Recrystallization and fission-gas-bubble swelling of U–Mo fuel

At high burnup, U–Mo fuel exhibits some form of recrystallization, by which fuel grains are subdivided. The effect of grain subdivision is to effectively enhance fission gas bubble (FGB) swelling due to increased grain boundaries. Inter-granular FGB swelling, i.e., FGB formation and growth at the grain boundaries, is much larger than the intra-granular FGB swelling. Recrystallized fuel volume fractions of U–Mo fuels irradiated to fission densities reaching 5.7 × 1021 f/cm3 were measured. Analytical expressions of recrystallization kinetics of U–Mo fuel during irradiation have been developed through the usage of the Avrami equation, a phenomenological equation which is also used to describe similar typical transformation reactions, such as new phase formation. In this work, we present a novel FGB swelling model of U–Mo fuel that is expressed in terms of Mo content, extent of cold work (fuel powder fabrication method), and fission density.