Urania fuel degradation to U3O8 and point defects and fission products studies.

Abstract

The conventional Urania (UO₂) fuel oxidizes to higher oxides compositions. In particular, there are about 67% more oxygen atoms per uranium atom in U₃O₈ than in Urania. In the P6̅2m hexagonal structure of U₃O₈, all uranium sites are equivalent; therefore, when assuming the exact symmetry for the electronic structure, the simple valency consideration leads to a non-integer ionic valency of 5.33 (1.33 valence increase from the quadrivalent ionic state in UO₂). Therefore, no band gap is formed unless symmetry restrictions are removed, and uranium ions are allowed to have different valences. This work discusses the point defects and fission product formation in the hexagonal phase of U₃O₈ utilizing density functional theory-based methodologies. In particular, the formation of uranium, oxygen, and antisite defects are studied. The incorporation of three gasses fission products, namely Kr, Xe, and He, and the precipitation of Zr are analyzed. The results suggest a much higher energy cost for forming a uranium vacancy compared to oxygen or antisite defects and a slight volume expansion due to oxygen (0.003 %) while a volume contraction with uranium (0.414%) or antisite (0.677%) vacancies. Further, we show that Kr forms gas bubbles causing fuel swelling, while Zr dissolves to the lattice.