a) 
@ } Blackburn 
a) (4) 
1° }Gronvold 
6 © Aronson 
(3) 
0/U Atom Ratio 
FIG. 1. Phase relationships of sys- 
tem UO.2-U;03 (2) 
SINTERED URANIUM DIOXIDE promises 
to be a most satisfactory fuel for many 
types of power reactors. Its advan- 
tages include: 
1. High melting point 
2. Chemical stability with most reac- 
tor coolants 
3. Compatibility with a wide variety 
of sheathing materials 
4. Ease of fabrication to high densi- 
ties 
5. Anisotropic structure stable under 
irradiation at core temperatures ap- 
proaching the melting point 
A disadvantage is its low thermal 
conductivity. Fortunately, however, 
the thermal conductivity in operating 
fuel elements appears to be little re- 
duced even after long irradiation. The 
release of fission gas may impose a 
limitation on future fuel-element de- 
signs, but there are not sufficient data 
available to say this with any certainty 
at present. 
Recent data on chemical and physi- 
cal properties, fabrication and irradia- 
tion behavior of UOz are reviewed in 
this article with the emphasis on those 
that aid in establishing the conditions 
required for the fabrication and oper- 
ation of an economic fuel of predictable 
performance. 
UO —Fabrication 
and Properties 
Thermal and irradiation stability make uranium 
dioxide a popular contender as a reactor fuel. 
However, uncertainties exist in such areas as the 
most economic fabricating methods, thermal 
conductivity and fission-gas release 
By O. J.C. RUNNALLS, Atomic Energy of Canada Limited 
Chalk River, Ontario, Canada 
Properties of UO, 
The properties of uranium oxides 
have been extensively reported (1-3). 
Only those properties that are of special 
interest in the design of fuel elements 
will be referred to here. 
Composition range. The phase re- 
lationships of the system UO2-U;0s (2) 
are summarized in Fig. 1. The phase 
boundaries appear to be reasonably 
well established in the UOz to UsO, 
region, although Roberts et al. (7) re- 
ported that the UO2,, phase extended 
to at least UOz.23 at 1,077° C, in marked 
disagreement with Fig. 1. The com- 
position limits of the U;Os phase are 
still in some doubt (2). 
The tetragonal U;07 phase, which is 
formed by the oxidation of UO2 powder 
or pellets below 300° C, is not shown in 
Fig. 1 because it appears to be a non- 
equilibrium structure. For example, 
samples in the composition range from 
UO..00 to UOx.25, which consisted of a 
mixture of cubic and tetragonal phases, 
transformed to cubic UO» and UsO9 
after four months at 140° C (8). 
The calculated density of UO2.0, de- 
termined from Gronvold’s measured 
lattice constant of a = 5.4704 A, is 
10.96 gm/cm? (4). Cubic UsO» has a 
higher calculated density, 11.30 gm/ 
cm, indicating that the excess oxygen 
atoms are accommodated interstitially 
in the UO.-like structure (5). From 
Gronvold’s high-temperature studies 
on the UOz,, phase, where it was ob- 
served that the lattice constant de- 
creased when the O/U ratio was in- 
creased, a similar conclusion can be 
Ross(2”) 
(36 observations) 
Englander'29) 
» (I5 observations) 
° 
& 
[o} 
oO 
Nm 
Thermal Conductivity (w/em/°C)} 
Eichenberg’s2) Hedge and 
(in-pile data) —__‘Fieldhouse'” 
0 400 800 1,200 1,600 
Temperature (°C) | 
FIG. 2. Thermal conductivity of UO» 
corrected to theoretical density, assum- 
ing linear dependence of conductivity 
on porosity 
59 
