RRFM 2009 Transactions - European Nuclear Society

Micro-XRD coupled with SEM+EDS characterization of U-Mo/Al-Si
diffusion couples
J. Allenou, H. Palancher, X. Iltis, E. Welcomme, Ph. Martin, F. Charollais, M.C. Anselmet
CEA/Cadarache, DEN/DEC, 13108 Saint Paul Lez Durance - France
M. Cornen, O. Tougait
Université de Rennes 1, UMR-CNRS 6226, Campus de Beaulieu, 35042 Rennes Cedex - France
R. Tucoulou
ESRF, 6 rue Jules Horowitz, 38042 Grenoble - France
P. Lemoine
CEA/Saclay, DEN/DSOE, 91191 Gif sur Yvette - France
ABSTRACT
A set of U-Mo7/Al-Si diffusion couples, with silicon content going from 2 wt% to 10
wt% were obtained by thermal annealing. These couples were systematically
characterized by SEM+EDS and by µ-XRD. Thanks to these characterizations, we
have shown that two main types of interaction layers (ILs) are evidenced,
depending on the silicon content in the aluminium alloy, with a threshold value of
about 5 wt%. Below the threshold value, the ILs are three-layered, with : a first sublayer,
close to U-Mo, containing the UAl 3 + U 6 Mo 4 Al 43 phases, an intermediate one
with the U(Al,Si) 3 phase and a third one, close to Al-Si, with the U(Al,Si) 3 +
UMo 2 Al 20 phases. Above this threshold, the ILs are mainly bi-layered, with an
U(Al,Si) 3 + U 3 Si 5 sub-layer, close to U-Mo, and an U(Al,Si) 3 + UMo 2 Al 20 sub-layer,
close to Al-Si. A mechanism is proposed for explaining the formation of these two
types of ILs.
1. Introduction
U-Mo/Al dispersed fuel is developed as an high-uranium-density for Materials Testing
Reactors (MTR) cores. In operating conditions, a reaction between U-Mo particles and the Al
matrix occurs and can lead to a poor irradiation behaviour [1]. The addition of Si into the Al
matrix seems to be a promising solution for solving this problem.
Microstructural examinations carried out on interaction layers (IL) formed between U-Mo and
Al-Si, after out-of-pile and in-pile tests, have shown that the decrease of the interaction rate
seems to be linked to a silicon accumulation process in the IL (see for example [2-9]).
However, these results differ in terms of Si accumulation level and Si gradient throughout the
IL (this IL being multi-layered, in certain cases), depending on the Si content of the Al-Si alloy
and the interaction conditions (temperature, irradiation or not, type of samples : diffusion
couples or fuel plates with dispersed U-Mo particles). Many questions remain also open
concerning the Si-rich phases present in the U-Mo/Al-Si interaction zones.
To our knowledge, the most detailed study concerning the Si-rich phases identification, in the
U-Mo/Al-Si ILs, was performed by Mirandou et al. [2-3], thanks to synchrotron
characterization of diffusion couples. The authors have shown that, at 550°C, the IL
developed between an U-7wt%Mo alloy and an Al-7.1wt%Si alloy has two sub-layers : the
first one, located close to the Al-Si alloy, comprises the U(Al,Si) 3 main phase and the second
one, more Si-enriched, is located close to the U-Mo side of the couple and comprises an
U 3 Si 5 phase, mixed with an unidentified one. At 340°C, on thinner ILs, U 3 Si 5 phase was also
found in a U-7wt%Mo/Al-0,6%Si couple.
Our previous work, also performed on U-Mo/Al-Si diffusion couples, has shown that two main
types of ILs could exist, depending on the Si content of the Al alloy [4-5]. This
complementary study is aimed to achieve a better knowledge of the microstructural
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