Effects of Charge State on Chromium Substitution and Oxygen Vacancy Segregation Energies at Grain Boundaries in Uranium Dioxide

Uranium dioxide is the main fuel used in light water reactors due to its relative radiation stability, high melting point, and chemical stability. However, fission gas release during reactor operations leads to degradation in thermal conductivity with an associated increase in thermal stresses and swelling. Promoting large grains during fuel sintering slows down fission gas diffusion to grain boundaries and increases fuel plasticity. Transition metal oxide dopants such as chromia are important for tuning this process. Previous work has shown that chromium solubility depends on its charge state, but there is disagreement about what charge states of chromium exist within the bulk versus at the grain boundary. Recent spectroscopic studies of chromia doped uranium dioxide suggest that the charge state of the chromium ion exists in a +3 charge state in the bulk, while a +2 charge state can be found in grain boundaries, challenging previous ab initio simulation work that points to a +2 charge state being energetically favorable in the bulk. To resolve the discrepancies, this work uses density functional theory calculations to identify the relationship between the charge state and the local atomic environment at the grain boundaries. Coincident site lattice grain boundary structures were generated, and chromium substitution energies were calculated while varying the charge state and defect position, allowing the analysis of the charge state’s effect on grain boundary segregation energy to inform dopant engineering. This work lays the foundation for a machine-learned interatomic potential to model the chemical behavior of transition metal dopants in actinide oxides beyond the time and length scales that can be modeled by density function theory.