The+2 oxidation state of Cr incorporated into the crystal lattice of UO2

Doping by Cr is used to improve the performance of uranium dioxide (UO2)-based nuclear fuel. However, the mechanism of structural incorporation of Cr remains unclear. Here, in order to understand this process on the atomic scale and the redox state of Cr in UO2-based nuclear fuel, we performed intensive ab initio atomistic simulations of the Cr doped UO2 matrix. We unexpectedly found that Cr in UO2 exists as Cr2+ species and not as the widely claimed Cr3+. We re-evaluated previously published x-ray absorption near edge structure spectroscopy data and confirmed the computed redox state of Cr. Thermodynamic consideration shows that the favorable structural arrangement of Cr in UO2 is given by a pair of associated Cr2+ and oxygen vacancy. The realism of this doping mechanism is further demonstrated by a match to the measured maximum Cr solubility and small lattice contraction.