Actinide Dioxides in Water: Interactions at the Interface

A comprehensive understanding of chemical interactions between water and actinide dioxide surfaces is critical for safe operation and storage of nuclear fuels. Despite substantial previous research, understanding the nature of these interactions remains incomplete. In this work, we combine accurate calorimetric measurements with first-principles computational studies to characterize surface energies and adsorption enthalpies of water on two fluorite-structured compounds, ThO2 and CeO2, that are relevant for understanding the behavior of water on actinide oxide surfaces more generally. We determine coverage-dependent adsorption enthalpies and demonstrate a mixed molecular and dissociative structure for the first hydration layer. The results show a correlation between the magnitude of the anhydrous surface energy and the water adsorption enthalpy. Further, they suggest a structural model featuring one adsorbed water molecule per one surface cation on the most stable facet that is expected to be a common structural signature of water adsorbed on actinide dioxide compounds.