RRFM 2009 Transactions - European Nuclear Society

Table 2. Measured compositions, in at%, at various locations in the interaction layer shown in Fig. 7.
Spot Al Si Mo U
A 77.8 11.4 1.9 8.9
B 66.9 12.1 4.7 16.3
C 68.7 5.1 6.9 19.3
D 40.1 3.8 12.1 44.0
E 20.9 0 23.9 56.7
F 50.7 4.4 8.8 36.0
G 84.1 7.3 0 9.7
H 93.7 6.0 0 0.2
4. Discussion
With respect to the observed crystallinity of the γ-phase U-7Mo particles and the presence of an
array of fission gas bubbles, the TEM characterization results reported in this paper showed
good agreement with what was reported for irradiated U-7Mo samples in [3]. Overall, the
U-7Mo remained γ-phase and did not become amorphous during irradiation, and the average
size of the fission gas bubbles was ~ 3 nm, with a bubble spacing between 6.5 and 8.3 nm
depending on the sample orientation
For the Si-rich interaction layers that originated between the U-7Mo particles and the Al-2Si
matrix during fuel fabrication and contained crystalline phases, they became amorphous during
irradiation. This was analogous to the irradiated U-7Mo/Al dispersion fuel reported in [3] where
the interaction layers also became amorphous. However, with respect to the fission gas
behavior in the interaction layers, the behavior seems to be quite different for the U-7Mo/Al-2Si
fuel compared to the U-7Mo/Al fuel. The interaction layers in the irradiated U-7Mo/Al
dispersion fuel did not retain fission gases during irradiation [3] while it is likely that the
interaction layer in the U-7Mo/Al-2Si fuel did. These fission gases appear to be retained in the
layers as small, stable fission gas bubbles up to moderate burnup. For irradiated U-7Mo
dispersion fuels with pure Al as the matrix, the fission gases do not remain in the interaction
layers during irradiation but instead migrate to the interaction layer/matrix interface, where they
agglomerate into large pores that can ultimately interconnect and cause failure of a fuel plate.
Based on OM and SEM analysis of fuel plate R2R010 [5], no large fission gas bubbles can be
observed at the interaction layer/matrix interface, and since the fission gases have to be present
somewhere, it appears, based on this work, that they are contained in the interaction layer. In
fact, SEM analysis has also been able to resolve fission gas bubbles in the interaction layer. In
some Si-depleted regions of the interaction layer, the fission gas bubbles had agglomerated and
become large enough that they could be resolved using an SEM. Future work will be performed
to identify what, if any, effects the ion polishing or presence of oxides had on the information
generated from the present TEM sample.
The presence of Si in the amorphous interaction layer seems to affect certain properties of the
material (e.g., viscosity) such that there is a propensity of the layer to retain the fission gases. It
has been proposed that for uranium-silicide fuels, which also go amorphous during irradiation,
the additional Si bonds in U3Si2 relative to U3Si results in an improvement in irradiation
performance [8]. These additional bonds reportedly have a large effect on the amount of free
437 of 455