The aim of this thesis is to provide insight into the structures and properties of the uranium oxides. As UO2 is easily oxidised during the nuclear fuel cycle it is important to have a detailed understanding of the structures and properties of the oxidation products. Experimental work over the years has revealed many stable oxides including UO2, U4O9, U3O7, U2O5, U3O8 and UO3, all with a number of different polymorphs. The oxides are broadly split into two categories, fluorite-based structures with stoichiometries in the range of UO2 to U2O5 and less dense layered-type structures with stoichiometries in the range of U2O5 to UO3. While UO2 is well characterised, both experimentally and computationally, there is a paucity of data concerning higher stoichiometry oxides in the literature. Experiments and simulations are emerging that deal with individual phases, however a comprehensive study that assesses the properties of all polymorphs and provides comparison over the full range of stoichiometries has been lacking from the literature First the nuclear fuel cycle is introduced, as well as UO2 as a nuclear fuel (Chapter 1), before the quantum mechanical methodology used throughout is explained (Chapter 2). Applying a number of different density functionals (including GGAs, meta-GGAs and hybrids) to UO2 in Chapter 3 it emerges that the PBE + U formalism reproduces the experimentally observed properties to a good degree of accuracy, and so is selected for the rest of the simulations. Following this Chapter 4 examines defect clusters in UO2, finding split interstitials to dominate at low stoichiometry (UO2 – UO2.0625), chains of 2:2:2 Willis clusters at higher stoichiometry (UO2.125 – UO2.25 (U4O9)) and split quad interstitials at higher stoichiometry (UO2.33 (U3O7)). Chapter 5 is an investigation of layered U2O5, where it emerges that the Np2O5 structure is more stable than δ-U2O5 and all uranium ions are in the U5+ oxidation state. Next Chapter 6 considers layered U3O8, which is structurally oxygen rich U2O5, where it is found that U5+ and U6+ ions exist in pentagonal bipyramidal and octahedral coordination respectively. The final set of results in Chapter 7 concern the polymorphs of UO3, where it is found that U6+ adopts a range of coordination environments and the predicted relative stability of each modification matches well with experiment. Finally the conclusions are presented in Chapter 8 along with plans for future work.

Uranium (U) exhibits a high temperature body-centered cubic (b.c.c.) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U-Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Metallic nuclear fuels swell under fission conditions, creating fission product gases such as helium, xenon and krypton. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. Two separate generalized gradient approximations of the exchange-correlation are used to calculate defect properties and are compared. The bulk modulus, the lattice constant, and the Birch-Murnaghan equation of state for the defect free b.c.c. uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr, He, Xe and Kr interstitial and substitutional defects. The results for vacancies agree very well with experimental and previous computational studies. The most probable self-interstitial site in γ-U is the <110> dumbbell and the most probable defect location for dilute Zr in γ-U is the substitutional site. The most likely position for Xe and Kr atoms in uranium is the substitutional site. Helium atoms are likely to be found in a wide variety of defect positions due to the comparable formation energies of all defect configurations analyzed.