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

Doping UO with Cr modifies the material's microstructure, enhancing its properties and making Cr-doped UO a promising candidate as accident-tolerant nuclear fuel (ATF). Numerous studies have examined the oxidation state and localization of Cr in UO but often yield inconsistent results, identifying either Cr or Cr as the most stable oxidation state. In the present study, DFT+U is employed to model the incorporation of Cr in the UO matrix, providing insights into the oxidation state of Cr in UO, in relation to the local atomic configurations. In particular, we investigate the O local configuration recently proposed by EPR and XANES experiments, alongside other theoretical configurations. Cr 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 UO, offering critical data for developing efficient and safer nuclear fuels.