Computational Study of Plutonium-Americium Mixed Oxides (Pu0.92Am0.08O2-X); Water Adsorption on {111}, {110} and {100} Surfaces

Hubbard U corrected generalized gradient approximation density functional theory is used to study the surface properties of plutonium-americium mixed dioxides (Pu-Am MOX). Stoichiometric and oxygen vacancy defect {111}, {110} and {100} surfaces are investigated, including the molecular and dissociative adsorptions of water. Comparison with previous work from our group on PuO2 and AmO2 leads to the conclusion that substituted Am gains electrons from the PuO2 slab. In agreement with previous experimental studies, when the oxygen to metal ratio of MOX decreases, substituted Am is reduced to the trivalent state before the reduction of Pu. The geometries of water adsorption are similar to adsorption on PuO2 and AmO2 surfaces, while the presence of Am in PuO2 surfaces promotes water adsorption. Higher temperature is required for dissociation of molecularly adsorbed water on Pu-Am MOX than on the corresponding PuO2 surfaces, while desorption of dissociatively adsorbed water on PuO2 is harder than on the corresponding MOX surfaces.