Trapping and diffusion of fission products in ThO2 and CeO2

The trapping and diffusion of Br, Rb, Cs and Xe in ThO2 and CeO2 have been studied using an Ab Initio total energy method in the local-density approximation of density functional theory. Fission products incorporated in cation mono-vacancy, cation-anion di-vacancy and Schottky defect sites are found to be stable, with the cation mono-vacancy being the preferred site in most cases. In both oxides, Rb and Cs are the most likely to be trapped, and Xe is more difficult to incorporate than other fission products. The energy barriers for migration of each species in ThO2 and CeO2 are also calculated. Alkali metals are relatively more mobile than other fission products, and bromine is the least mobile.