Ab initio Calculations of Defect Energetics in UO2

The behaviour of fission gases is an important parameter for the safety of nuclear fuel material. Vacancies are thought to be a major diffusion channel for fission gases in UO2. It is important to understand the nature of defects to investigate the diffusion mechanism in nuclear reactors. Much theoretical research has been devoted studying the diffusion of fission gases through vacancies. However, there have been some debates regarding the calculated results. Grimes et al. predicted that di-vacancy emits the lowest solution energy for xenon (Xe) and krypton (Kr) atoms in UO2 using the Mott-Littleton approximation with empirical potentials. Petit et al applied the density functional theory using the local density approximation (DFT-LDA) with a supercell containing 24 atoms, and suggested that tri-vacancy is the most stable solution site for Kr. In this work, we applied the density functional theory using the generalized gradient approximation (DFT-GGA) method and took advantage of a 98-atom supercell. We calculated the solution energy of Xe at various vacancy sites, and determined the most stable positions of Xe. From the obtained results, we suggest that tri-vacancy is the most preferable accommodation site of Xe and the most important defect site affecting the diffusion process of fission gases in UO2