Kinetic Study of the Oxidation of Uranium Dioxide

Oxidation of UO₂ to U₃O₈ in dry air and oxygen at temperatures of 160 to 350°C proceeds by a two‐step reaction. In the first step the tetragonal oxide of composition UO_2.34±0.03 is formed. In the second step this tetragonal phase is converted to orthorhombic U₃O₈. The rate controlling process in the first stage is the diffusion of oxygen through the uranium dioxide lattice. The value found for the diffusion coefficient D is $${\text{D=5.5×10}}^{\text{−3}}\exp \text{[−(26 300±1500)/RT],}$$ where the activation energy is 26.3±1.5 kcal/mole. The second stage of the reaction proceeds by a process of nucleation and growth.