Theoretical Investigation on Incorporation and Diffusion Properties of Xe in Uranium Mononitride

Abstract

The incorporation and diffusion behaviors of Xe in uranium mononitride (UN) have been studied using first-principles density functional theory calculations. The incorporation and binding energies of Xe located at different sites are calculated. Because of strain relief related to moving Xe atom from highly strained interstitial site into the large steric vacancy site, a stronger binding energy between the incorporated Xe and the large steric vacancy forms. Using ab initio molecular dynamics simulations and climbing-image nudged elastic band calculations, we found that the activation barrier of interstitial Xe in UN in the “kick-out” diffusion mechanism is lower than that in the direct interstitial mechanism, and the net Xe diffusion occurs with vacancies mediated; that is, once an interstitial Xe atom is trapped in a U vacancy site, it will be immobile without other uranium vacancies mediated.