Molecular dynamics study of grain boundary diffusion of fission gas in uranium dioxide

Among the various products originating from fission events, the noble gas elements Xe and, in a lesser extent, Kr present important fission yields. The accumulated gas inventory in its various states (e.g. atomically dissolved, precipitated in cavities or released from the fuel) has a strong impact on the performance of LWR fuel and is presently one of the limiting factors for fuel burnup extension. A more fundamental understanding of fission gas behaviour at the atomic scale would enable to improve the modelling of the various mechanisms ultimately leading to fission gas release and to refine conservative safety margins. Lots of efforts have already been undertaken using atomistic computer simulations, ab initio calculations and Empirical Potential Molecular Dynamics (EP-MD) techniques, that relate to the bulk behaviour. This article will discuss EP-MD investigations in nano-sized polycrystalline UO₂, constructed from Voronoi cells in a 3-D periodic environment. This study has focused on Xe diffusion at and close to grain boundaries.