Oxidation and phase separation of U3Si2 nuclear fuel in high-temperature steam environments

Abstract U3Si2 is being considered as a potential fuel candidate for water cooled power reactors. Herein, we examine the oxidation behavior and phase separation of U3Si2 under flowing steam to understand the fuel degradation mechanism in the event of a steam ingress through cracks in cladding. The weight gain, microstructure evolution, and phase change after isothermal annealing were investigated for spherical U3Si2 particles fabricated by centrifugal atomization. Equilibrium oxide products and their compositional changes with increasing degree of oxidation were predicted using thermodynamic calculations and compared with experimental results. It was seen that the oxidation reaction of U3Si2 in steam begins with dissociation into UO2 and Si, followed by Si being partially oxidized to amorphous SiO2. From these results, an oxidation mechanism of U3Si2 in steam is proposed. The theoretical volume expansion of U3Si2 during the steam oxidation was estimated and its impact on fuel performance is also discussed.