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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>Cr</mml:mi></mml:mrow> <mml:mrow><mml:mi>x</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msubsup> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>U</mml:mi></mml:mrow> <mml:mrow><mml:mn>1</mml:mn> <mml:mo>-</mml:mo> <mml:mi>x</mml:mi></mml:mrow> <mml:mrow><mml:mn>4</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msubsup> </mml:math> O_2-0.5x 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.