Paper No. 122-3
Presentation Time: 9:00 AM-6:30 PM
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 O2-, 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 O2- vacancy, and at a bisector between the U4+ and O2- 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.