| Abstract Scope |
UO2 can be engineered through doping to enhance grain growth, allowing for a purported enhancement of fission gas retention and plasticity. In this work, we investigate the behavior of Cr at UO2 grain boundaries and reveal a mechanism responsible for enhanced grain growth under environmental conditions representative of sintering. Grain boundaries are modeled using a combined approach of density functional theory and empirical potentials to explore the tendency of intrinsic defects, as well as Cr in various oxidation states, to segregate to grain boundaries as a function of temperature, with explicit consideration of space charge effects. Our new model sheds light on several important phenomena. We find a significant enhancement of uranium vacancy concentrations at grain boundaries, highlighting their critical role in mediating mass transport processes such as creep. Cr segregation to grain boundaries is thermodynamically favorable, and high grain boundary Cr concentrations are coupled with increased grain boundary electronic conductivity. At elevated temperatures, positively charged Cr interstitials preferentially segregate to the boundaries, which leads to an increase of negatively charged uranium vacancies. This, in turn, results in higher uranium diffusivity and accelerated grain growth kinetics. Overall, our findings provide new insight into the atomistic mechanisms by which Cr doping influences UO2 properties. |