Résumé
A good knowledge of the surface chemistry of uranium dioxide (UO) is critical for optimizing nuclear fuel reprocessing and ensuring long-term disposal of spent nuclear fuel. In this work, first-principles molecular dynamics (DFT-MD) simulations were used to investigate the interaction of water molecules with the three most stable UO2 surfaces: (111), (110), and (100). The stability of these surfaces was first assessed, confirming that the (111) surface is the most stable, followed by the (110) and finally the (100) surfaces, consistent with surface energy calculations. Water was gradually introduced to the surfaces, starting with single-molecule adsorption. It was found that water adsorption predominantly occurred in molecular form on the (111) and (110) surfaces, with partial dissociation, while the (100) surface exhibited full dissociation of the adsorbed molecules due to its higher reactivity. As water coverage increased, isosteric adsorption energies decreased, indicating that water molecules interacted with the surface through hydrogen bonding, forming structured layers. The simulations revealed that each surface influenced water molecules up to two or three layers, beyond which bulk behavior dominated. Notably, the (111) surface generated three well-defined water layers with strong molecular orientation, while the (110) surface showed fewer layers with less defined structure, and the (100) surface transitioned to bulk water behavior after a single layer. At equilibrium with water, the surfaces exhibited distinct hydration behavior: the (111) and (110) surfaces favored molecular adsorption (around 65%) with a slight preference for proton adsorption over hydroxyls, leading to a positive surface charge of around ~1 e.nm-2 and ~0.25 e.nm-2 for the (111) and (110) surfaces, respectively. On the other hand, the (100) surface was nearly neutral and was fully hydroxylated, leading to its early passivation.