Quantum-mechanical modeling of divalent cation incorporation into uranium dioxide

Abstract Quantum-mechanical calculations using density functional theory were employed to calculate the incorporation energies of divalent cations (e.g., Mg2+, Ca2+, Sr2+, and Ba2+) in uranium dioxide (UO2). Divalent cations are known to incorporate into the uranium dioxide system, albeit the incorporation mechanism remains unclear. In this work, three charge-balancing mechanisms were evaluated to achieve a net neutral system, including the substitution of (1) a divalent cation for a tetravalent uranium atom and oxygen atom; (2) two divalent cations for a tetravalent uranium atom; and (3) a divalent cation for a tetravalent uranium atom and associated oxidation of two additional tetravalent uranium atoms. Multiple incorporation scenarios were considered for each charge-balancing mechanism to determine the lowest energy case. For the energetically favorable incorporation mechanisms, Sr2+ incorporation is favored over Mg2+, Ca2+, and Ba2+ incorporation by 0.11 eV–0.97 eV. The coupled substitution of two divalent cations for one uranium cation is unfavorable. The mechanism where the substitution of a divalent cation for a U4+ atom is accompanied with the oxidation of two U4+ to U5+ (e.g., ΔEincorp (Sr2+) = −3.63 eV), is the most favorable incorporation mechanism compared to the other mechanisms investigated in this paper. In general, the most favorable case is where the oxidized uranium atoms are located at a first nearest neighbor distance (3.914 A) from the substituted divalent cation and are minimally separated (U5+-U5+ distance = 3.936 A).