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The stabilities of selected fission products—Xe, Cs, and Sr—are investigated as a function of non-stoichiometry x in UO 2±x. In particular, density functional theory (DFT) is used to calculate the incorporation and solution energies of these fission products at the anion and cation vacancy sites, at the divacancy, and at the bound Schottky defect. In order to reproduce the correct insulating state of UO 2, the DFT calculations are performed using spin polarization and with the Hubbard U term. In general, higher charge defects are more soluble in the fuel matrix and the solubility of fission products increases as the hyperstoichiometry increases. The solubility of fission product oxides is also explored. Cs 2 O is observed as a second stable phase and SrO is found to be soluble in the UO 2 matrix for all stoichiometries. These observations mirror experimentally observed phenomena.