Defect energetics and Xe diffusion in UO2UO2 and ThO2ThO2

We have performed ab initio total energy calculations to investigate the defect energetics and diffusion behavior of Xe in UO2UO2 and ThO2ThO2 matrices. All calculations have been carried out using density functional theory within the generalized gradient approximation and applying the projector-augmented-wave method. Our results show that the formation and migration energies of vacancy defects are more than twice as high in ThO2ThO2 compared with UO2UO2. Another notable difference between the two oxides is the role played by an oxygen vacancy in the movement of a cation vacancy. An vacancy enhances the movement of a uranium vacancy by lowering its migration energy by about 1 eV, but a similar effect is not observed in ThO2ThO2. The different behavior of cation vacancies in the two oxides strongly affects the mobility of fission gases and leads to differences in their respective diffusion behavior. We suggest that the strong resistance against oxidation of ThO2ThO2 prevents the creation and migration of defects, and results in a lower mobility of fission gases in ThO2 as compared to UO2UO2.