Microstructure characterization of as-fabricated and 475 °C annealed U–7 wt.% Mo dispersion fuel in Al–Si alloy matrix

Abstract

High-density uranium (U) alloys with an increased concentration of U are being examined for the development of research and test reactors with low enriched metallic fuels. The U–Mo fuel alloy dispersed in Al–Si alloy has attracted particular interest for this application. This paper reports our detailed characterization results of as-fabricated and annealed (475 °C for 4 h) U–Mo dispersion fuels in Al–Si matrix with a Si concentration of 2 and 5 wt.%, named as “As2Si”, “As5Si”, “An2Si”, “An5Si” accordingly. Techniques employed for the characterization include scanning electron microscopy and transmission electron microscopy with specimen prepared by focused ion beam in situ lift-out. Fuel plates with Al–5 wt.% Si matrix consistently yielded thicker interaction layers developed between U–Mo particles and Al–Si matrix, than those with Al–2 wt.% Si matrix, given the same processing parameters. A single layer of interaction zone was observed in as-fabricated samples (i.e., “As2Si”, “As5Si”), and this layer mainly consisted of U 3Si 3Al 2 phase. The annealed samples contained a two-layered interaction zone, with a Si-rich layer near the U–Mo side, and an Al-rich layer near the Al–Si matrix side. The U 3Si 5 appeared as the main phase in the Si-rich layer in “An2Si” sample, while both U 3Si 5 and U 3Si 3Al 2 were identified in sample “An5Si”. The Al-rich layer in sample “An2Si” was amorphous, whereas that in sample “An5Si” mostly consisted of crystalline U(Al,Si) 3, along with a small fraction of U(Al,Si) 4 and U 6Mo 4Al 43 phases. The influence of Si on the diffusion and reaction in the development of interaction layers in U(Mo)/Al(Si) is discussed in the light of growth-controlling mechanisms and irradiation performance.