Role of electronic effects on the incorporation of Cr at a Σ5 grain boundary in UO₂

Due to the important role of microstructure on nuclear fuel performance, it is desirable to obtain improved control of fuel microstructure. For example, Cr₂O₃ has been used as a grain growth promoter during the sintering of UO₂; however, the influence of Cr incorporation on the bonding within the grain boundary is not well understood. Here, we quantify the energetics associated with Cr incorporation and segregation in a model UO ₂ system with a Σ5 symmetric tilt grain boundary using density functional theory calculations. The results indicate that Cr prefers to reside in a U substitutional site and segregate to the grain boundary. The Bader charge analysis and the charge density difference analyses confirm that Cr forms bonds with neighboring O atoms that weaken the ionic nature of adjacent U-O bonds, especially when it occupies U substitutional site at the grain boundary. The implications of these findings for the usage of Cr₂O₃ as a growth promoter are discussed.