<i>Ab Initio</i> Atomistic Thermodynamics of Water Reacting with Uranium Dioxide Surfaces

Abstract

Using first-principles simulations, we study the temperature- and pressure-dependent adsorption reaction of water on the flat (111) and (211) and (221) stepped surfaces of uranium dioxide. Our calculations are based on the density functional theory (DFT) corrected for on-site Coulomb interactions (DFT+U) for describing the chemical interaction of water with UO2, in combination with ab initio molecular dynamics simulations to capture the temperature dependence of the reaction. We compute the pressure–temperature phase diagrams and establish the thermodynamic boundaries which govern the feasibility of water adsorption at these surfaces. Effects of water coverage on the surface adsorption reaction have been taken into account. We find that the dissociative adsorption reaction of water on stepped surfaces can be analyzed as two separated reactions, the dissociative water adsorption on the step edge and the water adsorption on the terrace. The most stable water adsorption upon modification of the water partial pressure and temperature is adsorption on the (211) step edge, followed by adsorption on the (221) step edge and being the least favorable for the (111) surface. We conclude that these UO2 surfaces will always react with water at room temperature and atmospheric pressure, leading to water dissociation and a modification of the step morphology.