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

• Published in: Journal of physical chemistry. C 2024-12, Vol.128 (50), p.21559-21571
• Main Authors: Neilson, William D., Rizk, Jason, Cooper, Michael W. D., Andersson, David A.
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.4c06580

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 UO2±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 UO2±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.