Molecular Dynamics Study on Grain Boundary Diffusion of Actinides and Oxygen in Oxide Fuels

Abstract

Diffusion phenomena of actinides and oxygen are relating to various properties of oxide fuels, e.g. actinide and oxygen redistribution, pore migration, sintering behavior. Although lots of experimental data have been obtained for U and O diffusions, these seem to be a little scattered among experiments. In addition, there are few studies of tran-suranium elements. Recently, we experimentally showed that diffusion coefficients of Am and Pu were almost comparable with that of U and the contribution of grain boundaries (GB) were relatively large in oxide fuels. On the other hand, the molecular dynamics (MD) calculation showed that U ions migrated via its vacancies and the O diffu-sion mechanism changed with temperature in bulk. In this study, the GB diffusion coefficients and the mechanism were systematically evaluated for U, Am and O in oxide fuels by MD calculations. For actinides and oxygen, their GB diffusion coefficients are significantly larger than bulk ones, and the diffusion mechanisms in GBs are almost in-dependent on temperature. Furthermore, the GB diffusion is strongly dependent on its structure and the interfacial potential energy. The lower oxygen-to-metal (O/M) ratio gives the larger GB diffusion coefficients for actinides and oxygen.