First-Principles Study on the Migration and Release Properties of Xe on the Surface of Uranium Mononitride

The fission gas uranium mononitride (UN) causes swelling and affects the properties of fission fuel. Since surface behavior is closely related to the release of gases, it is crucial to study the properties of Xe on the UN surface. Density functional theory was used to study the properties of Xe gas on the UN(001) surface and subsurface layers. Different bulk and surface models of UN were established, and the formation energies of bulk and surface defects, as well as the incorporation energy of surface Xe, were calculated. Differential charge density maps were generated, and the analysis revealed that the migration of Xe atoms on the surface predominantly occurs through a vacancy mechanism. Furthermore, Xe atoms located in the subsurface and interstitial positions are less likely to escape from the surface due to the influence of surrounding atoms. Finally, the Climbing Image Nudged Elastic Band method was employed to calculate migration pathways and the associated migration energies. The modelling results indicated that surface Xe atoms’ migration exhibits a vacancy-assisted mechanism, while surface and subsurface U-vacancies on the UN surface may promote the diffusion of fission gas atoms.