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

• Electronic structure and mechanical properties of UO2 and U3O8 are calculated.
• The transformation from UO2 to U3O8 results in a 35–42% volume increase.
• The implications of volume expansion on the linear expansion is discussed.
• Hexagonal and orthorhombic pseudo-hexagonal phases of U3O8 are compared.
• The effect of Hubbard U on the electron spin and charge density is shown.

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.

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