The corrosion of uranium dioxide: An atomic-scale investigation

Spent nuclear fuel is composed of ≈95% UO2, and upon storage in geologic repository the fuel itself is a barrier to the release of radionuclides. Therefore, it is important to understand the behavior of oxygen and water with UO2 surfaces in order to determine the long-term stability of spent fuel. UO2 corrodes faster in the presence of water and oxygen as compared with oxygen alone. Fortunately, the addition of thorium to UO2 fuels increases the resistance to oxidation and decreases dissolution rates. In this thesis, atomic-scale quantum-mechanical and empirical-potential modeling methods are used to develop a mechanistic understanding of the effect of surface structure and surface chemistry on the interaction of adsorbates with UO2 and ThO2 surfaces. Oxidation rates are calculated for atomic oxygen interacting with specific UO2 surfaces, and an atomicscale model is proposed for the enhancement of UO2 oxidation rates in the presence of water. Bulk oxidation mechanisms are also explored for UO2.; Surface energy calculations were performed to establish the relative reactivity of UO2 and ThO2 surfaces that are found in polycrystalline fuel pellets. Adsorption energy trends for water, oxygen, and combinations thereof were determined on the stable (111) surface and on the more reactive (110) surface of UO2 and ThO2. Oxidation rates for atomic oxygen interacting with the UO2 (111) surface are slow in comparison to the (110) surface, for which rates are nearly instantaneous. The presence of water on both surfaces significantly lowers the activation energy to oxidation, and low-spin oxygen is found to be a necessary precursor to uranium oxidation. The semi-conducting nature of UO2 enhances oxidation in the presence of water, a phenomenon not observed on the insulating ThO2 surface. Bulk oxidation mechanisms for UO2 were also explored and two U5+ ions are found to be more stable that one U6+ ion upon the addition of O2- to the UO2 structure to form U4O9. The presence of U5+ versus U6+ has implications for oxygen diffusion rates in bulk UO2. These results provide insight into the properties that make nuclear fuel more corrosion resistant and the atomic-scale mechanisms that control oxidation and corrosion.