QUANTUM-MECHANICAL CALCULATIONS OF CA2+-INCORPORATION INTO UO2

The mechanism of cation incorporation into solid phases can elucidate the thermodynamic stability and kinetic feasibility of the doped systems. Divalent cation incorporation into uranium dioxide (UO₂) is key for understanding daughter product (e.g., Pb²⁺) distribution in uraninite, fission product (e.g., Sr²⁺) distribution in used nuclear fuels, and impurity (e.g., Ca²⁺) controls on microstructural features in stored actinide oxides. In this study, quantum-mechanical calculations are used to evaluate the charge-balanced mechanisms of Ca²⁺ substitution for U⁴⁺ in a 2×2×2 UO₂ supercell. The Hubbard U formalism as implemented in VASP was used to properly treat the bonding characteristics of the uranium 5f electrons. Reaction energetics were determined for three charge-balanced substitution mechanisms: 1) Ca²⁺ substitution for U⁴⁺ with an associated loss of an O^2-, 2) Two Ca²⁺ substitution for one U⁴⁺, and 3) Ca²⁺ substitution for U⁴⁺ with oxidation of another U atom to 6+. For case 1, Ca²⁺ is substituted in the U⁴⁺ vacancy, in the O^2- vacancy, and at a bisector between the U⁴⁺ and O^2- vacancies. All initial positions converged to Ca²⁺ substitution in the U⁴⁺ vacancy, and the resulting incorporation energy based on oxide source and sink phases is ‑0.61 eV. For case 2, two Ca²⁺ are substituted on the faces of the cube formed by the oxygen coordinating the U⁴⁺ vacancy, and results in a highly unfavorable incorporation energy (5.22 eV). For case 3, the initial Ca²⁺ is substituted in the U⁴⁺ vacancy, and the charge balancing U⁶⁺ is positioned at various distances from the Ca²⁺ (3.91 Å ‑ 8.71 Å). The coupled oxidation of U with incorporation of Ca²⁺ was the most favorable incorporation mechanism (-2.00 eV). Interestingly, charge density analysis shows the presence of U⁵⁺, rather than U⁶⁺ in the optimized structures. Additional calculations will compare larger divalent cation (i.e. Pb²⁺) incorporation based on the aforementioned incorporation mechanism, as well as incorporation of divalent cations (e.g., Mg²⁺) in PuO₂.