. Steckl et al. [42] have successfully investigated the growth of P-SiC on silicon substrate

using silacyclobutane (C3H6SiH2 Or SCB) at the temperatures as low as 800 oC in a low pressure

CVD system. Lin et al. [43] have successfully deposited SiC film on sapphire substrate using

trimethylsilane (C3H9SiH or TMS) at 1100 oC in a low pressure CVD system. Kunstmann et al.

[44] have reported the growth of P-SiC films using MTS at 1200 oC. Madapura et al. [45] have

reported the growth of P-SiC using TMS at the temperature between 1 100 to 1200 oC.

Silicon carbide can also be produced by CVD process using separate precursors. Yagi et al.

[46] have used SiH2 12 and C2H2 to grow SiC on silicon substrate at 1020 oC in a low pressure

CVD system. Powell et al. [47] have used SiH4 and propane to grow SiC on silicon substrate at

1360 oC. The ratio of the gaseous precursors has to be chosen carefully to ensure the

stoichiometry of SiC.

Experiment and Result

In this research, both TMS and MTS were used to deposit a SiC layer on carbon coated

uranium oxide particles. The buffer carbon layer was deposited on uranium oxide particles by

decomposition of propane (C3Hs). The CVD process was carried out at 1300 oC in a Lindberg

Blue high temperature tube furnace as shown in Figure 6-1. Argon and 5% H2 gaS was used as

carrier gas at a constant flow rate of 140 cm3 per minute. The precursor, TMS or MTS, flowed









through the furnace at the flow rate of 5 cm3 per minute at the temperature of 1300 oC for 30

minutes. Figure 6-2 is the temperature profile of the CVD process

The powder after the CVD process is shown in Figure 6-3. Fourier Transform Infrared

Spectroscopy (FTIR) and X-ray Diffraction (XRD) were used to characterize the SiC coating.

FTIR was used to obtain information about the chemical bonding in the material; XRD was used

to identify the crystalline structure. The FTIR result is shown in Figure 6-4. The powder after

CVD process has a peak at the same position as P-SiC powder, which indicates the formation of

Si-C bond. The XRD result is shown in Figure 6-5. There are no SiC peaks found in Figure 6-5

and all of the peaks in Figure 6-5 are UO2 peaks.

Discussion

The possible reason for no SiC peaks found in XRD result was that the precursor, TMS or

MTS, had already decomposed before reaching the carbon coated UO2 particles. The furnace

tube was about four feet long and the UO2 pOwder was placed in the middle of the furnace tube,

so the gaseous precursor had to travel two feet before reaching the powder. TMS or MTS might

have already decomposed and deposited on the inside of the furnace tube. A colored layer can be

seen on the inside wall of the furnace tube in Figure 6-6. The peak in FTIR result might be some

contamination containing Si-C bond.

There was another possibility that P-SiC was not detected on the carbon coated uranium

oxide particles. The amount of SiC was small and uranium oxide is a strong X-ray absorber, so

the X-ray diffracted from P-SiC was absorbed in UO2 and the peaks of SiC were not shown in

the XRD result.

Even if there was a P-SiC layer formed on the carbon coated UO2 pOwder, the powder will

be hard to sinter. The powder can be seen as SiC in the sintering process because SiC is the

outside layer and there are two SiC layers between two particles. The low temperature sintering









method of UO2 Can nOt be used. Temperature above 1900 oC is required to achieve a high density

SiC pellet, which is above the temperature at which SiC reacts with UO2. The CVD process was

not further studied because of the potential problem for sintering and the success in making the

SiC whiskers-UO2 COmposite.




































Figure 6-1. Lindberg high temperature furnace.


1400-
TMS/MTS

1200-


1000-


800-


H 600-


400-


200-



0 100 200 300 400 500 600 700 800 900 1000
Timie (minute)


Figure 6-2. Temperature profile of the CVD process.






























Figure 6-3. Uranium oxide powder after CVD process.


111~


O OO
4000


] 2000
Wavenumbers (crn-1)


Figure 6-4. Fourier transform infrared spectroscopy result of the powder after the CVD process.












1.2







0.8-



S0.6-



0.4-



0.2-




10 20 30 40 50 60 70 80
26


Figure 6-5. X-ray diffraction result of the powder after CVD process.


Figure 6-6. Furnace tube after CVD process.









CHAPTER 7
SILICON CARBIDE COATINTG FROM PRECERAMIC POLYMER

Background

Silicon carbide (SiC) can be produced by a pre-ceramic polymer. Grigoriev et al. [54] have

successfully deposit an amorphous SiC coating on alumina and zirconia substrate using a ter-

polysilane polymer at 900 oC. Zheng et al. [55] have reported the growth of SiC whiskers by

Allylhydridopolycarbosilane (AHPCS) and SiC powder in the temperature range of 1250 oC to

1350 oC. Berton et al. [56] have used AHPCS to make carbon/ silicon carbide composite by

polymer infiltration and pyrolysis (PIP). Solomon et al. [10] have also used AHPCS in the PIP

process to make SiC/UO2 COmposite. The conversion of pre-ceramic polymer to ceramic is a

relatively easy and low cost process compared with chemical vapor deposition (CVD) process.

Experiments and Results

The SiC pre-ceramic polymer used in this research is AHPCS, also called "SMP-10" by

the manufacturer Starfires Systems Inc. AHPCS is a liquid with bright orange color, as shown in

Figure 7-1. According to the manufacturer, amorphous SiC forms at 850 oC with 75 to 82%

ceramic yield and nano-crystalline P-SiC forms at 1250 to 1700 oC with 75 to 80% yield.

AHPCS was first used to make a SiC pellet. The SiC powder (1 CL) purchased from Alfa

Aesar was mixed with 10 w% AHPCS in Hexane (C6H14). After Hexane was dried, the mixed

powder was cold pressed in a 13 mm die at 200 MPa. The green pellet was then sintered in a

Lindberg furnace (Figure 7-2) in argon atmosphere following the sintering procedure provided

by Starfires Systems Inc (Figure 7-3). The sintered pellet made froml CL powder is shown in

Figure 7-4 and the SEM image of the pellet is shown in Figure 7-5. The density measured by the

Archimedes method was 98.7%TD, which was far different from 66.7%TD, the density

calculated by mass and volume.









Uranium oxide (UO2.27) WAS mixed with 10 weight % AHPCS in hexane. After the hexane

evaporated, the mixed powder was cold pressed in an alumina die (%/ inch in diameter) at 200

MPa. Then both the pellet and the alumina die were heated in an argon atmosphere in the same

Lindberg furnace. During the sintering process, about 5 MPa pressure was applied on the pellet.

The pressure was provided by two springs located at the end of the furnace tube, which was

sealed by stainless steel end caps. The pellet after sintering broke into pieces when it was taken

out of the alumina die.

Discussion

The reason for the large difference between the density measured by the Archimedes

method and the density calculated by mass and volume is that the Archimedes method is not

suitable for measuring the density of a pellet with large open porosity. The density of a pellet

with large open porosity measured by the Archimedes method is the density of the "skeleton" of

the pellet, which can be close to the theoretical density. The density calculated by mass and

volume is relative close to the real value compare to the result by the Archimedes method.

There is an easy way to tell whether the pellet is suitable for the Archimedes method. If

there are a lot of bubbles coming out of the pellet when immersed into liquid, the pellet is porous

and not suitable for the Archimedes method; otherwise, the pellet is low in porosity and suitable

for the density measurement by the Archimedes method. The density of the SiC pellet sintered

by pressureless sintering is low because of the relative low yield of the pre-ceramic polymer.

There were large pores in the SiC pellet, as shown in Figure 7-5.

The reason for the break up of the pellet may be that the pre-ceramic polymer was oxidized

by the UO2.27 pOwder. The polymer precursor could be oxidized by the extra oxygen in uranium

oxide powder before it can be converted to silicon carbide. Because the extra oxygen is essential









for the low temperature sintering method of UO2, the oxidation of the pre-ceramic polymer is

inevitable and the process of pre-ceramic coating on uranium oxide particles is not successful.


































Figure 7-1. Allylhydridopolycarbosilane (AHPCS), the SiC pre-ceramic polymer.


Figure 7-2. Lindberg/Blue Mini-Mite 1100 oC furnace.












1200

1 hour
1000-
1 or3 C/nrin

800-
3 OC/nrin

S600-


S400 OC//nn


200-

2 OC/nrin


0 100 200 300 400 500 600 700 800 900 1000
Time (minute)


Figure 7-3. The temperature profile of sintering process.


Figure 7-4. Silicon carbide pellet made by SiC powder (1 C) and AHPCS.










































Figure 7-5. Scanning electron microscopy image of the SiC pellet made by SiC powder (1 C) and
AHPCS.


1









CHAPTER 8
SILICON CARBIDE WHISKERS URANIUM DIOXIDE COMPOSITE

Background

Silicon carbide (SiC) whiskers are usually used as a reinforce material to improve the

mechanical properties, such as strength and fracture toughness, of matrix materials. Wei et al.

[34] used SiC whiskers to reinforce aluminum oxide (Al203) to improve the fracture toughness.

Sun et al. [35] used SiC whiskers to improve the fracture toughness and high temperature

strength of molybdenum disilicide (MoSi2).

Silicon carbide whiskers are commonly made by either rice hull or vapor-liquid-solid

(VLS) process. The SiC whiskers produced from rice hull process are typically less than 1

micron in diameter and range from 10 to 50 micron in length; the SiC whiskers produced from

VLS process are typically 5-6 micron in diameter and up to 100 mm in length [57]. Silicon

carbide whiskers are single crystal, which means fewer flaws than polycrystalline, so the thermal

conductivity and strengths are very high.

The commercially available SiC whiskers are commonly in an agglomerated form. The

agglomeration must be broken before mixing with the matrix to ensure homogenously dispersion

of SiC whiskers. Wei et al. [34] reported that SiC whiskers and ceramic powder were mixed in

hexane in a blender and then dispersed using an ultrasonic homogenizer; the hexane was then

dried by evaporation with constant agitation under flowing air.

Hot pressing was found to be required to achieve a high density pellet with greater than 5

vol% SiC whiskers, because the SiC whiskers interfered with matrix particle rearrangement

during sintering. During hot press sintering, SiC whiskers are preferentially oriented in a plane

perpendicular to the hot pressing direction [7-9, 34].









Several studies have shown that SiC whiskers can also increase the thermal conductivity of

matrix materials. Russell et al. [7] reported the thermal conductivity of 30 vol% VLS SiC

whisker-mullite composite is three times higher at room temperature than that of single phase

mullite in perpendicular direction to the hot pressing direction and two times higher in parallel

direction. Johnson et al. [8] reported that the thermal conductivity of 30 vol% SiC whisker-

osumilite glass composite is four times higher at room temperature than that of single phase

mullite in perpendicular direction to the hot pressing direction and two times higher in parallel

direction. Hesselman et al. [9] showed the thermal conductivity of 30 vol% VLS SiC whisker-

lithium aluminosilicate glass composite is five times higher at room temperature than that of

lithium aluminosilicate glass in perpendicular direction to the hot pressing direction and three

times higher in parallel direction. Hesselman et al. [9] suggested that a SiC whisker "percolation"

pathway was formed and heat was conducted through SiC whiskers, bypassing the matrix.

Experiments and Results

Characterization of Silicon Carbide Whiskers

The silicon carbide whiskers are commercially available from Alfa Aesar (Alfa) and

Advanced Composite Materials (ACM). The whiskers from Alfa Aesar are 1.5 micron in

diameter and about 18 micron in length (no detailed information available); the Whiskers from

Advanced Composite Materials are 0.45-0.65 micron in diameter and 5-80 micron in length.

Both of the whiskers are single crystal P-SiC. The received SiC whiskers were in the form of

agglomerates. The scanning electron microscope (SEM) images of the received SiC whiskers

from Alfa Aesar are shown in Figure 8-1 and Figure 8-2. The X-ray diffraction pattern of SiC

whiskers from Alfa Aesar is shown in Figure 8-3. The SEM images of the received SiC whiskers

from Advanced Composite Materials are shown in Figure 8-4 and Figure 8-5. The X-ray

diffraction pattern of SiC whiskers from Advanced Composite Materials is shown in Figure 8-6.









Mixing of SiC Whiskers and Uranium Oxide Powder

The received SiC whiskers were blended with hexane in a blender for 3 minutes to break

the agglomeration. A small amount of the mixture was dropped on the SEM sample holder and

the hexane was left to evaporate, leaving only the separated SiC whiskers. Figure 8-8 and Figure

8-9 are the SEM images of dispersed SiC whisker from Alfa Aesar and Advanced Composite

Materials, respectively. Based on the SEM images, the SiC whiskers were successfully dispersed

by the blending. The separated SiC whiskers were then mixed with UO2.27 particles (25 CL to 45

CL) in an ultrasonic mixer for 10 minutes. After hexane was dried, the mixed powder was ground

by a mortar and a pestle.

Pressureless Sintering of SiC Whiskers and Uranium Oxide Powder

The mixed powder of UO2 and SiC whiskers from Advanced Composite Materials was

cold pressed in an alumina (Al203) die (1/4" in diameter) at 200MPa. The green pellet and the

Al203 die were sintered at 1200 oC in argon atmosphere for 1 hour. The pellet of UO2 with 5

vol% SiC is shown in Figure 8-9. The SEM images of the pellet are shown in Figure 8-10 and

Figure 8-11i. The density of the pellet was 98.6% TD, which was measured by the Archimedes

method. The pellet of UO2 with 10 vol% SiC is shown in Figure 8-12. The SEM images of the

pellet are shown in Figure 8-13 and Figure 8-14. The density of the pellet was 82.2% TD, which

was also measured by the Archimedes method. The density of the pellet decreases sharply with

increasing the amount of SiC whiskers.

Hot Press Sintering of SiC Whiskers and Uranium Oxide Powder

The mixed powder of UO2 and SiC whiskers from Advanced Composite Materials was

cold pressed in an alumina die at 200MPa. The alumina die was made by an alumina tube and

two alumina rods, as shown in Figure 8-15. After cold pressing, the mixed powder and alumina

die were placed in a sample holder surrounded by a graphite tube at the position of mixed










powder, as shown in Figure 8-16. The geometry of the alumina die and graphite tube was shown

in Figure 8-17. The mixed powder was then hot press sintered in a hot press sintering system,

which included a sintering chamber, a high voltage alternating current (AC) generator and a

pyrometer, as shown in Figure 8-19. The high voltage AC generator was connected to a copper

coil inside the sintering chamber. An alternating electromagnetic field was generated inside the

coil with AC current flowing through it. The changing electromagnetic field induced currents in

an electrical conductor as it was placed inside the coil. The induced currents (Eddy current)

generated heat inside the conductor due to the resistance. The alumina die and graphite tube was

placed inside the coil. Eddy currents were induced inside the graphite tube, which is an electrical

conductive material. Alumina and uranium oxide are poor electrical conductors, so no Eddy

currents were generated inside them. The surface temperature of the graphite tube was measured

by an optical pyrometer. Figure 8-18 shows the heated graphite observed through the pyrometer.

The pyrometer communicated with the high voltage AC generator to keep the surface

temperature of the graphite tube constant. The mixed powder was sintered at 1201.4 oC for hour

in argon atmosphere. (The detailed sintering temperature calculation is in Appendix B.) The

pressure applied by the lead bricks on top during the sintering process was about 10 MPa. The

pellets after hot process sintering were soaked in hydrogen atmosphere to reduce the oxygen to

uranium ratio to 2.0. The densities of the pellets after reduction, which were measured by the

Archimedes method, are shown in Table 8-1.

The pellet of UO2 with 5 vol% SiC is shown in Figure 8-20. The SEM images of the pellet

are shown in Figure 8-21 and Figure 8-22 and the XRD result is shown in Figure 8-23. The pellet

of UO2 with 10 vol% SiC is shown in Figure 8-24. The SEM images of the pellet are shown in

Figure 8-25 and Figure 8-26 and the XRD result is shown in Figure 8-27. The pellet of UO2 with









15 vol% SiC is shown in Figure 8-28. The SEM images of the pellet are shown in Figure 8-29

and Figure 8-30 and the XRD result is shown in Figure 8-31.

The pellet of UO2 (<25 CL) with 15 vol% SiC from Advanced Composite Materials (Figure

8-32) was made by hot press sintering to study the effect of UO2 particle size on the final

density. The SEM images of the pellet are shown in Figure 8-33 and Figure 8-34 and the XRD

result is shown in Figure 8-3 5. The density of the pellet measured by Archimedes method was

95.6% TD.

The pellet of UO2 (25 CL to 45 CL) with 15 vol% SiC from Alfa Aesar (Figure 8-36) was also

made by hot press sintering to study the effect of different SiC whiskers on the final density. The

SEM images of the pellet are shown in Figure 8-37 and Figure 8-38 and the XRD result is shown

in Figure 8-39. The density of the pellet measured by Archimedes method was 96.0% TD.

Discussion

The densities of the pellets, which were made by pressureless sintering, decreased sharply

with increasing the amount of SiC whiskers. The density of the pellet of UO2 with 10 vol% SiC

whiskers was only 82.2%, compared to 98.6% of the pellet of UO2 with 5 vol% SiC whisker.

The SiC whiskers hinder the process of UO2 particles j oining together to form large grains. It is

more difficult for pellet to densify with increasing amount of SiC whiskers. High density is

usually not possible with greater than 5 vol% SiC whiskers. The grain size of UO2 with 10 vol%

SiC whiskers in Figure 8-14 is smaller than that of UO2 with 5 vol% SiC whiskers in Figure 8-

11.

The densities of the pellets made by hot press sintering were all close to 95% TD. The

density of the pellet of UO2 with 10 vol% SiC whiskers was 94.7% TD, which was slightly less

than the density of the pellet of UO2 with 5 vol% SiC whiskers, 95.4% TD. It is reasonable

because the SiC whiskers hinder the process of UO2 particles j oining together to form large









grains. It is more difficult for pellet to densify with increasing amount of SiC whiskers. The

density of the pellet of UO2 with 15 vol% SiC whiskers was 95.9%, which was larger than the

density of the pellet of UO2 with 10 vol% SiC whiskers and even larger than the density of the

pellet of UO2 with 5 vol% SiC whiskers. The reason for the unusual increase in the density may

be that the SiC whiskers were not well dispersed in UO2. Some UO2 islands can be found in the

SEM image of the pellet of UO2 and 15 vol% SiC whiskers (Figure 8-29 and Figure 8-30). The

UO2 islands increased the density of the pellet. Some UO2 islands can also be found in the SEM

images of the pellet of UO2 and 5 vol% or 10 vol% SiC whisker. It is more difficult to disperse

SiC whiskers homogeneously in UO2 with increasing amount of SiC whiskers.

The density of the pellet of UO2 (<25 CL) with 15 vol% SiC from Advanced Composite

Materials was 95.6% TD. The density of the pellet of UO2 (25 CL to 45 CL) with 15 vol% SiC from

Alfa Aesar was 96.0% TD. The two densities were very close to the density of UO2 (25 CL to 45

CL) with 15 vol% SiC from Advanced Composite Materials, so the UO2 particle size and the SiC

whiskers are not decisive factors for the density of the pellet after hot press sintering.

Due to the nature of material transport and flow during hot press sintering, the SiC

whiskers exhibited a preferential orientation. The SEM images of the cross section of UO2 with

30 vol% SiC whiskers after hot press sintering are shown in Figure 8-40 and Figure 8-41. The

SiC whiskers are oriented in a plane which is perpendicular to the direction of hot pressing. This

preferential orientation of the SiC whiskers is desired because the preferential orientation

increases the probability of the contact of SiC whiskers, thus increase the probability of the

formation of the percolation pathway for heat to flow out of the pellet.









Table 8-1. Densities of the pellets of UO2 and SiC whiskers after hot press sintering.
UO2 with 5 vol% SiC UO2 with 10 vol% UO2 with 15 vol%
whiskers SiC whiskers SiC whiskers
Density (% TD) 95.4 94.7 95.9
























Figure 8-. Scannin elecromiroscoyiaeo i hsesasrcie rmAfea
(500X).


Figure 8-2. Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar
(2,000X).


~r. ;;,,e1


r _I~ICLLI
















1-



0.8-



~0.6-



0.4-



0.2-



0 *
10 20 30 40 50 60 70 80
26


Figure 8-3. X-ray diffraction pattern of SiC whiskers from Alfa Aesar.


Figure 8-4. Scanning electron microscopy image of SiC whiskers as received from Advanced
Composite Materials (500X).


























1-C

0.8 -


0.6' -r+i 1 :
0.4 -

0.2f -' i'f :~I1 i ~~~~=

u, i ~" 1~
10 20 30 40 50 60 70 8
CIC 26
Fiue86.Xrydifato ptenofSCwisesfo Adane CmoiteMtras
































Figure 8-7. Scanning electron microscopy image of SiC whiskers from Alfa Aesar after
dispersion (2,000X).


Figure 8-8. Scanning electron microscopy image of SiC whiskers from Advanced Composite
Materials after dispersion (2,000X).






































i.:.
.; .?r~L.....


Figure 8-9. The pellet of UO2 with 5 vol% SiC whiskers after pressureless sintering.


Figure 8-10. Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after
pressureless sintering, (2,000X).











































Figure 8-11i. Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after
pressureless sintering, (5,000X).


Figure 8-12. The pellet of UO2 with 10 vol% SiC whiskers after pressureless sintering.


....ii,.ii;;
,i
"1"" i

































Figure 8-13. Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after
pressureless sintering, (2,000X).


Figure 8-14. Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after
pressureless sintering, (5,000X).










































































Sample Holder


I


~~~Li.l.. ......-.'.....C------"-i~--


-- ---


~E~""' "


Position of SiC whisker and UO2.x7 mixture


+--Graphite Tube


Alurnina Die


Figure 8-16. Alumina die, graphite tube and sample holder.


~pl~f"'l'YSu''l111111Illrllllllrllllll~llrlllllll, ''jIlilllr~l IIilllllllj11!11 iil~ijh~l~ i~iP
1111 1111
81 91 'iui
'i '
!L:'""""
*
~i -~ ~~~~-lr~
!k~YR~I~ nrc~P~s~pL!sLlnPrAsrrC~7uLEn mIIW lallls
-L r_ -X_~~~_ C-LIC.. ~---1L-~~-L_~_~ 4LCL~lr-~~




Figure 8-15. Alumina die for hot press sintering.










1/ inch



br


Graphite tube


O


O
O

O


1200oC O
O


g O Mixed powder

Ot Coil

OAlumina tube




1/ inch


1 inch


Figure 8-17. Geometry of the alumina die and graphite tube.


Figure 8-18. Heated graphite tube observed through the optical pyrometer.























































Figure 8-19. Hot press sintering apparatus.









97


































Figure 8-20. The pellet of UO2 with 5 vol% SiC whiskers after hot press sintering.


Figure 8-21. Scanning electron microscopy image of UO2 with 5 vol% SiC whiskers after hot
press sintering, (2,000X).













































































10 20 30 40 50 60 70 80
26


Figure 8-23. X-ray diffraction result of UO2 with 5 vol% SiC whiskers after hot press sintering.


Figure 8-22. Scanning electron microscopy image of UO2 with 5 vol%
press sintering, (5,000X).


SiC whiskers after hot


0.8-



0.6-


0.2 1-


-
,



























Figure 8-24. The pellet of UO2 with 10 vol% SiC whiskers after hot press sintering.


Figure 8-25. Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after hot
press sintering, (2,000X).


.1. ;*I ;.~
~ib:.iiic~...i
i"x1 i ..ii IIt.t F
t





10 20 30 40 50 60 70 80
26

Figure 8-27. X-ray diffraction result of UO2 with 10 vol% SiC whiskers after hot press sintering.


Figure 8-26. Scanning electron microscopy image of UO2 with 10 vol% SiC whiskers after hot
press sintering, (5,000X).

1.2 1IIIII uI


0.8-


0.6-


0.2 -~

































Figure 8-28. The pellet of UO2 with 15 vol% SiC whiskers after hot press sintering.


Figure 8-29. Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers after hot
press sintering, (2,000X).











LI.I

;c~


Figure 8-30. Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers after hot
press sintering, (5,000X).


1.2







0.8-



S0.6-



0.4-



0.2-




10 20 30 40 50 60 70 80
26


Figure 8-31i. X-ray diffraction result of UO2 with 15 vol% SiC whiskers after hot press sintering.


































Figure 8-32. The pellet of UO2 (<25 CL) with 15 vol% SiC whiskers after hot press sintering.


Figure 8-33. Scanning electron microscopy image of UO2 (<25 CL) with 15 vol% SiC whiskers
after hot press sintering, (2,000X).

































Figure 8-34. Scanning electron microscopy image of UO2 (<25 CL) with 15 vol% SiC whiskers
after hot press sintering, (5,000X).












1.2







0.8-



S0.6-



0.4-



0.2-




10 20 30 40 50 60 70 80
26


Figure 8-35. X-ray diffraction result of UO2 (<25 CL) with 15 vol% SiC whiskers after hot press
sintering.



































































~ s
~pr;
W
rsl


c~'
~c .


'!SI; ';
li;;al""..;i. :.
;; .ii .I:,,
;.i.


;.'iii "r

.
Illl;lllll;lfiiiii '' ;..
i i i::..

II
i i.;.

..;..


- --- FII~-IC


__


''C ;
*
.""
'"'
'' '',Iii ~;lli;il~;.
; ; il ;;` i;

.. Cii''
..i
ii i.; ;;
:!!I:i.C;;;;;ii; II
i 1
'"*"''' C. "''"' '
r.:;il i .. i+r
'i ;; ..; '..iii""';ii..ii .;'.. *..;;" ii f
; ; ;"i;
i;i "
i; ;.iii *; II
i;;;;i;l; ;;;';;;;
;i; ;
i
i; i;;;;. ;
Ii i~ iii:,: ;;;;;
t;
;; ;;;'""";'; ll:;i
I'-
;i ,r'i, ,; .;
ii
I



I Irri~
7r '1' ~' r !I I


s g ~:


I


i


1'1


Figure 8-36. The pellet of UO2 with 15
smntening.


vol% SiC whiskers (from Alfa Aesar) after hot press


.4'W~~~ 4..raW


Figure 8-37. Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers (from Alfa
Aesar) after hot press sintering, (2,000X).




107



































Figure 8-38. Scanning electron microscopy image of UO2 with 15 vol% SiC whiskers (from Alfa
Aesar) after hot press sintering, (5,000X).







I I I I I I


0.8-

0.6-

0.4-


L~i,


0.2 -


10 20 30 40 50 60 70 80
26
Figure 8-39. X-ray diffraction result of UO2 with 15 vol% SiC whiskers (from Alfa Aesar) after
hot press sintering.


Lc~































Figure 8-40. Scanning electron microscopy image of the cross section of UO2 with 30 vol% SiC
whiskers after hot press sintering, (2,000X).


Figure 8-41. Scanning electron microscopy image of the cross section of UO2 with 30 vol% SiC
whiskers after hot press sintering, (5,000X).









CHAPTER 9
CONCLUSIONS AND FUTURE WORK

Conclusion

In this research, the silicon carbide (SiC) whisker-uranium dioxide (UO2) COmposite was

successfully fabricated by low temperature sintering method of UO2. High density pellets were

achieved at 1200 oC by pressureless sintering of UO2 with 5 vol% SiC whiskers and hot press

sintering of UO2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. Based on the work by Russell

et al. [7], Johnson et al. [8] and Hesselman et al. [9], The thermal conductivity of SiC whisker-

UO2 COmposite is higher than the thermal conductivity of UO2-

Two other methods to incorporate SiC into UO2, the chemical vapor deposition (CVD)

process to coat uranium oxide particles with SiC and the pre-ceramic polymer coating on

uranium oxide particles process, were not successful.

Because SiC whiskers replace UO2, there is a reactivity penalty at the end of life (EOL).

The neutronic calculation showed the K-infinity ofUO2 with 5 vol%, 10 vol% and 15 vol% SiC

are about 2.4%, 4.9% and 7.6% less than the K-infinity of UO2 fuel at 60 GWD/MTU burnup.

The amount of SiC whiskers was limited to 15 vol% in this research to ensure the neutronic

properties of the fuel pellet. The neutronic calculation also proved the UO2 with up to 15 vol%

SiC additives can be operate safely in a reactor core because of the negative Doppler coefficient

and moderator temperature coefficient.

During the fabrication process of SiC whisker UO2 pellet, the low temperature sintering

method of UO2 WAS used to avoid the reaction between silicon carbide and UO2 at the

temperature above 1377 oC. The density of the SiC whisker-UO2 pellet fabricated by hot press

sintering was not significantly affected by the particle size of uranium oxide particle and the type









of SiC whiskers. The SiC whiskers after hot press sintering exhibited a preferential orientation,

which is perpendicular to the direction of hot pressing.

Future Work

There are several aspects of this research can be improved or studied in the future work.

The thermal conductivity of the UO2-SiC pellets need to be measured to study the effect of SiC

whiskers on the thermal conductivity of UO2-

The process of mixing SiC whiskers and UO2 particles need to be improved to make a

homogeneous mixture. The UO2 islands in the SiC whiskers-UO2 pellet can cause local heat

spot, which is highly undesired in the operation of the reactor.

The effect of whiskers orientation, aspect ratio of whiskers, particle size of the matrix and

interfacial thermal barrier resistance, if any, on the thermal conductivity of the composite need to

be studied. Because the thermal conductivity of single crystal drops significantly after radiation,

the change of the thermal conductivity of SiC whiskers-UO2 pellet under radiation also need to

be studied.









APPENDIX A
CASMO INPUT FILES

This appendix contains all of the CASMO-3 input files used to determine the effect of

silicon carbide additives on the neutronic properties of uranium dioxide fuel.












The CASMO-3 Input File for Uranium Dioxiide Fuel

*Crystal River-3 15X15 Assembly UO2 Fuel
TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 10.5216/4.66 *FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 29.36
XEN 0
LPI

17
1 11
111 7




DEP 0.0 0.5 5 10 15 20 25 30 35 40 45 50 55 60
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.5 5 10 15 20 25 30 35 40 45 50 55 60
TFU 900 1000 1100
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.5 5 10 15 20 25 30 35 40 45 50 55 60
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 5 w% Silicon Carbide


*Crystal River-3 15X15 Assembly UO2 Fuel with 5 w% SiC
TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 9.4458/3.9023 92238=79.8384 8000=11.2593 6000=1.4963 14000=3.5037
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 34.43
XEN 0
LPI


1 11 1
11 11 7
1 17 11 1
11 11 11 1
1 11 11 11 1
DEP 0.0 0.59 5.86 11.73 17.59 23.45 29.31 35.18
41.04 46.90 52.76 58.63 64.49 70.35
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.59 5.86 11.73 17.59 23.45 29.31
41.04 46.90 52.76 58.63 64.49 70.35
TFU 900 1000 1100
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.59 5.86 11.73 17.59 23.45 29.31
41.04 46.90 52.76 58.63 64.49 70.35
TMO 563 588 613
NLI,STA
END


35.18




35.18












The CASMO-3 Input File for Uranium Dioxide Fuel with 10 w% Silicon Carbide


*Crystal River-3 15X15 Assembly UO2 Fuel with 10 w% SiC
TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 8.5696/3.6969 92238=75.6364 8000=10.6667 6000=2.9925 14000=7.0075
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 40.06
XEN 0
LPI


1 11 1
11 11 7
1 17 11 1
11 11 11 1
1 11 11 11 1
DEP 0.0 0.68 6.82 13.64 20.46 27.28 34.10 40.93
47.75 54.57 61.39 68.21 75.03 81.85
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.68 6.82 13.64 20.46 27.28 34.10
47.75 54.57 61.39 68.21 75.03 81.85
TFU 900 1000 1100
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.68 6.82 13.64 20.46 27.28 34.10
47.75 54.57 61.39 68.21 75.03 81.85
TMO 563 588 613
NLI,STA
END


40.93




40.93












The CASMO-3 Input File for Uranium Dioxide Fuel with 5 vol% Silicon Carbide

*Crystal River-3 15X15 Assembly UO2 Fuel with 5 vol% SiC
TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 10.156/4.0428 92238=82.7123 8000=11.6646 6000=0.4729 14000=1.1074
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 30.91
XEN 0
LPI

17
1 11
111 7




1 68 121 173 126 178 1 1 1

STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58
36.84 42.11 47.37 52.63 57.89 63.16
TFU 900 1000 1100
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58
36.84 42.11 47.37 52.63 57.89 63.16
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide


*Crystal River-3 15X15 Assembly UO2 Fuel with 10 vol% SiC
TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 9.7904/3.9730 92238=81.2850 8000=11.4633 6000=0.9812 14000=2.2975
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 32.63
XEN 0
LPI


1 11 1
11 11 7
1 17 11 1
11 11 11 1
1 11 11 11 1
DEP 0.0 0.56 5.56 11.11 16.67 22.22 27.78
38.89 44.44 50.00 55.56 61.11 66.67
STA


33.33


TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78
38.89 44.44 50.00 55.56 61.11 66.67
TFU 900 1000 1100
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78
38.89 44.44 50.00 55.56 61.11 66.67
TMO 563 588 613
NLI,STA
END


33.33




33.33












The CASMO-3 Input File for Uranium Dioxiide Fuel with 15 vol% Silicon Carbide

*Crystal River-3 15X15 Assembly UO2 Fuel with 15 vol% SiC
TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 9.4249/3.8978 92238=79.7470 8000=11.2464 6000=1.5288 14000=3.5800
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 34.55
XEN 0
LPI

17
1 11
111 7




DEP 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29
41.18 47.06 52.94 58.82 64.71 70.59
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29
41.18 47.06 52.94 58.82 64.71 70.59
TFU 900 1000 1100
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29
41.18 47.06 52.94 58.82 64.71 70.59
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 5 vol% Silicon Carbide at Lower
Fuel Teniperature

*Crystal River-3 15X15 Assembly UO2 Fuel with 5 vol% SiC
TIT TFU=800 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 10.156/4.0428 92238=82.7123 8000=11.6646 6000=0.4729 14000=1.1074
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 30.91
XEN 0
LPI

17
1 11
111 7




1 68 121 173 126 178 1 1 1

STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58
36.84 42.11 47.37 52.63 57.89 63.16
TFU 700 800 900
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58
36.84 42.11 47.37 52.63 57.89 63.16
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide at
Lonver Fuel Teniperature

*Crystal River-3 15X15 Assembly UO2 Fuel with 10 vol% SiC
TIT TFU=800 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 9.7904/3.9730 92238=81.2850 8000=11.4633 6000=0.9812 14000=2.2975
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 32.63
XEN 0
LPI


1 11 1
11 11 7
1 17 11 1
11 11 11 1
1 11 11 11 1
DEP 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33
38.89 44.44 50.00 55.56 61.11 66.67
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33
38.89 44.44 50.00 55.56 61.11 66.67
TFU 700 800 900
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33
38.89 44.44 50.00 55.56 61.11 66.67
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 15 vol% Silicon Carbide at
Lonver Fuel Teniperature

*Crystal River-3 15X15 Assembly UO2 Fuel with 15 vol% SiC
TIT TFU=800 TMO=588 BOR=0 IDE='P32' *Mark-B10E
FUE 1 9.4249/3.8978 92238=79.7470 8000=11.2464 6000=1.5288 14000=3.5800
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 34.55
XEN 0
LPI


1 11 1
11 11 7
1 17 11 1
11 11 11 1
1 11 11 11 1
DEP 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29
41.18 47.06 52.94 58.82 64.71 70.59
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41
41.18 47.06 52.94 58.82 64.71 70.59
TFU 700 800 900
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41
41.18 47.06 52.94 58.82 64.71 70.59
TMO 563 588 613
NLI,STA
END


35.29




35.29












The CASMO-3 Input File for Uranium Dioxide Fuel with 5 vol% Silicon Carbide at Lower
Fuel Temperature and Boron Let Down

*Crystal River-3 15X15 Assembly UO2 Fuel with 5 vol% SiC
TIT TFU=800 TMO=588 BOR=1400 IDE='P32' *Mark-B10E
FUE 1 10.156/4.0428 92238=82.7123 8000=11.6646 6000=0.4729 14000=1.1074
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 30.91
XEN 0
LPI

17
1 11
111 7


1 1 1 1
1 1 1 1 7
1 1 1 140


26.32 10

47.38 1400
63.16 10
DEP 0.0 0.53 5.26 10.53 15.79 21.05 26.32 26.33 26.74 30.53 34.74
38.95 43.16 47.37 47.38 47.68 50.53 53.68 56.84 60.0 63.16
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 26.33 26.74 30.53
34.74 38.95 43.16 47.37 47.38 47.68 50.53 53.68 56.84 60.0 63.16
TFU 700 800 900
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 26.33 26.74 30.53
34.74 38.95 43.16 47.37 47.38 47.68 50.53 53.68 56.84 60.0 63.16
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide at
Lower Fuel Temperature and Boron Let Down

*Crystal River-3 15X15 Assembly UO2 Fuel with 10 vol% SiC
TIT TFU=800 TMO=588 BOR=1400 IDE='P32' *Mark-B10E
FUE 1 9.7904/3.9730 92238=81.2850 8000=11.4633 6000=0.9812 14000=2.2975
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 32.63
XEN 0
LPI

17
1 11
111 7


1 1 1 1
1 1 1 1 7
1 17 11 140
1 1 1 1 1

50. 1400


66.67 10
DEP 0.0 0.56 5.56 11.11 16.67 22.22 27.78 27.79 28.22 32.22 36.67
41.11 45.56 50.0 50.01 50.33 53.33 56.67 60.0 63.33 66.67
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 27.79 28.22 32.22
36.67 41.11 45.56 50.0 50.01 50.33 53.33 56.67 60.0 63.33 66.67
TFU 700 800 900
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 27.79 28.22 32.22
36.67 41.11 45.56 50.0 50.01 50.33 53.33 56.67 60.0 63.33 66.67
TMO 563 588 613
NLI,STA
END












The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide at
Lower Fuel Temperature and Boron Let Down

*Crystal River-3 15X15 Assembly UO2 Fuel with 15 vol% SiC
TIT TFU=800 TMO=588 BOR=1400 IDE='P32' *Mark-B10E
FUE 1 9.4249/3.8978 92238=79.7470 8000=11.2464 6000=1.5288 14000=3.5800
*FUEL COMP. #, DENSITY/ENRICHMENT
PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN'
PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole
PWR 15 1.443 21.81 PWR with pitch 1.443
FUM 2.778-04 2
PDE 34.55
XEN 0
LPI

17
1 11
111 7


1 1 1 1
1 1 11 70
1 1 1 140
1 1 1 1 1

029 1400


70.59 10
DEP 0.0 0.59 5.88 11.76 17.65 23.53 29.41 29.42 29.88 34.12 38.82
43.53 48.24 52.94 52.95 53.29 56.47 60.0 63.53 67.06 70.59
STA
TIT *+ FUEL TEMP BRANCHES
RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 29.42 29.88 34.12
38.82 43.53 48.24 52.94 52.95 53.29 56.47 60.0 63.53 67.06 70.59
TFU 700 800 900
NLI,STA
TIT *+ MOD TEMP BRANCHES
RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 29.42 29.88 34.12
38.82 43.53 48.24 52.94 52.95 53.29 56.47 60.0 63.53 67.06 70.59
TMO 563 588 613
NLI,STA
END









APPENDIX B
HOT PRESS SINTERING TEMPERATURE CALCULATION

The surface temperature of the graphite tube is 1200 oC. The geometry of the alumina die

and the graphite tube is shown in Figure 8-17. The steady state temperatures of the alumina tube

and mixed powder were calculated base on Equation B-1 and Equation B-5.

For graphite tube,


l(r ) ~+ = 0 (B-1)
r dr dr k

Where q"'' is the rate of heat production per unit volume and K, is the thermal conductivity

of graphite.

The two boundary conditions of the graphite tubs are Equation 3-2 and Equation 3-3.

dT
= 0 at r = 0.25 inch (B-2)
dr

T (r = 0.5 inch) = 1200 oC (B-3)

The heat flux on the surface of the graphite tube can be calculated by Equation 3-4.

q"' = cof" (B-4)

Where E is the emissivity, which is 0.95 for graphite, o is the Stefan-Boltzmann constant,

which equals to 5.67x10s W/m2-K4

The calculated graphite temperature at r = 0.25 inch is 1201.4 oC.

For alumina tube,

1 d dT
(r ) = (B-5)
r dr dr

The two boundary conditions of the graphite tubs are Equation 3-6 and Equation 3-7.

dT
= 0 at r = 0.125 inch (B-6)
dr









T (r = 0.25 inch) = 1201.4 oC (B-7)

The calculated temperature of alumina tube is a constant, 1201.4 oC.

For mixed powder,

1 d dT
(r ) = (B-8)
r dr dr

The two boundary conditions of the graphite tubs are Equation 3-6 and Equation 3-7.

dT
= 0 at r = (B-9)
dr

T (r = 0. 125 inch) = 1201.4 oC (B-10)

The calculated temperature of mixed powder is also a constant, 1201.4 oC.










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BIOGRAPHICAL SKETCH

Jiwei Wang was born in 1978 in Anshan City, Liaoning Province, People's Republic of

China. His parents are Li Wang and Shaofen Liu. He has an older sister, Jihong Wang. Jiwei

graduated with a Bachelor of Science in nuclear engineering from Tsinghua University in July

2001. After that, he took the TOEFL and GRE tests and prepared to pursue a graduate degree in

United States. Jiwei enrolled in nuclear and radiological engineering's graduate program at the

University of Florida in January 2003. He received a non thesis Master of Science degree in

December 2006. He is scheduled to graduate with a Doctor of Philosophy degree in August

2008.





PAGE 1

DEVELOPING A HIGH THERMAL CONDUCTIVITY NUCLEAR FUEL WITH SILICON CARBIDE ADDITIVES By JIWEI WANG A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008 1

PAGE 2

2008 Jiwei Wang 2

PAGE 3

To my parents, Li Wang and Shaofen Liu, for their love and support. 3

PAGE 4

ACKNOWLEDGMENTS I would like to thank Professor James Tulenko, my advisor and the supervisory committee chair, for his guidance and support. I would also like to thank Dr. Ronald Baney, my supervisory committee co-chair, for his patience and instruction. Without their help, this work would not have been possible. I would like to thank Dr. Samim Anghaie for being on my supervisory committee and his kindness for providing the equipments at the Innovative Nuclear Space Powder and Propulsion Institute. I would also like to thank my supervisory committee member Dr. Edward Dugan for his help on the computer codes. I would like to thank Shirvan Daryoosh at Innovative Nuclear Space Powder and Propulsion Institute for his help on the experiment equipments. I would also like to thank Department of energy for the funding of this study. Finally, I would like to thank my family for their consistent support through the many years of my education. 4

PAGE 5

TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................7 LIST OF FIGURES .........................................................................................................................8 LIST OF ABBREVIATIONS ........................................................................................................13 ABSTRACT ...................................................................................................................................14 CHAPTER 1 INTRODUCTION..................................................................................................................16 2 LITERATURE REVIEW.......................................................................................................20 Properties of Uranium Dioxide...............................................................................................20 Properties of Silicon Carbide..................................................................................................22 3 NEUTRONIC CALCULATION............................................................................................30 Introduction.............................................................................................................................30 Methods and Results...............................................................................................................30 Discussion...............................................................................................................................31 4 REACTION BETWEEN URAINIUM OXIDE AND SILICON CARBIDE........................50 Background.............................................................................................................................50 Experiments and Results.........................................................................................................50 Oxygen to Uranium Ratio of Uranium Oxide Powder....................................................50 Particle Size Distribution of Uranium Oxide Powder.....................................................51 Sintering UO 2 and SiC at 1300 o C and 1650 o C..............................................................51 Discussion...............................................................................................................................52 5 LOW TEMPERATURE SINTERING OF URANIUM DIOXIDE.......................................60 Background.............................................................................................................................60 Experiments and Results.........................................................................................................61 Discussion...............................................................................................................................62 6 SILICON CARBIDE COATING BY CHEMICAL VAPOR DEPOSITION.......................68 Background.............................................................................................................................68 Experiment and Result............................................................................................................69 Discussion...............................................................................................................................70 5

PAGE 6

7 SILICON CARBIDE COATING FROM PRECERAMIC POLYMER................................75 Background.............................................................................................................................75 Experiments and Results.........................................................................................................75 Discussion...............................................................................................................................76 8 SILICON CARBIDE WHISKERS URANIUM DIOXIDE COMPOSITE........................81 Background.............................................................................................................................81 Experiments and Results.........................................................................................................82 Characterization of Silicon Carbide Whiskers................................................................82 Mixing of SiC Whiskers and Uranium Oxide Powder....................................................83 Pressureless Sintering of SiC Whiskers and Uranium Oxide Powder............................83 Hot Press Sintering of SiC Whiskers and Uranium Oxide Powder.................................83 Discussion...............................................................................................................................85 9 CONCLUSIONS AND FUTURE WORK...........................................................................111 Conclusion............................................................................................................................111 Future Work..........................................................................................................................112 APPENDIX A CASMO INPUT FILES........................................................................................................113 B HOT PRESS SINTERING TEMPERATURE CALCULATION........................................126 LIST OF REFERENCES.............................................................................................................128 BIOGRAPHICAL SKETCH.......................................................................................................132 6

PAGE 7

LIST OF TABLES Table page 2-1 Neutronic cross sections (barns)........................................................................................26 3-1 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel.............................34 3-2 Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel.....................................................................................................................................34 3-3 MTC (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel......................35 3-4 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature)................................................................................................................35 3-5 Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature)................................................................................36 3-6 MTC (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature).....................................................................................................36 3-7 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down..................................................................................37 3-8 K-infinity versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down..................................................................................37 3-9 Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down...................................................38 3-10 Doppler coefficient (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down..................................................38 3-11 MTC (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down.......................................................................39 3-12 MTC (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down.......................................................................39 3-13 Centerline temperature of UO 2 fuel compared to UO 2 with SiC.......................................40 4-1 Particle size distribution of received uranium oxide powder.............................................53 8-1 Densities of the pellets of UO 2 and SiC whiskers after hot press sintering.......................87 7

PAGE 8

LIST OF FIGURES Figure page 2-1 Thermal conductivity of UO 2 .............................................................................................27 2-2 Thermal conductivity of hyperstoichiometric UO 2 ............................................................27 2-3 Thermal conductivity of (U 0.8 Pu 0.2 )Ox..............................................................................28 2-4 Thermal conductivity of UO 2 before and after irradiation.................................................28 2-5 Thermal conductivity of single crystal SiC and polycrystalline SiC compared to UO 2 ....29 2-6 Thermal conductivity of SiC before and after irradiation compared to UO 2 .....................29 3-1 Crystal River 15X15 assembly design. .............................................................................41 3-2 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel.............................41 3-3 Doppler coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel..............42 3-4 Moderator temperature coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel..............................................................................................................................42 3-5 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel.............................43 3-6 Doppler coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel..............43 3-7 Moderator temperature coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel..............................................................................................................................44 3-8 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature)................................................................................................................44 3-9 Doppler coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature).......................................................................................45 3-10 Moderator temperature coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature).........................................................................45 3-11 Boron concentration versus burnup at boron let-down......................................................46 3-12 K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down..................................................................................46 3-13 K-infinity versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down..................................................................................47 8

PAGE 9

3-14 Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down...................................................47 3-15 Doppler coefficient (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down..................................................48 3-16 Moderator temperature coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down............................48 3-17 Moderator temperature coefficient (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down...........................49 4-1 Uranium oxide powders with different O/U ratio. A) UO 2.10 B) UO 2.27 C) U 3 O 8 D) UO 2 ....................................................................................................................................54 4-2 X-ray diffraction pattern of UO 2.10 powder.......................................................................54 4-3 X-ray diffraction pattern of UO 2.27 powder.......................................................................55 4-4 X-ray diffraction pattern of U 3 O 8 powder.........................................................................55 4-5 X-ray diffraction pattern of UO 2.0 powder.........................................................................56 4-6 Sieve and shaker for analyzing particle size distribution...................................................56 4-7 Particle size distribution of received uranium oxide powder.............................................57 4-8 X-ray diffraction pattern of 30nm -SiC from Alfa Aesar................................................57 4-9 Uranium dioxide-silicon carbide pellet after sintering at 1300 o C....................................58 4-10 X-ray diffraction pattern of UO 2 -SiC pellet after sintering at 1300 o C............................58 4-11 Uranium dioxide-silicon caribide pellet after sintering at 1650 o C...................................59 4-12 X-ray diffraction pattern of UO 2 -SiC pellet after sintering at 1650 o C.............................59 5-1 Oxygen to uranium ratio of UO 2 powder oxidized in air at 140 o C. .................................64 5-2 Uranium dioxide pellet sintered at 1650 o C.......................................................................64 5-3 Scanning electron microscopy image of UO 2 pellet sintered at 1650 o C (5,000X)...........65 5-4 Uranium dioxide pellet sintered at 1200 o C.......................................................................65 5-5 Scanning electron microscopy image of UO 2 pellet sintered at 1200 o C (5,000X)...........66 5-6 Uranium dioxide pellet sintered at 1200 o C with pressure................................................66 9

PAGE 10

5-7 Scanning electron microscopy image of UO 2 pellet sintered at 1200 o C with pressure (5,000X).............................................................................................................................67 6-1 Lindberg high temperature furnace....................................................................................72 6-2 Temperature profile of the CVD process...........................................................................72 6-3 Uranium oxide powder after CVD process........................................................................73 6-4 Fourier transform infrared spectroscopy result of the powder after the CVD process......73 6-5 X-ray diffraction result of the powder after CVD process................................................74 6-6 Furnace tube after CVD process........................................................................................74 7-1 Allylhydridopolycarbosilane (AHPCS), the SiC pre-ceramic polymer.............................78 7-2 Lindberg/Blue Mini-Mite 1100 o C furnace........................................................................78 7-3 The temperature profile of sintering process.....................................................................79 7-4 Silicon carbide pellet made by SiC powder (1 ) and AHPCS.........................................79 7-5 Scanning electron microscopy image of the SiC pellet made by SiC powder (1 ) and AHPCS...............................................................................................................................80 8-1 Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar (500X)................................................................................................................................88 8-2 Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar (2,000X).............................................................................................................................88 8-3 X-ray diffraction pattern of SiC whiskers from Alfa Aesar..............................................89 8-4 Scanning electron microscopy image of SiC whiskers as received from Advanced Composite Materials (500X)..............................................................................................89 8-5 Scanning electron microscopy image of SiC whiskers as received from Advanced Composite Materials (2,000X)...........................................................................................90 8-6 X-ray diffraction pattern of SiC whiskers from Advanced Composite Materials.............90 8-7 Scanning electron microscopy image of SiC whiskers from Alfa Aesar after dispersion (2,000X)............................................................................................................91 8-8 Scanning electron microscopy image of SiC whiskers from Advanced Composite Materials after dispersion (2,000X)...................................................................................91 8-9 The pellet of UO 2 with 5 vol% SiC whiskers after pressureless sintering.........................92 10

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8-10 Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after pressureless sintering, (2,000X).........................................................................................92 8-11 Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after pressureless sintering, (5,000X).........................................................................................93 8-12 The pellet of UO 2 with 10 vol% SiC whiskers after pressureless sintering.......................93 8-13 Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after pressureless sintering, (2,000X).........................................................................................94 8-14 Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after pressureless sintering, (5,000X).........................................................................................94 8-15 Alumina die for hot press sintering....................................................................................95 8-16 Alumina die, graphite tube and sample holder..................................................................95 8-17 Geometry of the alumina die and graphite tube.................................................................96 8-18 Heated graphite tube observed through the optical pyrometer..........................................96 8-19 Hot press sintering apparatus.............................................................................................97 8-20 The pellet of UO 2 with 5 vol% SiC whiskers after hot press sintering..............................98 8-21 Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after hot press sintering, (2,000X)....................................................................................................98 8-22 Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after hot press sintering, (5,000X)....................................................................................................99 8-23 X-ray diffraction result of UO 2 with 5 vol% SiC whiskers after hot press sintering.........99 8-24 The pellet of UO 2 with 10 vol% SiC whiskers after hot press sintering..........................100 8-25 Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after hot press sintering, (2,000X)..................................................................................................100 8-26 Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after hot press sintering, (5,000X)..................................................................................................101 8-27 X-ray diffraction result of UO 2 with 10 vol% SiC whiskers after hot press sintering.....101 8-28 The pellet of UO 2 with 15 vol% SiC whiskers after hot press sintering..........................102 8-29 Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers after hot press sintering, (2,000X)..................................................................................................102 11

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8-30 Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers after hot press sintering, (5,000X)..................................................................................................103 8-31 X-ray diffraction result of UO 2 with 15 vol% SiC whiskers after hot press sintering.....103 8-32 The pellet of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering............104 8-33 Scanning electron microscopy image of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering, (2,000X)...................................................................................104 8-34 Scanning electron microscopy image of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering, (5,000X)...................................................................................105 8-35 X-ray diffraction result of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering............................................................................................................................106 8-36 The pellet of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering............................................................................................................................107 8-37 Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering, (2,000X)........................................................................107 8-38 Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering, (5,000X)........................................................................108 8-39 X-ray diffraction result of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering............................................................................................................109 8-40 Scanning electron microscopy image of the cross section of UO 2 with 30 vol% SiC whiskers after hot press sintering, (2,000X)....................................................................110 8-41 Scanning electron microscopy image of the cross section of UO 2 with 30 vol% SiC whiskers after hot press sintering, (5,000X)....................................................................110 12

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LIST OF ABBREVIATIONS AC Alternating current AHPCS Allylhydridopolycarbosilane BOL Beginning of life CVD Chemical vapor deposition DSC Differential scanning calorimetry EOC End of cycle EOL End of life FTIR Fourier transform infrared spectroscopy LOCA Lost of coolant accident MTC Moderator temperature Coefficient MTS Methyltrichlorosilane PACVD Photo assisted chemical vapor deposition PECVD Plasma enhanced chemical vapor deposition SCB Silacyclobutane SEM Scanning electron microscopy SiC Silicon carbide TD Theoretical Density TMS Trimethylsilane XRD X-ray diffraction UO 2 Uranium dioxide 13

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy DEVELOPING A HIGH THERMAL CONDUCTIVITY NUCLEAR FUEL WITH SILICON CARBIDE ADDITIVES By Jiwei Wang August 2008 Chair: James Tulenko Major: Nuclear Engineering Sciences Uranium dioxide (UO 2 ) is the most common fuel material utilized in commercial nuclear power reactors. The main disadvantage of UO 2 is its low thermal conductivity. During a reactors operation, there is a large temperature gradient in the UO 2 fuel pellet, causing a very high centerline temperature, and introducing thermal stresses, which lead to extensive fuel pellet cracking. These cracks will add to the release of fission product gases after high burnup. The high fuel operating temperature also increases the rate of fission gas release and the fuel pellet swelling caused by both fission gases bubbles and thermal expansion. The amount of fission gas release and fuel swelling limits the life time of UO 2 fuel in reactor. The objective of this research is to increase the thermal conductivity of UO 2 while not significantly affecting the neutronic property of UO 2 The concept is to incorporate another high thermal conductivity material, silicon carbide (SiC), into the UO 2 pellet. Silicon carbide is expected to form a conductive percolation pathway in the UO 2 for heat to flow out of the fuel pellet, thus increasing the UO 2 thermal conductivity. Three methods were studied to incorporate SiC into UO 2 Firstly, chemical vapor deposition (CVD) process was used to coat UO 2 particles with a SiC layer prior to the low temperature sintering process of UO 2 Secondly, allylhydridopolycarbosilane (AHPCS), a pre14

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ceramic polymer, was used to generate a SiC coating on UO 2 particles prior to the low temperature sintering process of UO 2 Thirdly, silicon carbide whiskers were mixed with UO 2 particles prior to the low temperature sintering method of UO 2 Though pellets with high density were not achieved by the first and second methods, pellets of 95% TD were achieved by pressureless sintering of UO 2 with 5 vol% SiC whiskers and hot press sintering of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. The thermal conductivity of the pellets will be measured at Idaho National Laboratory. The thermal conductivity data will not be discussed in this dissertation. 15

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CHAPTER 1 INTRODUCTION Uranium dioxide (UO 2 ) is the most common fuel material in commercial nuclear power reactors. UO 2 has the advantages of a high melting point, good high-temperature stability, good chemical compatibility with cladding and coolant and resistance to radiation. The main disadvantage of UO 2 is its low thermal conductivity. During a reactors operation, because the thermal conductivity of UO 2 is very low, for example, about 2.8 W/m-K at 1000 o C [1], there is a large temperature gradient in the UO 2 fuel pellet, causing a very high centerline temperature and introducing thermal stresses, which lead to extensive fuel pellet cracking. These cracks will add to the release of fission product gases after high burnup. The high fuel operating temperature also increases the rate of fission gas release and the fuel pellet swelling caused by fission gases bubbles. The amount of fission gas release and fuel swelling limits the life time of UO 2 fuel in a reactor. The objective of this research is to increase the thermal conductivity of UO 2 while not significantly affecting the neutronic property of UO 2 The concept is to incorporate another material with high thermal conductivity into UO 2 It has been reported that a 50% increase in the thermal conductivity of UO 2 has been achieved by adding 5 w% molybdenum (Mo) at 1000 o C [2], however, Mo has a large thermal neutron absorption cross section, which negates its use in thermal reactors. It has also been reported that a 25% increase in the thermal conductivity of UO 2 has been achieved by adding 1.2 w% BeO [3], however, BeO is a very toxic material for handling. Silicon carbide (SiC) is chosen in this research because the thermal conductivity of single crystal SiC is sixty times higher than that of UO 2 at room temperature and thirty times higher at 16

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800 o C [4]. Silicon carbide also has the advantage of a low thermal neutron absorption cross section, high melting point, good chemical stability and good irradiation stability [5]. The neutronic properties of UO 2 will be affected by the addition of the SiC whiskers. The effect of SiC whiskers on the neutronic property of a UO 2 pellet was simulated by CASMO-3 [6], a multi-group two-dimensional transport theory code. The CASMO-3 result set the limit of the amount of SiC additives that could be added to UO 2 without significantly affecting the neutronic property of UO 2 Several studies have shown that SiC whiskers can increase the thermal conductivity of matrix materials. Russell et al. [7] reported the thermal conductivity of 30 vol% VLS SiC whisker-mullite composite was three times higher at room temperature than that of single phase mullite in the perpendicular direction to the hot pressing direction, and two times higher in the parallel direction to the hot pressing direction. Johnson et al. [8] reported that the thermal conductivity of 30 vol% SiC whisker-osumilite glass composite was four times higher at room temperature than that of single phase mullite in the perpendicular direction to the hot pressing direction, and two times higher in the parallel direction to the hot pressing direction. Hesselman et al. [9] showed the thermal conductivity of 30 vol% VLS SiC whisker-lithium aluminosilicate glass composite was five times higher at room temperature than that of lithium aluminosilicate glass in the perpendicular direction to the hot pressing direction, and three times higher in the parallel direction to the hot pressing direction. Hesselman et al. [9] suggested that a SiC whisker percolation pathway was formed, and heat was conducted through SiC whiskers, bypassing the matrix. Solomon et al [10, 11] have tried to increase the thermal conductivity of UO 2 by impregnating a SiC pre-ceramic polymer into a high dense UO 2 pellet with 10 to 12 vol% open 17

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porosity and transferring the SiC pre-ceramic polymer to a crystalline form at 1300 o C. There was no improvement in the thermal conductivity of UO 2 because the maximum density of SiC was 75% TD even after several impregnation, which due to the blockage of the open porosity by the SiC during the impregnation and the low crystallization temperature. Uranium dioxide pellets are usually produced by sintering the green pellets at about 1700 o C in hydrogen atmosphere. The high sintering temperature around 1700 o C is necessary to achieve the required high density of 95% TD. However, based on the study by Allen et al. [12], UO 2 reacts with SiC at the temperature above 1377 o C, evolving CO and SiO, forming uranium carbide, uranium silicide and U 3 C 3 Si 2 A two-stage low temperature sintering method of UO 2 was studied to avoid the reaction between UO 2 and SiC. Fuhrman et al. [13] have reported that UO 2 pellets of 95% to 97% theoretical density (TD) were achieved by sintering at 1200 o C in nitrogen for one hour followed by one hour reduction in hydrogen, using a uranium oxide powder with extra oxygen (O/U ~ 2.37). Langrod [14] has also achieved UO 2 pellets of above 95% TD using mixture of UO 2 and U 3 O 8 (O/U ~ 2.30) by sintering at 1300 o C in argon or nitrogen atmosphere for two hours followed by reduction in hydrogen. Three methods were studied to incorporate SiC into UO 2 Firstly, a chemical vapor deposition (CVD) process was used to coat UO 2 particles with a SiC layer prior to the low temperature sintering process. Secondly, a SiC pre-ceramic polymer, allylhydridopolycarbosilane (AHPCS), was used to coat UO 2 particles prior to the low temperature sintering process. Thirdly, SiC whiskers were mixed with UO 2 particles prior to the low temperature sintering process. Though pellets with high density were not achieved by the first and second methods, pellets of 95% TD were achieved by pressureless sintering of UO 2 with 5 vol% SiC whiskers and 18

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hot press sintering of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. The thermal conductivity of the pellets will be measured at Idaho National Laboratory. The thermal conductivity data will not be discussed in this dissertation. 19

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CHAPTER 2 LITERATURE REVIEW Properties of Uranium Dioxide The properties of uranium dioxide (UO 2 ) have been investigated for decades. UO 2 has a cubic fluorite (CaF 2 ) type crystal structure with a lattice parameter of 0.5468nm [15]. The theoretical density of UO 2 is 10.96 g/cm 3 [16]. The melting point of UO 2 is about 2850 o C [17]. UO 2 also has the advantages of good high temperature stability, good chemical compatibility with cladding and coolant and resistance to radiation [18]. The thermal conductivity is one of the most important properties of UO 2 because it determines the fuel temperature, thus directly affect the behavior and performance of fuel pellet in a reactor. Based on the experiment data, Fink [1] pointed out that the thermal conductive of 95% dense UO 2 can be calculated by Equation 2-1, which is plotted in Figure 2-1. )35.16exp(64006142.3692.175408.71002/52ttttk (2-1) Where t = T (K)/1000 and k is the thermal conductivity of 95% dense UO 2 in W/m-K. Uranium dioxide is easily oxidized in air. The uranium oxide with O/U ratio greater than 2.0 is called hyperstoichiometric UO 2 ; the uranium oxide with O/U ratio less than 2.0 is called hypostoichiometric UO 2 The oxidation of UO 2 in air is a two-step reaction: UO 2 U 3 O 7 /U 4 O 9 U 3 O 8 The intermediate oxidation products, U 4 O 9 and U 3 O 7 are derivatives of the fluorite structure in which clusters of interstitial oxygen atoms are centered on unoccupied cubic sites in the lattice, with accompanying displacement of neighboring U atoms [19]. U 3 O 8 has an orthorhombic lattice structure [20]. The density of U 3 O 8 is 8.38 g/cm3, which is 24% less than UO 2 The density decrease results in an undesirable volume increase in the fuel. 20

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The thermal conductivity of hyperstoichiometric UO 2 [21]is shown in Figure 2-2. The excess oxygen atoms act as phonon scattering centers, thus reduce the thermal conductivity of UO 2 The thermal conductivity of (U 0.8 Pu 0.2 )O x (x < 2.0) [22] is shown in Figure 2-3. The defects in the UO 2 crystal lattice also act as phonon scattering centers, thus reduce the thermal conductivity of UO 2 In addition, hypostoichiometric UO 2 could contain uranium metal which could be highly reactive with other materials; hyperstoichiometric UO 2 may have an oxygen partial pressure sufficient to cause interaction with other materials [23]. The O/U ratio of an unknown UO 2.x powder can be determined by measuring the weight difference of UO 2.x and U 3 O 8 oxidized by UO 2.x or UO 2.x and UO 2 reduced by UO 2.x (for hyperstoichiometric UO 2 ). The thermal conductivity of irradiated UO 2 is affected by the changes that take place in the fuel during irradiation. During irradiation, fission products accumulate in the UO 2 matrix and cause the fuel swelling. The fission products dissolved in the UO 2 lattice serve as phonon scattering centers, thus reduce the thermal conductivity of UO 2 fuel; the precipitated fission products have a much higher thermal conductivity than UO 2 and have a positive contribution to the thermal conductivity of UO 2 fuel. The fission product gases initially form in irradiated fuel as dispersed atoms within the UO 2 lattice, then form small bubbles. The small bubbles within the UO 2 lattice also serve as phonon scattering centers, thus reduce the thermal conductivity [24]. At temperature below 1000 o C, uranium dioxide retains essentially all the fission gases, but above this temperature the gases are released, little remains in those region of the fuel where the temperature exceeds 1800 o C [18]. Radiation damage from neutrons, -decay and fission products increase the number of lattice defects and consequently reduces the thermal conductivity of UO 2 fuel. The thermal conductivities of UO 2 before and after irradiation are shown in Figure 2-4 [24]. The radiation-induced decrease in the thermal conductivity of UO 2 is 21

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large at low temperature. Oxygen and uranium defects are known to anneal at around 500 K and 1000 K, respectively. This explains why most changes in the thermal conductivity of UO 2 are seen below 1000 K [24]. Properties of Silicon Carbide Silicon carbide (SiC) is a ceramic compound of silicon and carbon, which was discovered by Edward Goodrich Acheson around 1893. Several polytypes of SiC exist, the most common polytype is -SiC (6H-SiC) and the cubic form is -SiC (3C-SiC). Only -SiC is considered in this research because of its property of isotropic expansion when heated. The density of -SiC is 3.21 g/cm^3 [25]. The melting temperature of -SiC is about 2700 o C [5]. Silicon carbide has low thermal neutron absorption cross section. The neutron cross-section for silicon, carbon and other relative nuclides are shown in Table 2-1 [26]. One of the most attractive properties of SiC is its high thermal conductivity. The thermal conductivity of single crystal SiC measured by Slack at room temperature is 490 W/m-K [4], which is higher than copper, 398 W/m-K. The thermal conductivity of polycrystalline -SiC by CVD process is lower, about 70 W/m-K at room temperature [27]. Figure 2-5 shows the thermal conductivities of single crystal SiC and polycrystalline -SiC versus temperature. The single crystal SiC has higher purity and less defects than the polycrystalline -SiC. The boundaries in the polycrystalline -SiC can also serve as phonon scattering centers. Figure 2-6 shows the thermal conductivity of polycrystalline -SiC before and after irradiation [28]. The thermal conductivity of unirradiated SiC decreases with increasing temperature. The thermal conductivity of SiC is mainly controlled by the lattice vibration waves (Phonons). The phonon-phonon scatterings increase with increasing temperature, which decrease the phonon mean free path and consequently decrease the thermal conductivity. 22

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The thermal conductivity of SiC decreases by a factor between 3 and 9 at room temperature when SiC was irradiated with fast neutrons fluence of 2.7-7.7 21 n/cm 2 (E>0.18 MeV) at 550 o C-1100 o C [28]. During fast neutron irradiation, point defects are introduced in the SiC lattice structure. These defects act as the scattering centers for phonons, so the thermal conductivity is sharply reduced as a consequence. The phonon mean free path is determined by the mean free path of defects, so the thermal conductivity is almost independent of temperature after irradiation. Amorphous SiC was studied by Snead [29] using transmission electron microscopy (TEM) after irradiation of -SiC with fast neutron of 2.6 25 n/m 2 (E>0.1 MeV) at 53 o C. The thermal conductivity of amorphous SiC is only 5.4 W/m-K at 800 o C, only slightly higher than UO 2 The annealing effect was also observed by Snead. The SiC crystallites grow slowly in the ~800-1100 o C temperature range and that the crystallite growth rate was approximately linear with annealing temperature; the SiC crystallites grow rapidly in the temperature range of 1100-1150 o C, with both faster growth of the existing crystallites and rapid nucleation of new crystallites throughout the amorphous material. No amorphous SiC is found after annealing at 1150 o C for 30 min [29]. Silicon carbide has good chemical stability because of the protective silicon oxide (SiO 2 ) layer formed on it. Silicon carbide is not attacked by any acids or alkalis or molten salts at room temperature. Verral reported that the SiC lost 2% at 573 K in deoxygenated water of pH 10.3 after 90 days and less than 2% at 573K in deoxygenated water of pH 3 after 32 days [5]. Hirayama also reported the weight loss increased with increasing pH value. The weight loss in the oxygenated solution was more than that in the deoxygenated solution [30]. Verral reported that there was no significant interaction between SiC and Zircaloy-4 at 1273 K. At 1773 K there 23

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was a diffusion-based reaction to form ZrC and free SiC, but the SiC Zircaloy cladding interaction was no worse than the UO 2 -Zircaloy cladding interaction [5]. Silicon carbide has been used as a layer in tristructral isotropic (TRISO) fuel particles for gas cooled nuclear reactors. There are four layers on the spherical UO 2 particle of TRISO fuel: a porous pyrolytic carbon buffer layer, a dense pyrolytic carbon layer, a SiC layer and another pyrolytic carbon layer. The SiC layer acts as a diffusion barrier to fission products and a pressure vessel of the particle [31]. Silicon carbide has shown the capability to maintain its properties under high irradiation and temperature conditions. There is a slightly expansion at the fluences up to 5 26 n/m2 and irradiation temperature below 1000 o C; at higher temperature, irradiation creates voids that cause continuing expansion, but the structural integrity is not affected. The irradiation has a negligible effect on the strength of SiC [27]. Silicon carbide has also been used as electronic devices operating under extreme conductions of power and temperature because of its wide band gap, high breakdown electric field strength, high saturated electron drift velocity and high thermal conductivity. Because of high purity requirement of the semiconductor device, the SiC is usually produced by a chemical vapor deposition process. Silicon carbide is commonly manufactured by combining silica sand (SiO 2 ) and carbon at high temperature, between 1600 o C to 2500 o C. The purity of the SiC crystals produced is relatively low compared to the more expensive chemical vapor deposition (CVD) process. Pre-ceramic polymers can also used to produce crystalline silicon carbide. Allylhydridopolycarbosilane (AHPCS) was successfully converted to crystalline -SiC at 1600 o C by Zheng et al. [32]. Silicon carbide fibers were also produced from the pre-ceramic polymer [33]. The fibers were used to improve the mechanical properties of the matrix materials. 24

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Besides SiC fibers, SiC whiskers were also produced to improve the mechanical properties of the matrix material. The two main methods to produce SiC whiskers are Rice Hull method and vapor-liquid-solid (VLS) method. The SiC whiskers are single crystal. The strengths, Youngs module and thermal conductivity can be extremely high. The SiC whiskers were not only used to increase the mechanical properties [34, 35], but also the thermal conductivity of the matrix materials [7-9]. 25

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Table 2-1. Neutronic cross sections (barns) [26]. abs (th) s (th) s (epi) C 0.0032 4.8 4.66 Si 0.00019 4.2 3.7 O 0.13 5 3.4 Zr 0.18 8 6.2 Be 0.01 7 6.11 Mo 2.5 7 6.0 26

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Tem p erature ( K ) Thermal Conductivity (W/m-K) 0 300 600 900 1200 1500 1800 2100 2400 27003000 1 2 3 4 5 6 7 89 Figure 2-1. Thermal conductivity of UO 2 [1]. Tem p erature ( K ) Thermal Conductivity (W/m-K) 400 600 800 1000 1200 1400 1600 18002000 1.5 2 2.5 3 3.5 4 4.5 5 5.56 O/U=2.0 O/U=2.006 O/U=2.03 O/U=2.06 Figure 2-2. Thermal conductivity of hyperstoichiometric UO 2 [21]. 27

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Tem p erature ( K ) Thermal Conductivity (W/m-K) 500 1000 1500 20002500 1 1.5 2 2.5 3 3.5 4 4.5 55.5 O/M=2.0 O/M=1.98 O/M=1.93 Figure 2-3. Thermal conductivity of (U 0.8 Pu 0.2 )O x [22]. Tem p erature ( K ) Thermal Conductivity (W/m-K) 600 800 1000 1200 1400 16001800 0 1 2 3 4 5 67 Unirradiated Irradiated Figure 2-4. Thermal conductivity of UO 2 before and after irradiation [24]. 28

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Tem p erature ( K ) Thermal Conductivity (W/m-K) 300 400 500 600 700800 0 100 200 300 400500 Single Crystal SiC Polycrystalline SiC UO2 Figure 2-5. Thermal conductivity of single crystal SiC and polycrystalline SiC compared to UO 2 [1], [4], [27]. Tem p erature ( K ) Thermal Conductivity (W/m-K) 0 100 200 300 400 500 600700 0 10 20 30 40 50 60 70 Unirradiated SiC Irradiated SiC at 1100 oC Irradiated SiC at 550 oC UO2 Figure 2-6. Thermal conductivity of SiC before and after irradiation compared to UO 2 [1], [27], [28]. 29

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CHAPTER 3 NEUTRONIC CALCULATION Introduction The effect of silicon carbide (SiC) additives on the neutronic properties of uranium dioxide (UO 2 ) should be studied before the experiments. CASMO-3 [6], a multi-group two-dimensional transport theory code, was used to simulate the burnup of UO 2 fuel and UO 2 fuel with SiC additives. The simulation utilized a Framatome Mark-B 15X15 assembly design. The power of the assembly is 14.37 MW. There are 208 fuel rods, 16 guide tubes and 1 instrument tube per assembly. The cross section view of Mark-B assembly is shown in Figure 3-1. The enrichment of the fuel is 4.66%. Methods and Results The K-infinity versus burnup of UO 2 fuel compared to UO 2 with 5 w% and 10 w% SiC is shown in Table 3-1 and Figure 3-2. The K-infinity of UO 2 fuel is less than that of UO 2 with 5 w% and 10 w% SiC at the beginning of life (BOL) because SiC replaces uranium-238, which has a large resonance absorption cross section. As the fuel burns up, the K-infinity of UO 2 fuel decreases slower than that of UO 2 with 5 w% and 10 w% SiC because the thermal utilization factor in UO 2 decreases slower than in UO 2 with SiC. At the end of life (EOL), 60 GWD/MTU, the K-infinity of UO 2 with 5 w% SiC and 10 w% SiC are about 7% and 14% less than the K-infinity of UO 2 fuel, respectively. The Doppler coefficient and moderator temperature coefficient (MTC) versus burnup of UO 2 fuel compared to UO 2 with 5 w% and 10 w% SiC are shown in Table 3-2, Table 3-3, Figure 3-3 and Figure 3-4. The Doppler coefficient of UO 2 fuel is less than that of UO 2 with 5 w% and 10 w% SiC at BOC, but larger than that of UO 2 with SiC at the end of cycle (EOC). The MTC of UO 2 fuel is less than UO 2 with SiC throughout the burnup. All the Doppler coefficients and MTCs of UO 2 fuel and UO 2 with SiC fuel are negative. 30

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Because volume percentage is always used when considering SiC whiskers-matrix composite, the K-infinity versus burnup of UO 2 fuel compared to UO 2 with 5 vol%, 10 vol% and 15 vol% SiC is shown in Table 3-1 and Figure 3-5. The 5 vol% SiC is equal to 1.5 w% SiC, the 10 vol% SiC is equal to 3.2% SiC and 15 vol% SiC is equal to 4.9% SiC. At 60 GWD/MTU, the K-infinity of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2.1%, 4.5% and 7.2% less than the K-infinity of UO 2 fuel. The Doppler coefficient and moderator temperature coefficient (MTC) versus burnup of UO 2 fuel compared to UO 2 with 5 vol%, 10 vol% and 15 vol% SiC are shown in Table 3-2, Table 3-3, Figure 3-6 and Figure 3-7. Discussion In this research, the limit of the amount of SiC additives was set to 5 w% to limit the change of K-infinity at EOL to 7 %. Because the SiC additives will affect the thermal conductivity of the fuel, thus affecting the fuel temperature, the effect of fuel temperature change on the neutronic properties of the fuel has to be considered. Assuming there was 50% increase in thermal conductivity of UO 2 with SiC additives compared to UO 2 fuel, the centerline temperature of the fuel rod was calculated according to Equation 3-1. The results are shown in Table 3-13. The centerline temperature of UO 2 with SiC was 275 o C lower than the centerline temperature of UO 2 fuel. ])(12[2'cfsfccgfffmcltrhrkthkrrqTT (3-1) Where T cl is the fuel centerline temperature (K) T m is the moderator temperature (K) q is the linear power density (W/cm) r f is the radius of fuel pellet (cm) k f is the average thermal conductivity of fuel (W/(cm*K)) 31

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h g is the gap heat transfer coefficient (0.5---1.1) (W/(cm^2*K)) k c is the thermal conductivity of cladding (W/(cm*K) h s is the coefficient of convective heat transfer (2.8---4.5) (W/(cm^2*K) t c is the clad thickness (cm) Based on the centerline temperature calculation, a 200 o C decrease in fuel temperature of UO 2 with SiC was assumed in the CASMO-3 calculation. (Different amount of SiC will change the fuel temperature differently. In this research, the changes of fuel temperatures of UO 2 with different amount of SiC were assumed to be the same, 200 o C, for simplicity) The K-infinity versus burnup of UO 2 fuel compared to UO 2 with 5 vol%, 10 vol% and 15 vol% SiC, which are 200 o C lower in fuel temperature, is shown in Table 3-4 and Figure 3-8. At 60 GWD/MTU, the K-infinity of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2.4%, 4.9% and 7.6% less than the K-infinity of UO 2 fuel. The differences in K-infinity are larger than that of UO 2 with SiC without considering the temperature change. The Doppler coefficient of UO 2 with SiC is less than UO 2 as shown in Table 3-5 and Figure 3-9. The MTC of UO 2 with SiC is still larger than UO 2 as shown in Table 3-6 and Figure 3-10. All the Doppler coefficients and MTC of UO 2 fuel and UO 2 with SiC fuel are negative. The effect of SiC additives on the neutronic properties of UO 2 fuel at boron let down situation was also studied. Considering a core with three batches (equal amount of fuel), a batch of fresh fuel, a batch of fuel burned once and a batch of fuel burned twice. The power distribution of the three batches was 1.25:1:0.75. In steady state, the cycle burnup of batch one was 25 GWD/MTU; the cycle burnup of batch two was 20 GWD/MTU; and the cycle burnup of batch three was 15 GWD/MTU. The boron concentration is shown in Figure 3-11. During boron let down, the boron concentration changed from 1400 ppm at 0 GWD/MTU of fuel assembly to 32

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10 ppm at 25 GWD/MTU of fuel assembly, from 1400 ppm at 25.01 GWD/MTU of fuel assembly to 10 ppm at 45 GWD/MTU of fuel assembly and from 1400 ppm at 45.01 GWD/MTU of fuel assembly to 10 ppm at 60 GWD/MTU of fuel assembly. The K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel is shown in Table 3-7 and Figure 3-12. The K-infinity versus burnup of UO 2 core compared with UO 2 with SiC core is shown in Table 3-8 and Figure 3-13. At EOC, which is 25 GWD/MTU of the fresh fuel assembly, the K-infinity of the core of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2.1%, 4.5% and 7.2% less than the K-infinity of the core of UO 2 fuel. The Doppler coefficient of UO 2 with SiC is less than UO 2 as shown in Table 3-9 and Figure 3-14. The Doppler coefficient of the core of UO 2 with SiC is less than the core of UO 2 as shown in Table 3-10 and Figure 3-15. All the Doppler coefficients are negative. The MTC of UO 2 with SiC is still larger than UO 2 as shown in Table 3-11 and Figure 3-16. The MTC of UO 2 with 10 vol% and 15 vol% SiC are positive at the 0.5 GWD/MTU of the fuel assembly. However, all the MTCs of the core are negative, as shown in Table 3-12 and Figure 3-17. 33

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Table 3-1. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 w% SiC UO 2 + 10 w% SiC UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 1.41154 1.42005 1.42592 1.41447 1.41739 1.4202 0.5 1.35996 1.36314 1.36451 1.3611 1.36221 1.36321 5 1.30833 1.30575 1.30048 1.30778 1.30685 1.30565 10 1.2556 1.24631 1.23368 1.25296 1.2499 1.24615 15 1.20872 1.19317 1.17308 1.20426 1.19899 1.19276 20 1.16617 1.14419 1.11601 1.15981 1.15238 1.14363 25 1.12643 1.0977 1.06056 1.11807 1.10839 1.09698 30 1.08863 1.05264 1.00586 1.07824 1.06614 1.05181 35 1.05234 1.00885 0.95243 1.03983 1.02516 1.00776 40 1.01733 0.96639 0.90124 1.00261 0.98546 0.96512 45 0.98355 0.92576 0.85395 0.96676 0.94723 0.92432 50 0.95128 0.88752 0.8124 0.93259 0.91097 0.88604 55 0.9207 0.85269 0.7779 0.90049 0.87735 0.85108 60 0.89211 0.82183 0.75079 0.87075 0.84677 0.82025 Table 3-2. Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 w% SiC UO 2 + 10 w% SiC UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -1.56 -1.50 -1.47 -1.53 -1.52 -1.50 0.5 -1.58 -1.54 -1.51 -1.56 -1.55 -1.53 5 -1.65 -1.61 -1.59 -1.62 -1.62 -1.60 10 -1.79 -1.76 -1.76 -1.76 -1.76 -1.76 15 -1.94 -1.93 -1.95 -1.93 -1.92 -1.93 20 -2.09 -2.09 -2.13 -2.08 -2.08 -2.09 25 -2.23 -2.24 -2.28 -2.22 -2.22 -2.24 30 -2.35 -2.37 -2.44 -2.34 -2.35 -2.37 35 -2.46 -2.49 -2.56 -2.45 -2.45 -2.49 40 -2.54 -2.59 -2.67 -2.54 -2.56 -2.59 45 -2.62 -2.67 -2.75 -2.62 -2.64 -2.67 50 -2.69 -2.74 -2.82 -2.70 -2.71 -2.74 55 -2.74 -2.79 -2.86 -2.75 -2.77 -2.79 60 -2.79 -2.84 -2.87 -2.80 -2.81 -2.82 34

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Table 3-3. MTC (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 w% SiC UO 2 + 10 w% SiC UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -34.40 -31.02 -27.20 -33.63 -32.49 -30.93 0.5 -32.32 -29.18 -25.58 -31.63 -30.48 -29.10 5 -36.84 -34.10 -30.75 -36.31 -35.14 -34.01 10 -42.27 -39.80 -36.48 -41.84 -40.61 -39.71 15 -47.05 -44.66 -41.20 -46.69 -45.41 -44.57 20 -51.29 -48.91 -45.00 -50.98 -49.71 -48.86 25 -55.32 -52.68 -47.97 -54.98 -53.66 -52.60 30 -58.90 -55.94 -50.20 -58.53 -57.14 -55.83 35 -62.25 -58.66 -51.33 -61.76 -60.29 -58.55 40 -65.37 -60.96 -51.43 -64.67 -63.04 -60.76 45 -67.92 -62.66 -50.51 -67.15 -65.44 -62.47 50 -70.38 -63.67 -48.75 -69.20 -67.28 -63.50 55 -72.58 -64.21 -46.67 -70.87 -68.66 -63.92 60 -74.45 -64.29 -44.81 -72.24 -69.71 -64.02 Table 3-4. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature). Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 1.41154 1.42088 1.42376 1.42653 0.5 1.35996 1.36718 1.36825 1.36921 5 1.30833 1.31363 1.31263 1.31139 10 1.2556 1.25868 1.25552 1.2517 15 1.20872 1.20967 1.20427 1.1979 20 1.16617 1.16473 1.15711 1.14813 25 1.12643 1.12235 1.11237 1.10064 30 1.08863 1.08171 1.06922 1.05442 35 1.05234 1.0424 1.02721 1.00921 40 1.01733 1.00417 0.98639 0.96531 45 0.98355 0.96728 0.947 0.92324 50 0.95128 0.93209 0.90964 0.88379 55 0.9207 0.899 0.87501 0.84784 60 0.89211 0.8684 0.84362 0.8163 35

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Table 3-5. Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature). Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -1.56 -1.67 -1.65 -1.64 0.5 -1.58 -1.71 -1.69 -1.68 5 -1.65 -1.78 -1.76 -1.75 10 -1.79 -1.92 -1.92 -1.91 15 -1.94 -2.10 -2.10 -2.11 20 -2.09 -2.27 -2.27 -2.28 25 -2.23 -2.42 -2.42 -2.44 30 -2.35 -2.55 -2.56 -2.59 35 -2.46 -2.67 -2.68 -2.71 40 -2.54 -2.78 -2.79 -2.82 45 -2.62 -2.86 -2.89 -2.92 50 -2.69 -2.95 -2.97 -3.00 55 -2.74 -3.02 -3.04 -3.07 60 -2.79 -3.07 -3.09 -3.12 Table 3-6. MTC (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature). Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -34.40 -32.74 -31.54 -30.15 0.5 -32.32 -30.75 -29.62 -28.32 5 -36.84 -35.33 -34.34 -33.15 10 -42.27 -40.74 -39.81 -38.71 15 -47.05 -45.46 -44.54 -43.44 20 -51.29 -49.63 -48.77 -47.59 25 -55.32 -53.59 -52.58 -51.26 30 -58.90 -57.06 -55.93 -54.40 35 -62.25 -60.22 -58.85 -56.98 40 -65.37 -62.99 -61.33 -59.05 45 -67.92 -65.33 -63.39 -60.52 50 -70.38 -67.29 -64.76 -61.28 55 -72.58 -68.77 -65.67 -61.42 60 -74.45 -69.97 -66.18 -61.18 36

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Table 3-7. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down. Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 1.27265 1.27525 1.27131 1.26663 0.5 1.23342 1.23469 1.22975 1.22414 5 1.2104 1.21074 1.20511 1.1987 10 1.18497 1.18436 1.17774 1.17014 15 1.16305 1.16153 1.15393 1.14514 20 1.1442 1.14159 1.13303 1.12307 25 1.12757 1.12383 1.11428 1.1031 25.01 1.02681 1.01685 1.00175 0.98436 25.4 1.02499 1.01488 0.99968 0.98215 29 1.01752 1.00657 0.99079 0.97252 33 1.00981 0.99784 0.98129 0.96208 37 1.00246 0.98941 0.97203 0.95173 41 0.9957 0.98157 0.96336 0.94212 45 0.98974 0.97463 0.95579 0.93381 45.01 0.89612 0.87443 0.84991 0.82166 45.3 0.8952 0.87343 0.84887 0.82061 48 0.89583 0.87401 0.84983 0.82219 51 0.89699 0.87504 0.85112 0.82397 54 0.89835 0.87629 0.85257 0.82588 57 0.90022 0.87812 0.85472 0.82861 60 0.90282 0.88088 0.858 0.83272 Table 3-8. K-infinity versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down. Equivalent burnup of fresh fuel assembly (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 1.0966 1.0889 1.0761 1.0613 0.5 1.0794 1.0711 1.0578 1.0426 5 1.0675 1.0585 1.0449 1.0292 10 1.0546 1.0449 1.0306 1.0142 15 1.0433 1.0328 1.0180 1.0009 20 1.0337 1.0224 1.0069 0.9891 25 1.0254 1.0134 0.9974 0.9791 37

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Table 3-9. Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down. Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -1.75 -1.88 -1.88 -1.88 0.5 -1.77 -1.91 -1.91 -1.90 5 -1.79 -1.94 -1.93 -1.94 10 -1.90 -2.05 -2.05 -2.06 15 -2.03 -2.19 -2.19 -2.21 20 -2.14 -2.32 -2.32 -2.34 25 -2.23 -2.42 -2.42 -2.44 25.01 -2.46 -2.67 -2.70 -2.74 25.4 -2.46 -2.67 -2.71 -2.75 29 -2.50 -2.73 -2.76 -2.80 33 -2.54 -2.77 -2.80 -2.85 37 -2.58 -2.81 -2.84 -2.88 41 -2.60 -2.84 -2.86 -2.90 45 -2.60 -2.85 -2.86 -2.91 45.01 -2.87 -3.17 -3.23 -3.30 45.3 -2.87 -3.17 -3.22 -3.30 48 -2.86 -3.15 -3.19 -3.26 51 -2.84 -3.13 -3.17 -3.23 54 -2.82 -3.11 -3.14 -3.20 57 -2.80 -3.08 -3.11 -3.14 60 -2.77 -3.04 -3.07 -3.09 Table 3-10. Doppler coefficient (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down. Equivalent burnup of fresh fuel assembly (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -2.27 -2.47 -2.49 -2.52 0.5 -2.27 -2.48 -2.50 -2.54 5 -2.30 -2.51 -2.52 -2.56 10 -2.35 -2.56 -2.58 -2.62 15 -2.41 -2.63 -2.65 -2.68 20 -2.46 -2.68 -2.69 -2.73 25 -2.49 -2.72 -2.73 -2.76 38

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Table 3-11. MTC (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down. Equivalent burnup (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -5.21 -2.63 -0.21 2.48 0.5 -3.48 -0.97 1.38 4.00 5 -12.86 -10.44 -8.36 -5.90 10 -23.56 -21.19 -19.29 -17.11 15 -33.90 -31.66 -29.99 -28.04 20 -44.25 -42.20 -40.89 -39.25 25 -55.11 -53.38 -52.40 -51.09 25.01 -20.51 -16.52 -13.03 -8.73 25.4 -21.27 -17.31 -13.79 -9.50 29 -28.91 -25.03 -21.67 -17.54 33 -37.87 -34.15 -31.04 -27.00 37 -47.29 -43.83 -40.93 -37.23 41 -57.26 -54.15 -51.71 -48.49 45 -67.81 -65.27 -63.46 -60.92 45.01 -25.70 -18.81 -12.33 -3.93 45.3 -26.41 -19.54 -13.12 -4.72 48 -34.17 -27.70 -21.65 -13.84 51 -43.33 -37.24 -31.73 -24.59 54 -53.09 -47.49 -42.58 -36.20 57 -63.45 -58.49 -54.32 -48.84 60 -74.53 -70.45 -67.16 -62.87 Table 3-12. MTC (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down. Equivalent burnup of fresh fuel assembly (GWD/MTU) UO 2 UO 2 + 5 vol% SiC UO 2 + 10 vol% SiC UO 2 + 15 vol% SiC 0 -15.43 -11.30 -7.52 -2.86 0.5 -15.14 -11.06 -7.30 -2.68 5 -23.54 -19.62 -16.12 -11.77 10 -33.27 -29.52 -26.32 -22.28 15 -43.16 -39.67 -36.78 -33.14 20 -53.39 -50.25 -47.85 -44.73 25 -64.20 -61.61 -59.78 -57.31 39

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Table 3-13. Centerline temperature of UO 2 fuel compared to UO 2 with SiC. T m q' r f k f h g k c h s t c Tcl (K) Tcl ( o C) UO 2 with SiC 600 270 0.47 0.041 0.5 0.15 2.8 0.06731 1383 1110 1.1 4.5 1272 999 UO 2 600 270 0.47 0.027 0.5 0.15 2.8 0.06731 1648 1375 1.1 4.5 1538 1265 40

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Figure 3-1. Crystal River 15X15 assembly design. E q uivalent Burnu p ( GWD/MTU ) K-infinity 0 5 10 15 20 25 30 35 40 45 50 5560 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.41.5 0.8921 0.8218 0.7508 UO2 UO2 + 5 w% SiC UO2 + 10 w% SiC Figure 3-2. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. 41

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E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -3 -2.8 -2.6 -2.4 -2.2 -2 -1.8 -1.6-1.4 UO2 UO2 + 5 w% SiC UO2 + 10 w% SiC Figure 3-3. Doppler coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30-25 UO2 UO2 + 5 w% SiC UO2 + 10 w% SiC Figure 3-4. Moderator temperature coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. 42

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E q uivalent Burnu p ( GWD/MTU ) K-infinity 0 5 10 15 20 25 30 35 40 45 50 5560 0.7 0.8 0.9 1 1.1 1.2 1.3 1.41.5 0.8921 0.8708 0.8468 0.8203 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-5. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -3 -2.8 -2.6 -2.4 -2.2 -2 -1.8 -1.6-1.4 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-6. Doppler coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. 43

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E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30-25 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-7. Moderator temperature coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel. E q uivalent Burnu p ( GWD/MTU ) K-infinity 0 5 10 15 20 25 30 35 40 45 50 5560 0.7 0.8 0.9 1 1.1 1.2 1.3 1.41.5 0.8921 0.8684 0.8436 0.8163 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-8. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature). 44

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E q uivalnet Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -3.2 -3 -2.8 -2.6 -2.4 -2.2 -2 -1.8 -1.6-1.4 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-9. Doppler coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature). E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30-25 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-10. Moderator temperature coefficient versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature). 45

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E q uivalent Burnu p ( GWD/MTU ) Boron Concentration (ppm) 0 5 10 15 20 25 30 35 40 45 50 5560 0 200 400 600 800 1000 12001400 Figure 3-11. Boron concentration versus burnup at boron let-down. E q uivalent Burnu p ( GWD/MTU ) K-infinity 0 5 10 15 20 25 30 35 40 45 50 5560 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.251.3 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-12. K-infinity versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down. 46

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E q uivalent Burnu p of Fresh Fuel Assembl y ( GWD/MTU ) K-infinity 0 5 10 15 2025 0.96 0.98 1 1.02 1.04 1.06 1.081.1 1.0254 1.0134 0.9974 0.9791 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-13. K-infinity versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down. E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -3.4 -3.2 -3 -2.8 -2.6 -2.4 -2.2 -2 -1.8-1.6 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-14. Doppler coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down. 47

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E q uivalent Burnu p of Fresh Fuel Assembl y ( GWD/MTU ) pcm/K 0 5 10 15 2025 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3-2.2 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-15. Doppler coefficient (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down. E q uivalent Burnu p ( GWD/MTU ) pcm/K 0 5 10 15 20 25 30 35 40 45 50 5560 -75 -65 -55 -45 -35 -25 -15 -5 5 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-16. Moderator temperature coefficient (pcm/K) versus burnup of UO 2 fuel compared to UO 2 with SiC fuel (200 o C less in fuel temperature) at boron let-down. 48

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E q uivalent Burnu p of Fresh Fuel Assembl y ( GWD/MTU ) pcm/K 0 5 10 15 2025 -80 -60 -40 -200 UO2 UO2 + 5 vol% SiC UO2 + 10 vol% SiC UO2 + 15 vol% SiC Figure 3-17. Moderator temperature coefficient (pcm/K) versus burnup of UO 2 core compared to UO 2 with SiC core (200 o C less in fuel temperature) at boron let-down. 49

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CHAPTER 4 REACTION BETWEEN URAINIUM OXIDE AND SILICON CARBIDE Background There are very few papers about the reaction between uranium dioxide (UO 2 ) and silicon carbide (SiC). G. C Allen et al. [12] have reported that UO 2 reacts with SiC at a measurable rate above 1377 o C. The reactions were described as Equation 4-1 to Equation 4-5. UO 2 + 2SiC USi 2 + 2CO (4-1) UO 2 + 2SiC UC 2 + 2SiO (4-2) USi 2 USi 1.67 + 0.33Si (4-3) 5Si + 3UC 2 U 3 C 3 Si 2 + 3SiC (4-4) UO 2 + 3UC 2 4UC + 2CO (4-5) W. Lippmann et al. [36] have confirmed the reaction between UO 2 and SiC at the temperature above 1700 o C in a system where gaseous products were free to escape. However, they also found that there was no reaction up to 1800 o C in a system where gaseous products were sealed. Solomon et al. [10] also observed reactions between UO 2 and SiC at 1400 o C after fifteen hours. Experiments and Results UO 2 and SiC powder were sintered together at 1300 o C and 1650 o C to verify the literature result. Before the sintering process, the uranium oxide powder from Framatome/Areva was characterized for its oxygen to uranium ratio and particle size distribution. Oxygen to Uranium Ratio of Uranium Oxide Powder The powder was oxidized to U 3 O 8 and the weight difference was measured to determine the O/U ratio of the uranium oxide powder from Framatome/Areva. The powder was oxidized in air at 350 o C for 24 hours. The weight change indicated the O/U ratio of received uranium oxide 50

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powder was 2.10. The generated U 3 O 8 was reduced to UO 2.0 in argon and 5% hydrogen environment at 900 o C for 4 hours. The received uranium oxide powder was oxidized in air at 140 o C for 24 hours. The O/U ratio calculated by weight difference was 2.27. Figure 4-1 shows the uranium oxide powder with different O/U ratio. The received UO 2.10 powder was dark brown; the UO 2.27 was black; the U 3 O 8 powder was dark green; and the UO 2.0 powder was orange. The X-ray diffraction (XRD) results of the four uranium oxide powders are shown in Figure 4-2 to Figure 4-5. The XRD peaks of UO 2.10 and UO 2.27 were close to the peaks of UO 2.0 the extra oxygen in UO 2.10 and UO 2.27 slightly broadens and offsets the peaks of UO 2.0 The XRD peaks of U 3 O 8 were totally different from the peaks of UO 2.0 because the crystal structures were different. Particle Size Distribution of Uranium Oxide Powder The particle size of received UO 2.10 powder was characterized by sieve analysis. Ten gram UO 2.10 powder was sieved through a series of screens with standardized mesh size. The sieve was shaken for half an hour on an Octagon digital sieve shaker. The sieve and shaker are shown in Figure 4-6. The powder between two screens was weighed and recorded in Table 4-1. The experiments were repeated for three times. The average value was plotted in Figure 4-7. There was about a 4% powder loss in the sieving process. Sintering UO 2 and SiC at 1300 o C and 1650 o C The received uranium oxide powder was ball milled with the -SiC powders (30 nm in particle size, from Alfa Aesar) for half an hour. The weight ratio of SiC to UO 2 is one. Figure 4-8 shows the X-ray diffraction (XRD) pattern of the 30 nm -SiC. The mixture powders were cold pressed at 200MPa, then sintered at 1300 o C in argon atmosphere. The pellet after sintering is shown in Figure 4-9. The X-ray diffraction result (Figure 4-10) showed no new peaks at 1300 o C. 51

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When the sintering temperature was increased to 1650 o C, the X-ray diffraction result (Figure 4-12) showed that USi 1.88 was formed. The pellet after sintering is shown in Figure 4-11. Discussion The experiment result was different from Allens result [12] because the ratio of SiC to UO 2 used by Allen et al. was 2.5. Uranium dioxide pellets are usually produced by sintering the green pellets at about 1700 o C in hydrogen atmosphere. High temperature around 1700 o C is necessary to achieve the required high density of over 95% of theoretical. The reaction between UO 2 and SiC at 1377 o C has to be avoided to successfully incorporate SiC into UO 2 52

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Table 4-1. Particle size distribution of received uranium oxide powder. Particle Size (m) Weight (g) Weight (g) Weight (g) Average (g) <25 0.6767 0.5633 0.6769 0.6390 >25 and < 45 1.9094 1.9317 1.9308 1.9240 >45 and <53 1.0295 1.1097 1.0454 1.0615 >53 and <63 0.8636 0.8886 0.9009 0.8844 >63 and <90 2.4028 2.4171 2.3799 2.3999 >90 and <150 1.4126 1.4529 1.4846 1.4500 >150 and <250 0.4147 0.4447 0.4161 0.4252 >250 0.8482 0.8681 0.8153 0.8439 53

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Figure 4-1. Uranium oxide powders with different O/U ratio. A) UO 2 .10. B) UO 2.27 C) U 3 O 8 D) UO 2 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 4-2. X-ray diffraction pattern of UO 2.10 powder. 54

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2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 4-3. X-ray diffraction pattern of UO 2.27 powder. 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 4-4. X-ray diffraction pattern of U 3 O 8 powder. 55

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2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 4-5. X-ray diffraction pattern of UO 2.0 powder. Figure 4-6. Sieve and shaker for analyzing particle size distribution. 56

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Particle Size ( micron ) Relative Number 0 25 50 75 100 125 150 175 200 225250 0 0.2 0.4 0.6 0.8 11.2 Figure 4-7. Particle size distribution of received uranium oxide powder. 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 4-8. X-ray diffraction pattern of 30nm -SiC from Alfa Aesar. 57

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Figure 4-9. Uranium dioxide-silicon carbide pellet after sintering at 1300 o C. 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 SiC UO2 Figure 4-10. X-ray diffraction pattern of UO 2 -SiC pellet after sintering at 1300 o C. 58

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Figure 4-11. Uranium dioxide-silicon caribide pellet after sintering at 1650 o C. 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 SiC UO2 USi1.88 Figure 4-12. X-ray diffraction pattern of UO 2 -SiC pellet after sintering at 1650 o C. 59

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CHAPTER 5 LOW TEMPERATURE SINTERING OF URANIUM DIOXIDE Background Uranium dioxide (UO 2 ) pellets are usually produced by sintering the green pellets at about 1700 o C in hydrogen atmosphere [13, 14, 37-40]. The high sintering temperature around 1700 o C is necessary to achieve the required high density of over 95% of theoretical. However, sintering at a temperature around 1700 o C is expensive due to the furnace cost and maintenance cost. Several studies have shown that UO 2 pellets with high density can be achieved at lower sintering temperature. Fuhrman et al. [13] have reported that UO 2 pellets of 95 to 97% theoretical density (TD) were achieved by sintering at 1200 o C in nitrogen for 1 hour followed by 1 hour reduction in hydrogen, while using uranium oxide powder with extra oxygen (O/U ~ 2.37). They also mentioned that UO 2 pellets of 95% TD were achieved at temperatures as low as 1000 o C using the same method, though the result was not consistent. Langrod [14] has reported that UO 2 pellets of above 95% TD was achieved by sintering at 1300 o C in nitrogen atmosphere for 2 hours followed by reduction in hydrogen, using mixture of UO 2 and U 3 O 8 (O/U ~ 2.30). Ayaz et al. [39] have sintered UO 2 pellets of 95% TD at 1150 o C in CO 2 and water vapor atmosphere for 4 hour, followed by reduction in Ar+8% H 2 for 1hour, using uranium oxide powder with extra oxygen (O/U = 2.15). Williams et al. [41] have sintered uranium oxide with different O/U ratio in argon, nitrogen, carbon dioxide and vacuum and achieved UO 2 pellets of 94% TD at temperatures lower than 1400 o C in various gases. Excess oxygen was believed to be the key factor to decrease the sintering temperature. Fuhrman et al. [13] indicated that a minimum O/U ratio of 2.25 to 2.28 was required to achieve a density above 95% of theoretical. In the sintering process, the uranium oxide particles undergo solid state diffusion. Based on the theory by Williams et al. [41], the rate of the diffusion of 60

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uranium ions determines the rate of the sintering process. The diffusion of uranium ions in non-stoichiometric uranium oxides is more rapid than in stoichiometric UO 2 because the extra oxygen in non-stoichiometric uranium oxides lowers the lattice binding energies. The uranium oxide powder was oxidized in air to increase the oxygen to uranium ratio. The rate and degree of oxidation of UO 2 versus temperature studied by Langrod [14] is shown in Figure 5-1. Langrod [14] also found that the sintering behavior of UO 2 powder blended with U 3 O 8 was identical with the air oxidized UO 2 powder of the same O/U ratio. The non-stoichiometric pellets must be further processed to bring the oxygen to uranium ratio back to 2.0 by soaking the pellets in hydrogen environment. Fuhrman et al. [13] reported that 1100 o C was required to remove the excess oxygen in a reasonable time. The study by Fuhrman et al. [13] showed that the grain size of the UO 2 pellet sintered at low temperature is smaller than the grain size of the UO 2 pellet sintered at high temperature. The result was confirmed by Ayaz et al. [39]. Experiments and Results A UO 2 pellet was first sintered in hydrogen atmosphere at high temperature. The received uranium oxide powder was cold pressed in a 13 mm die at 200 MPa. The green pellet was sintered at 1650 o C for 4 hours in hydrogen (balanced with argon) atmosphere. The density of sintered pellet was 96.03% TD, which was measured by the Archimedes method (Archimedes principle). The pellet is shown in Figure 5-2. The grain size of the pellet is in the range of 5 to 20 micron, as shown in the scanning electron microscopy (SEM) image (Figure 5-3). The received uranium oxide powder was oxidized in air at 140 o C for 24 hours to increase the O/M ratio to UO 2.27 The UO 2.27 powder was then cold compacted in a 13mm die at 200 MPa pressure. The green pellet was sintered at 1200 o C for 1 hour in argon atmosphere, followed by the reduction in Ar+5% H 2 at 1200 o C for one hour. The picture of the sintered pellet is shown in 61

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Figure 5-4. The density of the pellet measured by the Archimedes method was 95.71%TD. The SEM image of the pellet is shown in Figure 5-5. The grain size of the pellet is in the range of 5 to 10 micron. Another UO 2 pellet was sintered at low temperature with pressure. The UO 2.27 powder was cold compacted in a 13mm die at 200 MPa pressure. The green pellet was then sintered at 1200 o C with 10 MPa pressure for 1 hour in argon atmosphere, followed by the reduction in Ar+5% H 2 at 1200 o C for one hour. (The detailed experiment apparatus of sintering process with pressure will be explained in chapter 8.) The picture of the sintered pellet is shown in Figure 5-6. The density of the pellet measured by the Archimedes method was 97.99%TD. The SEM image of the pellet is shown in Figure 5-7. The grain size of the pellet is in the range of 5 to 10 micron, but larger than the grain size of the UO 2 pellet sintered at low temperature without pressure. Discussion Uranium dioxide fuel pellets with high density (>95%TD) can be achieved at 1200 o C, which is lower than the temperature at which the reaction between UO 2 and SiC occurs. So the low temperature sintering method can be used to avoid the reaction between UO 2 and SiC. The density of the UO 2 pellet sintered at 1200 o C is less than the density of the UO 2 pellet sintered at 1650 o C; the density of the UO 2 pellet sintered at 1200 o C with pressure is larger than the density of the UO 2 pellet sintered at 1650 o C. The grain size of the UO 2 pellet sintered at low temperature is smaller than the grain size of the UO 2 pellet sintered at 1650 o C. The pressure applied during the sintering process increased the grain size. The grain size was still smaller than the grain size of the UO 2 pellet sintered at 1650 o C. A larger grain size is desired to lower the amount of fission gas release because the fission gas has to diffuse longer distance to the grain boundary. 62

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The rate of fission gas release is related to the fuel temperature. The SiC additives are expected to increase the thermal conductivity and lower the fuel temperature, thus lower the fission gas release and offset the negative effect of smaller grain of UO 2 pellet sintered at low temperature. 63

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Figure 5-1. Oxygen to uranium ratio of UO 2 powder oxidized in air at 140 o C [14]. Figure 5-2. Uranium dioxide pellet sintered at 1650 o C. 64

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Figure 5-3. Scanning electron microscopy image of UO 2 pellet sintered at 1650 o C (5,000X). Figure 5-4. Uranium dioxide pellet sintered at 1200 o C. 65

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Figure 5-5. Scanning electron microscopy image of UO 2 pellet sintered at 1200 o C (5,000X). Figure 5-6. Uranium dioxide pellet sintered at 1200 o C with pressure. 66

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Figure 5-7. Scanning electron microscopy image of UO 2 pellet sintered at 1200 o C with pressure (5,000X). 67

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CHAPTER 6 SILICON CARBIDE COATING BY CHEMICAL VAPOR DEPOSITION Background The chemical vapor deposition (CVD) is a chemical process to produce high purity solid materials by dissociation and /or chemical reactions of gaseous reactants (precursors) in an active (heat, light, plasma etc) environment. The CVD process has the advantage of producing highly dense and pure materials at relative low processing temperature. For example, SiC can be produced below 1400 o C [42-47], much lower than the melting point. There are several variants of CVD methods, which are initiated using different energy sources, such as plasma enhanced CVD (PECVD) [48], photo-assisted CVD (PACVD) [49] and microwave CVD [50]. Though the use of variant energy sources may requires sophisticated system and increase the cost of the process, the cost for conventional CVD technique is reasonable [51]. The CVD process has been used to deposit a SiC layer in tristructral isotropic (TRISO) fuel particles in a fluidized bed. In general, argon-hydrogen carrier gas is bubbled through a liquid precursor, and then passed through a fluidized bed of carbon coated fuel particles maintained at 1000 to 1800 o C. [52]. Methyltrichlorosilane (CH 4 SiCl 3 or MTS) was usually used as the precursor because it contains the same number of silicon and carbon. The deposition rate and microstructure of SiC were determined by the deposition temperature, MTS flow rate and argon to hydrogen ratio. Based on the study by Gulden [52], the deposition rate of SiC is independent of temperature and only depends on the flow rate of MTS. The microstructure of SiC is strongly dependent on temperature. A relative low dense laminar structure of SiC was formed at the temperature between 1200 o C to 1400 o C and almost full dense columnar structure of SiC was formed at the temperature above 1400 o C. However, by adjustment of argon to hydrogen ratio, 68

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theoretical dense SiC can be deposited at the temperature between 1200 to 1400 o C [53]. The crystalline size of SiC increases with increasing temperature and decreases with increasing argon to hydrogen ratio. The study by Ford et al. [53] showed that the crystalline size of SiC was 0.25 micron at 1700 o C, using only hydrogen and 0.03 micron at 1300 o C, using an argon-hydrogen mixture. The CVD process has also been used to produce SiC electronic devices in semiconductor industry. Steckl et al. [42] have successfully investigated the growth of -SiC on silicon substrate using silacyclobutane (C 3 H 6 SiH 2 or SCB) at the temperatures as low as 800 o C in a low pressure CVD system. Lin et al. [43] have successfully deposited SiC film on sapphire substrate using trimethylsilane (C 3 H 9 SiH or TMS) at 1100 o C in a low pressure CVD system. Kunstmann et al. [44] have reported the growth of -SiC films using MTS at 1200 o C. Madapura et al. [45] have reported the growth of -SiC using TMS at the temperature between 1100 to 1200 o C. Silicon carbide can also be produced by CVD process using separate precursors. Yagi et al. [46] have used SiH 2 Cl 2 and C 2 H 2 to grow SiC on silicon substrate at 1020 o C in a low pressure CVD system. Powell et al. [47] have used SiH 4 and propane to grow SiC on silicon substrate at 1360 o C. The ratio of the gaseous precursors has to be chosen carefully to ensure the stoichiometry of SiC. Experiment and Result In this research, both TMS and MTS were used to deposit a SiC layer on carbon coated uranium oxide particles. The buffer carbon layer was deposited on uranium oxide particles by decomposition of propane (C 3 H 8 ). The CVD process was carried out at 1300 o C in a Lindberg Blue high temperature tube furnace as shown in Figure 6-1. Argon and 5% H 2 gas was used as carrier gas at a constant flow rate of 140 cm 3 per minute. The precursor, TMS or MTS, flowed 69

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through the furnace at the flow rate of 5 cm 3 per minute at the temperature of 1300 o C for 30 minutes. Figure 6-2 is the temperature profile of the CVD process The powder after the CVD process is shown in Figure 6-3. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) were used to characterize the SiC coating. FTIR was used to obtain information about the chemical bonding in the material; XRD was used to identify the crystalline structure. The FTIR result is shown in Figure 6-4. The powder after CVD process has a peak at the same position as -SiC powder, which indicates the formation of Si-C bond. The XRD result is shown in Figure 6-5. There are no SiC peaks found in Figure 6-5 and all of the peaks in Figure 6-5 are UO 2 peaks. Discussion The possible reason for no SiC peaks found in XRD result was that the precursor, TMS or MTS, had already decomposed before reaching the carbon coated UO 2 particles. The furnace tube was about four feet long and the UO 2 powder was placed in the middle of the furnace tube, so the gaseous precursor had to travel two feet before reaching the powder. TMS or MTS might have already decomposed and deposited on the inside of the furnace tube. A colored layer can be seen on the inside wall of the furnace tube in Figure 6-6. The peak in FTIR result might be some contamination containing Si-C bond. There was another possibility that -SiC was not detected on the carbon coated uranium oxide particles. The amount of SiC was small and uranium oxide is a strong X-ray absorber, so the X-ray diffracted from -SiC was absorbed in UO 2 and the peaks of SiC were not shown in the XRD result. Even if there was a -SiC layer formed on the carbon coated UO 2 powder, the powder will be hard to sinter. The powder can be seen as SiC in the sintering process because SiC is the outside layer and there are two SiC layers between two particles. The low temperature sintering 70

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method of UO 2 can not be used. Temperature above 1900 o C is required to achieve a high density SiC pellet, which is above the temperature at which SiC reacts with UO 2 The CVD process was not further studied because of the potential problem for sintering and the success in making the SiC whiskers-UO 2 composite. 71

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Figure 6-1. Lindberg high temperature furnace. Timie ( minute ) Temperature 0 100 200 300 400 500 600 700 800 9001000 0 200 400 600 800 1000 1200 1400 TMS/MTS Figure 6-2. Temperature profile of the CVD process. 72

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Figure 6-3. Uranium oxide powder after CVD process. Figure 6-4. Fourier transform infrared spectroscopy result of the powder after the CVD process. 73

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2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 6-5. X-ray diffraction result of the powder after CVD process. Figure 6-6. Furnace tube after CVD process. 74

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CHAPTER 7 SILICON CARBIDE COATING FROM PRECERAMIC POLYMER Background Silicon carbide (SiC) can be produced by a pre-ceramic polymer. Grigoriev et al. [54] have successfully deposit an amorphous SiC coating on alumina and zirconia substrate using a ter-polysilane polymer at 900 o C. Zheng et al. [55] have reported the growth of SiC whiskers by Allylhydridopolycarbosilane (AHPCS) and SiC powder in the temperature range of 1250 o C to 1350 o C. Berton et al. [56] have used AHPCS to make carbon/ silicon carbide composite by polymer infiltration and pyrolysis (PIP). Solomon et al. [10] have also used AHPCS in the PIP process to make SiC/UO 2 composite. The conversion of pre-ceramic polymer to ceramic is a relatively easy and low cost process compared with chemical vapor deposition (CVD) process. Experiments and Results The SiC pre-ceramic polymer used in this research is AHPCS, also called SMP-10 by the manufacturer Starfires Systems Inc. AHPCS is a liquid with bright orange color, as shown in Figure 7-1. According to the manufacturer, amorphous SiC forms at 850 o C with 75 to 82% ceramic yield and nano-crystalline -SiC forms at 1250 to 1700 o C with 75 to 80% yield. AHPCS was first used to make a SiC pellet. The SiC powder (1 ) purchased from Alfa Aesar was mixed with 10 w% AHPCS in Hexane (C 6 H 14 ). After Hexane was dried, the mixed powder was cold pressed in a 13 mm die at 200 MPa. The green pellet was then sintered in a Lindberg furnace (Figure 7-2) in argon atmosphere following the sintering procedure provided by Starfires Systems Inc (Figure 7-3). The sintered pellet made from1 powder is shown in Figure 7-4 and the SEM image of the pellet is shown in Figure 7-5. The density measured by the Archimedes method was 98.7%TD, which was far different from 66.7%TD, the density calculated by mass and volume. 75

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Uranium oxide (UO 2.27 ) was mixed with 10 weight % AHPCS in hexane. After the hexane evaporated, the mixed powder was cold pressed in an alumina die ( inch in diameter) at 200 MPa. Then both the pellet and the alumina die were heated in an argon atmosphere in the same Lindberg furnace. During the sintering process, about 5 MPa pressure was applied on the pellet. The pressure was provided by two springs located at the end of the furnace tube, which was sealed by stainless steel end caps. The pellet after sintering broke into pieces when it was taken out of the alumina die. Discussion The reason for the large difference between the density measured by the Archimedes method and the density calculated by mass and volume is that the Archimedes method is not suitable for measuring the density of a pellet with large open porosity. The density of a pellet with large open porosity measured by the Archimedes method is the density of the skeleton of the pellet, which can be close to the theoretical density. The density calculated by mass and volume is relative close to the real value compare to the result by the Archimedes method. There is an easy way to tell whether the pellet is suitable for the Archimedes method. If there are a lot of bubbles coming out of the pellet when immersed into liquid, the pellet is porous and not suitable for the Archimedes method; otherwise, the pellet is low in porosity and suitable for the density measurement by the Archimedes method. The density of the SiC pellet sintered by pressureless sintering is low because of the relative low yield of the pre-ceramic polymer. There were large pores in the SiC pellet, as shown in Figure 7-5. The reason for the break up of the pellet may be that the pre-ceramic polymer was oxidized by the UO 2.27 powder. The polymer precursor could be oxidized by the extra oxygen in uranium oxide powder before it can be converted to silicon carbide. Because the extra oxygen is essential 76

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for the low temperature sintering method of UO 2 the oxidation of the pre-ceramic polymer is inevitable and the process of pre-ceramic coating on uranium oxide particles is not successful. 77

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Figure 7-1. Allylhydridopolycarbosilane (AHPCS), the SiC pre-ceramic polymer. Figure 7-2. Lindberg/Blue Mini-Mite 1100 o C furnace. 78

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Time (minute)Temperature (oC) 0 100 200 300 400 500 600 700 800 9001000 0 200 400 600 800 10001200 2 oC/min1 oC/min3 oC/min3 oC/min5 oC/min1 hour1 hour Figure 7-3. The temperature profile of sintering process. Figure 7-4. Silicon carbide pellet made by SiC powder (1 ) and AHPCS. 79

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Figure 7-5. Scanning electron microscopy image of the SiC pellet made by SiC powder (1 ) and AHPCS. 80

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CHAPTER 8 SILICON CARBIDE WHISKERS URANIUM DIOXIDE COMPOSITE Background Silicon carbide (SiC) whiskers are usually used as a reinforce material to improve the mechanical properties, such as strength and fracture toughness, of matrix materials. Wei et al. [34] used SiC whiskers to reinforce aluminum oxide (Al 2 O 3 ) to improve the fracture toughness. Sun et al. [35] used SiC whiskers to improve the fracture toughness and high temperature strength of molybdenum disilicide (MoSi 2 ). Silicon carbide whiskers are commonly made by either rice hull or vapor-liquid-solid (VLS) process. The SiC whiskers produced from rice hull process are typically less than 1 micron in diameter and range from 10 to 50 micron in length; the SiC whiskers produced from VLS process are typically 5-6 micron in diameter and up to 100 mm in length [57]. Silicon carbide whiskers are single crystal, which means fewer flaws than polycrystalline, so the thermal conductivity and strengths are very high. The commercially available SiC whiskers are commonly in an agglomerated form. The agglomeration must be broken before mixing with the matrix to ensure homogenously dispersion of SiC whiskers. Wei et al. [34] reported that SiC whiskers and ceramic powder were mixed in hexane in a blender and then dispersed using an ultrasonic homogenizer; the hexane was then dried by evaporation with constant agitation under flowing air. Hot pressing was found to be required to achieve a high density pellet with greater than 5 vol% SiC whiskers, because the SiC whiskers interfered with matrix particle rearrangement during sintering. During hot press sintering, SiC whiskers are preferentially oriented in a plane perpendicular to the hot pressing direction [7-9, 34]. 81

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Several studies have shown that SiC whiskers can also increase the thermal conductivity of matrix materials. Russell et al. [7] reported the thermal conductivity of 30 vol% VLS SiC whisker-mullite composite is three times higher at room temperature than that of single phase mullite in perpendicular direction to the hot pressing direction and two times higher in parallel direction. Johnson et al. [8] reported that the thermal conductivity of 30 vol% SiC whisker-osumilite glass composite is four times higher at room temperature than that of single phase mullite in perpendicular direction to the hot pressing direction and two times higher in parallel direction. Hesselman et al. [9] showed the thermal conductivity of 30 vol% VLS SiC whisker-lithium aluminosilicate glass composite is five times higher at room temperature than that of lithium aluminosilicate glass in perpendicular direction to the hot pressing direction and three times higher in parallel direction. Hesselman et al. [9] suggested that a SiC whisker percolation pathway was formed and heat was conducted through SiC whiskers, bypassing the matrix. Experiments and Results Characterization of Silicon Carbide Whiskers The silicon carbide whiskers are commercially available from Alfa Aesar (Alfa) and Advanced Composite Materials (ACM). The whiskers from Alfa Aesar are 1.5 micron in diameter and about 18 micron in length (no detailed information available); the Whiskers from Advanced Composite Materials are 0.45-0.65 micron in diameter and 5-80 micron in length. Both of the whiskers are single crystal -SiC. The received SiC whiskers were in the form of agglomerates. The scanning electron microscope (SEM) images of the received SiC whiskers from Alfa Aesar are shown in Figure 8-1 and Figure 8-2. The X-ray diffraction pattern of SiC whiskers from Alfa Aesar is shown in Figure 8-3. The SEM images of the received SiC whiskers from Advanced Composite Materials are shown in Figure 8-4 and Figure 8-5. The X-ray diffraction pattern of SiC whiskers from Advanced Composite Materials is shown in Figure 8-6. 82

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Mixing of SiC Whiskers and Uranium Oxide Powder The received SiC whiskers were blended with hexane in a blender for 3 minutes to break the agglomeration. A small amount of the mixture was dropped on the SEM sample holder and the hexane was left to evaporate, leaving only the separated SiC whiskers. Figure 8-8 and Figure 8-9 are the SEM images of dispersed SiC whisker from Alfa Aesar and Advanced Composite Materials, respectively. Based on the SEM images, the SiC whiskers were successfully dispersed by the blending. The separated SiC whiskers were then mixed with UO 2.27 particles (25 to 45 ) in an ultrasonic mixer for 10 minutes. After hexane was dried, the mixed powder was ground by a mortar and a pestle. Pressureless Sintering of SiC Whiskers and Uranium Oxide Powder The mixed powder of UO 2 and SiC whiskers from Advanced Composite Materials was cold pressed in an alumina (Al 2 O 3 ) die (1/4 in diameter) at 200MPa. The green pellet and the Al 2 O 3 die were sintered at 1200 o C in argon atmosphere for 1 hour. The pellet of UO 2 with 5 vol% SiC is shown in Figure 8-9. The SEM images of the pellet are shown in Figure 8-10 and Figure 8-11. The density of the pellet was 98.6% TD, which was measured by the Archimedes method. The pellet of UO 2 with 10 vol% SiC is shown in Figure 8-12. The SEM images of the pellet are shown in Figure 8-13 and Figure 8-14. The density of the pellet was 82.2% TD, which was also measured by the Archimedes method. The density of the pellet decreases sharply with increasing the amount of SiC whiskers. Hot Press Sintering of SiC Whiskers and Uranium Oxide Powder The mixed powder of UO 2 and SiC whiskers from Advanced Composite Materials was cold pressed in an alumina die at 200MPa. The alumina die was made by an alumina tube and two alumina rods, as shown in Figure 8-15. After cold pressing, the mixed powder and alumina die were placed in a sample holder surrounded by a graphite tube at the position of mixed 83

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powder, as shown in Figure 8-16. The geometry of the alumina die and graphite tube was shown in Figure 8-17. The mixed powder was then hot press sintered in a hot press sintering system, which included a sintering chamber, a high voltage alternating current (AC) generator and a pyrometer, as shown in Figure 8-19. The high voltage AC generator was connected to a copper coil inside the sintering chamber. An alternating electromagnetic field was generated inside the coil with AC current flowing through it. The changing electromagnetic field induced currents in an electrical conductor as it was placed inside the coil. The induced currents (Eddy current) generated heat inside the conductor due to the resistance. The alumina die and graphite tube was placed inside the coil. Eddy currents were induced inside the graphite tube, which is an electrical conductive material. Alumina and uranium oxide are poor electrical conductors, so no Eddy currents were generated inside them. The surface temperature of the graphite tube was measured by an optical pyrometer. Figure 8-18 shows the heated graphite observed through the pyrometer. The pyrometer communicated with the high voltage AC generator to keep the surface temperature of the graphite tube constant. The mixed powder was sintered at 1201.4 o C for 1hour in argon atmosphere. (The detailed sintering temperature calculation is in Appendix B.) The pressure applied by the lead bricks on top during the sintering process was about 10 MPa. The pellets after hot process sintering were soaked in hydrogen atmosphere to reduce the oxygen to uranium ratio to 2.0. The densities of the pellets after reduction, which were measured by the Archimedes method, are shown in Table 8-1. The pellet of UO 2 with 5 vol% SiC is shown in Figure 8-20. The SEM images of the pellet are shown in Figure 8-21 and Figure 8-22 and the XRD result is shown in Figure 8-23. The pellet of UO 2 with 10 vol% SiC is shown in Figure 8-24. The SEM images of the pellet are shown in Figure 8-25 and Figure 8-26 and the XRD result is shown in Figure 8-27. The pellet of UO 2 with 84

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15 vol% SiC is shown in Figure 8-28. The SEM images of the pellet are shown in Figure 8-29 and Figure 8-30 and the XRD result is shown in Figure 8-31. The pellet of UO 2 (<25 ) with 15 vol% SiC from Advanced Composite Materials (Figure 8-32) was made by hot press sintering to study the effect of UO 2 particle size on the final density. The SEM images of the pellet are shown in Figure 8-33 and Figure 8-34 and the XRD result is shown in Figure 8-35. The density of the pellet measured by Archimedes method was 95.6% TD. The pellet of UO 2 (25 to 45 ) with 15 vol% SiC from Alfa Aesar (Figure 8-36) was also made by hot press sintering to study the effect of different SiC whiskers on the final density. The SEM images of the pellet are shown in Figure 8-37 and Figure 8-38 and the XRD result is shown in Figure 8-39. The density of the pellet measured by Archimedes method was 96.0% TD. Discussion The densities of the pellets, which were made by pressureless sintering, decreased sharply with increasing the amount of SiC whiskers. The density of the pellet of UO 2 with 10 vol% SiC whiskers was only 82.2%, compared to 98.6% of the pellet of UO 2 with 5 vol% SiC whisker. The SiC whiskers hinder the process of UO 2 particles joining together to form large grains. It is more difficult for pellet to densify with increasing amount of SiC whiskers. High density is usually not possible with greater than 5 vol% SiC whiskers. The grain size of UO 2 with 10 vol% SiC whiskers in Figure 8-14 is smaller than that of UO 2 with 5 vol% SiC whiskers in Figure 8-11. The densities of the pellets made by hot press sintering were all close to 95% TD. The density of the pellet of UO 2 with 10 vol% SiC whiskers was 94.7% TD, which was slightly less than the density of the pellet of UO 2 with 5 vol% SiC whiskers, 95.4% TD. It is reasonable because the SiC whiskers hinder the process of UO 2 particles joining together to form large 85

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grains. It is more difficult for pellet to densify with increasing amount of SiC whiskers. The density of the pellet of UO 2 with 15 vol% SiC whiskers was 95.9%, which was larger than the density of the pellet of UO 2 with 10 vol% SiC whiskers and even larger than the density of the pellet of UO 2 with 5 vol% SiC whiskers. The reason for the unusual increase in the density may be that the SiC whiskers were not well dispersed in UO 2 Some UO 2 islands can be found in the SEM image of the pellet of UO 2 and 15 vol% SiC whiskers (Figure 8-29 and Figure 8-30). The UO 2 islands increased the density of the pellet. Some UO 2 islands can also be found in the SEM images of the pellet of UO 2 and 5 vol% or 10 vol% SiC whisker. It is more difficult to disperse SiC whiskers homogeneously in UO 2 with increasing amount of SiC whiskers. The density of the pellet of UO 2 (<25 ) with 15 vol% SiC from Advanced Composite Materials was 95.6% TD. The density of the pellet of UO 2 (25 to 45 ) with 15 vol% SiC from Alfa Aesar was 96.0% TD. The two densities were very close to the density of UO 2 (25 to 45 ) with 15 vol% SiC from Advanced Composite Materials, so the UO 2 particle size and the SiC whiskers are not decisive factors for the density of the pellet after hot press sintering. Due to the nature of material transport and flow during hot press sintering, the SiC whiskers exhibited a preferential orientation. The SEM images of the cross section of UO 2 with 30 vol% SiC whiskers after hot press sintering are shown in Figure 8-40 and Figure 8-41. The SiC whiskers are oriented in a plane which is perpendicular to the direction of hot pressing. This preferential orientation of the SiC whiskers is desired because the preferential orientation increases the probability of the contact of SiC whiskers, thus increase the probability of the formation of the percolation pathway for heat to flow out of the pellet. 86

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Table 8-1. Densities of the pellets of UO 2 and SiC whiskers after hot press sintering. UO 2 with 5 vol% SiC whiskers UO 2 with 10 vol% SiC whiskers UO 2 with 15 vol% SiC whiskers Density (% TD) 95.4 94.7 95.9 87

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Figure 8-1. Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar (500X). Figure 8-2. Scanning electron microscopy image of SiC whiskers as received from Alfa Aesar (2,000X). 88

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2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-3. X-ray diffraction pattern of SiC whiskers from Alfa Aesar. Figure 8-4. Scanning electron microscopy image of SiC whiskers as received from Advanced Composite Materials (500X). 89

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Figure 8-5. Scanning electron microscopy image of SiC whiskers as received from Advanced Composite Materials (2,000X). 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-6. X-ray diffraction pattern of SiC whiskers from Advanced Composite Materials. 90

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Figure 8-7. Scanning electron microscopy image of SiC whiskers from Alfa Aesar after dispersion (2,000X). Figure 8-8. Scanning electron microscopy image of SiC whiskers from Advanced Composite Materials after dispersion (2,000X). 91

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Figure 8-9. The pellet of UO 2 with 5 vol% SiC whiskers after pressureless sintering. Figure 8-10. Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after pressureless sintering, (2,000X). 92

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Figure 8-11. Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after pressureless sintering, (5,000X). Figure 8-12. The pellet of UO 2 with 10 vol% SiC whiskers after pressureless sintering. 93

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Figure 8-13. Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after pressureless sintering, (2,000X). Figure 8-14. Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after pressureless sintering, (5,000X). 94

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Figure 8-15. Alumina die for hot press sintering. Figure 8-16. Alumina die, graphite tube and sample holder. 95

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Figure 8-17. Geometry of the alumina die and graphite tube. Figure 8-18. Heated graphite tube observed through the optical pyrometer. Graphite tub e Alum ina tub e Mixed powder inch inch r Coil 1200 o C 1 inch 96

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Figure 8-19. Hot press sintering apparatus. 97

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Figure 8-20. The pellet of UO 2 with 5 vol% SiC whiskers after hot press sintering. Figure 8-21. Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after hot press sintering, (2,000X). 98

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Figure 8-22. Scanning electron microscopy image of UO 2 with 5 vol% SiC whiskers after hot press sintering, (5,000X). 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-23. X-ray diffraction result of UO 2 with 5 vol% SiC whiskers after hot press sintering. 99

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Figure 8-24. The pellet of UO 2 with 10 vol% SiC whiskers after hot press sintering. Figure 8-25. Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after hot press sintering, (2,000X). 100

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Figure 8-26. Scanning electron microscopy image of UO 2 with 10 vol% SiC whiskers after hot press sintering, (5,000X). 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-27. X-ray diffraction result of UO 2 with 10 vol% SiC whiskers after hot press sintering. 101

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Figure 8-28. The pellet of UO 2 with 15 vol% SiC whiskers after hot press sintering. Figure 8-29. Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers after hot press sintering, (2,000X). 102

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Figure 8-30. Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers after hot press sintering, (5,000X). 2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-31. X-ray diffraction result of UO 2 with 15 vol% SiC whiskers after hot press sintering. 103

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Figure 8-32. The pellet of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering. Figure 8-33. Scanning electron microscopy image of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering, (2,000X). 104

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Figure 8-34. Scanning electron microscopy image of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering, (5,000X). 105

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2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-35. X-ray diffraction result of UO 2 (<25 ) with 15 vol% SiC whiskers after hot press sintering. 106

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Figure 8-36. The pellet of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering. Figure 8-37. Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering, (2,000X). 107

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Figure 8-38. Scanning electron microscopy image of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering, (5,000X). 108

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2 Intensity 10 20 30 40 50 60 7080 0 0.2 0.4 0.6 0.8 11.2 Figure 8-39. X-ray diffraction result of UO 2 with 15 vol% SiC whiskers (from Alfa Aesar) after hot press sintering. 109

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Figure 8-40. Scanning electron microscopy image of the cross section of UO 2 with 30 vol% SiC whiskers after hot press sintering, (2,000X). Figure 8-41. Scanning electron microscopy image of the cross section of UO 2 with 30 vol% SiC whiskers after hot press sintering, (5,000X). 110

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CHAPTER 9 CONCLUSIONS AND FUTURE WORK Conclusion In this research, the silicon carbide (SiC) whisker-uranium dioxide (UO 2 ) composite was successfully fabricated by low temperature sintering method of UO 2 High density pellets were achieved at 1200 o C by pressureless sintering of UO 2 with 5 vol% SiC whiskers and hot press sintering of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC whiskers. Based on the work by Russell et al. [7], Johnson et al. [8] and Hesselman et al. [9], The thermal conductivity of SiC whisker-UO 2 composite is higher than the thermal conductivity of UO 2 Two other methods to incorporate SiC into UO 2 the chemical vapor deposition (CVD) process to coat uranium oxide particles with SiC and the pre-ceramic polymer coating on uranium oxide particles process, were not successful. Because SiC whiskers replace UO 2 there is a reactivity penalty at the end of life (EOL). The neutronic calculation showed the K-infinity of UO 2 with 5 vol%, 10 vol% and 15 vol% SiC are about 2.4%, 4.9% and 7.6% less than the K-infinity of UO 2 fuel at 60 GWD/MTU burnup. The amount of SiC whiskers was limited to 15 vol% in this research to ensure the neutronic properties of the fuel pellet. The neutronic calculation also proved the UO 2 with up to 15 vol% SiC additives can be operate safely in a reactor core because of the negative Doppler coefficient and moderator temperature coefficient. During the fabrication process of SiC whisker UO 2 pellet, the low temperature sintering method of UO 2 was used to avoid the reaction between silicon carbide and UO 2 at the temperature above 1377 o C. The density of the SiC whisker-UO 2 pellet fabricated by hot press sintering was not significantly affected by the particle size of uranium oxide particle and the type 111

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of SiC whiskers. The SiC whiskers after hot press sintering exhibited a preferential orientation, which is perpendicular to the direction of hot pressing. Future Work There are several aspects of this research can be improved or studied in the future work. The thermal conductivity of the UO 2 -SiC pellets need to be measured to study the effect of SiC whiskers on the thermal conductivity of UO 2 The process of mixing SiC whiskers and UO 2 particles need to be improved to make a homogeneous mixture. The UO 2 islands in the SiC whiskers-UO 2 pellet can cause local heat spot, which is highly undesired in the operation of the reactor. The effect of whiskers orientation, aspect ratio of whiskers, particle size of the matrix and interfacial thermal barrier resistance, if any, on the thermal conductivity of the composite need to be studied. Because the thermal conductivity of single crystal drops significantly after radiation, the change of the thermal conductivity of SiC whiskers-UO 2 pellet under radiation also need to be studied. 112

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APPENDIX A CASMO INPUT FILES This appendix contains all of the CASMO-3 input files used to determine the effect of silicon carbide additives on the neutronic properties of uranium dioxide fuel. 113

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The CASMO-3 Input File for Uranium Dioxide Fuel *Crystal River-3 15X15 Assembly UO2 Fuel TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 10.5216/4.66 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 29.36 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.5 5 10 15 20 25 30 35 40 45 50 55 60 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.5 5 10 15 20 25 30 35 40 45 50 55 60 TFU 900 1000 1100 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.5 5 10 15 20 25 30 35 40 45 50 55 60 TMO 563 588 613 NLI,STA END 114

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The CASMO-3 Input File for Uranium Dioxide Fuel with 5 w% Silicon Carbide *Crystal River-3 15X15 Assembly UO2 Fuel with 5 w% SiC TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 9.4458/3.9023 92238=79.8384 8000=11.2593 6000=1.4963 14000=3.5037 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 34.43 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.59 5.86 11.73 17.59 23.45 29.31 35.18 41.04 46.90 52.76 58.63 64.49 70.35 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.59 5.86 11.73 17.59 23.45 29.31 35.18 41.04 46.90 52.76 58.63 64.49 70.35 TFU 900 1000 1100 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.59 5.86 11.73 17.59 23.45 29.31 35.18 41.04 46.90 52.76 58.63 64.49 70.35 TMO 563 588 613 NLI,STA END 115

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The CASMO-3 Input File for Uranium Dioxide Fuel with 10 w% Silicon Carbide *Crystal River-3 15X15 Assembly UO2 Fuel with 10 w% SiC TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 8.5696/3.6969 92238=75.6364 8000=10.6667 6000=2.9925 14000=7.0075 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 40.06 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.68 6.82 13.64 20.46 27.28 34.10 40.93 47.75 54.57 61.39 68.21 75.03 81.85 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.68 6.82 13.64 20.46 27.28 34.10 40.93 47.75 54.57 61.39 68.21 75.03 81.85 TFU 900 1000 1100 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.68 6.82 13.64 20.46 27.28 34.10 40.93 47.75 54.57 61.39 68.21 75.03 81.85 TMO 563 588 613 NLI,STA END 116

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The CASMO-3 Input File for Uranium Dioxide Fuel with 5 vol% Silicon Carbide *Crystal River-3 15X15 Assembly UO2 Fuel with 5 vol% SiC TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 10.156/4.0428 92238=82.7123 8000=11.6646 6000=0.4729 14000=1.1074 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 30.91 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58 36.84 42.11 47.37 52.63 57.89 63.16 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58 36.84 42.11 47.37 52.63 57.89 63.16 TFU 900 1000 1100 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58 36.84 42.11 47.37 52.63 57.89 63.16 TMO 563 588 613 NLI,STA END 117

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The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide *Crystal River-3 15X15 Assembly UO2 Fuel with 10 vol% SiC TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 9.7904/3.9730 92238=81.2850 8000=11.4633 6000=0.9812 14000=2.2975 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 32.63 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33 38.89 44.44 50.00 55.56 61.11 66.67 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33 38.89 44.44 50.00 55.56 61.11 66.67 TFU 900 1000 1100 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33 38.89 44.44 50.00 55.56 61.11 66.67 TMO 563 588 613 NLI,STA END 118

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The CASMO-3 Input File for Uranium Dioxide Fuel with 15 vol% Silicon Carbide *Crystal River-3 15X15 Assembly UO2 Fuel with 15 vol% SiC TIT TFU=1000 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 9.4249/3.8978 92238=79.7470 8000=11.2464 6000=1.5288 14000=3.5800 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 34.55 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29 41.18 47.06 52.94 58.82 64.71 70.59 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29 41.18 47.06 52.94 58.82 64.71 70.59 TFU 900 1000 1100 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29 41.18 47.06 52.94 58.82 64.71 70.59 TMO 563 588 613 NLI,STA END 119

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The CASMO-3 Input File for Uranium Dioxide Fuel with 5 vol% Silicon Carbide at Lower Fuel Temperature *Crystal River-3 15X15 Assembly UO2 Fuel with 5 vol% SiC TIT TFU=800 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 10.156/4.0428 92238=82.7123 8000=11.6646 6000=0.4729 14000=1.1074 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 30.91 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58 36.84 42.11 47.37 52.63 57.89 63.16 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58 36.84 42.11 47.37 52.63 57.89 63.16 TFU 700 800 900 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 31.58 36.84 42.11 47.37 52.63 57.89 63.16 TMO 563 588 613 NLI,STA END 120

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The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide at Lower Fuel Temperature *Crystal River-3 15X15 Assembly UO2 Fuel with 10 vol% SiC TIT TFU=800 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 9.7904/3.9730 92238=81.2850 8000=11.4633 6000=0.9812 14000=2.2975 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 32.63 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33 38.89 44.44 50.00 55.56 61.11 66.67 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33 38.89 44.44 50.00 55.56 61.11 66.67 TFU 700 800 900 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 33.33 38.89 44.44 50.00 55.56 61.11 66.67 TMO 563 588 613 NLI,STA END 121

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The CASMO-3 Input File for Uranium Dioxide Fuel with 15 vol% Silicon Carbide at Lower Fuel Temperature *Crystal River-3 15X15 Assembly UO2 Fuel with 15 vol% SiC TIT TFU=800 TMO=588 BOR=0 IDE='P32' *Mark-B10E FUE 1 9.4249/3.8978 92238=79.7470 8000=11.2464 6000=1.5288 14000=3.5800 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 34.55 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 DEP 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29 41.18 47.06 52.94 58.82 64.71 70.59 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29 41.18 47.06 52.94 58.82 64.71 70.59 TFU 700 800 900 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 35.29 41.18 47.06 52.94 58.82 64.71 70.59 TMO 563 588 613 NLI,STA END 122

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The CASMO-3 Input File for Uranium Dioxide Fuel with 5 vol% Silicon Carbide at Lower Fuel Temperature and Boron Let Down *Crystal River-3 15X15 Assembly UO2 Fuel with 5 vol% SiC TIT TFU=800 TMO=588 BOR=1400 IDE='P32' *Mark-B10E FUE 1 10.156/4.0428 92238=82.7123 8000=11.6646 6000=0.4729 14000=1.1074 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 30.91 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 PVD 'BOR' 0 1400 26.32 10 26.33 1400 47.37 10 47.38 1400 63.16 10 DEP 0.0 0.53 5.26 10.53 15.79 21.05 26.32 26.33 26.74 30.53 34.74 38.95 43.16 47.37 47.38 47.68 50.53 53.68 56.84 60.0 63.16 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 26.33 26.74 30.53 34.74 38.95 43.16 47.37 47.38 47.68 50.53 53.68 56.84 60.0 63.16 TFU 700 800 900 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.53 5.26 10.53 15.79 21.05 26.32 26.33 26.74 30.53 34.74 38.95 43.16 47.37 47.38 47.68 50.53 53.68 56.84 60.0 63.16 TMO 563 588 613 NLI,STA END 123

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The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide at Lower Fuel Temperature and Boron Let Down *Crystal River-3 15X15 Assembly UO2 Fuel with 10 vol% SiC TIT TFU=800 TMO=588 BOR=1400 IDE='P32' *Mark-B10E FUE 1 9.7904/3.9730 92238=81.2850 8000=11.4633 6000=0.9812 14000=2.2975 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 32.63 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 PVD 'BOR' 0 1400 27.78 10 27.79 1400 50.0 10 50.01 1400 66.67 10 DEP 0.0 0.56 5.56 11.11 16.67 22.22 27.78 27.79 28.22 32.22 36.67 41.11 45.56 50.0 50.01 50.33 53.33 56.67 60.0 63.33 66.67 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 27.79 28.22 32.22 36.67 41.11 45.56 50.0 50.01 50.33 53.33 56.67 60.0 63.33 66.67 TFU 700 800 900 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.56 5.56 11.11 16.67 22.22 27.78 27.79 28.22 32.22 36.67 41.11 45.56 50.0 50.01 50.33 53.33 56.67 60.0 63.33 66.67 TMO 563 588 613 NLI,STA END 124

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The CASMO-3 Input File for Uranium Dioxide Fuel with 10 vol% Silicon Carbide at Lower Fuel Temperature and Boron Let Down *Crystal River-3 15X15 Assembly UO2 Fuel with 15 vol% SiC TIT TFU=800 TMO=588 BOR=1400 IDE='P32' *Mark-B10E FUE 1 9.4249/3.8978 92238=79.7470 8000=11.2464 6000=1.5288 14000=3.5800 *FUEL COMP. #, DENSITY/ENRICHMENT PIN 1 .4699 .4788 .5461/ '1' 'AIR' 'CAN' PIN 7 .632 .6731/ 'COO' 'CAN' *Center water hole PWR 15 1.443 21.81 PWR with pitch 1.443 FUM 2.778-04 2 PDE 34.55 XEN 0 LPI 7 1 1 1 1 7 1 1 1 1 1 1 1 1 7 1 1 7 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 PVD 'BOR' 0 1400 29.41 10 29.42 1400 52.94 10 52.95 1400 70.59 10 DEP 0.0 0.59 5.88 11.76 17.65 23.53 29.41 29.42 29.88 34.12 38.82 43.53 48.24 52.94 52.95 53.29 56.47 60.0 63.53 67.06 70.59 STA TIT *+ FUEL TEMP BRANCHES RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 29.42 29.88 34.12 38.82 43.53 48.24 52.94 52.95 53.29 56.47 60.0 63.53 67.06 70.59 TFU 700 800 900 NLI,STA TIT *+ MOD TEMP BRANCHES RES 'P32', 0.0 0.59 5.88 11.76 17.65 23.53 29.41 29.42 29.88 34.12 38.82 43.53 48.24 52.94 52.95 53.29 56.47 60.0 63.53 67.06 70.59 TMO 563 588 613 NLI,STA END 125

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APPENDIX B HOT PRESS SINTERING TEMPERATURE CALCULATION The surface temperature of the graphite tube is 1200 o C. The geometry of the alumina die and the graphite tube is shown in Figure 8-17. The steady state temperatures of the alumina tube and mixed powder were calculated base on Equation B-1 and Equation B-5. For graphite tube, 0''')(1gkqdrdTrdrdr (B-1) Where q is the rate of heat production per unit volume and K g is the thermal conductivity of graphite. The two boundary conditions of the graphite tubs are Equation 3-2 and Equation 3-3. 0drdT at r = 0.25 inch (B-2) T (r = 0.5 inch) = 1200 o C (B-3) The heat flux on the surface of the graphite tube can be calculated by Equation 3-4. q = T 4 (B-4) Where is the emissivity, which is 0.95 for graphite, is the Stefan-Boltzmann constant, which equals to 5.67x10 -8 W/m 2 K 4 The calculated graphite temperature at r = 0.25 inch is 1201.4 o C. For alumina tube, 0)(1drdTrdrdr (B-5) The two boundary conditions of the graphite tubs are Equation 3-6 and Equation 3-7. 0drdT at r = 0.125 inch (B-6) 126

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T (r = 0.25 inch) = 1201.4 o C (B-7) The calculated temperature of alumina tube is a constant, 1201.4 o C. For mixed powder, 0)(1drdTrdrdr (B-8) The two boundary conditions of the graphite tubs are Equation 3-6 and Equation 3-7. 0drdT at r = 0 (B-9) T (r = 0.125 inch) = 1201.4 o C (B-10) The calculated temperature of mixed powder is also a constant, 1201.4 o C. 127

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BIOGRAPHICAL SKETCH Jiwei Wang was born in 1978 in Anshan City, Liaoning Province, Peoples Republic of China. His parents are Li Wang and Shaofen Liu. He has an older sister, Jihong Wang. Jiwei graduated with a Bachelor of Science in nuclear engineering from Tsinghua University in July 2001. After that, he took the TOEFL and GRE tests and prepared to pursue a graduate degree in United States. Jiwei enrolled in nuclear and radiological engineerings graduate program at the University of Florida in January 2003. He received a non thesis Master of Science degree in December 2006. He is scheduled to graduate with a Doctor of Philosophy degree in August 2008. 132