Abstract
After their stay in the reactor, spent nuclear fuels (SNF) are still composed of more than 95 wt.% of uranium dioxide (UO2) and 1 wt.% plutonium dioxide, which constitute valuable secondary resources. In France, the SNF can be reprocessed in order to recover remaining uranium (and plutonium). The head-end step of this process deals with the dissolution of the SNF in hot and concentrated nitric acid, followed by the separation and purification of uranium and plutonium oxides. Besides, one of the potential solutions for long-term isolation of the end-of-life SNF is its direct disposal in deep geological repositories, where UO2 surfaces could be in contact with water with various possible physical-chemical conditions. Acquiring a thorough understanding of the stability of UO2 in various physical-chemical conditions is therefore of paramount interest to (1) assess the long-term behavior of the end-of-life SNF in geological repository conditions and (2) finely tune the UO2 dissolution stage in the reprocessing of the SNF. Here, for the first time, we employed ab initio molecular dynamics simulations to thoroughly characterize the hydration mechanisms of the (111) surface -the main cleavage plane- of UO2, and, particularly, the dynamic equilibrium of this surface in presence of pure water. The surface coverage was gradually increased from a single water molecule to the system where the vacuum above the surface was completely filled with water molecules (see picture). For a single water molecule, the molecular adsorption was significantly favored over the dissociated adsorption. However, for two water molecules both introduced in their molecular form, one adsorbed under its molecular form while the second spontaneously dissociated on the surface; the OH- anion adsorbed on an uranium atom of the surface while the H+ ion adsorbed on a surface oxygen atom. For higher surface coverages, we systematically observed that a significant part of the water molecules spontaneously dissociated on the surface while ΔHads tended towards the enthalpy of condensation of water with increasing coverages. When the cell was completely filled with water, about a third of the surface uranium atoms were hydroxylated, which meant that 33% of the water molecules of the monolayer dissociated on the UO2 (111) surface. A dynamic equilibrium between the bulk of water and the surface was established, with frequent recombination/dissociation of water molecules as well as adsorption/desorption of HO-, H+, or H2O. This work will serve as a basis for further studies, particularly about the dissolution mechanisms of UO2 in the presence of nitric acid.