Oxygen Potential, Uranium Diffusion, and Defect Chemistry in UO2±x: A Density Functional Theory Study

Abstract

Point defects play a crucial role in controlling the thermodynamic and kinetic properties of materials; in UO₂ nuclear fuel, they impact critical engineering-scale fuel performance properties, such as creep, fission gas release, and thermal conductivity. This work builds a point defect model informed using defect energies calculated by density functional theory (DFT) and vibrational entropies calculated by empirical potential calculations to predict point defect concentrations in UO_2±x. The DFT methodology uses large supercells and considers dispersion interactions, spin–orbit coupling, and noncollinear magnetic contributions. The result is a model that enables quantitative investigation of UO₂ defect chemistry over a wide range of conditions. Experimental validation is achieved in the deviation of x in UO_2±x as a function of temperature and partial pressure of oxygen, being facilitated by oxygen-type defects. By considering lattice thermal expansion when calculating the mobility of uranium vacancies, we have also been able to validate the model against experimentally measured uranium self-diffusivity; the calculated temperature dependence in the activation energy of uranium self-diffusion prompts interesting implications for future studies of defect transport in materials more generally.