Corrosion of UO2 and ThO2 : A quantum-mechanical investigation

The addition of Th to U-based fuels increases resistance to corrosion due to differences in redox-chemistry and electronic properties between UO2 and ThO2. Quantum-mechanical techniques were used to calculate surface energy trends for ThO2, resulting in (1 1 1) < (1 1 0) < (1 0 0). Adsorption energy trends were calculated for water and oxygen on the stable (1 1 1) surface of UO2 and ThO2, and the effect of model set-up on these trends was evaluated. Molecular water is more stable than dissociated water on both binary oxides. Oxidation rates for atomic oxygen interacting with defect-free UO2(1 1 1) were calculated to be extremely slow if no water is present, but nearly instantaneous if water is present. The semi-conducting nature of UO2 is found to enhance the adsorption of oxygen in the presence of water through changes in near-surface electronic structure; the same effect is not observed on the insulating surface of ThO2.