Defect energetics and Xe diffusion in UO₂ and ThO₂

We have performed ab initio total energy calculations to investigate the defect energetics and diffusion behavior of Xe in UO₂ and ThO₂ 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 ThO₂ compared with UO₂. 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 ThO₂. 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 ThO₂ prevents the creation and migration of defects, and results in a lower mobility of fission gases in ThO₂ as compared to UO₂.