Formation energy and diffusion barrier of point defects in three uranium compounds as nuclear fuels

During fission neutron irradiation, point defects are generated in uranium fuel, and the point defects inevitably have some effects on the properties of the uranium fuel. In particular, the point defects accumulate through diffusion, resulting in formation of body defects that degrade the properties of the fuel. Using the first principles calculation method, based on the density functional theory, we calculated the elastic properties of three kinds of uranium compounds (UO₂, UN, and UC). The results are in agreement with calculated and experimental results of others. On the basis of verifying the accuracy of the calculation model, we investigated formation energy and diffusion barrier of three kinds of point defects (vacancy, self-interstitial atom, and hydrogen/helium (H/He) atoms) in the three kinds of uranium compounds. The formation energy of the U vacancy is lower than that of the O, N, and C vacancy in the three kinds of uranium compounds, whereas the interstitial U atom has a higher formation energy than the interstitial O, N, and C atoms. The formation energy of the interstitial H atom is much lower than that of the interstitial He atom in the three kinds of uranium compounds, and the H atom prefers to occupy the O, N, and C vacancy while the interstitial He atom prefers to occupy the U vacancy. The diffusion of the interstitial H atom in UO₂ is easier than that of the interstitial He atom, whereas the diffusion of the interstitial He atom in UN and UC is easier than that of the interstitial H atom. The diffusion of the interstitial self-atom in UO₂ is more difficult than that of the self-atom in UN and UC. The work provides a fundamental understanding of the point defects formation and diffusion to develop uranium fuel by controlling defect formation.