Implication of volume changes in uranium oxides: A density functional study

In severe nuclear accident scenarios (in air environments and high temperatures) UO₂ fuel pellets oxidise to produce uranium oxides with higher oxygen content, e.g., U₄O₉ or U₃O₈. As a first step in investigating the microstructural changes following UO₂ oxidation to hexagonal high temperature phase of U₃O₈, density functional quantum mechanical calculations of the structure, elastic properties and electronic structure of U₃O₈ have been performed. The calculated properties of hexagonal phase of U₃O₈ are compared to those of the orthorhombic pseudo-hexagonal phase which is stable at room temperature. The total energy technique based on the local density approximation plus Hubbard U as implemented in the CASTEP code is used to investigate changes in the lattice constants. The first-principles calculations predict a 35–42% increase in volume per uranium atom as a result of the transformation from UO₂ to U₃O₈, in agreement with experimental data. The implications of this prediction on the linear expansion and fragmentation of fuel are discussed.