• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Previous work
 Experimental procedure
 Experimental data and results
 Discussion of results
 Conclusions
 Appendix A: Calculations
 Appendix B: Analysis for polymer...
 References
 Biographical sketch














Title: Fission product degradation and neutron moderating properties of fluorocarbons
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 Material Information
Title: Fission product degradation and neutron moderating properties of fluorocarbons
Physical Description: viii, 135 l. : illus. ; 28 cm.
Language: English
Creator: Scott, Thomas Howard, 1932-
Publisher: s.n.
Place of Publication: Gainesville
Publication Date: 1966
Copyright Date: 1966
 Subjects
Subject: Fluorocarbons   ( lcsh )
Radiochemistry   ( lcsh )
Nuclear Engineering Sciences thesis Ph.D   ( lcsh )
Dissertations, Academic -- Nuclear Engineering Sciences -- UF   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: l. 132-134.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097877
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000421890
oclc - 11021108
notis - ACG9888

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Table of Contents
    Title Page
        Page i
        Page i-a
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
    List of Tables
        Page iv
    List of Figures
        Page v
        Page vi
    Abstract
        Page vii
        Page viii
    Introduction
        Page 1
        Page 2
        Page 3
    Previous work
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
    Experimental procedure
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
    Experimental data and results
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
    Discussion of results
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
        Page 94
    Conclusions
        Page 95
        Page 96
        Page 97
        Page 98
    Appendix A: Calculations
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
        Page 119
        Page 120
        Page 121
        Page 122
        Page 123
        Page 124
        Page 125
        Page 126
        Page 127
        Page 128
        Page 129
    Appendix B: Analysis for polymer in irradiated samples
        Page 130
        Page 131
    References
        Page 132
        Page 133
        Page 134
    Biographical sketch
        Page 135
        Page 136
        Page 137
Full Text







FISSION PRODUCT DEGRADATION AND

NEUTRON MORDERATING PROPERTIES

OF FLUOROCARBONS


















By
I H'i:AS H'-,:'.; R) SCOTT










I* ['::L .T'.T.:N intr.F:i TF T THE GRADUATE COUNCIL OF
TiHf F lNr LFP: T' OF FLORIDA
1.i f AI.TLLL Fi.LI LLi lENT OF THE REQUIREMENTS FOR THE
Lii.:PL *:-F DOCTOR OF PHILOSOPHY









LN I'. ERSITY' OF FLORIDA


April, 1966












ACi: :rO'JLEDGENr'IT S


The author wishes to express his appreciation to

Dr. John A Wethinmton, Jr.. chairman of the supervi-

sory committee, for his untirinr guidance, assistance,

and encoura.emrent without which this work would have

nor. been Cossible.

The assistance ,f Mr L. D. Butterfield in the

performance of the reactor irradi.aions and the as-

sisrance of .Lr. J A. r'acLean in the performance of th.

,amrma irradiitions are acknou.ledred.

Dr. T. V. Reed provided the distillation equipment,

and Dr. F. F. Parkinson assisted in the neur.orn pulsin<

experiment .

The imars sp.ectrometric analy3se3 were performed

by Dr. iM. P. F3allatter and Dr. R. J. Hanrahin.

The nuclear frma;neric resonance measurement s were

made b" Dr. ';. 3. Prey, and the infrared specrra were

measured -," Dr H C Prown.

The author is indebted tc the Oak Rid,!e Institute

of Nuclear Studies and to the Collece of En-ineerinz of

the University of Florida for fellowship assistance.

Finally, the patience, enccurscement, and persever-

ance of the author's wife, Janet, during, the preparation

of this rranuscript are rratefully acknowledged.

ii














TABLE OF CONTENTS


ACKNOWLEDGMENTS . .

LIST OF TABLES . .

LIST OF FIGURES . .

ABSTRACT . . . .


Chapter

I.

II.

III.

IV.

V.

VI.


INTRODUCTION . . . . . .. 1

PREVIOUS WORK . . . . . ... 4

EXPERIMENTAL PROCEDURE . . . .. 18

EXPERIMENTAL DATA AND RESULTS ... 41

DISCUSSION OF RESULTS . . . ... 56

CONCLUSIONS . . . . .... . . 95


Appendices

A. CALCULATIONS . . . . .

B. ANALYSIS FOR POLYMER IN IRRADIATED
SAMPLES . . . . . . .

LIST OF REFERENCES . . . . . . .

BIOGRAPHICAL SKETCH . . . . . . .


Page

. . . . . . ii

. . . . . . iv

. . . . . . v

. . . . . . vii









LIST OF TAPL[S


Table Fa' e


1. Suimirar,' of .onlu3iii-ns in Feferenc:es 1
Through 20 . . . . . . . . 9

2. Results of Anal'.'ses of CgF16 . . . . 2

3. Decomposition Percentages and Gaseous
Product Distribution for Gamma Irradiation
of C F16 . . . . . . . . .

4. G Values for Gamma Irradiation of CgF1 . .7

5. Decomposition Percentages and Gaseous
Product Distribution for Reactor
Irradiation of CF16 . . . . . . L9

6. Decomposition Percentages and Gaseous
Product Distribution for F3actor
Irradiation of C8F16 and UF6 . . ... 1

7. G Values for Reactor Irradiation of CgF-'
and UF6 . . . . . . . .. .

8. Geometric Buckling and Decay Con3tantz for
Pulsed Neutron Experiment. . . ... :.

9. Data for Figure 11 . . . . . . .

10. Macroscopic Absorption Cross Sections of
Moderators . . . . . . . . 9L

11. Supporting Data for Figure 20 . . .. 107

12. Supporting Data for Figure 21 . 108

13. Supportin C Data for Fi,:ure 2 . .. ... 109









LIST OF FIGURES


Figure Page

1. Chromatogram of Crude C8F16 . . . 19

2. Chromatogram of Distilled C8F16 . 22

3. Metal Vacuum System . . . . . 24

4. Metal and Glass Vacuum Systems . . .. 25

5. Capsule Fluorination System . . . 28

6. Glass Vacuum System . . . .... 37

7. Neutron Pulsing Experimental Equipment 38

8. C8F16 Decomposed to Gas in Gamma
Irradiation . . . . . . ... .60

9. C8F16 Converted to Polymer in Gamma
Irradiation . . . . . . . 61

10. Log (N x 100) Vs. Dose for Gamma


NO
Irradiation of CgF6 . . . . . 64

11. Log ( x 100) Vs. Dose for Other
Fluorocarbons . . . . . . .. 68

12. Dose-Decomposition Relationship for
Fluorocarbons Corrected for Effects of
Lighter Products . . . . .... .72

13. C8F16 Decomposed to Gas in Reactor
Irradiation . . . . . . ... .74

14. C8F16 Decomposed to Gas in Gamma and
Reactor Irradiation . . . .... 75

15. Analysis of Pulsed Neutron Experimental
Data . . . . . . . . . . 86









FiF ure


Fa e


16. Counts Per Channel Vs. Time for CF~1 .F 1

17. Counts Per Channel Vs. Time for Empty'
TanR . . . . . . . . Q

19. Counts Per Channel for CFP i'inus Counrs
Per Channel for Empty Tank Vs. Time . 91

19. Decay Constants Vs. B2

20. Chromatogram of Cases from Sample No. 36 10L

21. Chromatogram of Sample No. 36 in Vapor
Phase . . . . . . . . . 10

22. Chromatogram of Sample No. 36 in Liquid
Phase . . . . . . . . . 10-













Abstract of Dissertation Presented to the Graduate Council
in Partial Fulfillment of the Requirements for
the Degree of Doctor of Philosophy

FISSION PRODUCT DEGRADATION
AND NEUTRON MODERATING PROPERTIES OF FLUOROCARBONS

By


Thomas Howard Scott

April, 1966


Chairman: Dr. John A. Wethington, Jr.
Major Department: Nuclear Engineering

The radiolysis of perfluorodimethylcyclohexane

by gamma radiation and by reactor radiation was studied.

The primary product was a dimeric material with small

amounts of gaseous products including CF4, C2F6, C2F60,

C3Fg, and C3F0O. The production of dimer and gaseous
products was found to be a linear function of the total

dose.

An expression was derived which related the frac-

tion of sample remaining after exposure to a known dose.

This expression was modified to correlate all available

data for the irradiation of alicyclic and aliphatic

fluorocarbons. A separate expression was found for

vii









aromatic fluorocarbons. The expressions hold over a

wide ranre of total doses, dose rates, and for several

different types of radiations.

The decomposition of CjFlI in a solution of CjFlI

and UW. exposed to reactor irradiation was investigated.

Techniques were developed for the preparation of solu-

tions and for the subsequent separation of the CF1,'

from the JUFr and the fission fraj.ments. The primary

product were CF4, C-.FP, and C-F,0 in roughly equal

proportions. lio dimeric material was found. The kinetics

of this reaction supported the assumption that fiZsion

fragments shatter the ,FIr, into fragments which are then

saturated with fluorine present in the system.


viii













CHAPTER I


INTRODUCTION


During the past ten years considerable effort has

been devoted by the Atomic Energy Commission towards

the development of a nuclear reactor in which a hydro-

carbon liquid is used as the working fluid. During the

past ten years there has also been a large scale devel-

opment of the fluorocarbons. The proposal has been made

that fluorocarbons might be used as the working fluid

in a nuclear reactor (1).

This proposal is based on several factors. First

of all, the hydrocarbons tend to form complex compounds

with the iron in the walls of the colder regions of the

reactor system (2). These carbon-iron compounds then

tend to be deposited on the heat transfer surfaces of

the reactor fuel and seriously retard the rate of heat

transfer from these surfaces. The fluorocarbons are

generally considered to be much less reactive than

the hydrocarbons; therefore, this corrosion problem

might be eliminated. Secondly, uranium hexafluoride

is soluble in fluorocarbons. Thus a possible reactor

concept could be developed in which the uranium fuel

is dissolved in a fluorocarbon working fluid.

1








The uranium hexafluoride-fluorocarbon reactor

would have several distinct advantages. First of all,

uranium hexafluoride is the uranium compound used in

the gaseous diffusion process for producing enriched

uranium. Thus, the material could be used without fur-

ther processing,and the conversion and fuel fabrication

costs for solid fuels could be eliminated. These costs

are a significant portion of present fuel cycle costs.

Secondly, by means of a side stream, the fuel could be

continuously reprocessed and new fuel added. The need

for lengthy shutdowns for refueling would be eliminated

and the offsite reprocessing of solid fuel elements

would not be necessary. A final possible advantage of

such a reactor mieht be the radiation induced pro-

duction of fluorocarbons which can not be produced by

conventional chemical processes because of the nonre-

active chemical properties of the fluorocarbons. The

fissioning of uranium in a solution would produce fission

fragments of high energy. The possible decomposition of

the fluorocarbon and the subsequent recombination of the

various decomposition products might produce new and

useful fluorocarbons. In this consideration,the uranium

hexafluoride-fluorocarbon reactor would be closely akin

to the chemonuclear reactor which has been discussed for

several years. The chemonuclear reactor uses radiation-








induced chemical reactions or dissociations to produce

desired chemical compounds.

Obviously, much information regarding the per-

formance of fluorocarbons and uranium hexafluoride-

fluorocarbon solutions in the presence of radiation

must be obtained before any use is made of these

materials in reactors. Among the types of information

needed are the rates of decomposition, the products

formed, the possible effects of the evolution of

fluorine or hydrogen fluoride, and the compatibility of

fluorocarbons and uranium hexafluoride with regard to

possible side reactions. It is also desirable to know

the neutron moderating and absorbing properties of the

fluorocarbons.

The purpose of this work is to investigate the

effects of ionizing radiation on the fluorocarbon,

perfluorodimethylcyclohexane (C8F16), the performance

of CgF16-UF6 solutions in a reactor flux, and the
neutron moderating and absorbing properties of C8F16.












CHAPTER II


PREVIOUS WORK


Radiation Chemistry of Fluorocarbons


Fluorocarbon chemistry is a relatively new branch

of chemical science. Before 1937, perfluoromethane and

perfluoroethane were the only positively known fluoro-

carbons. In 1937 (3) and in 1939 (4), Simons and

Bloch reported the preparation and properties of

fluorocarbons containing more than three carbon atoms.

These compounds, which ranged from perfluoropropane to

perfluoroheptane, were reported to be stable both chemi-

cally and thermally. During the early stages of the

Manhattan Project the need arose for materials which

could be used as sealants and coolants in mechanical

systems containing corrosive uranium hexafluoride. Since

all other materials which had been tested had been found

to react with uranium hexafluoride, Simons (5) suggested

in July, 19LO, that the recently developed fluorocarbons

might be suitable for these applications. In December,

1940. Simons sent a two-cubic centimeter sample of liquid

fluorocarbon, virtually all of the material available,

4






5


to Columbia University for testing. These tests showed

that fluorocarbons had the desired properties (6), and an

extensive research program was then started to find the

most desirable fluorocarbons for the various applications

and to develop processes for producing these fluorocar-

bons. Thus by the end of the second World War,

fluorocarbon chemistry was established as a distinct

branch of chemical science.

The best known of the fluorocarbons commercially

produced in the post-war era was Teflon. A series of

papers published in the period between 1953 and 1956

(7, 8, 9) indicated that Teflon had very poor radiation
resistance. Indeed Teflon was found to be severely

damaged at a dose of 3 x 106 roentgens (9), and unfor-

tunately this property was generally extended to all

fluorocarbons without any real experimental evidence on

fluorocarbons other than Teflon. An additional negative

factor was the liberation of fluorine during the irradia-

tion of Teflon (5). The corrosion problems related to

the presence of fluorine gas in a mechanical system would

seriously restrict, if not prohibit, the use of fluoro-

carbons in ionizing radiation fields.

Slowly, experimentaldata were published which showed

that all fluorocarbons were not as sensitive to radiation

as Teflon. In 1958, Feng reported that the gamma irradi-

ation of mixtures of partially fluorinated hydrocarbons








and hydrocarbons and mixtures of fluorocarbons and hydro-

carbons indicated that the organic fluorine compounds

showed a much greater resistance to radiation than did
the analogous chlorine, bromine, and iodine compounds (10).

Also in 1660, Wall, Florin, and Brown (11) reported that

C6F6 had radiation stability approaching that of C6H6

and suggested that C6F> be used for the preparation of

radiation resistant polymers. Mixtures of CjF6 with

hydrocarbons were less stable than the parent material,

and the difference was attributed to the production of

hydrogen fluoride in the hydrocarbon-fluorocarbon

mixtures.

The first reactor irradiations of fluorocarbons

were performed by Simons and Taylor (12) who irradiated

the saturated fluorocarbons--C-Fl6, CGF160, and (CF9)-N--

in a reactor flux of 5.5 x 1011 neutrons/cm.2 sec. for

four weeks. The G value (number of molecules produced

or destroyed per 100 ev. of energy absorbed) was

calculated to be 2 or 3 molecules transformed per

100 ev. absorbed with a total energy absorption of 2.2

x 10 ev. per gram. No fluorine or CFL were observed,

and there was no evidence of corrosion, since the

inner walls of each capsule were bright and clean after

irradiation.

Concurrently, Wall and Florin (13) found that the

tensile strength of irradiated Teflon decreased very









rapidly in the presence of oxygen, but the loss of ten-

sile strength in the absence of oxygen was hardly

perceptible for long periods of irradiation. This find-

ing suggested that the reported radiation instability

of Teflon was due to oxygen impurities.

Bloch, MacKenzie, and Wiswall (14) used electron,

gamma, and reactor sources to irradiate C6F6, C12F10,

and C10F18. The two aromatic compounds had overall

stabilities of the same order as their hydrocarbon

analogs. Even though the fluorocarbons tended to undergo

more reaction to form high molecular weight compounds

than the comparable hydrocarbons, the fluorocarbons were

found to be more stable to fragmentation leading to

gaseous products. The gas yields for the alicyclic com-

pounds were much higher than the aromatics, but polymer

yields were of the same order or possibly lower. This

observation was in direct contrast to the hydrocarbons

where the alicyclics are much less stable to irradiation

than are the aromatics. Gas chromatography was used to

analyze the radiolysis products.

In 1962, Heine (15), using doses of 5 x 108 rads,

studied the gamma radiolysis of C8F18, (C4Fg) N,

and c-CeF160 ; the stability of these fluorocarbons was

shown to be greater than the radiation stability of the

analogous hydrocarbons. During 1964 and 1965,five papers

were published which added greatly to the understanding









of the effects of radiation on fluorocarbons. The

conclusions given in references (14) through (20) are

summarized in Table 1.

MacKenzie, Bloch, and Wiswall (16) performed radi-

olysis experiments on a series of highly purified cyclic

fluorocarbons--C6 F, C12F10, and C10 F-- and on their satu-

rated analocs--CF12, C12-F-, and ClF18,. The materials

were irradiated to doses of the order of 10 rads in

nickel cells usinc a 1.5 Mev. Van de Graff electron

beam. The irradiations were carried out at temperatures

chosen so that the materials were in the liquid phase.

It was concluded that the radiation stability of the

aromatic compounds was less than that of the correspond-

ing hydrocarbons; whereas the alicyclic fluorocarbons

were more stable than their hydrocarbon analogs. The

total C values for destruction of starting material

were very similar for aromatic and alicyclic compounds

and ranged from 1.3 to 2.4. Considerable amounts of

eases and products of molecular weight lower than the

startinE material were found in the radiolysis of

allcyclic compounds; whereas polymeric material was

almost exclusively yielded in the irradiation of aro-

matic fluorocarbons,with only traces of gaseous and

low molecular weight compounds being formed. No free

fluorine was detected in any of the irradiations.

In an extension of the aforementioned work, Bloch











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and MracKenzie (1?) used a Co(- source to irradiate a series

of fluorocarbons at elevate.d temperatures generally y ahout

4500 C.) to study the combined effects )f thermal decompo-

sition and radiolytic decomposition. The results indicated

that for the aromatic series there was no marked increase

in decomposition with temperature, however, a Jose of less

than 10- rads at L,5', almost completely destro',yed C1 F',

(perfluorobicyclohe:.--'l). ,rhe only alicyclic 'luorccarbon

studied. It was noted that C1F-,-) mi-ht not be typical of

alicyclic fluorocarbons (i.e., the bond joininri the rinrs

seemed especially vulnerable), but it was supposed that

the results indicated that, as a class, the alicyclic

fluorocarbon would not stanJ the combined effects of hith

temperature and hich radiation Jose.

The performance of C12F,, (perfluorobiphenvl) was

particularly rood in that its radiolvtic decomposition was

still reasonably snail at 5.0:' C. For comparison,the hy-

droarb.on terphenyls hecan to show drastic radiolytic

decorrmposition at temperatures over 4,50: C.,and ar. 500 C.

the thermal decomposition rates were of the order of 5.

per cent per day.

The radiolysis of C-F. usinr a gamma source has been

studied by h:evan and Hamlet (19) who made the following,

comparison between the radiolsis of saturated fluoro-

carbons and hydrocarbons.

(a) C-.,F was about one-fifth as sensitive to radia-

tion decomposition as was C,-H,. The G value for C-,Fe








was 1.9 while the G value for C2H6 was 9. The causes for
this apparent radiation stability included the absence of

F abstraction in fluorocarbons, the possibility of back

reactions of free radicals to reform C2F6, and the absence

of excited molecule decompositions which would have given

molecular fluorine. Molecular ejection of hydrogen from

excited molecules was shown to make important contribu-

tions to product formation in the alkanes. It was

suggested that similar excited molecule reactions in

perfluoroalkanes were unimportant.

(b) Hydrogen was unreactive in alkane radiolysis;

whereas fluorine was a reactive product in perfluoro-

alkane radiolysis. This dissimilarity may have been

caused by the great difference in bond strength and by

the exothermic reaction of fluorine with perfluoroalkyl

radicals.

(c) Linear alkanes were characterized by large yields

of hydrogen while perfluoroalkanes were characterized by

large yields of CF .

(d) The radiolysis of linear fluorocarbons may

produce cyclic compounds; however, no cyclic compounds

are produced in the radiolysis of linear hydrocarbons.

The radiation chemistry of C6F12 and c-C F8 was in-

vestigated by Fallgatter and Hanrahan (19). The gamma

doses used were from 0.2 Mrads to about 8 Krads. The

purpose of using doses lower than those used by previous








invest iLators was to study the initial products and the

initial product yields. At low doses dimeric fluorocar-

bons were found to te the predominant products of the

ra~nia radiolvis of CAF%2. The initial C value for CF 12

for products lighter than the parent compound was Eiven

as C0.', and the '3 value for products heavier than the

parent compound was civen as 3.1. The latter '3 value was

broken down into C = 2.2 for C12F2- (perfluorobicyclohexyl)

3 = 0.54. for C,1 F,2 (Ferfluoroc'clohe:-,lhexene), and C =

0.22 for ,-122 (perfluorocyclche:'lIhexane) These total

C values compared favorably with those given by MIacKenzie

et al. (1 ) of C0.. for lighter products and 2.1 for heav-

ier products. Portions of the analysis for this work

were performed on a F"as chromatocraph-mass spectrometer

combination. The nethod of separating a specific ras

front' the radiolysis products by means ,of the r'as chroma-

tograFh and then injecting the separated cas into the mass

spectromieter for analysis was discussed.

Although Fallcatter and Hanrahan felt that extended

speculation about mechanism was premature, it was

possible to make some generalizations. The treat strength

of the C-F bond compared to the C-H bond and the weak-

ness of the F-F bond as com-pared to the H-h bond were two

of the major differences between the fluorocarbons and hy-

drocarrons; however, since more than .?0 per cent. of the

observed hond rupture in the radiolysis of liquid CF12

involved C -F rather than C-C bonis, it was apparent








that the greater strength of the C-F bond was no deterrent

to its rupture. It was pointed out that C6F12 was

probably not a typical compound since C6H12 showed far

less C-C bond rupture than hydrocarbons in general.

Even so, the assumption that the radiation chemistry of

perfluorocarbons was centered in C-C bonds rather than

C-F bonds was to be avoided.

Abstraction from the fluorocarbon substrate by F

atoms to make F2 was essentially impossible because of

the weakness of the F-F bond. Consideration of this

factor alone showed that the radiation chemistry of fluoro-

carbons should be much different than that of hydrocarbons.

The lack of C6F10 in the radiolysis products tended to

confirm the lack of fluorine abstraction. Most proposed

mechanisms predicted the production of fluorine and
C 6Fo, and even though the fluorine might have reacted with

other products or the walls of the system, the C6F10

would not. A consideration of the reactions to produce

fluorine and C6F10 showed that the reactions were endo-

thermic and were quite unfavorable thermodynamically.

Reed, Mailen, and Askew (20) irradiated a series of

pure fluorocarbons using three different radiation

sources. Fluorocarbons ranging from CF4 to 2,3-(CF3)2C Fg

were irradiated in a Co60 gamma source, in the Low In-

tensity Test Reactor at Oak Ridge National Laboratory, and








in the Oak Ridge Graphite Reactor. In addition, the

radiolysis of mixtures of CFL and C2F6 was studied.
The gamma irradiations gave G values for the fluoro-

carbons which were considerably less than those of the
analogous hydrocarbons especially for the lower members
--CFL, C2F6, and C3F3. The highly branched structure

2,3-(CF3)2C F8 was the only exception, and it had a
total G value comparable to those of the hydrocarbon
alkanes.


Radiation Chemistry of Uranium Hexafluoride

The radiation stability of UF6 must also be consid-

ered. The use of UF6 in gaseous diffusion plants

necessitated studies on the critical sizes of various
configurations containing UF6. Snell and Rush (21)
determined the multiplication factor for 300 lb. product

drums containing 1.1 and 0.536 per cent U235. Grueling et al.
(22) studied the critical dimensions of water tamped

slabs and spheres of UF6, and Bull (23) calculated the
critical mass of a UF. core with reflectors of D20, Be,
and C. However, all of these efforts were concerned

with potential criticality incidents and not with the

use of UF6 in a reactor as fuel. Bernhardt, Davis, and
Shiflett (24) irradiated UF, with alpha particles and

found that intermediate solid fluorides and fluorine








were formed. It was also found that these products tended

to recombine,giving a net G value of one molecule of UF6
decomposed per 100 ev. of energy absorbed. Dmitrievskii
and Migachev (25) reported that the neutron irradiation
of UF6 yielded UF5 and free fluorine with a G value of

0.5 molecule of UF6 decomposed per 100 ev. of energy
absorbed in the gas. An equilibrium was found to exist
between the UFg and the UF5 and fluorine products with
the equilibrium concentrations being a function of the
dose rate. A further significant result was the fact that

the decomposition of the UF6 was completely prevented by
the addition of 25.5 per cent fluorine to the UF6. This

result had been previously predicted by Goodman (26).
Thus it appears that the radiation stability of UF6 may
not be good, but it can be brought up to acceptable

levels by the addition of fluorine to the UF6. In fact,
the operation of a UF6 reactor has been described by
Kikoin et al. (27) in which the decomposition of the
UF6 was controlled by the addition of chlorine tri-
fluoride to the fuel to serve as a source of fluorine.












CHAPTER III


EXPERIMENTAL PROCEDURE


Distillation

Approximately 50 liters of COF,6 were obtained

from the Oak Ridge rational Laboratory. No information

was available on the previous history of the CRFl6 or

on its purity. Chromatozraphic analysis of the crude

CgF16 indicated that there were several impurities as

shown in Figure 1. In addition, the material had a

yellowish cclor and a distinct odor which are not

characteristic of fluorocarbons. The material was also

analyzed using nuclear magnetic resonance measurements

and infrared spectra measurements. The results for

these analyses are shown in Table 2.

The impurities in the CpF16 were removed by a

batch-type distillation using a distillation apparatus

consisting of a 2.5 liter pot with a packed column 100

cm. hich. The column had a diameter of 1.5 cm. and was

packed with stainless steel helixes, and two indepen-

dently controlled electric heaters were used to maintain

the column temperature at the desired levels.


































0
4-) i
C.r
4 ~ ~
to


CL.




C-,
0





H a0


E
m0






too

N






0
\ E c.














0H
r-is






4-))
4-4











x0
I~' I I




r-1l

c4)-.

r(4

0 C..













TABLE 2


RESULTS OF ANALYSES OF COFI,


Material

Distilled C;?F


Crude CFl6


Infrared ;Measurements

Wave Length
(microns)


;. 7


C-H BonIs
(Relative units)

1.5

- ,'


Nuclear Magnetic Resonance Measurements

1o. of Hydrogen IHydrogen
Material Atoms per molecule (Wei-ht per cent)


Distilled CF'


0. 01


0.25


Material

Distilled CF16


Distilled CdF1,


Elemental Analysis

Hydrogen Fluorine
(ieicht per cent) (Weipht per cent)

2 .12 76T.16


2L.01 f7.94









A 1.5 liter batch was charged into the distilla-

tion pot at room temperature, and the pot temperature

was raised until the charge began to boil. Concurrent-

ly the column temperature was increased to 1000 C. for

the upper half of the column and to 1040 C. for the

lower half of the column. The product was taken off

at a rate of about 10 drops a second until the tem-

perature of the product reached 95.00 C., and the pot

temperature increased to about 1150 C. The product

taken in this temperature range consisted of two im-

miscible liquids. The lighter phase was yellowish and

had an odor somewhat like benzene while the heavier

phase was colorless and had an almost undetectable

odor.

The next cut, taken between product temperatures

of 100.70 C. and 102.1 C. at a flow rate of 4 drops

per second, was the purified C8F16 which was used in the

experimental work. The pot temperature increased from

1190 C. to 1220 C. during this cut,with approximately

1.0 liter of purified material being obtained from

each 1.5 liter of raw materials charged to the pot.

The distillation significantly decreased the amount of

impurities as shown by the chromatogram in Figure 2.

The analyses of the distilled material are given in

Table 2.




































































Pt





x I'


4, ~




.2 -


LL

C:












r,

t'



C,

0


F- L.
.C





33

LC.

I-










C









Fluorination


Since UF6 was to be used in some experiments, it

was necessary to fluorinate all surfaces which were to

be exposed to UF6 with fluorine gas in order to form a

protective fluoride film. This film prevents the reac-

tion of the UF6 with the metal surfaces to produce UF4

which would adhere to the surfaces and cause difficulties

in sample transfers and in material balance calculations.

The vacuum system shown in Figures 3 and 4 was fluo-

rinated using fluorine gas from a 1-pound fluorine gas

cylinder obtained from the Matheson Chemical Company. All

components except the bellows seal valves of the sys-

tem were carefully washed in carbon tetrachloride in

order to decrease all surfaces which were to be in

contact with fluorine. The bellows seal valves were

Nupro type B-4H bellows valves which were specially de-

signed for systems in which no leakage could be permitted.

These valves were cleaned by the manufacturer. The sys-

tem was pressure tested three times with a nitrogen

pressure of 60 psig. and vacuum tested five times for

periods from one to twelve hours. For fluorination, the

sodium fluoride traps were replaced with sodium chloride

traps. The outlet of the sodium chloride trap used for

capsule analysis was led to a glass bubbler containing

a solution of 20 per cent potassium hydroxide which was

vented to the hood.

















a i



L






I j I
Q-


0


12

E
0 L
s: L
'-i 2-


< ',
.-0


- I


3E

> .
cV) fj
- ^ -- tx -

r-'


- -

o
Cua ar





4rr,














co





L.
L ,


L



















am
T-
o -
2 G
a -
C r-
















L *l l r


L.'
E-C



o -'-4

U,
- -I
c?


CuL-
















- a a -- -f


E

4.)










cl







0g


4-)









The fluorine gas was introduced into the vacuum

system through a bellows seal valve on the vacuum rack.

A specially designed fluorine regulator (Matheson model

# 15F-670) was used to control the flow of fluorine.

This valve permitted the addition of small quantities of

fluorine to a carrier gas which in this case was nitro-

gen. A sodium fluoride trap was placed between the

regulator and the vacuum system to remove any hydrogen

fluoride which was present. The sodium fluoride trap

was heated under vacuum for one hour with a Eas-air

torch to activate the surfaces of the sodium fluoride

pellets. A tape heater was used to maintain the trap

at a temperature of 100 C. during the fluorination.

The fluorination operation was commenced with es-

tablishment cf a steady nitrogen flow. The rate of flow

was adjusted so that the flow of nitrogen in the bubbler

was as large as possible without causing the potassium

hydroxide solution to bubble out of the bubbler. The

nitrogen pressure on the system was about g psig.

The fluorine regulator valve was opened so that the

fluorine pressure between the fluorine regulator and

the fluorine control valve was about 12 psi. The

control valve was then opened for one-second intervals

approximately every two minutes for thirty minutes. The

interval for which the control valve was open was slowly









increased during the next hour so that at the end of

the hour the interval was fifteen seconds. The fluorine
control valve was then opened for intervals of one min-

ute every five minutes for thirty minutes. During the

operation, the sodium chloride trap temperature was mon-

itored with a slow increase in temperature being observed.

The system was purged with nitrogen and allowed to stand

overnight. Final purging was accomplished by removing

the glass bubbler, filling the entire vacuum system to

50 psig., and then allowing the nitrogen to blow off

quickly.

The vacuum system was then evacuated so that the

nitrogen pressure in the system was about 0.5 atmos-

phere, and the glass bubbler was reconnected. The

fluorine control valve was then opened, and the pressure

in the vacuum system was brought up to about 1.0 atmos-

pheres. The system was then completely isolated and

allowed to stand for thirty minutes. A very low nitro-

gen flow was established for thirty minutes after which

the pressurizing and blow off procedure described above

was repeated.

A system which could be used to fluorinate capsules,

valves, and other fittings was built as shown in Figure 5.

All items were carefully degreased with carbon

tetrachloride,and, in addition, the capsules were immersed

























CL


j. E

!-11


L






OJ
0E
Cl)









three times in boiling water to remove any remaining

silver solder flux. The capsules and fittings were

fluorinated in the reaction vessel while the valves were

fluorinated by being inserted into the nitrogen-fluorine

stream between the reaction vessel and the sodium

chloride trap.

The system was pressure tested to 50 psig. with

nitrogen for one hour. The pressure was quickly re-

leased with the glass bubbler disconnected to purge air

from the system. The pressurization and release operation

was repeated three times. A steady flow of nitrogen

was established with the bubbler in place, and the

fluorine pressure on the regulator outlet was raised

to 12 psig. Fluorine was introduced into the system

by opening the outlet valve for a period of one second

at intervals of one minute. After thirty minutes the

period during which the outlet valve was opened was

increased to five seconds with the intervals that the

valve was closed still being one minute. After another

thirty minutes fluorine was introduced for a one-minute

period, and the system was completely isolated and

allowed to stand for one hour. The purge was then re-

established for fifteen minutes. The glass bubbler was

disconnected, and the previously described pressuriza-

tion and release operation was carried out three times.










Capsule Fabrication


Capsules for Ganmma Irradiation3

The capsules for the pamma irradiations were fab-

ricated frru a ?-in. section of one-half in. copper

tubing. One end of the capsule was sealed h:, crimpin1t

and silver solderinr the copper tubing, and the other

end was sealed with a brass *'ite.: valve (1-VNA)

attached to the tube with a brass Swazelok fitting-.



Capsules for Reactor Irradiations

The capsules for the reactor irradiation were

fabricated from a 5-in. section of one-half in. alumi-

num tubing. One end of the capsule was sealed ,,by

crimping and heliarc welding the aluminum tubing,and

the other end was sealed with an alumiinum Whitey valve

(I-W'Al) attached to the tuting with an aluminum

Swagelok fitting. All reactor capsules were tested wi.h

a nitrogen pressure of 300 psic. All components f

the capsule were decreased with carbon tetrachloride

and fluorinated.








Sample Preparation

Preparation of C8F16 Samples

The C8F16 samples were prepared using the vacuum

system shown in Figure 3. The capsules were attached

to the system and both pressure and vacuum tested. The

calibrated burette was filled with C F16 which had been

degassed by vacuum transfer from one glass bulb to an-

other glass bulb for a total of three times. After each

transfer the dissolved gases were pumped to vacuum. The

C8F16 sample was vacuum transferred to the capsule, and
the size of the sample was determined by difference on

the calibrated burette.

A gold wire weighing 1 mg. was wrapped in tissue

paper and attached with tape to each capsule which was

to be exposed to reactor radiation. The gold wires had

previously been weighed to the nearest 0.001 mg.


Preparation of CgF16 UF6 Samples

Samples containing CgF16 and UF6 were also prepared

with the vacuum system. The UF6 was obtained from the

Allied Chemical Company and was found to contain some

dissolved air. Most of the air was removed by vacuum

transferring the UF6 from the storage tank to the cali-

brated tank and back to the storage tank. During each

transfer the UF6 was sublimed and recondensed,with the









dissolved air bein7 partially released. After each

transfer,the tank containing the UF6 was pumped to vacuum

while liquid nitrogen was still on the bottom of the

tank.

The volume of the calibrated tank was found by

filling the tank with water in a constant temperature

bath (250 C.) and then weighing the tank and water.

Subtraction of the empty tank weight from the full tank

weight gave the i.ei,:ht of the water in the tank. The

weight of the water was then converted to volume using

the density of water at 25' C., and the volume was found

to be 758 cc.

The calibrated tank was filled with UF5. by opening

valves as necessary, anJ allowing the system to equili-

brate for four minutes at room temperature. The valve

on the calibrated tank was then closed, and liquid

nitrogen was applied to the nipple on the storage tank

to recondense all UFP gas in the system other than that

in the calibrated tank. Using the vapor pressure of

UF. as given in Reference 28 for the observed room tem-

perature, the weight of the UF6 sample was calculated

by the ideal gas law. After five minutes the manometer

valve was cracked to determine that all of the UF, had

been condensed in the tank, and the valve on the storage

tank was then closed.









The UF6 was transferred to the capsule by opening

the capsule and the calibrated tank valves and applying

liquid nitrogen to the bottom one-half inch of the cap-

sule for two minutes. The manometer valve was opened,

and the residual gas pressure (generally about 3 mm.)

was checked to ascertain that the transfer had been

completed. The calibrated tank valve and the capsule

valves were closed, and the system was pumped to vacuum

with the liquid nitrogen still on the bottom of the

capsule. The vacuum valve was closed, and the cali-

brated burette valve was opened so that the C8F16 could

be transferred to the capsule. After the desired

amount of C8F16 had been transferred to the capsule, the

capsule valve was closed, and liquid nitrogen was applied

to the calibrated glass burette. After the manometer in-

dicated that the CsF16 had been condensed, the calibrated

burette valve was closed, and the burette was allowed to

come to room temperature. The amount of C8Fl6 trans-

ferred was then determined by difference.

A gold wire weighing about 1 mg. was attached to

the capsule as discussed in the previous section.








UFTR Irradiation

The reactor irradiations were performed in the

University of Florida Training Reactor which has been

described by Boynton (29).

The plastic handles on the capsule valves were re-

moved, and the capsules to be irradiated were inserted

into an aluminum cannister which was 10J in. long with

a 3-in. outside diameter. A 1-ft. section of a graphite

stringer with a L-in. x i-in. cross section was removed

from the thermal column of the UFTR. This stringer was

immediately adjacent to the core. The aluminum cannister

was inserted in the 1-ft. void, and the remainder of the

stringer and shielding blocks was reinserted.

Upon removal the cannister was monitored and stored

in a fuel storage pit until the radiation level had

dropped to levels which permitted transfer of the capsule

to the vacuum system.



Gamma Irradiation

The gamma irradiations were performed in the Univer-

sity of Florida pool-type food irradiator. The 30,000

curie Co60 source was in the form of 96 plates which

were placed so that a relatively flat gamma flux was

obtained. The capsules were lowered into the containers

within the irradiator and exposed to the desired dose.









The containers were filled with water. Only one capsule

of the entire series of gamma capsules was found to have

leaked water. The gamma dose was measured by the Fricke

dosimetry method (30).


Analysis

The analysis of the samples was performed with a

Model 700 F & M Chromatograph which has a thermal con-

ductivity detector. The conditions used for the various

analyses and the associated calculations are given in

the appendix.

The capsule containing the sample to be analyzed

was attached to the vacuum system. The vacuum system

was both pressure and vacuum tested up to the capsule

valve. The sample was then transferred to a glass bulb

in the vacuum system using the vacuum transfer technique.

The samples which contained UF6 were passed through the

sodium fluoride trap to remove the UF6 and fission frag-

ments. If the sample transferred to the glass bulb was

discolored, the sample was transferred back through the

sodium fluoride trap to the capsule. The sample was then

transferred once more through the sodium fluoride trap

to the glass bulb so that the sample was passed through

the trap for a total of three times. No sample required

more than three passes.









The bulb was surrounded by a mixture of ice and

water and allowed to stand for one hour so that equilib-

rium could be established. The gases over the sample
were then chromatographed several times using the

chromatofraph gas sampling valve with either a 1 milli-

liter, a 2 milliliter, or a 10 milliter gas sample loop.

A silica rel column was used for this analysis.

The volume of the sample was measured in the glass

burette on the vacuum system. The sample was then trans-

ferred to the 12 liter flask of the glass vacuum system

and vaporized. The lrass vacuum system is shown in Firure

6. Chromatographs were made of the completely vaporized

sample using a silicone gum rubber column. Finally, the

sample was recondensed, and liquid samples were chromato-

graphed with a silicone gum rubber column to determine

the amount of polymer.



Neutron Pulsing

The experimental equipment used for the neutron

pulsing is shown in Figure 7. The neutron generator

was a Cockcroft-'''alton type accelerator, Texas Nuclear

Corporation Model 150-1H. The neutrons were produced

by bombarding the tritium target with deuterons. The

enerr.' of the neutrons was about 14 Mev. The neutron

source was roughly isotropic and in this experiment was


























02


m



E e


El
0 u





O d

J ^



0
]P tfl
U1 0
























r4

t- L




L- C~
-c c
0-


c


C
L.



.&-
a,

ii













In
I-',
CT



L ..















Ct









placed under the tank containing the fluorocarbon. The

tank was lined with cadmium to prevent neutrons which

leave the system at higher energies from returning at

lower energies. The detector was placed just inside the

upper boundary of the system. The detector was a LiI(Eu)

scintillation detector which was designed to detect

thermal neutrons. The signal from the detector was fed

into an amplifier and then to a scaler where the total

counts were recorded. The signal was also fed into the

Technical Measurements Corporation, Model CN-110, 256

channel time analyzer.

The experimental procedure was as follows. A pulse

generator produced a signal which would initiate a steady

neutron beam. After the neutron beam had reached steady

state operation, the neutron beam was turned off, and the

decay of the neutron population in the tank was recorded

on the 256 channel time analyzer. The neutron pulse

width was 50 microseconds and the channel width for the

analyzer was 2 microseconds. Thus, the first 25 channels

showed the neutron pulse, and the remaining 231 channels

recorded the decay of the neutron population in the

fluorocarbon. After the 256 channel time analyzer had

finished counting, a new pulse was introduced, and the

cycle was repeated. The pulse repetition rate was 980






40


cycles per second. This procedure was followed until
sufficient counts for a reasonable statistical analysis

had been obtained. The moveable top was then changed to

a different height, and the volume of the fluorocarbon

was adjusted so that the fluorocarbon liquid level was

at the moveable top. This adjustment cave the system a

new geometricc buckling. The pulsing was then repeated

at the new liquid level.












CHAPTER IV


EXPERIMENTAL DATA AND RESULTS


The experimental data and results are tabulated

in Tables 3 through 8. Table 3 presents the data for

the gamma irradiation of CgF16 in the Food Irradiator.

The following explanation is given for the various side

headings. The doses, in terms of rads, were determined

by Fricke dosimetry. The per cent C8F16 converted to

gaseous products is the percentage of molecules in the

original sample converted to any gaseous product; like-

wise, the per cent CgF16 converted to dimer is the

percentage of molecules in the original sample converted

to any dimeric product. The calculations for these

percentages are shown in the appendix. The per cent

C8F16 converted to dimer chromatographh) gives the per-
centage of the original sample converted to dimer based

on the chromatographic analysis assuming each molecule

of dimer to be equivalent to two molecules of starting

material. The per cent CgF16 converted to dimer

(material balance) represents the percentage of the

original sample converted to dimer based on the differ-
ence in the volumes of the original and irradiated








samples.

Table L shows the G values for the gamma irradia-

tion of CFl The C value is defined as the number of

molecules of a given material produced per 100 ev. of

enerrm absorbed in the sample. For example, for Sample

Io. .3, G(CFI) is 0.129, and accordingly, 0.129 mole-

cules of CFP are produced per 100 ev. of energy absorbed

in the sample. The G dimerr) values are given for the

amount of dimer produced as indicated by both the

chromatographic analysis and the material balance loss.

The disappearance of CgF,l, is -iven as G(-CsF,) which

is the number of molecules of CF16 lost per 100 ev. of

enerrv absorbed. The calculations for the G values are

rPiven in the appendix.

Table c presents the data and results for the

reactor irradiation of CF1,' in the UFTR. The method

for calculating the integrated flux is given in the ap-

pendix. The per cent CFi6 converted to gaseous products

is the same as in Table 3. An aJditional sample, No.

11, was irradiated with an integrated flux of 59 x 101L

neutrons/cm.2. Liquid samples were taken directly from

this irradiated sample for dimer analysis,and the per

cent of the original sample converted to dimer was found

to be O.40 per cent. This sample was the only one in









this series that was checked for dimer. No gas analysis

was made.

The data and results for the reactor irradiation of

C8F16 and UF6 are presented in Tables 6 and 7. The cal-
culations for the data are given in the Appendix.

Table 8 presents the data and results for the pulsed

neutron experiment. The symbols used in Table 8 and the

method for calculating the decay constants are described

in Chapter V.

The range of error for the decomposition percentages

given in the tables is estimated to be + 10 per cent.

The range of error for the G values is estimated to be

+ 15 per cent.























0
O
?-I
E-

1-i rr-
c: O
E--4 ) 0

0 0,
l4 *J O 0

Cr -
U- r"f *. x 0


E-,^
SOU


O





4. C4
0






U0 rI

ii r-. O1 0 I 0

rrC 0 0 0









00 E3
09) O cu
O L- O 0 ,

O 0 O C
C ty i C ~0
m 0a






















Sr) U r S a
S 0 E r. X 0U
cc 0 o C
Lf>bz. r 1 -2 0



I-I















Ca -. ^ \- *-. ^















I,-
0

\o X Ot
to O O-
H *-
CV 0



0



-
Ni












0
rl









0 H 0
O










a oO
o I o
3u\ 0 r m
S.0 0 X 0 ?'

C tO 0 l
C0 \0
















H 0 0 0 o S4









H H H
W t t
iO- O














S0 0 0 ;4








O< o o o e
0a H '0 ^






K H -F '0 i a


CV0 0 0 C
OT ca 'e^ 's. U- '
















TO--1
to
C,








1-4











ro







0
-r ri











0- -a
rt










Wt


en

-





c:
CI
.-' CU
U C



r 0 t-
G .3 -'
'* .- .




Dl 0t'

0-


rj ~ U- L.

-0 ri (







N N en to

C-. '-4 -1


o rl





cl -4 N-




o -O~


I I
I I


C O
.*0 0 Ta to
Cr. C. ~ C4 C.
C.) 0 i C
0000U









TABLE 4


G VALUES FOR GANMA IRRADIATION OF C8716

SAMPLE NO. 33 38 39

Dose radss) 2.4 x 107 4.3 x 107 4.3 x 107


G Values

CF4 0.129

C2F6 0.020

C2F60 0.004


C3F ---

C3F0 --

G (Gas) Total 0.153



G (Dimer)

Chromatograph 0.40

Material 1.00
Balance


0.176

0.017

0.004

0.001

0.001

0.199






1.57

1.68


0.197

0.037

0.002

0.001

0.002

0.239






1.60

1.68


34

5.9 x 107




0.109

0.017

0.002

0.005

0.005

0.156






0.55

1.12


2.21 2.75


3.25 1.88


G (-c8F16)













SAT'LE IN0.


Dose radss)


'3 Values




CFn
C F,.
C2F0

CIF,

C 3FO

C (Cas) To~tal




'3 (Dimer)


Chromatoc-raph


TI.aterial Balance


G (-CFI,)


TABLE 4 --Continued

:. 12

h.P x 10 12.2 x 10'


O. 1O



0.001


0.001



0. 20,







1 -.L


1.99


0 1L


0.000.


0. O01

0.0007


0. 000L.

1'.1, ;










1 3


2. 2.. .F


13


1L.2 x 10


0.1"2


0.007


0.000 O


0.0006


0.000.?

0.133







1.12


0.9,7




1.7.


























H
E-






0
E-i




O
co o

t0
Ei-t


<0
HO
CO


0
a


















H



0
Ior

























0
03 M











a:










Q


u r-t
-l 0 uH




cv-



(\






100 0

r^j


x


NO -
~- 0
x O
0


--t

. H
CV
x


r-i -4t
r 0 to -0
S C \ 0


0 X


0\
oI -


to-t
f\ 0
0 O
C; x


S--t -.o
0 0 0'-A

00
0s


s o
0)

0 1 01


S0 0 o0

o H \o w0
4- X- -- co
























(1 0: C' I
tocI
















C- C C











1 ?:







U- 0 0
Ui 'I I I

'1


S-i

6)


'0
cr u
O c



if 0 It



en vC


U ---


c

C '4- '0
CjC,



















0
i O0
N


0
H







3H





0 QM
E-H




0









00


OH

CQ






M0
a
E<<





a \o
o





















O
00
C::4
^0 MS
H~ 0 1-










wC

MM
&
M&
E-
1-
oT
0,


0
o
r O











o O




0 0




0
H 0
C.-'


-~ 0
* '
H


*-r-
1-s
0


\0 x












0

,4-




,-

O

rr

0
0


0 -.4 C9-. O0
"a o

'o IV >
u 0c 0
00 0 0 r
0 M 0 4 0
S cp o o
0 O %
)O 4. 0 C 0o
)bf to r0t
Hc 3 e h 0 ) 0
0 U I) *0 0
co o f; u Jc c o
OT > ; ft; M *- *e U








5'i














C- .9 rr .r.






-,j C- 'C 2` C-





-2 'Q 4 ~ C













-- C- .-i r i -C
't 1 r


'-4f


I I I
I I


301-
4,

UC

"O *r-4
Lr)r-



. 3 C 0
dL. to
-a L. Cl. Cr. Cl.




0 'l to
tC Cc
u















r-4
0











0
,-I



















N -
tto

X
.-I







,.-t




0- -1


zx
0 0







00
n i--
W O-




0

E-' X

rlmc
c 0
0 r
0
< t


\0 N um 0 C- 0
\0 r\ M 0 0
0 0 0 0 0 0







N m \0 CO N
'r^ CT\ '0 i- UN '0
C- 0 0 0 0 '-4








0 ( 0 00 '0
O O O O H


tO O

c- 0


I 1 -0
O


0 0
0 o I'

f. 4L :3 \0C o to to 6
- hO -b. 3 O to
C r N Nr ,N m m 0
4 > C 0 0 0 0 U
< C -
CO H 0


N
















-2





'*'
I-I







S C
f-




C-






I-
r-



C C
-4






C
0



I

E--4



1'

C-
1-1


-3 CO 5 03 (
o o C c -I











=4 0 10 C. r
-4 -4 "I -- C











-4 r C* r


-- 0 0C 0 0C










- -4


7rC


-cc
U

C. t' 1
)c n A.


.- L m 4 L. CI. C. [t
4 l U C* U (2 '
< CC
U, (2'

































0.
0





rrr:
Co



o-



c4














~0
1HZ


u C

1-Io

04a
0
C-,


-k -d' -Zl
0 00
H- H H





0~ '.















tol 0 0 0








rm
m 0 0 0









C N)
C'.) oN





c'-l H H




C.) '. '0 '.
o o




('4 () (N












CHAPTER V


DISCUSSION OF RESULTS


Radiation Studies


Gamma Irradiations

The gamma irradiation of C vFo1 yielded two groups

of products. One group was composed of gaseous pro-

ducts. and the second croup was composed of heavy

polymeric products. The ratio of CAF1, converted to

polymer to CFi16 converted to gaseous products was

111 to 1. This value is the average of the data given

in Table 2 for per cent CgFI1 converted to dimer and

per cent CF1,6 converted to gaseous products.

The principal gaseous products of the gamma irra-

diation of CoF16 are seen from Tables 3 and 4 to he CFL

and C2F6 in the ratio of about L to 1. Smaller amounts

of C2F6,O and CF0 were also found,as well as a compound
tentatively identified as C3FgC'. Identification of

C3FCO was based on the chromatographic retention time of

a similar compound as reported in Reference 20.

The polymeric material was found by chromatoRraphic

analysis, and based on mass spectrometric analysis using








the equipment described in Reference 19, was identi-
fied as being dimeric. The dimeric material was
composed of at least two different molecules. The
first molecule had a molecular weight range of 750-
760, and one possible isomer is shown below. The
second molecule had a molecular weight range of 805-
815, and one possible isomer is also shown below.


CF3
CF
F2C CF--CF3
F2C CF
F2C CF


CF2
2


F3
CF
F2C F-CF3
I I


--- FC


/CF
CF2
2


Product, Molecular Weight, 750-760


CF-CF3
I3


CF3 CF
( 3


CF- CF --CF2- CF CF-CF----CF
S/2 2 2 3
CF2


Product, Molecular Weight, 805-815








The average C values for the production of gas-

eous products and of polymer are in good agreement with

the results of other investigators as shown below.

Compound C (Gas) G (Polymer) Investigators

C- .F0.3 .1 Fallrratter and
Hanrahan (19)
CAFA 0.3 1.1 Bloch, MacKenzie,
and Wiswall (i,)
C(F16 0.2 2.5 This study

A mechanism may be proposed which accounts for
the various products formed d-uring the gamma radiolysis

of CpF1A. A gamma photon strikes a COF,2 molecule and

produces a CF8F + ion and a F- ion. This reaction and

the subsequent reactions may be represented by the

following equations:

+
CgF16 4 CgF15 + F- (1)

C1 F + F- -> Cp F + F" (2)

CFo.lF + CyFli -1 > C1630

C F5 + F- -> CF + CF* ()

CF,- F" CFL (.)


CF1F + C F15- -)- C15F2


ClF,28 + 2F --> CF30









Primary chemical change occurs in reactions (1) and

(2). Subsequent products are then produced by the re-

maining reactions. Dimer is produced by the reactions

shown in equation (3) and in equations (4), (6), and

(7). The major gaseous product, CF4, is produced by
reactions (4) and (5). The other gases are produced
by various recombination reactions of the perfluoro-

methyl radical. The oxygen source for the production

of C2F60 and C3Fg0 was probably dissolved and ad-
sorbed air.

The production of gaseous products and dimer are

shown to be linear with respect to total dose by the

curves in Figures 8 and 9. The equation for the

curve in Figure 8 is

log -- Nx 100 = 0.025 + 0.53 x 10-9D



where NO N
-N-- x 100 = per cent of original sample con-
verted to gas, and
D = total dose in rads.


The equation for the

log P x 100

N N
where N0 N x 100
-N- 0


curve in Figure 9 is

= 0.29 + 0.91 x 10-7 D



per cent of original sample con-

verted to dimer, and



















N.







N








N


r- c



C-
L.










Cd
'-I0




cr-





0



L 0
0
0-
tc









-
Cr,
*- C


0 TO r.i
- 0 0 0 0


D
C 00

0r 0

*- o
i c% E3LI o
0cW J f


c,

C
0
C







61









4)
0 (



0 H 0


r:4
0 C
\rl (fl
4.() '









OU
\ 0
to
o (a)



00
l 0 H
\ cs

S


X H

I o\






0
t











o 0 0
O \ O


\ C,






C\)





\ I

*tO 'C -i- ('4 O TO -O (' 0




G -o



P-a C-) Q








D = total dose in rads.
The chemical structure of CpF1( shown below indi-
cates that there are sixteen possible ways for a CgF I

molecule to lose one fluorine atom.
CF
I 3

F2C CF--CF3

1 ]
FC, CF.
CF-
F'e rf 1luo rod imet h ylv.'c 1 ohe xane


Since two CFi molecules are required to form one

dimer molecule, there are 16 :. 16 or 256 possible ways
to form a molecule of dimer from two molecules of
CF1Fl. There are two ways for one CF-, group to be sep-

arated from a parent molecule, or,since two molecules

were considered in the dimer formation, there are 2 x 2
or 4 wayv in which a CF3 proup may be separated from
tw. CeF1, molecules. Thus the ratio of possible ways

to form dimer to the possible ways to form ;,as is 256
to I. or '-. to 1 as opposed to the eyperimentally found
ratio of 111 to 1.

The foregoinrn discussion has considered the decom-
position of CgF1. by ganjma radiation in terms of the

products yielded and in terms of the fraction of









original material converted to other materials. A
different analysis may be made by considering the

fraction of starting material remaining after a given

exposure to gamma radiation.

Let dN be the number of parent molecules decom-

posed by a dose dD on the sample which contained N

molecules. Since the number of molecules decomposed

is proportional to the number of molecules present and

to the dose, the proportionality can be given as

dNo. NdD,

and letting k be the proportionality constant, the

relationship becomes

dN = kNdD

where the minus sign indicates that dN is the number of

molecules lost. Rearrangement gives

dN = -kdD
J--

which can be integrated from N= NO to N = N and from

D = 0 to D = D. The result is

n N = -kD or


log N = -KD
0

k
where K =
2.303

The results for the gamma irradiation of C8F16 are

plotted in Figure 10 as log (N x 100) versus D. The
0











































/








//


,/


S






/

S




/


o *rr TC, Lr -7 ,^ *",j -i
C O' '- 0 o 0 0 0 0j


0



'I1


.-

1-



0
0




L




0
0
1o












0o -



X -I













L
sa'
n i













i-i
j
L.
t
C1.
0r x









curve is a straight line, and K is found to be 0.042

rad -. Therefore, it is possible to predict the num-

ber of molecules of CgF16 remaining in a sample with

a known number of starting molecules after the sample

has been given a dose of gamma radiation. Since the

dimensionless ratio N/NO is used, the units might be

grams or any other convenient unit which is related to

the number of molecules.

The question arises as to whether or not this re-

lationship holds only for CgF16 and only for the range

of doses used in this investigation. The data avail-

able from the literature at the present time for the

decomposition of fluorocarbons by gamma irradiation

are given in Table 9 and shown in Figure 11. Examina-

tion of Table 9 shows that the data used in Figure 11

cover a wide variety of compounds including alicyclic,

aromatic, and aliphatic fluorocarbons. Other variables

include type of radiation, dose rate, total dose, and

experimental conditions.

There are two distinct curves in Figure 11. One

curve relates data for aromatic fluorocarbons, and the

second curve correlates data for alicyclic and aliphatic

fluorocarbons. The decomposition of aromatic fluorocar-

bons is considerably less than the decomposition of

aliphatic fluorocarbons and alicyclic fluorocarbons,


























r







o,



a r
Cu
L 0 0-


t-



L.:
E -
t
E-





C r

+-'
4'-

> 'o
E- 'U
iX,


S- '


. C 0 0 00


c- *: )T I. .


C


' U


C C
c C
L L

ct c
Lt* (,
I t
-1 l


C C
C C.
L L
fJ ft
.' -'
t' O>
*- e--I
t* I.


C *'4 to
S ,O .' --i ''. ..
',. Er. L. Li' "--, a.
Ct Ci C r. C r, [r '.,










, .. .3 '
rl"-4 -, -7 C


2' C,
"


L
C, 03


C-

c.







67










0 l -. -t CI- Do
ON 0 0GN 0 0 0 0o







~to
0
4,- --t C Y QN to N N
07 N -rf -4 L' '0 N -4r
oX








0 X
4, L)
)N4)> -) 4J 4-) 4-) 4) 4)

0,

ONO
o B E E~c~la


40



E-4 4-1 dd t
Cd Cd o tv c




LJ, b.0 W bD hC O W3
co





\0 '0 \0 \0 '0 \-0 \0




0 rz t
0. t CC to
E 0 0 0UU
0









C-,
0 x
4, E
























































































0, 0 n: r- ,,



'I I-





rC-


0


-' ri ril -


J1
C
ri
.C
L

0







4,



0










Vl
L.








C,
r..





U)

C,
0










in



0:


C,-,
0








L


'1'
ci-









and the curve for aromatic fluorocarbons is a straight

line over the dose range for which data are available.

The curve for aliphatic fluorocarbons and alicy-

clic fluorocarbons is reasonably straight in the lower

dose range; however, the curve tends to slope downwards

at higher doses. This result is unexpected since it

might imply that the gamma radiation becomes more effec-

tive at higher doses. A better explanation is that the

number of small decomposition product molecules becomes

very large at high doses. These small molecules absorb

a large fraction of the gamma energy, but because of

their more stable structure, do not decompose. The

energy absorbed by the small molecules is transferred

to the heavier molecules, and the actual energy ab-

sorbed by the remaining parent molecules is greater

than that due to the absorption of gamma radiation

alone; therefore, the previously developed relation-

ship must be modified to take into consideration the

build-up of CF4 and other light gaseous products.

The number of molecules decomposed is proportional

to the number of parent molecules present plus the num-

ber of CFI molecules generated. The number of CF4

molecules present after some arbitrary exposure is

given by the following expression:









r!o. of CF, molecules present

no. of CgFI, molecules decomposeJ

1 molecule of CSFiP decomposed to ras
111 molecules COF1 decomposed
g molecules CF, produced
molecule ",F1 decomposed to ras

The number of molecules of 2FFl,. decomposed is simply

the number of molecules of CF1 originally present
minus the number cf C:F1, molecules remaining after

the exposure. If the number of molecules is iv'en in

terms of fractions with Ir-, = 1, then the following re-

lationship can be found for the number of CF, molecules
pre sent

r!o. of CF molecules present =

(1 N) < ~< x

.0;? (1 ) .

The original relati :nship becomes
jlu = .\ldD k [:.0-2(l I!]dD,

and it can be rewritten as


Sd = kdD
,.- + 0.0

Integration between the limits of II = I and N = !,-, and

of D = D and D = 0 ,iv es








1 0o.928N + 0.072
0.928 i 0.928NO + 0.072 = -
0.928k
By letting K = 02- k and NO = 1, the expression
2.303
becomes

log (0.928N + 0.072) = -KD .

Figure 12 is a plot of log (0.928N + 0.072) versus
D, and the constant K was found by a least squares fit
to have a value of 2.13 x 10- rad- for the aromatic
fluorocarbon curve and 6.64 x 10-10 rad-1 for the ali-
phatic and alicyclic fluorocarbon curve. Both curves

are straight lines and can be used to predict the
fraction of a sample which would be decomposed by a

known dose of radiation over the wide range of param-

eters given in Table 9.


Reactor Irradiations

Irradiation of C8F16.-- The reactor irradiations of

C8F16 gave results very similar to the results for the
gamma irradiation of C8F16. The principal gaseous pro-

ducts were CF4 and C2F6 in the ratio of about 4 to 1.

This finding is in agreement with data obtained by Davjs
for the irradiation of C7F16 in the Low Intensity Test

Reactor (31). Chromatographic analysis indicated that

there were also dimeric materials similar to those
found in gamma irradiated samples.





















/ -)
a / c




0 0
S/ L.




/ 00

/ -_

,/ ':*,
/' 0'L
C*







S / C.,/
'- f.- .r '


,, 'c. a :


SO- C C


,-. L 0

I,/ L i, C
l ~L,









", C-
I c









The per cent decomposition of the parent compound

to gaseous products is plotted versus integrated flux

in Figure 13. The curve is a straight line over the

range of integrated fluxes used. The equation for the

curve is

logg N x 107 = -13.2 + 1.1 log nvt


for the range 1012 < nvt < 1016

where NO N x 107 = per cent of original sample con-
NO
verted to gas x 105


The relationship between the neutron flux and the

gamma flux in a graphite moderated reactor was studied by

Richardson, Allen, and Boyle (32). Expressions were

derived which permit the conversion of integrated

neutron fluxes in terms of neutrons per cm.2 into ab-

sorbed gamma and neutron doses in terms of rads. These

conversion factors were used to convert the integrated

neutron fluxes for the reactor irradiations into rads.

(See appendix.)

The data for the decomposition of CgF16 to gaseous

products by reactor irradiation were combined with the

data for the decomposition of CgF16 to gaseous products

by gamma irradiation. The combined data are plotted in

Figure 14, and the continuity between the two sets of

data is very good. The equation for the curve is




































































101? 1014 1015 1016

Integrated Neutron Flux (neutrons/cm.2)


Figure 13 CSF1', Decomposed to Gas

for Reactor Irradiation.


V)

cc
C-1
0

10-

0
0.




00-

fu
U
L






C-)


IL)

cL.






75



100




10-1
io0





to
S10-3








CO







10-4
e Gamma Irradiation

E Reactor Irradiation



10-5 I I 1 1 I
o103 104 105 106 107 108

Dose radss)

Figure 14. C8F16 Decomposed to Gas
for Gamma And Reactor Irradiation.
U










*P Gam Irdto
















for Gamma And Reactor Irradiation .









I-j x lu00 = 0.007 x 0.72 / 10 -QD
Ti0

where = fraction of sample converted to gas, and


D = total dose in rads.

An analysis of the work by Richardson, Allen, and Boyle

reveals that the enerry deposited by the gamma flux in

a reactor of the type bein- used in this work greatly

exceeds the energy deposited by the neutrons which are

primarily thermal neutrons. Since most of the energy

deposited in the sample results from g amma photon

interactions, the reaction mechanism and rate equations

which can be applied rn this case are identical with

those developed in the previous section.

There was no evidence of any fluorine or hydrogen

fluoride. Some C-.FaC was found in the mixture. The

oxyr,en required to produce this compound was probably

either adsorbed on the walls of the :apsules or dis-

solved in the fluorocarbon.

A calculation was made to compare the decomposi-

tion of C1gFl, in a reactor radiation field with the

decomposition of the coolant used in the organic cooled

reactor. (See appendix.) This coolant is a mixture of

hydrocarbons including biphenyl and terphenyls.

The data for the Piqua Nuclear Power Facility were

used for comparison of the coolants (33). If CSFl6









were to be used as the coolant in the PNPF, the decom-

position rate for the CgFl6 would be about 0.50 per

cent per hour as compared with a calculated value of

3.0 per cent per hour for the organic coolant. In the
light of available data C8F16 would be a better coolant

for the PNPF than the currently used organic coolant.

Other important advantages of the C8F16 as a

coolant would be its higher thermal stability and its

lower corrosiveness as compared to the organic coolant.

Possible disadvantages of using CgFl6 would be its high-

er cost and the poorer heat transfer properties

generally ascribed to fluorocarbons (1 ).


Irradiation of C8Fl6 and UF6.--The results of the

reactor irradiation of CsF16 and UF6 differ significant-

ly from the reactor irradiation of CgF16 and from the

gamma irradiation of C8Fl6. The most significant differ-

ence is the absence of polymeric material in the reactor

irradiated solution. An extensive analytical effort

which is described in the appendix was directed towards

the confirmation of this finding. No evidence of poly-

meric material was found by any method. A second

significant difference was the different distribution

of the gaseous products. For the reactor irradiations

of C8F16 and UF6, roughly equal amounts of CF4, C2F6,









and C2F60 were found with lesser amounts of C Fg and

C3F~O; whereas in the gamma and reactor irradiations

of CFl., the major gaseous products were CF4 and C2F6

in the ratio of about four to one with only small amounts

of C2F60, C3F., and C F80.

A mechanism which accounts for these differences

can be proposed. In the reactor irradiation of CsF16

and UF, the U235 nuclei fission into two fragments.

These two fragments dissipate their kinetic oneriv--

about 195 Nev.-- into the surrounding medium. These two

fragments strike the fluorocarbon molecules and shatter

the ring at three or more bonds,producing three or

more fluorocarbon radicals. Since each fission also

releases six fluoride ions or fluorine atoms, an excess
of fluorine is available for recombination with the

fluorocarbon radicals. In addition it has been pre-

viously reported (25) that the neutron irradiation of

UF6 yields LFU5 and iree fluorine with a C value of 0.5

molecules of TUF, decomposed per 100 ev. of energy ab-

sorbed; consequently there is another source of fluorine

in the system. In addition to these sources of fluorine,

the compound UFP, is itself a potent fluorinating agent.

The processes can be represented as follows:









0n1 + U235F6 -> 2 fission fragments
+ 6 fluorine atoms or ions

+ neutrons gammas.

Collision of fission fragments with the surrounding

fluorocarbon molecules shatters the molecules and

produces various fluorocarbon radicals such as CF3*,

C2F5*, and C3F7' The excess of fluorine immediately

combines with the radicals to form small saturated

fluorocarbons such as CFi, C2F6, and C3F8. The oxygen

containing compounds are formed by the recombination

of necessary radicals with oxygen which is present in
the UF6; i.e., two CF3 radicals could combine with one

oxygen atom or ion to form C2F60, and likewise one CF3

radical and one C2F5 radical could recombine with one

oxygen atom or ion to form C3F0O.

The reactions which have been discussed are demon-

strative and do not necessarily include all possible

reactions; however, the "proposed mechanism" does

account for the lack of dimer because the parent mole-

cule is shattered into small fragments by the primary

act,and these small fragments are immediately saturated

with fluorine.

Consideration of the G values for the gamma irradi-

ated CgF16 and for the reactor irradiated CgF6 UF6

confirms the proposed shattering of the fluorocarbon









molecule. The G values for cas production from the

solution of CF16l and IJF, are higher by a factor of ten

than the C values for gas production from the CAFI6

alone when irradiated with gamma photons. In the reac-

tor irradiations of CF,16 and LWF6,the major products

are CF', C2F,, and C2FAG: and one molecule of CqFl6

thus produces eight CFL molecules, four C2F,t' molecules,

four CF,0 molecules, or some combination of these

molecules. In the pamma irradiation,a sizeable portion

of the energy deposited goes into the production of

pol'vmeric material. In this case one molecule of C1Fi6

produces only 0.5 molecule of dimer, and the number

of molecules produced per unit of energy absorbed is

less than when cases are formed. A comparison of the

C values for the decomposition of C1Fl6 indicates that

the energy required to decompose a molecule of C~F1

by gamma radiation is the same order of magnitude as

the energy required for decomposition by fission

fragments.

An expression may be derived which relates the

number of CaFI6 molecules converted to gas with the

integrated neutron flux. The number of molecules dN

converted to gas is proportional to the number of mole-

cules present II. the neutron flux nv. and the time

increment dt.









The proportionality can be written as

-dNo(Nnvdt ,

and, if k is the proportionality constant,

dN = kNnvdt.

Examination of the data from Table 6 shows that the

total decomposition is very small so that

N =- NO,


and the expression


Integration from N

gives


can be written as

-dN = knvNodt

= NO to N = N and


from t = 0 to t = t


-(N NO) = knvNt ,


N0 N
N knvt
NO


The term on the left is simply the fraction of molecules
converted to gas. The data are plotted in Figure 13

and the points fall along a straight line as expected.

The equation for the C8F16 + UF6 curve is

logN N -N x 107 = -9.80 + 0.92 log nvt


for the range 1012 < nvt < 1016


where NO N x 107 = per cent of original sample
0
converted to gas x 105, and









nvt = integrated neutron flux in neutrons/cm.2

Several other experimental results are worthy of

note. Solutions of IUF6 and CgFI were prepared and were

found to be homogeneous with no apparent side reactions.

The solubility of UF6 in CF16 was found to be about

0.65 gm./ml. The fluorocarbon was successfully sepa-

rated from the fission fragments and the IlF6 by passing

the samples through a sodium fluoride trap with the

samples beinr in the vapor phase. The largest number of

passes required was three with some separations being

complete after one pass. A light film of greenish

white powder remained in the capsule. This powder was

probably UF, which had been chemically reduced to UFL.

The radioactivity of the separated fluorocarbon was

barely detectable. Thus, separation of fluorocarbons

from UFF6 and fission products has been found to be a

simple process. There was no evidence of elemental

fluorine or hydrogen fluoride in the capsules.

A calculation was made to ascertain the decompo-

sition of a CjF16- UFT solution if the solution were to

be used in a reactor similar to the Homogeneous Reactor

Experiment I, HRE-1 (3L). Assuming the same concentra-

tion of U233 per unit volume and the same flux level

as the HKE-1, a typical decomposition rate for the

CFI16 solvent was found to be 93 per cent per hour.









This rate of decomposition and the cost of make-up

feed would counterbalance the fact that the solution is

electrically nonconducting and hence noncorrosive. The

decomposition rate was obtained by extrapolating the

decomposition rate an order of magnitude higher than the

highest data point. This assumption of linearity of

decomposition with dose may not be completely justified.

Another factor which should be considered before this

result is extended to other fluorocarbons is whether

or not C8F16 is a good representative material. In pre-

vious investigations, the alicyclic fluorocarbons, of

which C8F16 is a member, have been found to be consid-

erably less radiation resistant than have the aromatic

fluorocarbons.









Ileutron Phy'sics Parameters


In order to evaluate the fluorocarbons as worrini.

fluids in a nuclear reactor it is necessary that their

neutron absorbin( and scattering properties be known.

Conventional static experiments for measuring these

Properties require large amounts of material, but simi-

lar information can be obtain-d with a small sample by

m-easurin- neutron decay with the pulsed neutron tech-

nique. The basis of this technique can best be shown by

examinin!, the time dependent equation for the neutron

flux in a nonrultiplyin, mrediiume. The neutron flux as a

function of space anJd ime ) (r,t) is riven by


) (r,t) =I)(r)e-( a + Dv ,)L


where the neutron flux ) is the product of the number

of neutrons per unit rolume and the velocity,. of the

neutrons. The velocity of the neutrons is v, ani

S, is the macroscopic absorption cross section
for the medium. The macroscocic absorption cross section,

a property of the medium, is a measure of the medium's

tendercy to absorb neutrons by (n,f ) reactions and

thus remove the neutrons from the neutron population.

The diffusion coefficient D is an inverse measure of

the medium's ability to scatter neutrons by neutron









interactions with the nuclei of the medium. The mater-

ial buckling B2g for a cylinder is given by

B2 2.405 2 + TT 2
R H

where R = radius + 2 D

and H = height + 4 D
The spatial dependence of the neutron flux can be

removed if the flux is considered at one specific point

in the system. In this case ln((t) (v a + DvB2g)t

if the natural logarithms of both sides are taken. If

the neutrons are assumed to be monoenergetic, all of

the terms inside the parenthesis are constants for this

case, and these constants can be represented by a single
term X Substituting the decay constant in the previ-
ous relationship gives the simple expression In (t)'- -t,
and A the decay constant, can be determined by measur-

ing the neutron flux as a function of time.

If the physical dimensions of the system are

varied and the decay constant measured for each size

system, the macroscopic absorption cross section and the

diffusion coefficient can be determined as is shown by

Figure 15. The plot of \ versus B2 has as its inter-
cept with the ordinate, the quantity via, and by

knowing the neutron velocity, it is quite simple to

calculate Z a. Likewise, the slope is Dv which will





















S= v f. + vDB.
d '


B2s (cnr,2)


Figure 15. Analysis of Pulsed Neutror
Experimental DaLa.


X (sec-I)









yield the diffusion coefficient if the neutron velocity

is known. As was apparent from the mathematical defi-

nition of buckling, the buckling decreases as the

physical size increases. Therefore, in order to get

a good value of the intercept via, it is important to

have points on the curve near the ordinate, or in other

words,the system should be as large as possible. This

is an expected result since B2g = 0 actually represents

an infinite system which would lose no neutrons to the

surroundings.

The results of a typical run are shown in Figure

16 which is a plot of counts per channel versus channel

number or time. The curve has no region where the slope

is constant for more than 2 or 3 microseconds. This

result was characteristic of all of the runs and led to

the conclusion that there was a steady state contribu-

tion to the neutron decay which was independent of the

fluorocarbon. A run was made with all equipment

arranged as before with the exception that there was no

fluorocarbon in the tank. As shown in Figure 17, there

is a decay of the neutron population in the empty tank

which is significant when compared to the decay of the

neutron population in the fluorocarbon. This decay could

possibly be attributed to neutrons leaking from the sys-

tem, being moderated and reflected in the room, and

being returned at energies slightly above the cadmium















End of Neutron Fulse


V,


Y.


Data from Fun 1o. 12
H = 12.7 cm. C 8Fi, in Tank
E = 0.0'1, cm. -
6C, ,


S 12 1's .

Time (,-sec. )


Figure 1'. Counts Per Channel Vs. Time for CF1,. -


c. x 10,


Counts

Perl

Channel


2 10.'


0 .


-20 2 ." 2


I I










5 x 104


4


3



2





Counts

Per 104

Channel


2 x 103


0 4 8 12 16
Time (/ sec.)


20 24 28


Figure 17. Counts Per Channel Vs. Time for Empty
Tank.









cut-off. Another possible cause would be moderation of
the neutrons by the air in the tank.

The result of a channel by channel subtraction of

the decay of the neutron population in the empty tank

from the decay of the full tank is shown in Figure 18.

A reasonably straight region for which a decay constant

can be determined was civen by the subtraction of the

two curves. The decay constant for this curve was cal-

culated usint a least square fit of the data. This

subtraction procedure was repeated for other runs with

similar straight regions being found. It is to be noted

that the subtraction was performed channel by channel

rather than smooth curve from smooth curve. This channel

by channel subtraction was, of course, a much more severe

treatment of the data.

Figure 19 presents the plot of \ versus B'K. for

C8F16. The data were least square fitted by the lion

Linear Least Squares (TILLS) computer code. The points

are in a reasonable straight line: however, the point

for the lowest buckling, that is the maximum volume, is

not too close to the axis when it is realized that the

intercept is of the order of .O00 and the first point

has a value of about 3.1. The computer code gave a

value for the intercept of 0.0073 sec.-. A statistical

analysis showed that the deviation with a 95 per cent
























A= 6.651 x 104 sec.-1


Run No. 12 -Run No. 10
H = 12.7 cm.
Channel by Channel Subtraction
B2 = 0.0461 cm.-2
g


S I i I I


0 4 8 12 16 18 24 28

Time (q sec.)

Figure 18. Counts Per Channel for CgF16 Minus
Counts Per Channel for Empty Tank Vs. Time .


5 x 103


Counts

Per

Channel


103


5x102 F


SI I n


-




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