Chromium Accommodation at UO ₂ Grain Boundaries: Defect Chemistry Insights into Cr-Induced Grain Growth

The mechanism by which chromium is accommodated in UO2 and its role in accelerating grain growth remain subjects of debate. This work presents an atomistic analysis of chromium incorporation in both bulk UO2 and grain boundaries, explicitly accounting for space-charge effects. A Σ9 symmetric tilt and an asymmetric boundary are modeled within a thermochemical framework to evaluate defect behavior. We predict hyperstoichiometric, negatively charged grain boundary cores, compensated by hole polarons in adjacent space-charge layers. Chromium is accommodated as Cr2+ in the bulk and segregates to grain boundaries in multiple states, with interstitial Cr1+ dominating under sintering conditions. This enhances grain boundary uranium vacancy concentrations, promoting densification and grain growth. The effect is predicted to occur within an oxygen potential window consistent with experimental observations of enhanced grain growth. Chromium also increases polaron concentrations at grain boundaries and raises the electron potential barrier across the boundary, reducing conductivity. The findings help clarify how dopants are accommodated in metal oxides and the consequent impacts on their chemical, electrical, and mechanical behavior.