Phase transformation of metastable cubic γ-phase in U–Mo alloys

Over the past decade considerable efforts have been put by many fuel designers to develop low enriched uranium (LEU < 20%U235) base U–Mo alloy as a potential fuel for core conversion of existing research and test reactors which are running on high enriched uranium (HEU > 85%U235) fuel and also for the upcoming new reactors. U–Mo alloy with minimum 8 wt% molybdenum shows excellent metastability with cubic γ-phase in cast condition. However, it is important to characterize the decomposition behaviour of metastable cubic γ-uranium in its equilibrium products for in reactor fuel performance point of view. The present paper describes the phase transformation behaviour of cubic γ-uranium phase in U–Mo alloys with three different molybdenum compositions (i.e. 8 wt%, 9 wt% and 10 wt%). U–Mo alloys were prepared in an induction melting furnace and characterized by X-ray diffraction (XRD) method for phase determination. Microstructures were developed for samples in as cast condition. The alloys were hot rolled in cubic γ-phase to break the cast structure and then they were aged at 500 °C for 68 h and 240 h, so that metastable cubic γ-uranium will undergo eutectoid decomposition to form equilibrium phases of orthorhombic α-uranium and body centered tetragonal U2Mo intermetallic compound. U–Mo alloy samples with different ageing history were then characterized by XRD for phase and development of microstructure.