U–Mo/Al–Si interaction: Influence of Si concentration

Within the framework of the development of low enriched nuclear fuels for research reactors, U–Mo/Al is the most promising option that has however to be optimised. Indeed at the U–Mo/Al interfaces between U–Mo particles and the Al matrix, an interaction layer grows under irradiation inducing an unacceptable fuel swelling. Adding silicon in limited content into the Al matrix has clearly improved the in-pile fuel behaviour. This breakthrough is attributed to an U–Mo/Al–Si protective layer around U–Mo particles appeared during fuel manufacturing. In this work, the evolution of the microstructure and composition of this protective layer with increasing Si concentrations in the Al matrix has been investigated. Conclusions are based on the characterization at the micrometer scale (X-ray diffraction and energy dispersive spectroscopy) of U–Mo7/Al–Si diffusion couples obtained by thermal annealing at 450 °C. Two types of interaction layers have been evidenced depending on the Si content in the Al–Si alloy: the threshold value is found at about 5 wt.% but obviously evolves with temperature. It has been shown that for Si concentrations ranging from 2 to 10 wt.%, the U–Mo7/Al–Si interaction is bi-layered and the Si-rich part is located close to the Al–Si for low Si concentrations (below 5 wt.%) and close to the U–Mo for higher Si concentrations. For Si weight fraction in the Al alloy lower than 5 wt.%, the Si-rich sub-layer (close to Al–Si) consists of U(Al, Si) 3 + UMo 2Al 20, when the other sub-layer (close to U–Mo) is silicon free and made of UAl 3 and U 6Mo 4Al 43. For Si weight concentrations above 5 wt.%, the Si-rich part becomes U 3(Si, Al) 5 + U(Al, Si) 3 (close to U–Mo) and the other sub-layer (close to Al–Si) consists of U(Al, Si) 3 + UMo 2Al 20. On the basis of these results and of a literature survey, a scheme is proposed to explain the formation of different types of ILs between U–Mo and Al–Si alloys (i.e. different protective layers).