Simulating the Behaviour of Inert Gases in UO2

Abstract

The behaviour of He, Ne, Ar, Kr and Xe in U0 2 has been investigated using the semi-classical Molt-Littleton simulation technique. The interactions of the gas atoms with the lattice show two extremes of behaviour characterised by the properties of Xe and He. Xenon is very insoluble-its retention in U0 2 is a consequence of the large activation energy necessary for diffusion (>3eV). The most stable solution site for Xe is a function of stoichiometry: in U02-x• the solution energy is lowest at a IIi-vacancy; in U0 2 , at a di-vacancy and in U0 2 +x' at a uranium vacancy. In comparison with xenon, helium is small and readily accommodated at either a defect or interstitial site (solution energy-O.leV). Despite this, in the perfect lattice, He exhibits a barrier of-4eV to diffusion between interstitial sites. However, in the presence of defects, diffusion barriers are reduced to-0.2e V. These results are discussed with respect to the range of vacancy and vacancy clusters (solution sites) of the type expected in a highly defective or radiation damaged material.