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Enhancement of Uranium Dioxide Thermal and Mechanical Properties by Oxide Dopants

Permanent Link: http://ufdc.ufl.edu/UFE0022454/00001

Material Information

Title: Enhancement of Uranium Dioxide Thermal and Mechanical Properties by Oxide Dopants
Physical Description: 1 online resource (67 p.)
Language: english
Creator: Dooies, Brett
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: afci, alumina, chromia, dioxide, dopant, niobia, scandia, titania, uo2, uranium, vanadia, yttria
Nuclear and Radiological Engineering -- Dissertations, Academic -- UF
Genre: Nuclear Engineering Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The Advanced Fuel Cycle Initiative (AFCI) program is funding the development of high-burnup fuels for use in current and future reactors. These fuels have the capacity to reduce the rate at which spent fuel is created and thus reduce the amount of spent fuel requiring long-term geologic disposal. One of the factors limiting the burnup of current reactor fuel is the buildup of internal pin pressure due to the release of gaseous fission products from the fuel matrix. Theoretical calculations have shown that increasing the grain size of uranium dioxide fuel could help to reduce the release of fission products and thus slow the buildup of internal pin pressure. This theory has been proven by numerous irradiation studies of large-grained UO2 which have shown a decreased release rate of fission product gases. Obtaining large-grained fuel can be accomplished by several methods including doping UO2 with a small amount ( < 1 wt%) of another metal oxide. In this research, UO2 was doped with oxides of niobium (Nb), aluminum (Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and titanium (Ti) at a concentration of 0.5 wt%. The pellets were sintered to greater than 93% of their theoretical density and microstructural analysis was performed to ascertain the effects of the dopants on the sintered pellets. The grain sizes were measured after chemically etching polished sections of the samples. Titania showed the most substantial promotion of larger grain sizes with a 281% increase over undoped UO2. Other dopants that showed potential for grain size increase were niobia, alumina, chromia, and vanadia. Scandia had no significant effect on the grain size of the fuel. The grain size of the yttria-doped fuel was unable to be determined due to inconsistencies with the chemical etching process. Overall, the ability of a small amount of dopant to promote larger grains in uranium dioxide fuel was shown, with titania having the most potential.
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 Brett Dooies.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Anghaie, Samim.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022454:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022454/00001

Material Information

Title: Enhancement of Uranium Dioxide Thermal and Mechanical Properties by Oxide Dopants
Physical Description: 1 online resource (67 p.)
Language: english
Creator: Dooies, Brett
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: afci, alumina, chromia, dioxide, dopant, niobia, scandia, titania, uo2, uranium, vanadia, yttria
Nuclear and Radiological Engineering -- Dissertations, Academic -- UF
Genre: Nuclear Engineering Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The Advanced Fuel Cycle Initiative (AFCI) program is funding the development of high-burnup fuels for use in current and future reactors. These fuels have the capacity to reduce the rate at which spent fuel is created and thus reduce the amount of spent fuel requiring long-term geologic disposal. One of the factors limiting the burnup of current reactor fuel is the buildup of internal pin pressure due to the release of gaseous fission products from the fuel matrix. Theoretical calculations have shown that increasing the grain size of uranium dioxide fuel could help to reduce the release of fission products and thus slow the buildup of internal pin pressure. This theory has been proven by numerous irradiation studies of large-grained UO2 which have shown a decreased release rate of fission product gases. Obtaining large-grained fuel can be accomplished by several methods including doping UO2 with a small amount ( < 1 wt%) of another metal oxide. In this research, UO2 was doped with oxides of niobium (Nb), aluminum (Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and titanium (Ti) at a concentration of 0.5 wt%. The pellets were sintered to greater than 93% of their theoretical density and microstructural analysis was performed to ascertain the effects of the dopants on the sintered pellets. The grain sizes were measured after chemically etching polished sections of the samples. Titania showed the most substantial promotion of larger grain sizes with a 281% increase over undoped UO2. Other dopants that showed potential for grain size increase were niobia, alumina, chromia, and vanadia. Scandia had no significant effect on the grain size of the fuel. The grain size of the yttria-doped fuel was unable to be determined due to inconsistencies with the chemical etching process. Overall, the ability of a small amount of dopant to promote larger grains in uranium dioxide fuel was shown, with titania having the most potential.
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 Brett Dooies.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Anghaie, Samim.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022454:00001


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ENHANCEMENT OF URANIUM DIOXIDE THERMAL AND MECHANICAL
PROPERTIES BY OXIDE DOPANTS





















By

BRETT JAMESON DOOIES


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2008




































2008 Brett Jameson Dooies


































To my mom









ACKNOWLEDGMENTS

I would first like to acknowledge my graduate advisor, Dr. Anghaie for his continued

encouragement. I am lucky to be his student. Also, I thank the other members of my committee,

Dr. Dugan and Dr. Sigmund. Special thanks goes out to Dr. Dugan for being an excellent

teacher and undergraduate advisor.

This work was performed under the Department of Energy's Advanced Fuel Cycle

Initiative Fellowship. The funding they provided enabled me to finish this research in a timely

manner. I would like to thank Dr. James Bresee and Dr. Tom Ward, for their technical feedback

on my research proposal. I would also like to thank Cathy Dixon and Donna Knight, for the

excellent work that they do for this program.

I would like to acknowledge the facilities that I used to perform my research. My samples

were made at the Innovative Nuclear Space Power and Propulsion Institute at the University of

Florida. Sample characterization was performed at the Major Analytical Instrumentation Center

at UF. I extend thanks to Dr. Travis Knight for his extremely helpful insights and to Dr. Jiwei

Wang for sharing his materials and equipment.

Finally, thanks go out to my family, my girlfriend, and my friends, whose support has

meant everything.









TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ..............................................................................................................4

L IST O F T A B L E S .......................................................................... 7

LIST OF FIGU RE S ................................................................. 8

ABSTRACT ............................................ .. ......... ........... 10

CHAPTER

1 IN TR OD U CTION ......................................................... ................. .. ........ 12

1.1 B background Inform action ..................................................... ................................... 12
1.1.1 A advanced Fuel Cycle Initiative...................................... ........ ............... 12
1.1.2 N ear-Term G oals of A FCI ..................................................... .......... ..... 13
1.1.3 L ong-Term G oals of A FC I ............................................... ............... .... 13
1.2 Waste Reduction in Current and Generation III+ Reactors ................ ...... .......... 13
1.3 Objectives and Scope ............ .... ..... .... .... ............ .... ........ ... ............... 14
1.3.1 Objectives of Research........................ ................... ............... 14
1.3.1.1 Grain size modification ofUO2 ........ ............. ........................14
1.3.1.2 D oping U O 2 ....................................... ...... ............ ............. 14
1.3.2 Scope of Research .................. .......... ............ .... .. ................. 15

2 L ITE R A TU R E R E V IE W ......... ............... ......................................... ................................ 16

2.1 Theoretical Foundation ............................ ................. .. ............................ 16
2.2 Previous Experim ental W ork ......... ............... ..................................... ............... 16
2.2.1 Production of Doped Fuel Pellets ................................... .................17
2.2.2 R results of Previous Studies........................................ ........................... 18

3 M A TER IA L S A N D M ETH O D S ........................................ .............................................24

3 .1 P ro c e ssin g ............................................................................................................... 2 4
3 .1.1 P ow ders ............... .......... ................................................................. 24
3.1.2 C old U niaxial Pressing........................................................... ............... 25
3.1.3 Pellet Sintering ....................... ............ ... ............... 26
3.1.3.1 Induction furnace .......................... ............. ........ .. ... .......... 26
3.1.3.2 Sintering ......... ............................. ......... .... ...... .. ........ .... 27
3.2 C haracterization ................................................................27
3.2.1 Preparation for A nalysis......................................................... ............... 27
3.2.1.1 Scanning electron microscopy........... ............ .. .................27
3.2.1.2 X -ray diffraction ...................................................... .... ........... 29
3.2.2 A analysis of Sam ples........... ............................. ................ ............... 29
3.2.2.1 D ensity m easurem ents .............. ................................ .................. 29









3.2.2.2 Scanning electron microscopy and energy dispersive
sp e ctro sc o p y ................................................................................. 2 9
3.2.2.3 Grain size determination...................... ... ........................ 30
3.2.2.4 X-ray diffraction ............................ ................. .................. 30

4 R E SU L TS A N D D ISCU SSIO N ..................................................................... ..................38

4.1 D density M easurem ents ........ ........................................................... ............... 38
4 .2 P ellet C characterization ......................................................................... ...................38
4.2.1 O optical M icroscopy .......... ................. .............. ................. ............... 38
4.2.2 Scanning Electron M icroscopy ........................................ ...... ............... 39
4.2.3 Electron D ispersive Spectroscopy................................... ...................... 43
4 .2 .4 X -R ay D iffraction ..................................................................... ..................43

5 CONCLUSIONS AND FUTURE WORK.................... ................ ............... 60

5 .1 P ellet P ro c e ssin g ..................................................................................................... 6 0
5.2 G rain Size A naly sis........... ................................................................ ......... ....... 60
5.3 F future W ork .............. ............................................................................... 6 1
5.4 Final Comments .................................. ... .. .......... ............... 62


APPENDIX POWDER CHARACTERISTICS ............................................ ............... 64

B IO G R A PH IC A L SK E T C H ........................ ............................................ ................................67



























6









LIST OF TABLES

Table page

2-1 Grain sizes obtained for doped U02 in previous studies.....................................................22

3-1 Sum m ary of dopant pow ders .......................................................................... ..................32

3-2 Masses of UO2 and dopant powders in sample mixtures. ................... ............................. 32

3-3 G riding and polishing sequence. ........................................ ............................................32

4-1 TD 's of doped sam ples ....................................... ... ............. .............. .. 45

4-2 Pellet m asses and densities. ........................................................................... ....................45

4-3 Results of statistical analysis for pellet grain sizes...................................... ............... 45

4-4 Lattice param eters of sintered pellets .............................................................................. 46

5-1 Sum m ary of grain size analysis ............ ......... ... .......................................... .. ......... ...... 63









LIST OF FIGURES

Figure page

3-1 Sartorius R180D analytic balance used for mass measurements..................................33

3-2 Hydraulic press used for cold uniaxial pressing of pellets. .............................................33

3-3 Stainless steel die set used for cold pressing powders....................................... .......... 34

3-4 Base used to hold pellet during sintering ........................................ ....................... 34

3-5 Induction furnace, controller, pyrometer, and vacuum chamber.......................................35

3-6 LECO VC-50 precision low-speed diamond saw.................... ........................ 35

3-7 LECO Spectrum System 1000 grinder/polisher with semi-automatic head....................36

3-8 Setup for measuring apparent mass of sample while immersed in water........................36

3-9 JEOL JSM 6400 scanning electron microscope. .................................... .................37

3-10 Philips APD 3720 x-ray diffractometer. ........................................ ......................... 37

4-1 Optical images of pellet surfaces post-sinter .......................... ...... ... .............. 47

4-2 U ndoped U O 2 ......................................................................................................... ...... 48

4-3 Nb20O -doped U 02 ............... .......................... ......... .............. ........... 49

4-4 A l20 3-doped U O 2 ......................................................................... ........ ...... 50

4 -5 C r20 3-d o p ed U O 2 .................................................................................................... 5 1

4-6 Sc203-doped U O2 ............. .. .. ... ...... .. ....... ............................. 52

4-7 Y203-doped U02 .......... ....... ....... ..... ............................ 53

4-8 V205-doped U02 .......... ....... ....... ..... ............................ 54

4-9 TiO2-doped U02................... ............................. ............ 55

4-10 ED S spectrum ofundoped UO2 sample....................................... .......................... 56

4-11 EDS spectrum of Nb2O5-doped sample. ............... .......... ........................ 56

4-12 EDS spectrum of A1203-doped sample. ..... ...............................57

4-13 EDS spectrum of Cr203-doped sample. .................................. ....... .................. 57


8









4-14 EDS spectrum of Sc203-doped sample. .......................................................................58

4-15 EDS spectrum of Y203-doped sample ....................................... ................................58

4-16 EDS spectrum ofV205-doped sample. ............................................................................59

4-17 ED S spectrum of TiO 2-doped sam ple ..................................................................... .....59

A-1 Particle size distribution of received uranium dioxide powder. .............. ...................64









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science


ENHANCEMENT OF URANIUM DIOXIDE THERMAL AND MECHANICAL
PROPERTIES BY OXIDE DOPANTS

By

Brett Jameson Dooies

August 2008

Chair: Samim Anghaie
Major: Nuclear Engineering Sciences

The Advanced Fuel Cycle Initiative (AFCI) program is funding the development of high-

burnup fuels for use in current and future reactors. These fuels have the capacity to reduce the

rate at which spent fuel is created and thus reduce the amount of spent fuel requiring long-term

geologic disposal. One of the factors limiting the burnup of current reactor fuel is the buildup of

internal pin pressure due to the release of gaseous fission products from the fuel matrix.

Theoretical calculations have shown that increasing the grain size of uranium dioxide fuel could

help to reduce the release of fission products and thus slow the buildup of internal pin pressure.

This theory has been proven by numerous irradiation studies of large-grained U02 which have

shown a decreased release rate of fission product gases. Obtaining large-grained fuel can be

accomplished by several methods including doping U02 with a small amount (< 1 wt%) of

another metal oxide. In this research, UO2 was doped with oxides of niobium (Nb), aluminum

(Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and titanium (Ti) at a

concentration of 0.5 wt%. The pellets were sintered to greater than 93% of their theoretical

density and microstructural analysis was performed to ascertain the effects of the dopants on the

sintered pellets. The grain sizes were measured after chemically etching polished sections of the









samples. Titania showed the most substantial promotion of larger grain sizes with a 281%

increase over undoped U02. Other dopants that showed potential for grain size increase were

niobia, alumina, chromia, and vanadia. Scandia had no significant effect on the grain size of the

fuel. The grain size of the yttria-doped fuel was unable to be determined due to inconsistencies

with the chemical etching process. Overall, the ability of a small amount of dopant to promote

larger grains in uranium dioxide fuel was shown, with titania having the most potential.









CHAPTER 1
INTRODUCTION

1.1 Background Information

The United States currently has 104 operating nuclear reactors. In 2007 these reactors

safely produced over 806 billion kWh of electricity. Unlike many other major energy sources,

nuclear energy does not emit any greenhouse gases into the atmosphere. Currently, about one-

third of energy produced in the US is from carbon-free sources, and nuclear power makes up

about 70% of that energy (NEI, 2008).

The majority of the reactors currently operating utilize an 18-month refueling cycle, with

average fuel assembly discharge burnups in the range of 35 45 GWd/kgU. With new

Generation III+ reactors on the horizon, many utilities are hoping to switch to a 24-month

refueling cycle. This would be beneficial economically and from a fuel efficiency standpoint,

reducing the overall amount of spent fuel and high level waste being produced. As such, reactor

fuel will soon be pushed to new, higher burnups and will be subjected to increasingly rigorous

operating conditions. The reliability of high-burnup nuclear fuel must be assured in order for

these goals to be realized.

1.1.1 Advanced Fuel Cycle Initiative

The Advanced Fuel Cycle Initiative (AFCI) is a program of the United States Department

of Energy. The mission of AFCI is "to develop fuel cycle technologies that will meet the need

for economic and sustained nuclear energy production while satisfying requirements for

controlled, proliferation-resistant nuclear materials management system." (DOE, 2008) AFCI is

meant to develop new technologies to aid the current reactor fleet, as well as future Generation

III+ and Generation IV reactors. Mission success would result in a reduction of the amount of









high level radioactive waste requiring geologic disposal, a reduction of the plutonium content of

civilian spent fuel, and increased energy extraction from the fuel.

1.1.2 Near-Term Goals of AFCI

The near-term goals of AFCI are to develop and demonstrate a proliferation-resistant spent

fuel recycling program. This pursuit would reduce the volume and heat content of high level

waste that requires long-term storage, and exploit the large amount of fissile material still present

in spent fuel. The Secretary of Energy is required to advise Congress on the need for an

additional geologic repository (to follow Yucca Mountain) by 2010. This recommendation will

be driven by the ability to establish a viable spent fuel recycling program for commercial spent

nuclear fuel.

1.1.3 Long-Term Goals of AFCI

The long-term goals of AFCI are to develop a system for recycling spent fuel that would

separate the fuel and allow for the destruction of actinides and other long-lived fission products

in fast reactors through transmutation. The removal of these products would increase the

capacity of the planned Yucca Mountain repository up to fifty-fold due to the decrease in the

heat load of the remaining spent fuel. By successfully increasing the technical capacity by such

a large factor, Yucca Mountain would be sufficient for the storage of all current spent fuel as

well as all the fuel produced in the next century (DOE, 2008).

1.2 Waste Reduction in Current and Generation III+ Reactors

The long-term goals of the AFCI could take upwards of 30 years to be completely

developed and implemented. Until the technology for reprocessing is available, current reactors

(and those currently being planned for construction) must be concerned with the waste that they

are producing now. Many reactor sites have recently run out of space for the storage of spent

fuel on site, and with the opening of Yucca Mountain still years away, dry cask storage is being

13









used all over the country to alleviate the overcrowded spent fuel pools. Increased fuel cycle

length for Generation III+ reactors will reduce the rate at which spent fuel is produced. This is

directly in line with meeting the goals of the AFCI program. Current research is supporting the

development of reliable, high-bumup fuels for immediate deployment.

1.3 Objectives and Scope

1.3.1 Objectives of Research

One of the factors limiting fuel burnup is the accumulation of internal pin pressure due to

the release of fission gases from the fuel and into the gap region during irradiation. It was first

shown by Turnbull (Turnbull, 1974) that an increase in the fuel's grain size can slow the fission

gas release (FGR) and fuel swelling rates of irradiated fuel. By limiting the release of the fission

gases, the buildup of internal pin pressure is slowed and burnup can be increased. Thus, the goal

of this research is to increase the burnup capabilities of uranium dioxide fuel by increasing the

grain size of the fuel.

1.3.1.1 Grain size modification of UO2

There are several methods available for increasing the grain size of sintered UO2 fuel. One

method is to subject the sintered fuel to a long heat treatment to promote continued grain growth.

This has the disadvantages of being time consuming and expensive. Another method involves

"doping" the U02 powder with a small amount (typically < 1 wt%) of another oxide powder.

The dopants can promote grain growth through several different mechanisms and can facilitate

an increase in the burnup capabilities of the fuel.

1.3.1.2 Doping UO2

This research project aims to investigate the effects of several oxide dopants on the grain

sizes of sintered uranium dioxide fuel. This research will require the production of eight

different uranium dioxide fuel samples. Seven different oxide dopants will be considered and

14









one undoped pellet will be produced for comparison. The dopants that will be used are oxides of

niobium (Nb), aluminum (Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and

titanium (Ti). The dopant concentration for each dopant will be set at 0.5 wt%. The average

grain sizes will be determined and the dopants that are most effective at increasing grain size will

be recommended for further consideration. This study represents the first investigation of

scandia and yttria as dopants for U02. For the remaining dopants, a comparison will be made

with results found in other publications.

1.3.2 Scope of Research

This research aims to characterize the grain sizes of doped UO2 fuel using equivalent

conditions. The dopant concentrations and the sintering process will be equivalent for each

sample. The goal is exclusively to identify each dopant's ability to promote grain size growth

during sintering. Future research should aim to optimize the parameters for a specific dopant,

including the concentration of the dopant, the oxygen potential of the sintering atmosphere, and

the sintering temperature. Ideally, this study will allow future research to focus on the dopants

that show the most promise of increasing the grain size of UO2.









CHAPTER 2
LITERATURE REVIEW

2.1 Theoretical Foundation

The diffusion of fission gases in fuel under irradiation was first described by A. H. Booth

(1957). The expression for F, the fractional release of stable fission gas from the fuel, is shown

in (2-1).


6a2 1 exp(2 -n22Dt
F= 1- exp( 2 (2-1)
/4Dt =, n4

In this equation, D is the diffusion coefficient, t is the time, and a is the radius of a hypothetical

spherical volume. For a small gas release, F oc 1/a. If the hypothetical volume is assumed to be a

grain in the fuel, increasing the grain size can reduce the fraction of fission gas released from the

fuel matrix into the pin volume. This model is the basis for the proceeding experimental

research.

The Booth model was later modified to include the effects of gas re-solution from

intragranular bubbles and interlinked porosity. A simple relation for low quantity gas release, as

proposed by Killeen (1975), is given in (2-2).

S 4 Dt
V 3 (2-2)

In this case, the release fraction is related to the surface-to-volume ratio of the sample.

Again, this relationship demonstrates that by increasing the grain size (and thus decreasing the

surface-to-volume ratio), the fission gas release rate will decrease.

2.2 Previous Experimental Work

Several of the dopants investigated in this study have been researched previously,

specifically oxides of niobium, aluminum, chromium, vanadium, and titanium. Scandia and









yttria have not previously been used as dopants for U02. It is desirable to review the results of

previous research in order to identify the parameters that must be considered when doping UO2.

Prior experiments can be split into pre-irradiation and post-irradiation results. In this

research project, no irradiation testing was conducted. Therefore, this review will focus on

sample production methods and pre-irradiation analyses.

2.2.1 Production of Doped Fuel Pellets

The process for producing U02 fuel pellets is well established and is used in wide scale

production throughout the world. The only steps typically required are cold pressing the powder

into a green pellet and sintering the pellet to achieve densification. Binders and lubricants are

sometimes used, though not usually necessary. Sintering is typically performed in a reducing

environment at around 1700C for approximately four hours. When working with doped UO2,

an initial step of mixing the powders is also necessary. The effects of manipulating these

variables, as well as dopant concentrations, have been investigated in several studies of doped

UO2 fuel (Ainscough et al., 1974; Bourgeois et al., 2001).

Dopant mixing. The large majority of doping studies have integrated the dopant powder

into the U02 powder by a mechanical mixing method. This is typically done without organic

additions by rotary ball milling with steel balls for two to 72 hours. In some cases this was

preceded or followed by sieving of the powder mixture.

While mechanical mixing is the most common method of mixing the dopant and U02

powders, one other noteworthy method was used by Bourgeois et al. (2001) for doping U02 with

Cr203. In order to more homogeneously mix the dopant into the powder, spray-drying of a

suspension containing U02 powder in solution with distilled water and (NH4)2CrO4 was

performed. The spray-drying was followed by calcination of the powders in argon to transform

the chromate into Cr203. This process did not disturb the specific surface area of the powders,

17









nor their O/U ratios. It was found that this mixing method resulted in larger final grain sizes for

Cr203-doped UO2 than previous studies, presumably due to the improved homogeneity of the

mixing.

Cold pressing. The method used almost exclusively for cold pressing doped U02 powders

is uniaxial pressing, often bilaterally. Typical pressures for this process are from 100-350 MPa.

Green densities are normally on the order of 50-60 %TD. Due to the high compressibility of the

powders, large differences in cold pressures do not yield large differences in green densities. A

study by Bourgeois et al. (2001) found that a 5-6% difference in green density resulted in less

than 1% difference in sintered density. Due to the relative unimportance of the cold pressure and

green pellet density on the final sintered density of samples, the effects of varying the cold

pressure were not studied in this project.

Other parameters. There are several other parameters that play an important role in the

final sintered pellet characteristics. Specific examples include the sintering temperature, the

sintering time, the oxygen potential of the sintering environment, and any heat treatments that

may be applied after sintering. Table 2-1 shows several variables from previous research and

illustrates the large variety of options available. Some of the effects of changing these

parameters will be discussed with regards to the results that follow.

2.2.2 Results of Previous Studies

There have been a multitude of studies concerning the effects of dopants on the

characteristics of sintered UO2 pellets. The majority of these studies have focused on the

abilities of different dopants to promote an increase in grain size. Table 2-1 lists grain sizes

found for doped-UO2 pellets in previous studies that have used Cr203, TiO2, Nb205, A1203 and

V205, all of which were investigated in this research. The results of these studies vary widely

depending on the dopant concentrations and the sintering conditions used. Additionally, most

18









studies do not state whether they have reported 2-D grain sizes or have corrected for and reported

3-D grain sizes. The far right column shows the relative percent increase of the doped samples

over the control (undoped) samples in each study. Dashes indicate that the control grain size was

not reported. Since different studies have different powders and parameters, the percent increase

in grain size provides a better measure for comparison of different studies than the absolute grain

sizes.

There is no published literature on the use of Sc203 or Y203 as dopants for U02. Thus,

there are no previous results with which to compare.

Chromia. For Cr203-doped UO2, the average grain sizes quoted vary between 12 and 126

tm. Using a concentration of 0.5 wt% and similar sintering conditions as the current research,

the reported grain sizes vary between 50 and 126 am, or an average relative increase of

approximately 550%. The grain size increase due to doping with Cr203 seems to be limited to a

dopant concentration of 0.3-0.5 wt%. At concentrations above 0.5 wt%/, no additional grain size

increase is seen

Solubility limits have been quoted by several sources. Leenaers et al. (2003) found the

solubility at 1600, 1660, and 1760C to be 0.065, 0.086, and 0.102 wt%, respectively. Kashibe

et al. (1998) found that the solubility at 17500C was only 0.012 wt%, although in this case a very

small amount (0.065 wt%) was initially added to the powder. Bourgeois et al. (2001) found the

solubility to be 0.07 wt% in the range of 1500-17000C. Solubility varies with both temperature

and oxygen potential, which could explain some of the differences above.

The actual amount of Cr203 added is well above these limits in most research published.

Above the solubility limit, Cr and Cr203 form a eutectic at around 1550C and the liquid phase

enhances grain growth and densification. This provides some explanation for the mechanism of









increased grain size in Cr203-doped UO2. It is also seen in Table 2-1 that for pellets sintered

below 1550C, the grain sizes achieved are generally much smaller than for those sintered at

temperatures above 16000C.

Titania. Titania-doped UO2 grain sizes range from 5 to 133 tm. For a concentration of

0.5 wt% and similar sintering conditions to the current research, the grain sizes vary from 70 to

133 atm, or an average relative increase of approximately 350%.

Ainscough et al. (1974) studied the effect of sintering temperature and additive content on

grain size and found that for concentrations of 0.03-0.33 wt% TiO2, grain sizes varied from 5-45

atm at 15500C sintering temperature. At 0.13 wt% dopant, the maximum grain sizes were

obtained. At 1650 and 17500C, the samples containing >0.13 wt% TiO2 experienced enhanced

grain growth due to the formation of a U02-TiO2 eutectic that separated to the grain boundaries

during sintering. The eutectic was never formed for concentrations of 0.07 wt% or less and was

occasionally formed for 0.13 wt%. This led to the conclusion that TiO2 solubility in UO2 is

between 0.07 and 0.13 wt% in the range of 1650-17500C.

Niobia. Niobia-doped UO2 grain sizes range from 7 to 56 tm. For a concentration of 0.5

wt% and similar sintering conditions to the current research, the grain sizes vary from 40 to 56

atm, or an average relative increase of approximately 230%.

The solubility of Nb205 in UO2 has been quoted as being approximately 0.5 wt% in several

studies. Harada (1996) showed the variation of the lattice parameter as the niobia content is

increased and found that the variation halted very near 0.5 wt%. Harada (1996) also showed the

variation of the lattice parameter as a function of temperature and reported that a solid solution

occurs between 1400 and 17000C.









Alumina. There is significantly less published research using alumina as a dopant for U02

than there is for chromia, titania, and niobia. The range of grain sizes found in the two studies

shown in Table 2-1 is from 10 to 30 tm. Both of the studies have dopant concentrations lower

than the 0.5 wt% used in this research.

Vanadia. Vanadia was shown in the study by Amato et al. (1967) to reduce the grain size

of sintered U02 pellets when used as a dopant. On the other hand, Radford and Pope (1983) and

Aybers et al. (2004) found that grain size increased with increasing V205 concentration up to 1.0

mol%. Without additional studies, it is hard to say whether or not vanadia is effective at

promoting increased grain size. This research will try to provide further insight on this dopant.











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CHAPTER 3
MATERIALS AND METHODS

The methodologies employed in this research focused on the processing of the doped U02

pellets and the characterization of the sintered pellets. The materials required for this research

included the uranium dioxide powder, the dopant powders, the cold uniaxial press, the induction

furnace and associated parts, the equipment used to prepare the samples for analysis, and the

analysis equipment.

3.1 Processing

Pellet processing was performed at the Applied Ultra High Temperature Research

Laboratory at the Innovative Nuclear Space Power and Propulsion Institute at the University of

Florida. The production of the doped fuel pellets entailed several steps. The first step was to

measure and mix each of the dopants with the uranium dioxide powder. Next, the powder

mixtures were cold pressed into semi-dense compacts (green pellets). Then the green pellets

were sintered to achieve final densification.

3.1.1 Powders

The powders used in this research were the uranium dioxide powder and the seven dopant

powders. The U02 powder was comprised of depleted uranium (DU) obtained from

Framatome/AREVA. Appendix A shows some properties of the initial U02 powder. Table 3-1

shows a summary of the dopant powders used in this research.

Powder mixing. The dopants were added to the U02 powder at an optimum concentration

of 0.5 wt%. Table 3-2 shows the actual measured values for the U02 and dopant powder masses

prior to mixing. The powder masses were measured using a Sartorius R180D analytic balance

seen in Figure 3-1.









The powder mixtures were combined in HDPE bottles and six stainless steel shots were

added. The powder mixing was performed in air at atmospheric pressure for all of the dopants

except for Cr203 and V205, which were mixed under argon at atmospheric pressure due to their

volatility. The mixtures were shaken for thirty minutes in a Spex Certiprep 8000M Mixer/Mill.

The stainless steel shots were added to help break up the powder particles during shaking and to

induce good mixing.

3.1.2 Cold Uniaxial Pressing

The cold press used in this research was a Carver model #3912 hydraulic press, shown in

Figure 3-2. The cold uniaxial pressing of powders requires the use of a punch and die. Initially,

a simple stainless steel rod and tube were used for cold pressing the powders. One rod was

inserted into the tube, powder was poured in, and a second rod was inserted above. The

collection was then punched in the hydraulic press. This initial process did not produce green

pellets of sufficient quality to be sintered. It was evident that the ends of the rods were not

polished well enough, as the green pellets were sticking and breaking. Additionally, the tubes

were not strong enough to withstand the outward pressure exerted by the powder upon being

pressed. This resulted in bowing of the sides and poor compaction of the powder.

Due to the failure of the stainless steel punch and die, a polished die set made specifically

for cold pressing pellets was obtained. The set was an International Crystal Laboratories 13mm

KBr Die Set consisting of a base, a stainless steel die, and two stainless steel anvils for punches,

shown in Figure 3-3. The process for cold pressing a pellet using this die set was as follows:

* Measure 3.5-4.5 g of doped U02 powder

* Clean each piece of the die set and attach the die to the base

* Coat the inside of the die with stearic acid solution, which acts as a lubricant, using the
long stainless steel punch









* Coat the first anvil in stearic acid solution and insert it into the die, polished side up

* Pour the powder into the die

* Coat the second anvil in stearic acid solution and insert it into the die, polished side down

* Place the punch on top of the second anvil and press in the hydraulic press for ten minutes

* Remove the punch and separate the die from the base, removing the first (bottom) anvil

* Flip the die over and reinsert the punch

* Place a metal ring on top of the die and use the hydraulic press to punch out the pellet
through the top

Typical pressures for cold pressing range between 200 and 350 MPa. Each pellet in this

study was pressed at approximately 300 MPa and green pellet densities were all measured to be

between 50% and 60% TD. The green pellet densities were calculated using their measured

dimensions to estimate their volume.

3.1.3 Pellet Sintering

3.1.3.1 Induction furnace

The furnace used for heating and sintering the green pellets was a 20 kW Taylor Winfield

Thermonic generator, model CE2000. The heating was accomplished with a water-cooled

copper coil inside of a vacuum chamber inductively heating a tungsten tube that surrounded the

sample. The pellet sat on a piece of tantalum carbide foam which sat atop a boron nitride base.

The setup is shown in Figure 3-4. Both the BN base and the TaC foam were used for their high

temperature capabilities and chemical stability. At the bottom of the BN stand was a hole that

allowed gas to enter and flow up through the TaC foam, across the pellet, and out the top of the

apparatus. A mixture of Ar-5%H2 was used to reduce the samples during sintering.

Temperature was controlled using a Maxline IRCON active controller and measured using a

two-color infrared pyrometer. The furnace system is shown in Figure 3-5.









3.1.3.2 Sintering

The pellets were sintered at 17000C for approximately four hours while exposed to a

mixture of Ar 5%H2. The heat up and cool down rates were controlled manually and were thus

subject to some amount of variation. Due to this variation, several of the pellets experienced

cracking during sintering. It is surmised that excessively fast heating up or cooling down was

the cause of the cracking. This did not affect the results of this study since microstructural

characterization was still possible despite the cracking. It is not presumed that the cracking

indicates any real difficulties that would be associated with producing doped pellets.

3.2 Characterization

Characterization of the sintered pellets was performed in order to compare the

microstructures and grain sizes of the doped pellets. Density measurements, optical microscopy,

and preparations for other analyses were performed at the Applied Ultra High Temperature

Research Laboratory at the Innovative Nuclear Space Power and Propulsion Institute at the

University of Florida. SEM, EDS, and XRD were performed at the Major Analytical

Instrumentation Center at the University of Florida.

3.2.1 Preparation for Analysis

3.2.1.1 Scanning electron microscopy

Cutting. The sintered fuel pellets first needed to be cut into cross-sectional pieces to allow

for internal microstructural examination. Cutting was performed with a LECO VC-50 precision

low-speed diamond saw shown in Figure 3-6. Samples were cut into eighths and the slices were

approximately 1-3 mm in thickness.

Setting in epoxy. After cutting the samples, the slices needed to be set in epoxy to allow

for grinding and polishing. The epoxy is a 10:2 mixture of LECO epoxy resin and LECO

hardener. This was poured over the slices and allowed to set overnight.

27









Grinding and polishing. The samples were ground and polished using a LECO Spectrum

System 1000 grinder/polisher with semi-automatic head shown in Figure 3-7. The

grinder/polisher head attachment enabled semi-automatic operation for six samples at a time with

constant applied pneumatic pressure. Table 3-3 shows the grinding and polishing parameters.

Chemical etching. After grinding and polishing, a chemical etch was used to reveal the

grain structure of the sample. There are a wide variety of etchants that have been reported in the

literature (Petzow, 1978). The most common etchant for UO2 seems to be a 1:1:1 mixture of

H2SO4, H202, and H20. This was used for several of the samples in this study. The etchant was

applied with a cotton tipped applicator, allowed to sit for anywhere from one minute to two

hours, and then brushed off with the cotton swab. The samples were ultrasonically cleaned

following the etching process. This etchant was mildly effective but did not work for all of the

samples.

Another etchant that was used was a 1:1 mixture of distilled water and 90% nitric acid.

This etchant was applied to the surface and allowed to sit for five seconds to 2 minutes before

being brushed off with a cotton swab. The reasons for different etchants yielding different

results are unknown. Many other methods for etching could have been tried, possibly to greater

effect, including thermal etching and electrolytic etching techniques.

Conductive paint. Investigation by SEM requires that the sample be electrically

conductive, lest a charge build up in the sample and cause burning and image distortion. Thus, a

coating of conductive graphite paint was applied to the epoxy region surrounding the samples to

establish an electrical contact with the specimen holder.









3.2.1.2 X-ray diffraction

A small amount of each sample was ground into powder and prepared for investigation by

XRD. The powder was placed on slides and adhered using a solution of one part collodian and

seven parts amyl acetate.

3.2.2 Analysis of Samples

3.2.2.1 Density measurements

The density of each sample was measured after sintering. Density measurements were

performed using an immersion technique. First, the mass of the sample in air was measured.

Then the apparent mass of the sample while submersed in water was measured. This

measurement was made by filling a beaker with water so that the sample holder was immersed,

tearing the scale, and then placing the pellet on the holder. It is important to tear the scale after

immersing the sample holder in order to account for its change in apparent mass. Though small,

this could introduce a significant discrepancy in the measurement if done improperly. The red

platform kept the beaker isolated from the scale. Figure 3-8 shows the apparatus used for

measuring the apparent mass while submersed in water.

Since the density of water at room temperature is 1 g/cc, the difference in the mass in air

and the mass in water is equal to the sample volume. In this way, the densities of the samples

were calculated by dividing the mass in air by the volume.

3.2.2.2 Scanning electron microscopy and energy dispersive spectroscopy

Secondary and backscatter electron images were taken using a JEOL JSM 6400 SEM with

Link ISIS digital image capturing system shown in Figure 3-9. Secondary electron images

reveal topographical information while backscatter electron images are used to show

compositional contrast. Images were taken of both polished and etched samples to reveal

microstructural characteristics and grain structures.

29









Energy dispersive spectroscopy was also performed with the JEOL JSM 6400 SEM. This

gives a qualitative assessment of the elements present in the sample. Under ideal conditions,

EDS can reveal elements at concentrations down to 0.1 wt%. Thus, it is theoretically possible to

see the dopant elements (present as oxides at 0.5 wt% nominally) as long as their spectra do not

overlap with the uranium spectrum.

3.2.2.3 Grain size determination

The most desirable method for analyzing the grain size of a material is to use a computer

program to automate the process. This is only possible for very high quality images of grain

structure as the programs identify the grain boundaries based on contrast within the image. None

of the images of etched samples were of sufficient quality to be analyzed by software. Instead,

grain sizes were calculated using a linear intercept method (Abrams, 1971). This involved

placing multiple lines at different orientations over images of the etched samples and counting

the number of intercepts along the line. Each grain that is crossed is counted as 1 and if the test

length ends inside of a grain it is counted as 12. For each sample, at least two images were used,

with approximately 15 test lengths used on each image to determine the grain size of the

specimen. For each image, the total number of intercepts counted was greater than 50. The

mean grain size result found in each image was averaged and the standard deviation was

computed.

3.2.2.4 X-ray diffraction

Samples were investigated using a Philips APD 3720 x-ray diffractometer, shown in

Figure 3-10. The XRD was operated at a voltage of 40 kV and current of 20 mA. The resultant

peaks were compared with known data for information about the constituents and phases present.

XRD spectra were taken of both the mixed powders prior to sintering and of the sintered pellets









(after pulverizing a small portion of them into powder). Lattice parameters were found for each

of the sintered pellets and compared to undoped U02.









Table 3-1. Summary of dopant powders.
Dopant # Material Supplier
1 Nb205 Sigma Aldrich
2 A1203 Alfa
3 Cr203 Sigma Aldrich
4 Sc203 Alfa
5 Y203 Fisher Scientific
6 V205 Sigma Aldrich
7 TiO2 Sigma Aldrich


Purity


99.99%
99.60%
99.90%
99.90%
99.99%
99.99%
99.99%


Table 3-2. Masses ofUO2 and dopant powders in sample mixtures.
Pellet # Dopant Measured Mass U02 (g) Measured Mass Dopant (g) Dopant wt%
1- 9.16463 0 0.000%
2Nb205 4.13159 0.02076 0.500%
3 A1203 9.10194 0.04579 0.501%
4 Cr203 9.39934 0.04716 0.499%
5 Sc203 9.11687 0.04580 0.500%
6 Y203 9.11591 0.04576 0.499%
7V205 9.11214 0.04576 0.500%
8 TiO2 9.10587 0.04540 0.496%


Table 3-3. Grinding and polishing sequence.
Grit/Micron size Time (minutes) Pressure (psi)
120 grit 2-4
400 grit 2
600 grit 2
1200 grit 1.5
6 micron 1.5
1 micron 1.5
0.5 micron 1
*Manual pressure applied


40
35
35
25
Variable*
Variable*
Variable*





























Figure 3-1. Sartorius R180D analytic balance used for mass measurements.


Figure 3-2. Hydraulic press used for cold uniaxial pressing of pellets.








I


Figure 3-3. Stainless steel die set used for cold pressing powders.


Figure 3-4. Base used to hold pellet during sintering.































Figure 3-5. Induction furnace, controller, pyrometer, and vacuum chamber.


Figure 3-6. LECO VC-50 precision low-speed diamond saw.

























Figure 3-7. LECO Spectrum System 1000 grinder/polisher with semi-automatic head.


Figure 3-8. Setup for measuring apparent mass of sample while immersed in water.














I L
A I,


4 v


Figure 3-9. JEOL JSM 6400 scanning electron microscope.


Figure 3-10. Philips APD 3720 x-ray diffractometer.









CHAPTER 4
RESULTS AND DISCUSSION

4.1 Density Measurements

The theoretical density of the fuel is slightly affected by the presence of the dopant species.

Thus, calculations of the theoretical densities of the samples were made by mass averaging the

densities of the UO2 and the dopant. The theoretical densities for each sample are presented in

Table 4-1.

The measured masses and calculated densities of the final sintered pellets are shown in

Table 4-2. The average density of the pellets is 10.39 g/cc. None of the samples had a density

lower than 93.35 %TD. Thus, the processing method used was successful in producing dense

pellets suitable for further analysis.

4.2 Pellet Characterization

4.2.1 Optical Microscopy

Optical microscopy was used to image the surface of the pellets after sintering. Figure 4-1

shows all eight pellets.

It is seen that the chromia-doped, scandia-doped, and titania-doped pellets each had a large

crack that propagated from the center to the edge of the pellet. The vanadia-doped pellet also

had a crack but it was less severe. These cracks are most likely due to excessively quick heat up

and cool down rates during the sintering process. Future processing methods must consider this

problem to avoid cracking the pellets, as cracked pellets would not be mechanically sound for

use in reactors. However, since this research was only concerned with microstructural

characterization, the cracked pellets are not a major issue for this project.









The surfaces of many of the pellets appear to contain multiple scratches and other flaws.

These arose from handling the pellets before and after sintering. All microstructural

investigation was performed on ground and polished internal cross-sections of the pellets.

4.2.2 Scanning Electron Microscopy

Images were taken of the samples before and after chemical etching. Prior to chemically

etching the samples, topographical and compositional features could be investigated to look for

any unexpected features or phases. After etching, the grain structure was revealed and grain

sizes could be analyzed. Figures 4-2 through 4-9 present the results.

Backscatter electron images showed no signs of compositional heterogeneity in any of the

doped samples. This does not mean that the mixture was perfectly homogeneous throughout the

sample as only small sections were imaged. In any case, it does not seem that any of the dopants

segregated severely within the samples.

Undoped U02. For the undoped U02 pellet, the microstructure reveals a low porosity

sample as expected from the high density measured. The lines seen across the sample in Figure

4-2 A are remnants of the grinding and polishing process and not a microstructural feature.

Figure 4-2 B shows two regions of the etched surface with grains revealed. A sulfuric acid

solution was used to etch the sample. The etchant did not work exceptionally well on the

undoped sample as seen by the roughness of the grain structure. Despite the poor etching, it was

still possible to measure the grain sizes with this sample. The average grain size of the undoped

UO2 sample was 2.73 [tm with a standard deviation of 0.35 [tm. This is significantly smaller

than typical grain sizes for undoped UO2 quoted in the literature, which average nearly 10 atm in

the studies shown in Table 2-1. The reason for this discrepancy is not known but could be a

result of differences in initial powder size distribution, processing parameters, or sintering

procedures. Due to this difference, the relative increase in grain size with each dopant will be









used as a more meaningful measurement, rather than a direct comparison of grain sizes with

other studies.

Nb2Os-doped UO2. The niobia-doped pellet seen in Figure 4-3 shows a low porosity

microstructure as well. This sample was etched with sulfuric acid and shows a more defined

grain structure than the undoped U02. It is seen that some of the grains appear to have bumps on

the surface. This occurred with several samples and is thought to be a consequence of the

chemical etching process. The average grain size of the niobia-doped pellet was 5.94 tm with a

standard deviation of 0.58 tm. This is a relative increase of 117% over undoped U02, a very

substantial increase. However, it is less than the average grain size increase found for other

studies using 0.5 wt% niobia, which resulted in increases of nearly 230% under similar

processing conditions. This could again be due to differences in powders or methodologies.

However, despite being less than the changes quoted by other studies, 117% increase in grain

size is significant for a 0.5 wt% addition ofNb20s.

Al203-doped UO2. Figure 4-4 shows the alumina-doped pellet. The topography is again

seen to be low porosity with no unexpected features present. The etchant used was sulfuric acid.

Figure 4-4 B shows the etched regions with a relatively well defined grain structure. The

bumpiness noted for the niobia-doped pellet is also seen here. The average grain size of this

sample was 5.14 [tm with a standard deviation of 0.50 tm. This is a relative increase of 88%

over the undoped U02 pellet. Previous studies found an average increase of approximately 80%

with alumina, although these were done with lower concentrations than 0.5 wt%.

Cr203-doped UO2. Figure 4-5 A shows the chromia-doped U02 prior to etching at two

magnifications. The topography of this sample is slightly more porous than other samples. This

pellet had the second lowest measured density at 93.61 %TD. Figure 4-5 B shows the etched









regions of the chromia-doped pellet. Sulfuric acid was used to etch this sample. The average

grain size of the pellet was 4.77 [tm with a standard deviation of 0.58 tm. This is a 74% increase

over the undoped sample, significantly smaller than the increase seen by other studies using

chromia as a dopant. Studies using 0.5 wt% chromia and similar processing tactics found an

average relative increase in grain size of 550%. Previous studies have found evidence of a Cr-

Cr203 eutectic forming above 1550C, which creates a liquid phase that enhances grain growth

during sintering. There has been evidence of Cr-rich spots appearing in the sintered pellet as a

result (Bourgeois et al., 2001). These Cr-rich spots were not found in this study using BSE

compositional contrast imaging, however EMPA may better elucidate this sort of phenomenon.

Sc203-doped UO2. No previous results for scandia-doped U02 have been reported in the

literature. Figure 4-6 shows the microstructure of this pellet. In Figure 4-6 A, no unusual

microstructural features are observed. Figure 4-6 B shows the etched microstructure for this

sample. The etching was attempted with sulfuric acid four times, with the acid allowed to sit

different lengths of time varying from 10 seconds to 2 hours. None of these attempts provided

sufficient quality images to reveal grain structures. The images seen in Figure 4-6 B were

acquired after etching with a 1:1 mixture of 90% nitric acid and distilled water for approximately

20 seconds. While the quality of these images is still poor, grain size analysis could be executed.

The average grain size measured for the scandia-doped sample was 2.63 t-m with a

standard deviation of 0.35 tm. This is a 4% decrease from the undoped UO2 sample. This

difference, however, is not statistically significant. As such, it is concluded that scandia-doping

had no significant effect on the grain size of UO2.

Y203-doped UO2. Figure 4-7 shows the yttria-doped pellet. It is seen in Figure 4-7 B that

the chemical etching failed to reveal the grain structure for this pellet. Both sulfuric and nitric









acid etchings were tried, but to no avail. As such, no grain size analysis can be performed on this

sample.

V20s-doped UO2. Figure 4-8 shows the vanadia-doped UO2 sample. Again, no

unexpected microstructural features are seen in Figure 4-8 A. The grain structure in Figure 4-8

B was revealed by sulfuric acid etching. The average grain size of this sample was 3.41 [tm with

a standard deviation of 0.13 [tm. This is an increase over undoped UO2 of 25%. Previous

studies have reported results ranging from 50% decreases up to nearly 350% increases in grain

size due to vanadia additions. This study finds that the increase is modest at 25%, but it is a

statistically significant increase. This lends more evidence to the argument that vanadia could be

used as a dopant to increase grain sizes in UO2.

TiO2-doped UO2. Figure 4-9 shows the titania-doped pellet. It is seen in Figure 4-9 A

that the polish of this sample was not as successful as some of the others. However, it is still

possible to conclude that no unusual features are present. Figure 4-9 B shows the etched regions

of the sample. The nitric acid etchant was used on the pellet and the results are the best of any of

the etched samples. The grain boundaries are well defined and the grain structure is easily

visible throughout the images. It is not clear why the nitric acid was so successful in this

instance. The average grain size of this sample was 10.37 atm with a standard deviation of 1.19

[tm. This is a 279% relative increase in grain size over the undoped U02 pellet, by far the largest

increase seen for any of the dopants in this study. This is still slightly less than the average

increase seen by similar studies of 330%, but it is reasonably close.

Grain size statistical analysis. The doped samples' grain sizes found in this study were

subjected to statistical analysis to confirm that they were significantly different from the undoped

sample's grain size. This was done by using a two-sample t-test assuming equal population









variances. The assumption of equal variances is justified because while the grain sizes differ, the

distribution of sizes is probably relatively constant for each sample. Each doped sample was

tested against the undoped sample. The null hypothesis was that the mean grain size of the

doped sample was equal to the mean grain size of the undoped sample. The alternative

hypothesis was that the doped sample's mean grain size was greater than that of the undoped

sample. In each case, the pooled standard deviation was used. This was calculated using (4-1).

s2 = ((ni-1)*s12 + (n2-1)*s22) / (n + n2 2) (4-1)

The results of the statistical analysis showed that the differences in mean grain sizes were

significant for the niobia-doped, alumina-doped, chromia-doped, vanadia-doped, and titania-

doped samples with respect to the undoped sample at a confidence level of 95%. The scandia-

doped sample was not significantly different. The results of this analysis are summarized in

Table 4-3.

4.2.3 Electron Dispersive Spectroscopy

EDS was used to identify composition of the samples after sintering. The spectra are

shown in Figures 4-10 through 4-17. EDS spectra were taken at an operating voltage of 25 kV

and operated for a live time of 300 seconds. All of the dopants are at least marginally visible in

the spectra except for vanadium. As mentioned earlier, the minimum elemental concentration

that can be detected by EDS is 0.1 wt%. The dopants are present at 0.5 wt% as oxides, with

metal contents therefore being even lower. It is thus very impressive that the dopants are seen in

these spectra. As EDS is only a qualitative tool, no quantitative information can be found from

these spectra.

4.2.4 X-Ray Diffraction

Powder XRD measurements were made before and after sintering. Several of the pre-

sinter measurements showed small traces of the dopant powders. Additionally, the U02 powder









was slightly hyperstoichiometric prior to sintering, as expected from the powder analysis shown

in Appendix A which measured the received powder to be UO2.10. The post-sinter results show

only stoichiometric U02 peaks for all of the samples, signifying that the powder was reduced by

the hydrogen in the sintering environment and that the dopant powders were integrated into the

matrix. Any amount of dopant that remained segregated, if any, was too small an amount to be

detected using XRD. The lattice parameters were calculated from the spectra using measured d-

values from the (331) plane of UO2. The lattice parameter ao is related to the d-value and (hkl)

plane in cubic lattices by (4-2) (Brundle et al., 1992).

ao = dhk X (h2 + k2 + 12)1/2 (4-2)

The lattice parameters are shown in Table 4-4. In all of the doped samples, the lattice

parameter decreased compared to undoped U02. This change in lattice parameter further

confirmed that species other than pure U02 are present in the samples.









Table 4-1. TD's of doped samples
Sample Dopant Dopant Density (g/cc)
1 Undoped
2Nb205
3 A1203
4 Cr203
5 Sc203
6 Y203
7 V205
8 TiO2


TD of doped sample (g/cc)


4.60
3.96
5.22
3.86
5.03
3.35
4.23


Table 4-2. Pellet masses and densities.
Sample # Dopant Mass in air (g) Mass in water (g) Volume [
1Undoped 4.47979 4.05572
2Nb205 3.59458 3.24769
3A1203 4.33465 3.91698
4 Cr203 3.98003 3.59108
5 Sc203 3.61022 3.2562
6Y203 4.55315 4.12305
7V205 4.09821 3.70537
8 TiO2 3.89407 3.51828


10.960
10.928
10.925
10.931
10.925
10.930
10.922
10.926


Am] (cc) Density (g/cc) %TD
0.42407 10.56 96.39%
0.34689 10.36 94.82%
0.41767 10.38 94.99%
0.38895 10.23 93.61%
0.35402 10.20 93.35%
0.43010 10.59 96.85%
0.39284 10.43 95.52%
0.37579 10.36 94.84%


Table 4-3. Results of statistical analysis for pellet grain sizes
Sample Dopant Pooled St Dev p-value (95% confidence) Confidence Interval
2Nb205 0.474 0.0000 (2.39, 4.03)
3 A1203 0.431 0.0001 (1.66, 3.15)
4 Cr203 0.416 0.0024 (1.04, 3.04)
5 Sc203 0.347 0.6200 (-0.93, 0.73)
7V205 0.259 0.0051 (0.28, 1.13)
8 TiO2 0.796 0.0000 (6.08, 9.21)









Table 4-4. Lattice parameters of sintered pellets
Sample Dopant ao (A)
1 Undoped 5.46706
2Nb205 5.46689
3 A1203 5.46275
4 Cr203 5.46214
5 SC203 5.46214
6 Y203 5.45939
7 V205 5.45721
8 TiO2 5.45756

















































Figure 4-1. Optical images of pellet surfaces post-sinter. Pellet labels in top left corners.














47



















4" 1

S..v.,


a, U.
jf 1.ft -% 1 p.~~
ii


Figure 4-2. Undoped U02. A) Before etching. B) After etching. Images on the right are at higher
magnifications than the images to their left.
















M.. As.

tr

'%e
r.... .... .
hi 4~



*1 U.


Figure 4-3. Nb20O-doped U02. A) Before etching. B) After etching. Images on the right are at
higher magnifications than the images to their left.










































Figure 4-4. Al203-doped U02. A) Before etching. B) After etching. Images on the right are at
higher magnifications than the images to their left.











































Figure 4-5. Cr203-doped U02. A) Before etching. B) After etching. Images on the right are at
higher magnifications than the images to their left.
















ff .o .



b -* .!

... _. .2 ""- "'"...

.

-
ii
rr


&*


b ..'. j

7&.
.4. 1

.9t .!


a




S' ,*


Figure 4-6. Sc203-doped U02. A) Before etching. B) After etching. Images on the right are at

higher magnifications than the images to their left.


"





































'A ;'-0,' .-g.,9 p-... t1

B

Figure 4-7. Y203-doped UO2. A) Before etching. B) After etching. Images on the right are at
higher magnifications than the images to their left.










































Figure 4-8. V205-doped UO2. A) Before etching. B) After etching. Images on the right are at
higher magnifications than the images to their left.


I v

























I


jT e


S.




*


* .


.




E: *


Figure 4-9. TiO2-doped U02. A) Before etching. B) After etching. Images on the right are at

higher magnifications than the images to their left.





















Coun


100






150


40


20


ts


'0-


'0-


00-
0
C0- c


'0 --


)0-
tO l* l l ..


Figure 4-10. EDS spectrum ofundoped UO2 sample.








Counts


Figure 4-11. EDS spectrum of Nb20O-doped sample.


20
Energy (keV)


20
Energy keV)


















Counts


5 10 15 20
Energ' (keV)


Figure 4-12. EDS spectrum of A1203-doped sample.


Energy (keV)


Figure 4-13. EDS spectrum of Cr203-doped sample.




















Count


400



30C



200



100


0-



0-

0-


0 U C
ISo l SU U
0-I 1 ,
0 5 10 15 20
Energy (keV)


Figure 4-14. EDS spectrum of Sc203-doped sample.









Count


0-




0-




0-


U U

0 -'
0 5 10 15 20
Energy (kelV


Figure 4-15. EDS spectrum of Y203-doped sample.


600




40C




200






















Counts


20
Energy (keV)


Figure 4-16. EDS spectrum of V205-doped sample.







Croun


Energy keV)


Figure 4-17. EDS spectrum of TiO2-doped sample.









CHAPTER 5
CONCLUSIONS AND FUTURE WORK

5.1 Pellet Processing

The processing method used in this research was successful in making dense pellets

suitable for microstructural examination. The process was streamlined and repeatable and the

resulting bulk pellet properties did not vary significantly. The initial cold pressing was

unsuccessful as the stainless steel pieces were neither polished enough nor strong enough to

create green pellets of acceptable quality. The ICL KBr die set remedied this issue, producing

high quality green pellets from the powders. The sintering process caused cracking in several

pellets, most likely due to excessively quick heat up and cool down rates. This was not a

hindrance for the remaining characterization but would be unacceptable for reactor fuel.

5.2 Grain Size Analysis

The etching process for revealing grain boundaries in UO2 was wildly inconsistent

between samples, even for different sections of the same sample. It was found that an excellent

polish is required to have any chance of revealing grains, but even this does not guarantee a

successful etch. Both the sulfuric acid and nitric acid solutions were used successfully, but both

of them failed more often than they succeeded. The discovery of an etchant that could provide

repeated, high quality results would be extremely useful for this kind of analysis.

Table 5-1 summarizes the results of the grain size analysis. As mentioned above, all of the

average grain sizes measured in this study are smaller than typical values found in other studies.

The relative increases in grain sizes for each dopant are typically smaller as well. However, the

increases do show that a very small addition of a dopant to U02 powder can significantly

increase the grain size of the sintered pellet, which in turn can improve fission gas retention and

swelling properties. This result should be kept in mind when designing high-burnup fuel for









current and next generation reactors that utilize uranium dioxide as their fuel. The dopants that

seemed to be most effective in this study were niobia and titania, with titania showing the largest

grain size increase of 279% over the undoped sample. The yttria-doped pellet did not yield grain

size measurements due to ineffective etching. However, this dopant cannot be discounted as it

has never been investigated. Scandia proved to have no significant effect on the grain size,

therefore it should not be considered further.

5.3 Future Work

There is no shortage of further work to be done in the study of doped fuel. Factors that

could affect the results include powder mixing methods, dopant concentrations and solid

solubility limits, cold pressing methods and pressures, and sintering conditions. For mixing

methods, it has been suggested by the work of Bourgeois et al. (2001) that more homogeneous

mixing can lead to further increases in grain size due to the better distribution of the dopant

within the matrix of the material. The ultimate attempt at homogenous mixing may be to coat

the UO2 powder particles with the dopant, either through chemical vapor deposition or atomic

layer deposition. This would provide further insight into the mechanisms of grain size increase

due to the dopants.

The obvious next step in this research is irradiation testing of doped fuel samples.

Previous studies that have looked at irradiating doped fuel have in many cases confirmed a

decrease in fission gas release and fuel swelling during irradiation, making possible higher

burnups (Arborelius et al., 2006; Delafoy et al., 2007; Harada, 1996; Kashibe et al., 1998;

Killeen, 1975; Turnbull, 1974; Yuda et al., 1997). Other improvements due to the presence of

dopants have been noted as well. Delafoy et al. (2007) found that doping U02 with chromia

improved the pellet-clad interaction properties of the fuel.









Several possible drawbacks should also be looked for when investigating doped fuel under

irradiation. Niobia, while being a good promoter of grain size increase, has also been found to

increase the rare gas diffusion coefficient in U02 (Killeen, 1975). This causes the fission gases

to diffuse more quickly within the fuel matrix, offsetting the benefits of the increased grain size.

5.4 Final Comments

The possibility of low quantity dopant additions providing an avenue to a high burnup UO2

based fuel is extremely attractive. Uranium dioxide is the most qualified fuel on the planet and

the ability to improve upon it by such a small modification must be appreciated. A fuel

qualification program for doped-UO2 would not be nearly as intensive as one for a new fuel

form. Thus, doped UO2 could improve fuel cycle economy for existing and future reactors in

less than a decade. This is an option that should be pursued with further research to fulfill its

enormous potential.









Table 5-1. Summary of grain size analysis.
Sample Dopant Mean Grain Size ([tm) St Dev ([tm)
1Undoped 2.73 0.35
2Nb205 5.94 0.58
3 A1203 5.14 0.50
4 Cr203 4.77 0.58
5 Sc203 2.63 0.35
6 Y203
7V205 3.41 0.13
8 TiO2 10.37 1.19


% Error
12.6%
9.7%


9.8%
12.1%
13.4%


3.7%
11.4%


% Change


117%
88%
74%
-4%


25%
279%










APPENDIX
POWDER CHARACTERISTICS

The initial powder information was obtained from Dr. Jiwei Wang who analyzed the U02

powder received from Framatome/AREVA. The initial oxygen-to-uranium ratio was found by

oxidizing the powder to U308 and measuring the weight change. The change indicated an initial

O/U ratio of 2.10.

The particle size distribution of the received UO2.10 powder was characterized by sieve

analysis. Ten grams of the powder were sieved through a series of screens, which were then

weighed. The analysis was done three times and the average values are plotted in Figure A-1.


I I I I I I I I I


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

Figure A-1. Particle size distribution of received uranium dioxide powder.









LIST OF REFERENCES


Abrams, H., 1971. Grain size measurement by the intercept method. Metallography 4, 59-78.

Ainscough, J., Raven, L., Sawbridge, P., 1978. Int. Symp. on water reactor fuel fabrication with
special emphasis on its effect on fuel performance. Prague: IAEA-SM233.

Ainscough, J., Rigby, F., Osborn, S., 1974. The effect of titania on grain growth and
densification of sintered U02. J. Nucl. Mater. 52, 191-203.

Amato, I., Colombo, R., Petruccioli Balzari, 1966. Grain growth in pure and titania-doped
uranium dioxide. J. Nucl. Mater. 18, 252-260.

Amato, I., Ravizza, M., 1967. The effect of vanadium oxide additions on sintering and grain
growth of uranium dioxide. J. Nucl. Mater. 23, 103-106.

Arborelius, J., Backman, K., Hallstadius, L., Limback, M., Nilsson, J., Rebensdorff, B., et al.,
2006. Advanced doped U02 pellets in LWR applications. J. Nucl. Sci. Tech. 43 (9), 967-976.

Assmann, H., Dorr, W., Gradel, G., Maier, G., Peehs, M., 1981. Doping U02 with niobia -
beneficial or not? J. Nucl. Mater. 98, 216-220.

Aybers, M., Aksit, A., Akbal, S., Ekinci, S., Yayli, A., Colak, L., et al., 2004. Grain growth in
corundum-oxides doped uranium dioxide and effects of grain growth to the mechanical
properties of uranium dioxide such as elasticity determined by ultrasonic methods. Key Eng.
Mater. 264-268, 985-988.

Booth, A., 1957. A method of calculating fission gas diffusion from U02 fuel and its application
to the X-2-f test loop. AECL-496.

Bourgeois, L., Dehaudt, Ph., Lemaignan, C., Hammou, A., 2001. Factors governing
microstructure development of Cr203-doped U02 during sintering. Journal of Nuclear Materials
297, 313-326.

Brundle, C., Evans, C., Wilson, S., 1992. Encyclopedia of Materials Characterization.
Greenwich: Manning.

Delafoy, C., Dewes, P., Miles, T., 2007. AREVA NP Cr203-doped fuel development for BWRs.
Proceedings of the 2007 International LWR Fuel performance Meeting. San Francisco, CA.

DOE, 2008. Advanced Fuel Cycle Initiative. Retrieved from U. S. Department of Energy:
http://www.ne.doe.gov/afci/neAFCI.html

Harada, Y., 1996. Sintering behaviour of niobia-doped large grain U02 pellet. J. Nucl. Mater.
238, 237-243.









Kashibe, S., Une, K., 1998. Effect of additives (Cr203, A1203, Si02, MgO) on diffusional release
of Xe-133 from UO2 fuels. J. Nucl. Mater. 254, 234-242.

Killeen, J., 1980. Fission gas release and swelling in U02 doped with Cr203. J. Nucl. Mater. 88,
177-184.

Killeen, J. 1975. The effect of additives on the irradiation behaviour ofUO2. J. Nucl. Mater. 58,
39-46.

Leenaers, A., de Tollenaere, L., Delafoy, Ch., Van den Berghe, S., 2003. On the solubility of
chromium sesquioxide in uranium dioxide fuel. J. Nucl. Mater. 317, 62-68.

NEI, 2008. Resources and Stats. Retrieved from U.S. Nuclear Power Plants:
http://www.nei.org/resourcesandstats/nuclearstatistics/usnuclearpowerplants/

Ohai, D., 2003. Large grain size UO2 sintered pellets obtaining used for burnup extension.
Transactions of the 17th International Conference on Structural Mechanics in Reactor
Technology. Prague, Czech Republic.

Petzow, 1978. Metallographic Etching. Metals Park, Ohio: American Society of Metals.

Radford, K., Pope, J., 1983. U02 fuel pellet microstrcuture modification through impurity
additions. J. Nucl. Mater. 116, 305-313.

Sawbridge, P., Reynolds, G., Burton, B., 1981. The creep of U02 fuel doped with Nb20s. J.
Nucl. Mater. 97, 300-308.

Tumbull, J., 1974. The effect of grain size on the swelling and gas release properties of U02
during irradiation. J. Nucl. Mater. 50, 62-68.

Yuda, R., Harada, H., Hirai, M., Hosokawa, T., Une, K., Kashibe, S., et. al., 1997. Effects of
pellet microstructure on irradiation behavior of U02 fuel. J. Nucl. Mater. 248, 262-267.









BIOGRAPHICAL SKETCH

Brett Jameson Dooies was born in West Palm Beach, FL, on March 3, 1984. In 2002, he

graduated from the Alexander W. Dreyfoos, Jr. School of the Arts ninth in his class. He attended

the University of Florida, as an honors student, from 2002 to 2006, where he participated in both

academic and leadership activities. Brett was awarded a Bachelor of Science degree in nuclear

and radiological engineering in December 2006, graduating cum laude. He spent the next year on

a graduate assistantship with the University of Florida, beginning the initial stages of this

research project. He was then awarded funding from the Advanced Fuel Cycle Initiative of the

Department of Energy, a generous fellowship that allowed him to complete this research and

present his findings at national conferences. From 2007-2008, Brett served as the student

conference proposal chair for the UF chapter of the American Nuclear Society, an endeavor that

secured the annual ANS student conference for UF in 2009. In August 2008, he received his

Master of Science degree in nuclear engineering sciences from the University of Florida. In July

2008, Brett started as a member of General Electric's Edison Engineering Development Program

in Wilmington, NC.





PAGE 1

1 ENHANCEMENT OF URANIUM DI OXIDE THERMAL AND MECHANICAL PROPERTIES BY OXIDE DOPANTS By BRETT JAMESON DOOIES A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008

PAGE 2

2 2008 Brett Jameson Dooies

PAGE 3

3 To my mom

PAGE 4

4 ACKNOWLEDGMENTS I would first like to acknowledge my gradua te advisor, Dr. Anghaie for his continued encouragement. I am lucky to be his student. Also, I thank the other members of my committee, Dr. Dugan and Dr. Sigmund. Special thanks goes out to Dr. Dugan for being an excellent teacher and undergraduate advisor. This work was performed under the Depart ment of Energy’s Advanced Fuel Cycle Initiative Fellowship. The funding they provided en abled me to finish this research in a timely manner. I would like to thank Dr. James Bresee and Dr. Tom Ward, for their technical feedback on my research proposal. I would also like to thank Cathy Dixon and Donna Knight, for the excellent work that they do for this program. I would like to acknowledge the f acilities that I used to perf orm my research. My samples were made at the Innovative Nuclear Space Power and Propulsion Institute at the University of Florida. Sample characterization was performed at the Major Analytical Instrumentation Center at UF. I extend thanks to Dr. Travis Knight for his extremely helpful insights and to Dr. Jiwei Wang for sharing his mate rials and equipment. Finally, thanks go out to my family, my gi rlfriend, and my friends, whose support has meant everything.

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ............................................................................................................... 4LIST OF TABLES ................................................................................................................ ...........7LIST OF FIGURES ............................................................................................................... ..........8ABSTRACT ...................................................................................................................... .............10 CHAPTER 1 INTRODUCTION ................................................................................................................ ..121.1Background Information ...............................................................................................121.1.1Advanced Fuel Cycle Initiative .........................................................................121.1.2Near-Term Goals of AFCI ................................................................................131.1.3Long-Term Goals of AFCI ...............................................................................131.2Waste Reduction in Current and Generation III+ Reactors ..........................................131.3Objectives and Scope ....................................................................................................141.3.1Objectives of Research ......................................................................................141.3.1.1Grain size modification of UO2 ..........................................................141.3.1.2Doping UO2 ........................................................................................141.3.2Scope of Research .............................................................................................152 LITERATURE REVIEW .......................................................................................................16 2.1Theoretical Foundation .................................................................................................162.2Previous Experimental Work ........................................................................................162.2.1Production of Doped Fuel Pellets .....................................................................172.2.2Results of Previous Studies ...............................................................................183 MATERIALS AND METHODS ...........................................................................................243.1Processing .................................................................................................................... .243.1.1Powders .............................................................................................................243.1.2Cold Uniaxial Pressing ......................................................................................253.1.3Pellet Sintering ..................................................................................................263.1.3.1Induction furnace ................................................................................263.1.3.2Sintering ..............................................................................................273.2Characterization ............................................................................................................273.2.1Preparation for Analysis ....................................................................................273.2.1.1Scanning electron microscopy ............................................................273.2.1.2X-ray diffraction .................................................................................293.2.2Analysis of Samples ..........................................................................................293.2.2.1Density measurements ........................................................................29

PAGE 6

6 3.2.2.2Scanning electron microscopy and energy dispersive spectroscopy .......................................................................................293.2.2.3Grain size determination .....................................................................303.2.2.4X-ray diffraction .................................................................................304 RESULTS AND DISCUSSION .............................................................................................384.1Density Measurements ..................................................................................................384.2Pellet Characterization ..................................................................................................384.2.1Optical Microscopy ...........................................................................................384.2.2Scanning Electron Microscopy .........................................................................394.2.3Electron Dispersive Spectroscopy.....................................................................434.2.4X-Ray Diffraction .............................................................................................435 CONCLUSIONS AND FUTURE WORK .............................................................................605.1Pellet Processing ...........................................................................................................605.2Grain Size Analysis .......................................................................................................605.3Future Work ..................................................................................................................6 15.4Final Comments ............................................................................................................62 APPENDIX POWDER CHARACTERISTICS .......................................................................64BIOGRAPHICAL SKETCH .........................................................................................................67

PAGE 7

7 LIST OF TABLES Table page 2-1 Grain sizes obtained for doped UO2 in previous studies ........................................................223-1 Summary of dopant powders. ............................................................................................... ..323-2 Masses of UO2 and dopant powders in sample mixtures. ......................................................323-3 Grinding and polishing sequence. ......................................................................................... .324-1 TD’s of doped samples .................................................................................................... .......454-2 Pellet masses and densities. ............................................................................................. .......454-3 Results of statistical an alysis for pellet grain sizes.................................................................454-4 Lattice parameters of sintered pellets ................................................................................... ..465-1 Summary of grain size analysis. .......................................................................................... ...63

PAGE 8

8 LIST OF FIGURES Figure page 3-1 Sartorius R180D analytic bala nce used for mass measurements. ......................................33 3-2 Hydraulic press used for co ld uniaxial pressing of pellets. ...............................................33 3-3 Stainless steel die set us ed for cold pressing powders. ......................................................34 3-4 Base used to hold pellet during sintering. ..........................................................................34 3-5 Induction furnace, controller, pyrometer, and vacuum chamber. ......................................35 3-6 LECO VC-50 precision low-speed diamond saw. .............................................................35 3-7 LECO Spectrum System 1000 grinder/pol isher with semi-automatic head. .....................36 3-8 Setup for measuring apparent mass of sample while immersed in water. .........................36 3-9 JEOL JSM 6400 scanning electron microscope. ...............................................................37 3-10 Philips APD 3720 x-ray diffractometer. ............................................................................37 4-1 Optical images of pe llet surfaces post-sinter .....................................................................47 4-2 Undoped UO2. ....................................................................................................................48 4-3 Nb2O5-doped UO2 ..............................................................................................................49 4-4 Al2O3-doped UO2 ...............................................................................................................50 4-5 Cr2O3-doped UO2. ..............................................................................................................51 4-6 Sc2O3-doped UO2 ...............................................................................................................52 4-7 Y2O3-doped UO2 ................................................................................................................53 4-8 V2O5-doped UO2 ................................................................................................................54 4-9 TiO2-doped UO2.................................................................................................................55 4-10 EDS spectrum of undoped UO2 sample. ............................................................................56 4-11 EDS spectrum of Nb2O5-doped sample. ............................................................................56 4-12 EDS spectrum of Al2O3-doped sample. .............................................................................57 4-13 EDS spectrum of Cr2O3-doped sample. .............................................................................57

PAGE 9

9 4-14 EDS spectrum of Sc2O3-doped sample. .............................................................................58 4-15 EDS spectrum of Y2O3-doped sample. ..............................................................................58 4-16 EDS spectrum of V2O5-doped sample. ..............................................................................59 4-17 EDS spectrum of TiO2-doped sample. ...............................................................................59 A-1 Particle size dist ribution of received uran ium dioxide powder. ........................................64

PAGE 10

10 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ENHANCEMENT OF URANIUM DI OXIDE THERMAL AND MECHANICAL PROPERTIES BY OXIDE DOPANTS By Brett Jameson Dooies August 2008 Chair: Samim Anghaie Major: Nuclear Engineering Sciences The Advanced Fuel Cycle Ini tiative (AFCI) program is fundi ng the development of highburnup fuels for use in current and future reactors These fuels have the capacity to reduce the rate at which spent fuel is cr eated and thus reduce the amount of spent fuel requiring long-term geologic disposal. One of the f actors limiting the burnup of current reactor fuel is the buildup of internal pin pressure due to th e release of gaseous fission prod ucts from the fuel matrix. Theoretical calculations have shown that increasi ng the grain size of uran ium dioxide fuel could help to reduce the releas e of fission products and thus slow th e buildup of internal pin pressure. This theory has been proven by numerous irradiation studies of large-grained UO2 which have shown a decreased release rate of fission product gases. Obtaining large-grained fuel can be accomplished by several methods including doping UO2 with a small amount (< 1 wt%) of another metal oxide. In this research, UO2 was doped with oxides of niobium (Nb), aluminum (Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and titanium (Ti) at a concentration of 0.5 wt%. The pellets were sint ered to greater than 93 % of their theoretical density and microstructural analys is was performed to ascertain the effects of the dopants on the sintered pellets. The grain sizes were measured after chemically etching polished sections of the

PAGE 11

11 samples. Titania showed the most substantial promotion of larger grain sizes with a 281% increase over undoped UO2. Other dopants that showed pot ential for grain size increase were niobia, alumina, chromia, and vanadia. Scandia had no significant effect on the grain size of the fuel. The grain size of the yttria-doped fuel wa s unable to be determined due to inconsistencies with the chemical etching proce ss. Overall, the ability of a small amount of dopant to promote larger grains in uranium dioxi de fuel was shown, with titani a having the most potential.

PAGE 12

12 CHAPTER 1 INTRODUCTION 1.1 Background Information The United States currently has 104 operating nuclear reactors. In 2007 these reactors safely produced over 806 billion kWh of electri city. Unlike many other major energy sources, nuclear energy does not emit any greenhouse gases into the atmosphere. Currently, about onethird of energy produced in the US is from carbon-free sources, and nuclear power makes up about 70% of that energy (NEI, 2008). The majority of the reactors currently operati ng utilize an 18-month re fueling cycle, with average fuel assembly discharge burnups in the range of 35 – 45 GWd/kgU. With new Generation III+ reactors on th e horizon, many utilities are ho ping to switch to a 24-month refueling cycle. This would be beneficial ec onomically and from a fuel efficiency standpoint, reducing the overall amount of sp ent fuel and high level waste being produced. As such, reactor fuel will soon be pushed to new, higher burnups and will be subjected to increasingly rigorous operating conditions. The reliabil ity of high-burnup nuclear fuel must be assured in order for these goals to be realized. 1.1.1 Advanced Fuel Cycle Initiative The Advanced Fuel Cycle Ini tiative (AFCI) is a program of the United States Department of Energy. The mission of AFCI is “to develop fuel cycle technologi es that will meet the need for economic and sustained nuclear energy pr oduction while satisfy ing requirements for controlled, proliferation-resistan t nuclear materials management system.” (DOE, 2008) AFCI is meant to develop new technologies to aid the curr ent reactor fleet, as well as future Generation III+ and Generation IV reactors. Mission succes s would result in a reduc tion of the amount of

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13 high level radioactive wast e requiring geologic disposal, a redu ction of the plutonium content of civilian spent fuel, and increased energy extraction from the fuel. 1.1.2 Near-Term Goals of AFCI The near-term goals of AFCI are to develop a nd demonstrate a prolifer ation-resistant spent fuel recycling program. This pursuit would re duce the volume and heat content of high level waste that requires long-term storage, and exploit the large amount of fissile material still present in spent fuel. The Secretary of Energy is re quired to advise Congress on the need for an additional geologic repository (to follow Yu cca Mountain) by 2010. This recommendation will be driven by the ability to establish a viable spent fuel recycling progr am for commercial spent nuclear fuel. 1.1.3 Long-Term Goals of AFCI The long-term goals of AFCI are to develop a system for recycling sp ent fuel that would separate the fuel and allow for the destruction of actin ides and other longlived fission products in fast reactors through transmutation. The removal of these products would increase the capacity of the planned Yucca Mountain repository up to fifty-fold due to the decrease in the heat load of the remaining spent fuel. By su ccessfully increasing the t echnical capacity by such a large factor, Yucca Mountain would be sufficient for the storage of all current spent fuel as well as all the fuel produced in the next century (DOE, 2008). 1.2 Waste Reduction in Current and Generation III+ Reactors The long-term goals of the AFCI could take upwards of 30 years to be completely developed and implemented. Until the technology for reprocessing is available, current reactors (and those currently being planned for construction) must be concerned with the waste that they are producing now. Many reactor sites have recen tly run out of space for the storage of spent fuel on site, and with the opening of Yucca Mountai n still years away, dry cask storage is being

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14 used all over the country to alle viate the overcrowded spent fuel pools. Increased fuel cycle length for Generation III+ reactors will reduce the ra te at which spent fuel is produced. This is directly in line with meeting th e goals of the AFCI program. Cu rrent research is supporting the development of reliable, high-burnup fuels for immediate deployment. 1.3 Objectives and Scope 1.3.1 Objectives of Research One of the factors limiting fuel burnup is the acc umulation of internal pin pressure due to the release of fission gases from the fuel and into the gap region during irra diation. It was first shown by Turnbull (Turnbull, 1974) th at an increase in the fuel’s grain size can slow the fission gas release (FGR) and fuel swelli ng rates of irradiated fuel. By limiting the release of the fission gases, the buildup of internal pin pressure is sl owed and burnup can be in creased. Thus, the goal of this research is to increase the burnup capabil ities of uranium dioxide fuel by increasing the grain size of the fuel. 1.3.1.1 Grain size modification of UO2 There are several methods available for in creasing the grain size of sintered UO2 fuel. One method is to subject the sintered fuel to a long he at treatment to promote continued grain growth. This has the disadvantages of being time cons uming and expensive. Another method involves “doping” the UO2 powder with a small amount (typically < 1 wt%) of another oxide powder. The dopants can promote grain growth through seve ral different mechanisms and can facilitate an increase in the burnup cap abilities of the fuel. 1.3.1.2 Doping UO2 This research project aims to investigate th e effects of several oxide dopants on the grain sizes of sintered uranium dioxide fuel. This research will require the production of eight different uranium dioxide fuel samples. Seve n different oxide dopants will be considered and

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15 one undoped pellet will be produced for comparison. The dopants that will be used are oxides of niobium (Nb), aluminum (Al), chromium (Cr), s candium (Sc), yttrium (Y), vanadium (V), and titanium (Ti). The dopant concentration for each dopant will be set at 0.5 wt%. The average grain sizes will be determined and the dopants that are most effective at increasing grain size will be recommended for further consideration. This study represents the fi rst investigation of scandia and yttria as dopants for UO2. For the remaining dopants, a comparison will be made with results found in other publications. 1.3.2 Scope of Research This research aims to charac terize the grain sizes of doped UO2 fuel using equivalent conditions. The dopant concentrations and the si ntering process will be equivalent for each sample. The goal is exclusively to identify each dopant’s ability to promote grain size growth during sintering. Future research should aim to optimize the pa rameters for a specific dopant, including the concentration of the dopant, the oxygen potential of the sintering atmosphere, and the sintering temperature. Ideally, this study wi ll allow future research to focus on the dopants that show the most promise of increasing the grain size of UO2.

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16 CHAPTER 2 LITERATURE REVIEW 2.1 Theoretical Foundation The diffusion of fission gases in fuel under irra diation was first desc ribed by A. H. Booth (1957). The expression for F the fractional release of stable fission gas from the fuel, is shown in (2-1). 222 442 161 11exp()nanDt F Dtna (2-1) In this equation, D is the diffusion coefficient, t is the time, and a is the radius of a hypothetical spherical volume. For a small gas release, F 1/ a If the hypothetical volume is assumed to be a grain in the fuel, increasing the grain size can re duce the fraction of fission gas released from the fuel matrix into the pin volume. This model is the basis for the proceeding experimental research. The Booth model was later modified to incl ude the effects of gas re-solution from intragranular bubbles and in terlinked porosity. A simple relati on for low quantity gas release, as proposed by Killeen (1975), is given in (2-2). 4 3 SDt F V (2-2) In this case, the release fraction is related to the surface-to-volume ratio of the sample. Again, this relationship demonstr ates that by increasing the grai n size (and thus decreasing the surface-to-volume ratio), the fission ga s release rate will decrease. 2.2 Previous Experimental Work Several of the dopants investig ated in this study have be en researched previously, specifically oxides of niobium, aluminum, chro mium, vanadium, and titanium. Scandia and

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17 yttria have not previously been used as dopants for UO2. It is desirable to review the results of previous research in order to identify the parameters that mu st be considered when doping UO2. Prior experiments can be split into pre-irradi ation and post-irradiati on results. In this research project, no irradiation testing was c onducted. Therefore, this review will focus on sample production methods and pr e-irradiation analyses. 2.2.1 Production of Doped Fuel Pellets The process for producing UO2 fuel pellets is well establis hed and is used in wide scale production throughout the world. The only steps typi cally required are cold pressing the powder into a green pellet and sintering the pellet to ac hieve densification. Binders and lubricants are sometimes used, though not usually necessary. Si ntering is typically performed in a reducing environment at around 1700C for approximately four hours. When working with doped UO2, an initial step of mixing the powders is also necessary. Th e effects of manipulating these variables, as well as dopant con centrations, have been investigat ed in several studies of doped UO2 fuel (Ainscough et al., 1974; Bourgeois et al., 2001). Dopant mixing. The large majority of doping studies have integrated the dopant powder into the UO2 powder by a mechanical mixing method. Th is is typically done without organic additions by rotary ball milling with steel ball s for two to 72 hours. In some cases this was preceded or followed by sieving of the powder mixture. While mechanical mixing is the most common method of mixing the dopant and UO2 powders, one other noteworthy method was used by Bourgeois et al. (2001) for doping UO2 with Cr2O3. In order to more homogeneously mix the dopant into the powder, spray-drying of a suspension containing UO2 powder in solution with distilled water and (NH4)2CrO4 was performed. The spray-drying was followed by calc ination of the powders in argon to transform the chromate into Cr2O3. This process did not disturb the sp ecific surface area of the powders,

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18 nor their O/U ratios. It was found that this mixi ng method resulted in larg er final grain sizes for Cr2O3-doped UO2 than previous studies, presumably due to the improved homogeneity of the mixing. Cold pressing. The method used almost exclus ively for cold pressing doped UO2 powders is uniaxial pressing, often bilatera lly. Typical pressures for this process are from 100–350 MPa. Green densities are normally on the order of 50–60 %T D. Due to the high compressibility of the powders, large differences in cold pressures do no t yield large differences in green densities. A study by Bourgeois et al. (2001) found that a 5–6% difference in green dens ity resulted in less than 1% difference in sintered density. Due to the relative unimportance of the cold pressure and green pellet density on the final si ntered density of samples, th e effects of varying the cold pressure were not studied in this project. Other parameters. There are several other parameters that play an important role in the final sintered pellet characteristics. Specific ex amples include the sintering temperature, the sintering time, the oxygen potential of the sintering environment, and any heat treatments that may be applied after sintering. Table 2-1 shows several variables from previous research and illustrates the large variety of options availabl e. Some of the effects of changing these parameters will be discussed with re gards to the results that follow. 2.2.2 Results of Previous Studies There have been a multitude of studies concerning the effects of dopants on the characteristics of sintered UO2 pellets. The majority of these studies have focused on the abilities of different dopants to promote an increase in grain si ze. Table 2-1 lists grain sizes found for doped-UO2 pellets in previous st udies that have used Cr2O3, TiO2, Nb2O5, Al2O3 and V2O5, all of which were investigated in this research. The results of these studies vary widely depending on the dopant concentrations and the sintering conditions used. Additionally, most

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19 studies do not state whether they have reported 2D grain sizes or have corrected for and reported 3-D grain sizes. The far right column shows th e relative percent increas e of the doped samples over the control (undoped) samples in each study. Da shes indicate that the control grain size was not reported. Since different st udies have different powders and parameters, the percent increase in grain size provides a better measure for comparis on of different studies than the absolute grain sizes. There is no published lite rature on the use of Sc2O3 or Y2O3 as dopants for UO2. Thus, there are no previous results with which to compare. Chromia. For Cr2O3-doped UO2, the average grain sizes qu oted vary between 12 and 126 m. Using a concentration of 0.5 wt% and simila r sintering conditions as the current research, the reported grain sizes vary between 50 and 126 m, or an average relative increase of approximately 550%. The grain size increase due to doping with Cr2O3 seems to be limited to a dopant concentration of 0.3–0.5 wt%. At concentrat ions above 0.5 wt%, no additional grain size increase is seen. Solubility limits have been quoted by severa l sources. Leenaers et al. (2003) found the solubility at 1600, 1660, and 1760C to be 0.065, 0.086, and 0.102 wt%, respectively. Kashibe et al. (1998) found that the solubi lity at 1750C was only 0.012 wt%, although in this case a very small amount (0.065 wt%) was initially added to th e powder. Bourgeois et al. (2001) found the solubility to be 0.07 wt% in th e range of 1500–1700C. Solubility varies with both temperature and oxygen potential, which could explai n some of the differences above. The actual amount of Cr2O3 added is well above these limits in most research published. Above the solubility limit, Cr and Cr2O3 form a eutectic at around 1550C and the liquid phase enhances grain growth and densification. This provides some explanation for the mechanism of

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20 increased grain size in Cr2O3-doped UO2. It is also seen in Table 2-1 that for pellets sintered below 1550C, the grain sizes achieved are genera lly much smaller than for those sintered at temperatures above 1600C. Titania. Titania-doped UO2 grain sizes range from 5 to 133 m. For a concentration of 0.5 wt% and similar sintering conditions to the cu rrent research, the grai n sizes vary from 70 to 133 m, or an average relative incr ease of approximately 350%. Ainscough et al. (1974) studied the effect of sintering temp erature and additive content on grain size and found that for c oncentrations of 0.03–0.33 wt% TiO2, grain sizes varied from 5–45 m at 1550C sintering temperat ure. At 0.13 wt% dopant, the maximum grain sizes were obtained. At 1650 and 1750C, the samples containing 0.13 wt% TiO2 experienced enhanced grain growth due to the formation of a UO2-TiO2 eutectic that separate d to the grain boundaries during sintering. The eutectic was never formed for concentrations of 0.07 wt% or less and was occasionally formed for 0.13 wt%. This led to the conclusion that TiO2 solubility in UO2 is between 0.07 and 0.13 wt% in the range of 1650–1750C. Niobia. Niobia-doped UO2 grain sizes range from 7 to 56 m. For a concentration of 0.5 wt% and similar sintering condition s to the current research, the grain sizes vary from 40 to 56 m, or an average relative increase of approximately 230%. The solubility of Nb2O5 in UO2 has been quoted as being approximately 0.5 wt% in several studies. Harada (1996) showed the variation of the lattice parameter as the niobia content is increased and found that the variation halted very near 0.5 wt%. Harada (1996) also showed the variation of the lattice paramete r as a function of temperature a nd reported that a solid solution occurs between 1400 and 1700C.

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21 Alumina. There is significantly less published re search using alumina as a dopant for UO2 than there is for chromia, titania, and niobia. The range of grain sizes found in the two studies shown in Table 2-1 is from 10 to 30 m. Both of the studies have dopant concentrations lower than the 0.5 wt% used in this research. Vanadia. Vanadia was shown in the study by Amato et al. (1967) to reduce the grain size of sintered UO2 pellets when used as a dopant. On th e other hand, Radford and Pope (1983) and Aybers et al. (2004) found that gr ain size increased with increasing V2O5 concentration up to 1.0 mol%. Without additional studies, it is hard to say whether or not vanadia is effective at promoting increased grain size. This research wi ll try to provide furthe r insight on this dopant.

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22 Ta bl e 2 1 G ra i n s i zes o b ta i ne d f or d ope d U O 2 i n prev i ous stu di es Dopant Reference Dopant Concentra tion Sintering Atmosphere Temperature (C) Time (h) Grain Size ( m) % Increase Killeen (1980) 0.5 wt% Not quoted Not quoted Not quoted 50–55 767% Ohai (2003) 0.1 wt% Hydrogen 1700 4 45–60 420% 0.3 wt% Hydrogen 1700 4 65–110 770% 0.5 wt% Hydrogen 1700 4 80–126 930% 1.0 wt% Hydrogen 1700 4 80–126 930% Cr2O3 Aybers et al. (2004) 100 ppm 90% Ar + 10% H2 1700 4 12–20 60% 1000 ppm 90% Ar + 10% H2 1700 4 13–23 80% 2000 ppm 90% Ar + 10% H2 1700 4 13–25 90% 3000 ppm 90% Ar + 10% H2 1700 4 15–27 110% Kashibe et al. (1998) 0.065 wt% Hydrogen 1750 2 15 0% Ainscough et al. (1978) 0.3 wt% Not quoted Not quoted Not quoted 80 Bourgeois et al. (2001) 0.05–0.7 wt% H2 + 1 vol% H2O 1525 4 15–28 250% 0.05–0.7 wt% H2 + 1 vol% H2O 1625 4 29–50 388% 0.05–0.7 wt% H2 + 1 vol% H2O 1700 4 15–87 410% Delafoy et al. (2007) 0.16 wt% Not quoted Not quoted Not quoted 50–60 588% Arborelius et al. (2006) 1,000 ppma H2/CO2 1800 14 40–55 336% Amato et al. (1966) 0.5 wt% Hydrogen 1400 + heat treatb 1 4.4–51.5 250% Aiscough et al. (1974) 0.03–0.33 wt% Hydrogen 1550 125 20–45 0.03–0.33 wt% Hydrogen 1650 16 10–105 1040% 0.03–0.33 wt% Hydrogen 1750 4 10–90 TiO2 Radford and Pope (1983) 0.05–1.0 mol% metal Hydr ogen Six steps from 925 1780 1 hour at each step 10.9–69.7 410% Yuda et al. (1997) 0.2 wt% Not quoted Not quoted Not quoted 68 656% Ohai (2003) 0.05–1.0 wt% metal Hydrogen 1700 4 65–133 880% Aybers et al. (2004) 100 ppm 90% Ar + 10% H2 1700 4 11–19 50% 1000 ppm 90% Ar + 10% H2 1700 4 15–33 140% 2000 ppm 90% Ar + 10% H2 1700 4 23–43 230% 3000 ppm 90% Ar + 10% H2 1700 4 31–59 350%

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23 Table 2-1. Continued Dopant Reference Dopant Concentra tion Sintering Atmosphere Temperature (C) Time (h) Grain Size ( m) % Increase Killeen 0.1 at% Not quoted Not quoted Not quoted 7 0% 1.0 at% Not quoted Not quoted Not quoted 28 300% Sawbridge 0.25 mol% Not quoted Not quoted Not quoted 24–30 0.4 mol% Not quoted Not quoted Not quoted 28–32 0.5 mol% Not quoted Not quoted Not quoted 43 0.6 mol% Not quoted Not quoted Not quoted 28–32 0.8 mol% Not quoted Not quoted Not quoted 28–32 Nb2O5 1.0 mol% Not quoted Not quoted Not quoted 25 0.3 wt% Reducing 1750 2 30–35 220% Assmann 0.5 wt% Reducing 1750 2 40–50 350% 0.5 wt% Oxidative 1100 1 2–15 -20% 0.05–0.5 mol% metal Hydrogen Six steps fro m 925 1780 1 hour at each step 20.1–50.5 343% Radford 0.3 wt% Wet hydrogen 1700 3 30 500% Harada 0.05–1.0 wt% metal Hydrogen 1700 4 14–56 250% Ohai 0.05–1.0 wt% metal Hydrogen 1600 4 9–31 100% Kashibe 0.076 wt% Hydrogen 1750 2 30 100% Al2O3 Aybers 100 ppm 90% Ar + 10% H2 1700 4 10–24 60% 1000 ppm 90% Ar + 10% H2 1700 4 12–26 90% Amato 0.68 wt% Hydrogen 1400–1700 0.5–10 2.5–24.0 -32% 1.38 wt% Hydrogen 1400–1700 0.5–10 2.5–17.0 -40% 2.10 wt% Hydrogen 1400–1700 0.5–10 2.5–11.2 -50% V2O5 Radford 0.05 – 1.0 mol% metal Hydrogen Six steps from 925 1780 1 hour at each step 12.4–57.1 343% Aybers 100 ppm 90% Ar + 10% H2 1700 4 14–22 80% 1000 ppm 90% Ar + 10% H2 1700 4 16–30 130% 2000 ppm 90% Ar + 10% H2 1700 4 14–32 130% a Also included 1000 ppm alumina additive b Heat treatments were in hydrogen atmo sphere from 1450 1630 C for 0.5 10 hours

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24 CHAPTER 3 MATERIALS AND METHODS The methodologies employed in this research focused on the processing of the doped UO2 pellets and the characterization of the sintered pellets. The materials required for this research included the uranium dioxide powder, the dopant powders, the cold uniaxial press, the induction furnace and associated parts, the equipment used to prepare the samples for analysis, and the analysis equipment. 3.1 Processing Pellet processing was performed at the A pplied Ultra High Temperature Research Laboratory at the Innovative Nuclear Space Power a nd Propulsion Institute at the University of Florida. The production of the doped fuel pellets entailed several steps. The first step was to measure and mix each of the dopants with the uranium dioxide powder. Next, the powder mixtures were cold pressed into semi-dense co mpacts (green pellets). Then the green pellets were sintered to achieve final densification. 3.1.1 Powders The powders used in this research were th e uranium dioxide powder and the seven dopant powders. The UO2 powder was comprised of depleted uranium (DU) obtained from Framatome/AREVA. Appendix A shows some properties of the initial UO2 powder. Table 3-1 shows a summary of the dopant powde rs used in this research. Powder mixing. The dopants were added to the UO2 powder at an optimum concentration of 0.5 wt%. Table 3-2 shows the actual measured values for the UO2 and dopant powder masses prior to mixing. The powder masses were measur ed using a Sartorius R180D analytic balance seen in Figure 3-1.

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25 The powder mixtures were combined in HDPE bottles and six stainless steel shots were added. The powder mixing was performed in air at atmospheric pressure for all of the dopants except for Cr2O3 and V2O5, which were mixed under argon at atmospheric pressure due to their volatility. The mixtures were shaken for thirty minutes in a Spex Cer tiprep 8000M Mixer/Mill. The stainless steel shots were added to help br eak up the powder particles during shaking and to induce good mixing. 3.1.2 Cold Uniaxial Pressing The cold press used in this research was a Carver model #3912 hydraulic press, shown in Figure 3-2. The cold uniaxial pressing of powders requires the use of a punch and die. Initially, a simple stainless steel rod and tube were us ed for cold pressing the powders. One rod was inserted into the tube, powder was poured i n, and a second rod was inserted above. The collection was then punched in the hydraulic pre ss. This initial process did not produce green pellets of sufficient quality to be sintered. It was evident that the ends of the rods were not polished well enough, as the green pellets were sticking and breaking. Additionally, the tubes were not strong enough to withstand the outwa rd pressure exerted by the powder upon being pressed. This resulted in bowing of the sides and poor compaction of the powder. Due to the failure of the stainless steel punch and die, a polished die set made specifically for cold pressing pellets was obtained. The set was an International Crystal Laboratories 13mm KBr Die Set consisting of a base, a stainless steel die, and two stainless st eel anvils for punches, shown in Figure 3-3. The process for cold pressi ng a pellet using this die set was as follows: Measure 3.5–4.5 g of doped UO2 powder Clean each piece of the die set a nd attach the die to the base Coat the inside of the die with stearic acid solution, which acts as a lubricant, using the long stainless steel punch

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26 Coat the first anvil in stearic acid solution a nd insert it into the di e, polished side up Pour the powder into the die Coat the second anvil in stearic acid solution and insert it into the die, polished side down Place the punch on top of the second anvil and press in the hydraulic press for ten minutes Remove the punch and separate the die from the base, removing the first (bottom) anvil Flip the die over and reinsert the punch Place a metal ring on top of the die and use the hydraulic press to punch out the pellet through the top Typical pressures for cold pressing range be tween 200 and 350 MPa. Each pellet in this study was pressed at approximately 300 MPa and gr een pellet densities were all measured to be between 50% and 60% TD. The green pellet de nsities were calculated using their measured dimensions to estimate their volume. 3.1.3 Pellet Sintering 3.1.3.1 Induction furnace The furnace used for heating and sintering the green pellets was a 20 kW Taylor Winfield Thermonic generator, model CE2000. The h eating was accomplished with a water-cooled copper coil inside of a vacuum chamber inductiv ely heating a tungsten tu be that surrounded the sample. The pellet sat on a piece of tantalum car bide foam which sat atop a boron nitride base. The setup is shown in Figure 3-4. Both the BN base and the TaC foam were used for their high temperature capabilities and chemical stability. At the bottom of the BN stand was a hole that allowed gas to enter and flow up through the TaC foam, across the pellet, and out the top of the apparatus. A mixture of Ar-5%H2 was used to reduce the sa mples during sintering. Temperature was controlled using a Maxline IRCON active controller and measured using a two-color infrared pyrometer. The furnace system is shown in Figure 3-5.

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27 3.1.3.2 Sintering The pellets were sintered at 1700C for a pproximately four hours while exposed to a mixture of Ar 5%H2. The heat up and cool down rates we re controlled manually and were thus subject to some amount of variat ion. Due to this variation, se veral of the pellets experienced cracking during sintering. It is surmised that excessively fast heating up or cooling down was the cause of the cracking. This did not affect the results of this study since microstructural characterization was still possibl e despite the cracking. It is not presumed that the cracking indicates any real difficulties that would be associated with pr oducing doped pellets. 3.2 Characterization Characterization of the sintered pellets was performed in order to compare the microstructures and grain sizes of the doped pelle ts. Density measurements, optical microscopy, and preparations for other analyses were pe rformed at the Applied Ultra High Temperature Research Laboratory at the I nnovative Nuclear Space Power a nd Propulsion Institute at the University of Florida. SEM, EDS, and XR D were performed at the Major Analytical Instrumentation Center at th e University of Florida. 3.2.1 Preparation for Analysis 3.2.1.1 Scanning electron microscopy Cutting. The sintered fuel pellets first needed to be cut into cross-sectional pieces to allow for internal microstructural examination. Cu tting was performed with a LECO VC-50 precision low-speed diamond saw shown in Figure 3-6. Sample s were cut into eighth s and the slices were approximately 1–3 mm in thickness. Setting in epoxy. After cutting the samples, the slices needed to be set in epoxy to allow for grinding and polishing. The epoxy is a 10:2 mixture of LECO epoxy resin and LECO hardener. This was poured over the slic es and allowed to set overnight.

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28 Grinding and polishing. The samples were ground and polished using a LECO Spectrum System 1000 grinder/polisher with semi-aut omatic head shown in Figure 3-7. The grinder/polisher head attachment enabled semi-aut omatic operation for six samples at a time with constant applied pneumatic pressure. Table 3-3 shows the grinding and polishing parameters. Chemical etching. After grinding and polishing, a chem ical etch was used to reveal the grain structure of the sample. There are a wide vari ety of etchants that have been reported in the literature (Petzow, 1978). The most common etchant for UO2 seems to be a 1:1:1 mixture of H2SO4, H2O2, and H2O. This was used for several of the samples in this st udy. The etchant was applied with a cotton tipped appl icator, allowed to sit for anywhere from one minute to two hours, and then brushed off with the cotton sw ab. The samples were ultrasonically cleaned following the etching process. This etchant was m ildly effective but did not work for all of the samples. Another etchant that was used was a 1:1 mixtur e of distilled water and 90% nitric acid. This etchant was applied to the surface and allowe d to sit for five seconds to 2 minutes before being brushed off with a cotton swab. The reas ons for different etchan ts yielding different results are unknown. Many other methods for etchi ng could have been tried, possibly to greater effect, including thermal etching and electrolytic etchi ng techniques. Conductive paint. Investigation by SEM requires th at the sample be electrically conductive, lest a charge build up in the sample and cause burning and image distortion. Thus, a coating of conductive graphite paint was applie d to the epoxy region su rrounding the samples to establish an electrical cont act with the specimen holder.

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29 3.2.1.2 X-ray diffraction A small amount of each sample was ground into powder and prepared for investigation by XRD. The powder was placed on slides and adhere d using a solution of one part collodian and seven parts amyl acetate. 3.2.2 Analysis of Samples 3.2.2.1 Density measurements The density of each sample was measured af ter sintering. Density measurements were performed using an immersion technique. First, the mass of the sample in air was measured. Then the apparent mass of the sample while submersed in water was measured. This measurement was made by filling a beaker with water so that the sample holder was immersed, tearing the scale, and then placing the pellet on the holder. It is important to tear the scale after immersing the sample holder in or der to account for its change in apparent mass. Though small, this could introduce a significan t discrepancy in the measurement if done improperly. The red platform kept the beaker isolated from the s cale. Figure 3-8 shows the apparatus used for measuring the apparent mass while submersed in water. Since the density of water at room temperatur e is 1 g/cc, the difference in the mass in air and the mass in water is equal to the sample volum e. In this way, the densities of the samples were calculated by dividing the mass in air by the volume. 3.2.2.2 Scanning electron microscopy and energy dispersive spectroscopy Secondary and backscatter electron images were taken using a JEOL JSM 6400 SEM with Link ISIS digital image capturing system show n in Figure 3-9. Secondary electron images reveal topographical information while backs catter electron images are used to show compositional contrast. Images were taken of both polished and etched samples to reveal microstructural characteristics and grain structures.

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30 Energy dispersive spectroscopy was also perf ormed with the JEOL JSM 6400 SEM. This gives a qualitative assessment of the elements present in the sample. Under ideal conditions, EDS can reveal elements at concentrations down to 0.1 wt%. Thus, it is th eoretically possible to see the dopant elements (present as oxides at 0.5 wt% nominally) as long as their spectra do not overlap with the uranium spectrum. 3.2.2.3 Grain size determination The most desirable method for analyzing the grai n size of a material is to use a computer program to automate the process. This is only possible for very high quality images of grain structure as the programs identify the grain boundaries based on contra st within the image. None of the images of etched samples were of sufficie nt quality to be analyzed by software. Instead, grain sizes were calculated usi ng a linear intercept method (A brams, 1971). This involved placing multiple lines at different orientations over images of the etched samples and counting the number of intercepts along the li ne. Each grain that is crossed is counted as 1 and if the test length ends inside of a grain it is counted as For each sample, at least two images were used, with approximately 15 test lengths used on each image to determine the grain size of the specimen. For each image, the total number of intercepts counted was greater than 50. The mean grain size result found in each image wa s averaged and the standard deviation was computed. 3.2.2.4 X-ray diffraction Samples were investigated using a Philips APD 3720 x-ray diffractometer, shown in Figure 3-10. The XRD was operated at a voltage of 40 kV and current of 20 mA. The resultant peaks were compared with known data for informati on about the constituents and phases present. XRD spectra were taken of both the mixed powders prior to sintering and of the sintered pellets

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31 (after pulverizing a small portion of them into powder). Lattice parameters were found for each of the sintered pellets and compared to undoped UO2.

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32 Table 3-1. Summary of dopant powders. Dopant # Material Supplier Purity 1 Nb2O5 Sigma Aldrich 99.99% 2 Al2O3 Alfa 99.60% 3 Cr2O3 Sigma Aldrich 99.90% 4 Sc2O3 Alfa 99.90% 5 Y2O3 Fisher Scientific 99.99% 6 V2O5 Sigma Aldrich 99.99% 7 TiO2 Sigma Aldrich 99.99% Table 3-2. Masses of UO2 and dopant powders in sample mixtures. Pellet # Dopant Measured Mass UO2 (g)Measured Mass Dopant (g) Dopant wt% 1 9.164630 0.000% 2 Nb2O5 4.131590.02076 0.500% 3 Al2O3 9.101940.04579 0.501% 4 Cr2O3 9.399340.04716 0.499% 5 Sc2O3 9.116870.04580 0.500% 6 Y2O3 9.115910.04576 0.499% 7 V2O5 9.112140.04576 0.500% 8 TiO2 9.105870.04540 0.496% Table 3-3. Grinding an d polishing sequence. Grit/Micron size Time (minutes)Pressure (psi) 120 grit 2–440 400 grit 235 600 grit 235 1200 grit 1.525 6 micron 1.5Variable* 1 micron 1.5Variable* 0.5 micron 1Variable* *Manual pressure applied

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33 Figure 3-1. Sartorius R180D analytic balance used for mass measurements. Figure 3-2. Hydraulic press used fo r cold uniaxial pr essing of pellets.

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34 Figure 3-3. Stainless st eel die set used for cold pressing powders. Figure 3-4. Base used to hold pellet dur ing sintering.

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35 Figure 3-5. Induction furnace, controlle r, pyrometer, and vacuum chamber. Figure 3-6. LECO VC-50 preci sion low-speed diamond saw.

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36 Figure 3-7. LECO Spectrum System 1000 grinde r/polisher with semi-automatic head. Figure 3-8. Setup for measuri ng apparent mass of sample while immersed in water.

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37 Figure 3-9. JEOL JSM 6400 s canning electron microscope. Figure 3-10. Philips APD 3720 x-ray diffractometer.

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38 CHAPTER 4 RESULTS AND DISCUSSION 4.1 Density Measurements The theoretical density of the fuel is slightly affected by the pr esence of the dopant species. Thus, calculations of the theoretic al densities of the samples were made by mass averaging the densities of the UO2 and the dopant. The theoretical densit ies for each sample are presented in Table 4-1. The measured masses and calculated densities of the final sintered pellets are shown in Table 4-2. The average density of the pellets is 10.39 g/cc. None of the samples had a density lower than 93.35 %TD. Thus, the processing method used was successful in producing dense pellets suitable for further analysis. 4.2 Pellet Characterization 4.2.1 Optical Microscopy Optical microscopy was used to image the surfa ce of the pellets after sintering. Figure 4-1 shows all eight pellets. It is seen that the chromia-doped, scandia-dope d, and titania-doped pellets each had a large crack that propagated from the center to the edge of the pellet. The va nadia-doped pellet also had a crack but it was less severe. These cracks ar e most likely due to excessively quick heat up and cool down rates during the sintering process. Future processing methods must consider this problem to avoid cracking the pellets, as crac ked pellets would not be mechanically sound for use in reactors. However, since this rese arch was only concerned with microstructural characterization, the cracked pellets are not a major issue for this project.

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39 The surfaces of many of the pell ets appear to contain multiple scratches and other flaws. These arose from handling the pellets before and after sintering. All microstructural investigation was performed on ground and polishe d internal cross-sectio ns of the pellets. 4.2.2 Scanning Electron Microscopy Images were taken of the samples before and after chemical etching. Prior to chemically etching the samples, topographical and compositiona l features could be investigated to look for any unexpected features or phase s. After etching, the grain st ructure was revealed and grain sizes could be analyzed. Figures 4-2 through 4-9 present the results. Backscatter electron images showed no signs of compositional heteroge neity in any of the doped samples. This does not mean that the mixture was perfectly homogeneous throughout the sample as only small sections were imaged. In a ny case, it does not seem that any of the dopants segregated severely within the samples. Undoped UO2. For the undoped UO2 pellet, the microstructure reveals a low porosity sample as expected from the high density measur ed. The lines seen across the sample in Figure 4-2 A are remnants of the grinding and polishing process and not a microstructural feature. Figure 4-2 B shows two regions of the etched su rface with grains revealed. A sulfuric acid solution was used to etch the sample. The etchant did not work exceptionally well on the undoped sample as seen by the roug hness of the grain structure. Despite the poor etching, it was still possible to measure the grain sizes with th is sample. The averag e grain size of the undoped UO2 sample was 2.73 m with a standard deviation of 0.35 m. This is significantly smaller than typical grain sizes for undoped UO2 quoted in the literature, which average nearly 10 m in the studies shown in Table 2-1. The reason for this discrepancy is not known but could be a result of differences in initi al powder size distribu tion, processing parameters, or sintering procedures. Due to this difference, the relativ e increase in grain size with each dopant will be

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40 used as a more meaningful measurement, rather than a direct comparison of grain sizes with other studies. Nb2O5-doped UO2. The niobia-doped pellet seen in Figure 4-3 shows a low porosity microstructure as well. This sample was etch ed with sulfuric acid a nd shows a more defined grain structure than the undoped UO2. It is seen that some of th e grains appear to have bumps on the surface. This occurred with several samp les and is thought to be a consequence of the chemical etching process. The average gr ain size of the niobia-doped pellet was 5.94 m with a standard deviation of 0.58 m. This is a relative in crease of 117% over undoped UO2, a very substantial increase. However, it is less than the average grain size increase found for other studies using 0.5 wt% niobia, which resulted in increases of nearly 230% under similar processing conditions. This coul d again be due to differences in powders or methodologies. However, despite being less than the changes quoted by other studies, 117% increase in grain size is significant for a 0.5 wt% addition of Nb2O5. Al2O3-doped UO2. Figure 4-4 shows the alumina-doped pellet. The topography is again seen to be low porosity with no une xpected features present. The et chant used was sulfuric acid. Figure 4-4 B shows the etched regions with a relatively well defined grain structure. The bumpiness noted for the niobia-doped pellet is also seen here. Th e average grain size of this sample was 5.14 m with a standard deviation of 0.50 m. This is a relative increase of 88% over the undoped UO2 pellet. Previous studies found an average increase of approximately 80% with alumina, although these were done with lower concentrations than 0.5 wt%. Cr2O3-doped UO2. Figure 4-5 A shows the chromia-doped UO2 prior to etching at two magnifications. The topography of this sample is slightly more porous than other samples. This pellet had the second lowest measured density at 93.61 %TD. Figure 4-5 B shows the etched

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41 regions of the chromia-doped pellet Sulfuric acid was used to etch this sample. The average grain size of the pellet was 4.77 m with a standard deviation of 0.58 m. This is a 74% increase over the undoped sample, significan tly smaller than the increase seen by other studies using chromia as a dopant. Studies using 0.5 wt% ch romia and similar proc essing tactics found an average relative increase in grai n size of 550%. Previous studies have found evidence of a CrCr2O3 eutectic forming above 1550C, which creates a liquid phase that enhances grain growth during sintering. There has been evidence of Cr-rich spots appearing in the sintered pellet as a result (Bourgeois et al., 2001). These Cr-rich spots were not found in this study using BSE compositional contrast imaging, however EMPA may better elucidate this sort of phenomenon. Sc2O3-doped UO2. No previous results for scandia-doped UO2 have been reported in the literature. Figure 4-6 shows the microstructure of this pellet. In Figure 4-6 A, no unusual microstructural features are obser ved. Figure 4-6 B shows the etched microstructure for this sample. The etching was attempted with sulfuric acid four times, with the acid allowed to sit different lengths of time varying from 10 seconds to 2 hours. None of these attempts provided sufficient quality images to reveal grain struct ures. The images seen in Figure 4-6 B were acquired after etching with a 1:1 mixture of 90% n itric acid and distilled water for approximately 20 seconds. While the quality of these images is still poor, grain size analysis could be executed. The average grain size measured fo r the scandia-doped sample was 2.63 m with a standard deviation of 0.35 m. This is a 4% decrease from the undoped UO2 sample. This difference, however, is not statistically significan t. As such, it is concluded that scandia-doping had no significant effect on the grain size of UO2. Y2O3-doped UO2. Figure 4-7 shows the yttria-doped pell et. It is seen in Figure 4-7 B that the chemical etching failed to reveal the grain stru cture for this pellet. Bo th sulfuric and nitric

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42 acid etchings were tried, but to no avail. As su ch, no grain size analysis can be performed on this sample. V2O5-doped UO2. Figure 4-8 shows the vanadia-doped UO2 sample. Again, no unexpected microstructural features are seen in Figure 4-8 A. The grain structure in Figure 4-8 B was revealed by sulfuric acid etching. Th e average grain size of this sample was 3.41 m with a standard deviation of 0.13 m. This is an increase over undoped UO2 of 25%. Previous studies have reported results ranging from 50% d ecreases up to nearly 350% increases in grain size due to vanadia additions. This study finds th at the increase is mode st at 25%, but it is a statistically significant increase. This lends more evidence to the argument that vanadia could be used as a dopant to increase grain sizes in UO2. TiO2-doped UO2. Figure 4-9 shows the titania-doped pe llet. It is seen in Figure 4-9 A that the polish of this sample was not as successful as some of the others. However, it is still possible to conclude that no unusua l features are present. Figure 4-9 B shows the etched regions of the sample. The nitric acid etchant was used on the pellet and the results are the best of any of the etched samples. The grain boundaries are well defined and the grai n structure is easily visible throughout the images. It is not clear why the nitric acid was so successful in this instance. The average grain size of this sample was 10.37 m with a standard deviation of 1.19 m. This is a 279% relative incr ease in grain size over the undoped UO2 pellet, by far the largest increase seen for any of the dopants in this study. This is still slightly less than the average increase seen by similar studies of 33 0%, but it is reas onably close. Grain size statistical analysis. The doped samples’ grain sizes found in this study were subjected to statistical analysis to confirm that they were significantly different from the undoped sample’s grain size. This was done by using a two-sample t-test assuming equal population

PAGE 43

43 variances. The assumption of equa l variances is justified because while the grain sizes differ, the distribution of sizes is probabl y relatively constant for each sample. Each doped sample was tested against the undoped sample. The null hyp othesis was that the mean grain size of the doped sample was equal to the mean grain size of the undoped sample. The alternative hypothesis was that the doped sample’s mean grai n size was greater than that of the undoped sample. In each case, the pooled standard deviati on was used. This was calculated using (4-1). s2 = ((n1-1)*s1 2 + (n2-1)*s2 2) / (n1 + n2 2) (4-1) The results of the statistical analysis showed that the differences in mean grain sizes were significant for the niobia-doped, alumina-dope d, chromia-doped, vanadia-doped, and titaniadoped samples with respect to th e undoped sample at a confidence level of 95%. The scandiadoped sample was not significantly different. The results of this analysis are summarized in Table 4-3. 4.2.3 Electron Dispersive Spectroscopy EDS was used to identify composition of the samples after sintering. The spectra are shown in Figures 4-10 through 4-17. EDS spectra were taken at an opera ting voltage of 25 kV and operated for a live time of 300 seconds. All of the dopants are at least marginally visible in the spectra except for vanadium. As mentione d earlier, the minimum elemental concentration that can be detected by EDS is 0.1 wt%. The dopants are present at 0.5 wt% as oxides, with metal contents therefore being even lower. It is thus very impre ssive that the dopants are seen in these spectra. As EDS is onl y a qualitative tool, no quantitativ e information can be found from these spectra. 4.2.4 X-Ray Diffraction Powder XRD measurements were made before and after sintering. Several of the presinter measurements showed small traces of the dopant powders. Additionally, the UO2 powder

PAGE 44

44 was slightly hyperstoichiometric pr ior to sintering, as expected from the powder analysis shown in Appendix A which measured the received powder to be UO2.10. The post-sinter results show only stoichiometric UO2 peaks for all of the samples, sign ifying that the powder was reduced by the hydrogen in the sintering environment and that the dopant powders were integrated into the matrix. Any amount of dopant that remained segr egated, if any, was too small an amount to be detected using XRD. The lattice parameters were calculated from the spectra using measured dvalues from the (331) plane of UO2. The lattice parameter a0 is related to th e d-value and (hkl) plane in cubic lattices by (4-2) (Brundle et al., 1992). a0 dhkl h2 + k2 + l2)1/2 (4-2) The lattice parameters are shown in Table 4-4. In all of the doped samples, the lattice parameter decreased compared to undoped UO2. This change in la ttice parameter further confirmed that species other than pure UO2 are present in the samples.

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45 Table 4-1. TD’s of doped samples Sample Dopant Dopant Density (g /cc) TD of doped sample (g/cc) 1 Undoped -10.960 2 Nb2O5 4.6010.928 3 Al2O3 3.9610.925 4 Cr2O3 5.2210.931 5 Sc2O3 3.8610.925 6 Y2O3 5.0310.930 7 V2O5 3.3510.922 8 TiO2 4.2310.926 Table 4-2. Pellet masses and densities. Sample # Dopant Mass in air (g) Mass in water (g)Volume [= m] (cc) Density (g/cc) %TD 1 Undoped 4.47979 4.055720.4240710.56 96.39% 2 Nb2O5 3.59458 3.247690.3468910.36 94.82% 3 Al2O3 4.33465 3.916980.4176710.38 94.99% 4 Cr2O3 3.98003 3.591080.3889510.23 93.61% 5 Sc2O3 3.61022 3.25620.3540210.20 93.35% 6 Y2O3 4.55315 4.123050.4301010.59 96.85% 7 V2O5 4.09821 3.705370.3928410.43 95.52% 8 TiO2 3.89407 3.518280.3757910.36 94.84% Table 4-3. Results of statistical analysis for pellet grain sizes Sample Dopant Pooled St Dev p-value (95% confidence) Confidence Interval 2 Nb2O5 0.4740.0000(2.39, 4.03) 3 Al2O3 0.4310.0001(1.66, 3.15) 4 Cr2O3 0.4160.0024(1.04, 3.04) 5 Sc2O3 0.3470.6200(-0.93, 0.73) 7 V2O5 0.2590.0051(0.28, 1.13) 8 TiO2 0.7960.0000(6.08, 9.21)

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46 Table 4-4. Lattice paramete rs of sintered pellets Sample Dopant a0 () 1 Undoped 5.46706 2 Nb2O5 5.46689 3 Al2O3 5.46275 4 Cr2O3 5.46214 5 Sc2O3 5.46214 6 Y2O3 5.45939 7 V2O5 5.45721 8 TiO2 5.45756

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47 Figure 4-1. Optical images of pe llet surfaces post-sinte r. Pellet labels in top left corners.

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48 A B Figure 4-2. Undoped UO2. A) Before etching. B) After etchi ng. Images on the right are at higher magnifications than the images to their left.

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49 A B Figure 4-3. Nb2O5-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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50 A B Figure 4-4. Al2O3-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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51 A B Figure 4-5. Cr2O3-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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52 A B Figure 4-6. Sc2O3-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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53 A B Figure 4-7. Y2O3-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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54 A B Figure 4-8. V2O5-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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55 A B Figure 4-9. TiO2-doped UO2. A) Before etching. B) After et ching. Images on the right are at higher magnifications than the images to their left.

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56 Figure 4-10. EDS spectrum of undoped UO2 sample. Figure 4-11. EDS spectrum of Nb2O5-doped sample.

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57 Figure 4-12. EDS spectrum of Al2O3-doped sample. Figure 4-13. EDS spectrum of Cr2O3-doped sample.

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58 Figure 4-14. EDS spectrum of Sc2O3-doped sample. Figure 4-15. EDS spectrum of Y2O3-doped sample.

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59 Figure 4-16. EDS spectrum of V2O5-doped sample. Figure 4-17. EDS spectrum of TiO2-doped sample.

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60 CHAPTER 5 CONCLUSIONS AND FUTURE WORK 5.1 Pellet Processing The processing method used in this resear ch was successful in making dense pellets suitable for microstructural examination. The pr ocess was streamlined and repeatable and the resulting bulk pellet properties did not vary significantly. The initial cold pressing was unsuccessful as the stainless steel pieces we re neither polished e nough nor strong enough to create green pellets of accepta ble quality. The ICL KBr die set remedied this issue, producing high quality green pellets from the powders. The sintering process caused cracking in several pellets, most likely due to excessively quick he at up and cool down rates. This was not a hindrance for the remaining characterization but would be unacceptable for reactor fuel. 5.2 Grain Size Analysis The etching process for rev ealing grain boundaries in UO2 was wildly inconsistent between samples, even for different sections of the same sample. It was found that an excellent polish is required to have any chance of rev ealing grains, but even th is does not guarantee a successful etch. Both the sulfuric acid and nitr ic acid solutions were us ed successfully, but both of them failed more often than they succeeded. The discovery of an etchant that could provide repeated, high quality results would be extremely useful fo r this kind of analysis. Table 5-1 summarizes the results of the grain si ze analysis. As mentioned above, all of the average grain sizes measured in th is study are smaller than typical values found in other studies. The relative increases in grain si zes for each dopant are typically smaller as well. However, the increases do show that a very small addition of a dopant to UO2 powder can significantly increase the grain size of the sintered pellet, wh ich in turn can improve fission gas retention and swelling properties. This result should be kept in mind when designing high-burnup fuel for

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61 current and next generation reactors that utilize uranium dioxide as their fuel. The dopants that seemed to be most effective in this study were ni obia and titania, with tit ania showing the largest grain size increase of 279% over th e undoped sample. The yttria-dop ed pellet did not yield grain size measurements due to ineffective etching. Ho wever, this dopant cannot be discounted as it has never been investigated. Scandia proved to have no significant effect on the grain size, therefore it should not be considered further. 5.3 Future Work There is no shortage of further work to be done in the study of doped fuel. Factors that could affect the resu lts include powder mixing methods, dopant concentrations and solid solubility limits, cold pressing methods and pr essures, and sintering conditions. For mixing methods, it has been suggested by the work of B ourgeois et al. (2001) th at more homogeneous mixing can lead to further increases in grain si ze due to the better dist ribution of the dopant within the matrix of th e material. The ultimate attempt at homogenous mixing may be to coat the UO2 powder particles with the d opant, either through chemical vapor deposition or atomic layer deposition. This would provide further insight into the mechanisms of grain size increase due to the dopants. The obvious next step in this research is irradiation testing of doped fuel samples. Previous studies that have looked at irradiat ing doped fuel have in many cases confirmed a decrease in fission gas releas e and fuel swelling during irra diation, making possible higher burnups (Arborelius et al., 2006; Delafoy et al ., 2007; Harada, 1996; Kashibe et al., 1998; Killeen, 1975; Turnbull, 1974; Yuda et al., 1997). Other improvements due to the presence of dopants have been noted as well. De lafoy et al. (2007) found that doping UO2 with chromia improved the pellet-clad inter action properties of the fuel.

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62 Several possible drawbacks should also be l ooked for when investigating doped fuel under irradiation. Niobia, while being a good promoter of grain size in crease, has also been found to increase the rare gas diffusion coefficient in UO2 (Killeen, 1975). This ca uses the fission gases to diffuse more quickly within the fuel matrix, offs etting the benefits of the increased grain size. 5.4 Final Comments The possibility of low quantity dopant addi tions providing an ave nue to a high burnup UO2 based fuel is extremely attractive. Uranium dioxi de is the most qualified fuel on the planet and the ability to improve upon it by such a small modification must be appreciated. A fuel qualification program for doped-UO2 would not be nearly as intensive as one for a new fuel form. Thus, doped UO2 could improve fuel cycle economy fo r existing and future reactors in less than a decade. This is an option that should be pursued with furthe r research to fulfill its enormous potential.

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63 Table 5-1. Summary of grain size analysis. Sample Dopant Mean Grain Size ( m) St Dev ( m) % Error % Change 1 Undoped 2.730.3512.6% 2 Nb2O5 5.940.589.7% 117% 3 Al2O3 5.140.509.8% 88% 4 Cr2O3 4.770.5812.1% 74% 5 Sc2O3 2.630.3513.4% -4% 6 Y2O3 --7 V2O5 3.410.133.7% 25% 8 TiO2 10.371.1911.4% 279%

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64 APPENDIX POWDER CHARACTERISTICS The initial powder information was obtaine d from Dr. Jiwei Wang who analyzed the UO2 powder received from Framatome/AREVA. Th e initial oxygen-to-uran ium ratio was found by oxidizing the powder to U3O8 and measuring the weight change. The change indicated an initial O/U ratio of 2.10. The particle size distribution of the received UO2.10 powder was characterized by sieve analysis. Ten grams of the powder were sieved through a series of screens, which were then weighed. The analysis was done three times and th e average values are pl otted in Figure A-1. Particle Size ( micron ) Relative Number 0 25 50 75 100 125 150 175 200 225250 0 0.2 0.4 0.6 0.8 1 1.2 Figure A-1. Particle size distributi on of received uranium dioxide powder.

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65 LIST OF REFERENCES Abrams, H., 1971. Grain size measurement by th e intercept method. Metallography 4, 59-78. Ainscough, J., Raven, L., Sawbridge, P., 1978. Int. Sy mp. on water reactor fuel fabrication with special emphasis on its effect on fuel performance. Prague: IAEA-SM233. Ainscough, J., Rigby, F., Osborn, S., 1974. The effect of titania on grain growth and densification of sintered UO2. J. Nucl. Mater. 52, 191-203. Amato, I., Colombo, R., Petruccioli Balzari, 1966. Grain growth in pure and titania-doped uranium dioxide. J. Nucl. Mater. 18, 252-260. Amato, I., Ravizza, M., 1967. The effect of vanadi um oxide additions on sintering and grain growth of uranium dioxide. J. Nucl. Mater. 23, 103-106. Arborelius, J., Backman, K., Hallstadius, L., L imback, M., Nilsson, J., Rebensdorff, B., et al., 2006. Advanced doped UO2 pellets in LWR applications. J. Nucl. Sci. Tech. 43 (9), 967-976. Assmann, H., Dorr, W., Gradel, G., Maier, G., Peehs, M., 1981. Doping UO2 with niobia beneficial or not? J. Nucl. Mater. 98, 216-220. Aybers, M., Aksit, A., Akbal, S., Ekinci, S., Ya yli, A., Colak, L., et al., 2004. Grain growth in corundum-oxides doped uranium dioxide and eff ects of grain growth to the mechanical properties of uranium dioxide such as elastic ity determined by ultrasonic methods. Key Eng. Mater. 264-268, 985-988. Booth, A., 1957. A method of calculati ng fission gas diffusion from UO2 fuel and its application to the X-2-f test loop. AECL-496. Bourgeois, L., Dehaudt, Ph., Lemaigna n, C., Hammou, A., 2001. Factors governing microstructure development of Cr2O3-doped UO2 during sintering. Journa l of Nuclear Materials 297, 313-326. Brundle, C., Evans, C., Wilson, S., 1992. Ency clopedia of Materials Characterization. Greenwich: Manning. Delafoy, C., Dewes, P., Miles, T., 2007. AREVA NP Cr2O3-doped fuel development for BWRs. Proceedings of the 2007 International LWR Fuel performance Meeting. San Francisco, CA. DOE, 2008. Advanced Fuel Cycle In itiative. Retrieved from U. S. Department of Energy: http://www.ne.doe.gov/afci/neAFCI.html Harada, Y., 1996. Sintering behaviou r of niobia-doped large grain UO2 pellet. J. Nucl. Mater. 238, 237-243.

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66 Kashibe, S., Une, K., 1998. Effect of additives (Cr2O3, Al2O3, SiO2, MgO) on diffusional release of Xe-133 from UO2 fuels. J. Nucl. Mater. 254, 234-242. Killeen, J., 1980. Fission gas release and swelling in UO2 doped with Cr2O3. J. Nucl. Mater. 88, 177-184. Killeen, J. 1975. The effect of additiv es on the irradiation behaviour of UO2. J. Nucl. Mater. 58, 39-46. Leenaers, A., de Tollenaere, L., Delafoy, Ch., Van den Berghe, S., 2003. On the solubility of chromium sesquioxide in uranium dioxide fuel. J. Nucl. Mater. 317, 62-68. NEI, 2008. Resources and Stats. Retrieve d from U.S. Nuclear Power Plants: http://www.nei.org/resourcesandstats/nuc lear_statistics/usnuclearpowerplants/ Ohai, D., 2003. Large grain size UO2 sintered pellets obtaining used for burnup extension. Transactions of the 17th Inte rnational Conference on Structural Mechanics in Reactor Technology. Prague, Czech Republic. Petzow, 1978. Metallographic Etch ing. Metals Park, Ohio: American Society of Metals. Radford, K., Pope, J., 1983. UO2 fuel pellet microstrcuture modification through impurity additions. J. Nucl. Mater. 116, 305-313. Sawbridge, P., Reynolds, G., Bu rton, B., 1981. The creep of UO2 fuel doped with Nb2O5. J. Nucl. Mater. 97, 300-308. Turnbull, J., 1974. The effect of grain size on the swelling and gas release properties of UO2 during irradiation. J. Nucl. Mater. 50, 62-68. Yuda, R., Harada, H., Hirai, M., Hosokawa, T., Une, K., Kashibe, S., et. al., 1997. Effects of pellet microstructure on i rradiation behavior of UO2 fuel. J. Nucl. Mater. 248, 262-267.

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67 BIOGRAPHICAL SKETCH Brett Jameson Dooies was born in West Palm Beach, FL, on March 3, 1984. In 2002, he graduated from the Alexander W. Dreyfoos, Jr. School of the Arts ninth in his class. He attended the University of Florida, as an honors student from 2002 to 2006, where he participated in both academic and leadership activities. Brett was aw arded a Bachelor of Sc ience degree in nuclear and radiological engineering in December 2006, graduating cum laude He spent the next year on a graduate assistantship with th e University of Florida, beginning the initial stages of this research project. He was then awarded funding from the Advanced Fuel Cycle Initiative of the Department of Energy, a generous fellowship that allowed him to complete this research and present his findings at national conferences. From 2007 – 2008, Brett served as the student conference proposal chair for the UF chapter of the American Nucl ear Society, an endeavor that secured the annual ANS student conference for UF in 2009. In August 2008, he received his Master of Science degree in nuclear engineering sc iences from the University of Florida. In July 2008, Brett started as a member of General Electric’s Edison E ngineering Development Program in Wilmington, NC.


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