A model for evolution of oxygen potential and stoichiometry deviation in irradiated UO2 fuel

Highlights: ► A model for redistribution of oxygen in irradiated UO2+x under temperature gradients is developed. ► On the local scale of the grain size, irradiated fuel is considered as a multi-phase system. ► Intraganular diffusion of fission products is accompanied by local thermochemical equilibrium. ► On the global scale, changes of oxygen potential in temperature profile define thermal diffusion. ► The model results for high burnup fuel are in satisfactory agreement with experimental data. - Abstract: A model for radial redistribution of oxygen in irradiated UO2 fuel under conditions of temperature and fission rate gradients has been developed. The oxygen transport in irradiated fuel is considered as a two-scale problem. On the local scale defined by the grain size, irradiated fuel is considered as a multi-phase system including solid solution of fission products in UO2 matrix, solid precipitates (metal phase, grey phase of complex ternary compounds, the phase of condensed CsI) formed at the gas/solid interface and the gas phase in the intergranular bubbles. Intraganular transport of fission products is described by a set of diffusion equations which are supplemented by the condition of partial thermochemical equilibrium in the subsystem "precipitates and gas phase". The boundary conditions are formulated basing on thermochemical equilibrium on the interface of subsystems "solid solution" and "precipitates and gas phase". Calculation of the partial thermochemical equilibrium yields local values of the oxygen chemical potential and the deviation from fuel stoichiometry. On the global scale defined by the fuel pellet size, spatial variations of the oxygen potential caused by the temperature gradients or the presence of sources/sinks at the pellet boundary determine thermal diffusion fluxes resulting in redistribution of oxygen. The whole set of equations describing local equilibration and the transport in the local and global scales is solved in a self-consistent manner. The model results for radial distribution of oxygen potential of UO2 calculated for typical reactor operating conditions and the fuel burnup up ∼100 MW d/kg HM are in satisfactory agreement with experimental data.