DFT+U investigation of local configurations and oxidation states of Cr in Cr-doped UO2

Doping UO2 with Cr modifies the material's microstructure, enhancing its properties and making Cr-doped UO2 a promising candidate as accident-tolerant nuclear fuel (ATF). Numerous studies have examined the oxidation state and localization of Cr in UO2 but often yield inconsistent results, identifying either Cr2+ or Cr3+ as the most stable oxidation state. In the present study, DFT+U is employed to model the incorporation of Cr in the UO2 matrix, providing insights into the oxidation state of Cr in UO2, in relation to the local atomic configurations. In particular, we investigate the Cr_x³⁺ U_1-x⁴⁺ O2−0.5x local configuration recently proposed by EPR and XANES experiments, alongside other theoretical configurations. Cr3+ is found to be the most favorable oxidation state in this configuration, agreeing with the most recent experimental data. This work clarifies the controversy over Cr oxidation states and incorporation sites within UO2, offering critical data for developing efficient and safer nuclear fuels. Doping UO2 with Cr enhances its properties and makes it a promising accident-tolerant nuclear fuel, yet the stable oxidation state of Cr in Cr-doped UO2 remains debated. Here, the authors use DFT+U modeling to reveal Cr3+ as the most favorable oxidation state in Cr-doped UO2, aligning with the most recent experimental results.