Study of Ba and Zr stability in UO2±x by density functional calculations

Barium and zirconium behavior in nuclear fuels is investigated using density functional theory (DFT). More particularly, incorporation and solution energies of Ba and Zr in preexisting trap sites of UO2 (vacancies, interstitials, U-O divacancy, and Schottky trio defects) are calculated using the projector-augmented-wave method as implemented in the Vienna ab initio simulation package (VASP). Correlation effects are taken into account within the DFT+U approach. Our results are discussed in relation to those based on conventional functionals and with available experimental data. For both functionals, zirconium is found to be much more soluble than barium. However, the most favorable solution site depends on the treatment of correlation effects, the difference between DFT+U and DFT based results being more pronounced for Zr than for Ba. We also demonstrate that the solution process for more complex phases like BaO, ZrO2, BaUO3, and BaZrO3 are very sensitive to the correct description of correlation effects in UO2.