An advanced model for intragranular bubble diffusivity in irradiated UO2 fuel

An advanced model for intragranular bubble diffusivity in irradiated UO₂ fuel is developed. Three various (surface, volume and gas-phase) mechanisms for the gas-filled bubbles diffusivity are reconsidered. It is shown that the bubble mobility by the volume diffusion mechanism can be strongly enhanced under irradiation conditions. Influence of the two-phase interface kinetics at a bubble surface on the volume diffusion, evaporation/condensation and surface migration mechanisms that can strongly suppress diffusivity of small nanometre bubbles, is additionally studied using a non-linear adsorption law derived for the van-der-Waals gas in the bubbles. The improved model is implemented in the MFPR code and validated against measurements of the small bubbles mobility.