Observed limit of melting 
DB 
| 
L'(@)d0 
- “Surface 
aul 
= 474 fn(«a,r) 
08 
Fuel Radius,r (cm) 
FIG. 8. Determination of integral from 
observed radii of melting for specimens 
CR and DB. Both specimens were 
0.72 in. diameter and contained uranium 
enriched to 4.82 wt% U*** Tests 
were made in hydraulic rabbit 
UO. lattice. The oxygen self-diffu- 
sion rate is higher in UO.,4, than in 
UO... (3); thus grain growth should 
proceed at lower temperatures in non- 
stoichiometric oxides. In addition, the 
thermal conductivity of UO», is 
known to be lower at 60° C and hence 
is suspected to be lower at elevated 
temperatures. The liberation of gase- 
ous UO; by the disproportionation of 
UO.4, might also produce changes. 
The available irradiation results indi- 
cate that stoichiometric oxide should be 
chosen for optimum performance. If 
an element were to become defected in 
service, however, the core could be oxi- 
dized by coolants such as water or CO». 
Thus, it is desirable to obtain irradia- 
tion experience on purposely defected 
test specimens, particularly at high heat 
ratings. Tests under way at Chalk 
River indicate that defected Zircaloy-2- 
clad UO. can be operated safely in 
pressurized-water coolant at values of 
center ; 
es a k(@) dé as high as 50 w/cm. 
Fission-gas release. The release of 
fission gases from irradiated UO: has 
been studied extensively (57, 58, 65- 
67). However, the factors influencing 
gas release and the actual escape mech- 
anism are still not clearly understood. 
Markedly different amounts of gas were 
66 
evolved from samples of similar density 
prepared by different fabrication meth- 
ods (66). Even for specimens prepared 
by virtually the same fabrication tech- 
nique in the same laboratory and re- 
ceiving similar irradiations, the fission- 
gas release has varied appreciably. 
Where grain growth has been ob- 
served, the amount of gas released in 
long irradiations has always been sub- 
stantial; e.g., 35% of the fission-prod- 
uct xenon was released from rod 12 of 
the CR-V-e test in which ~45% of the 
oxide exhibited grain growth during an 
irradiation of 7,000 Mwd/tonne U. 
Even with the most pessimistic as- 
sumptions, fission-gas release would 
not limit the performance of the pres- 
ent UO, core in the Shippingport reac- 
tor (65). In reactors operating under 
lower pressures using elements with 
larger ratios of diameter to sheath 
thickness, however, the buildup of in- 
ternal gas pressure might prove to be a 
severe limitation. On the other hand, 
Davies (68) has calculated that the 
fission-gas pressure within a typical 
fuel element may reach a limiting value 
at less than 100 atmospheres due to a 
“knock-on” process where free gas 
atoms are kinetically excited by fission 
fragments and reenter the oxide lat- 
tice. An experimental program is 
under way at Chalk River to determine 
the validity of the theory. 
* * * 
I am indebted to A. S. Bain, G. H. Chalder, 
W. Evans, R. G. Hart, J. A. L. Robertson, 
A. M. Ross and M. B. Watson for supplying 
information in advance of publication. I am 
particularly grateful to J. A. L. Robertson 
for many valuable discussions and for 
Figs. 6 and 8. 
This article is based on a paper presented 
before the First International Symposium on 
Nuclear Fuel Elements at Columbia Uni- 
versity, Jan. 28-29, under the sponsorship 
of Columbia University and Sylvania-Corning 
Nuclear Corp. 
BIBLIOGRAPHY 
1. J. J. Katz, E. Rabinowitch, ‘‘ The Chemistry 
of Uranium,” National Nuclear Energy 
Series VIII-5 (McGraw-Hill Book Co., New 
York, 1951) 
J. Belle, B. Lustman, WAPD-184 (1957); 
also issued in TID-7546, p. 442 (1958) 
3. J. Belle, 2nd Int. Conf. on Peaceful Uses of 
Atomic Energy, Paper P/2404 (1958) 
4. P. E. Blackburn, Westinghouse 
100FF942-P1 (1957) 
§. F. Gronvold, J. Inorg. & Nuclear Chem. 1, 
357 (1955) 
6. S. Aronson, J. Belle, WAPD-T-573 (1957) 
7. L. E. J. Roberts et al., 2nd Int. Conf., 
Paper P/26 (1958) 
8. P. Perio, Bull. Soc. Chem. 20, 256 (1953) 
9. J. S. Anderson et al., J. Chem. Soc., 3946 
(1955) 
10. L. C. Watson, CRL-45 (1957); also issued in 
TID-7546, p. 384 (1958) 
11. P. Murray et al., TID-7546, p. 432 (1958) 
12. J. D. Eichenberg, WAPD-167 (1957) 
13. B. Lustman, Westinghouse Atomic Power 
Div., private communication (1957) 
% 
Report 
58. 
. E. Friederich, L. Sittig, Z. 
. L. M. Pidgeon, 
. J. Glatter et al., 
. D. L. Paterson, 
2. J. A. L. Robertson, A. S. 
) J. A. L. Robertson, 
. J. E. Antill et al, AERE Report M/M 168 
(1957) 
R. W. Nichols, Nuclear Eng. 8, 327 (1958) 
3. M. W. Mallett et al., BMI-1028 (1955) 
A. L. Eiss, SCNC-257 (1958) 
. L. M. Pidgeon, J. M. Toguri, University of 
Toronto, unpublished work (1958) 
. O. Ruff, D. Goecke, Z. angew. Chem. 24, 1459 
(1911) 
Anorg. u. allgem. 
Chem. 146, 127 (1925) 
W. A. Lambertson, M. H. Mueller, J. Am. 
Ceram. Soc. 36, 329 (1953) 
R. J. Ackermann, ANL-5482 (1955) 
L. G. Wisnyi, 8. Pijanowski, KAPL-1702 
(1957) 
T. C. Ehlert, J. L. Margrave, J. Am. Ceram. 
Soc. 41, 330 (1958) 
F. A. Halden, Stanford Research Institute, 
private communication (1958) 
University 
private communication (1958) 
of Toronto, 
. A. M. Ross, AECL Report CRFD-817, to be 
published 
W. D. Kingery et al., J. Am. Ceram. Soc. 87, 
107 (1954) 
. M. Englander, French Report CEA-79 (1951) 
. R. W. Scott, AERE Report M/R 2526 (1958) 
. J. C. Hedge, I. B. Fieldhouse, Armour Re- 
search Foundation Report GO22 D3 (1956) 
2.°J. D. Eichenberg, WAPD-200 (1958) 
. P. Murray, 
D. T. Livey, ‘Progress in 
Nuclear Energy,"’ vol V-1, p. 448 (Pergamon 
Press, London, 1956) 
. J. Thewlis, Acta Cryst. 5, 790 (1952) 
. M. D. Burdick, H. S. Parker, J. Am. Ceram. 
Soc. 39, 181 (1956) 
5. J.S. Anderson, AERE Report C/R 886 (1954) 
. E. A. Evans, HW-52729 (1957); also issued 
in TID-7546, p. 414 (1958) 
. C. D. Harrington, TID-7546, p. 369 (1958) 
>» G. H. Chalder et al., 2nd Int. Conf., Paper 
P/192 (1958) 
. G. H. Chalder, AECL*Report UK/C6/115 
(1957) 
2nd Int. Conf., Paper 
P/2380 (1958) 
. A. H. Webster, N. F. H. Bright, Canadian 
Dept. of Mines and Technical Surveys Report 
MD-223 (1957) 
. L. C. Watson et al., AECL Report CRCE-716, 
Parts I-III (1958) 
. Eldorado Mining and Refining Co. Ltd., Port 
Hope, Ontario, Catalogue of 
Uranium Products (1958) 
Canadian 
. A. Allison, W. K. Duckworth, BMI-1009 
(1955) 
. D. R. Stenquist, R. J. Anicetti, ‘‘ Nuclear 
Metallurgy,”’ vol. 5 (A.I.M.E., 1958) 
G. H. Chalder, 
Report CRFD-759, to be published 
AECL 
. R. W. Thackray, P. Murray, AERE Report 
M/R 614 (1950) 
. R. Scott, J. Williams, Trans. Brit. Ceram. 
Soc. 57, 199 (1958) 
C. A. Arenberg, C. Jahn, J. Am. Ceram. Soc. 
41, 179 (1958) 
. U. Runfors et al., 2nd Int. Conf., Paper P/142 
(1958) 
. N. Schonberg et al., 2nd Int. Conf., Paper 
P/182 (1958) 
53. W. B. Lewis, AECL Report DL-33 (1958) 
. J. Briola, 2nd Int. Conf., Paper P/1161 (1958) 
. W. L. Wyman, W. I. Steinkamp, HW-55667 
(1958) 
R. 8S. Ambartsumyan et al., 2nd Int. Conf., 
Paper P/2196 (1958) 
. J. D. Eichenberg et al., WAPD-183 (1957); 
also issued in TID-7546, p. 616 (1958) 
J. A. L. Robertson et al., 2nd Int. Conf., 
Paper P/193 (1958) 
. W. B. Lewis, AECL Report DM-44 (1957) 
60. W. G. Morison, Atomic Energy of Canada 
Ltd., unpublished work (1958) 
. A. S. Bain, AECL Report UKE-CR-1006 
(1958) 
Bain, AECL 
Report CRFD-825 (1959) 
Atomic Energy of 
Canada Ltd., private communication (1958) 
. A. S. Bain, J. A. L. Robertson, letter, sub- 
mitted to J. of Nuclear Materials (1959) 
. B. Lustman, WAPD-173 (1957) 
. A. H. Booth, G. T. Rymer, AECL Report 
CRDC-720 (1958) 
. A. H. Booth, AECL Report CRDC-721 
(1957) 
. J. A. Davies, Atomic Energy of Canada Ltd., 
private communication (1958) 
