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 UO2 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.