Computational Study of Plutonium–Americium Mixed Oxides (Pu_0.92Am_0.08O_2–x); Water Adsorption on {111}, {110}, and {100} Surfaces

Abstract

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 PuO 2 and AmO 2 leads to the conclusion that substituted Am gains electrons from the PuO 2 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 PuO 2 and AmO 2 surfaces, while the presence of Am in PuO 2 surfaces promotes water adsorption. Higher temperature is required for dissociation of molecularly adsorbed water on Pu-Am MOX than on the corresponding PuO 2 surfaces, while desorption of dissociatively adsorbed water on PuO 2 is harder than on the corresponding MOX surfaces.

DFT + U Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control

“…Due to the high radioactivity of PuO 2 , experiments are very challenging, and hence theoretical simulations play a particularly valuable role in its study. Density functional theory (DFT) with a Hubbard U correction is widely used ,, as it gives reasonable predictions at the lowest computational costs. However, the ability of the DFT + U approach to correctly identify the NM magnetic ground state of PuO 2 remains an issue.…”

Electron correlation and relativistic effects on the electronic properties of a plutonium and americium mixed oxide (PuAmO4): from single-particle approximation to dynamical mean-field theory

“…6 On the experimental side, besides the scarcity of available materials, the high radiotoxicity, activity, and toxicity pose a huge challenge for the direct observation of (Pu, Am) MOX. 1,9 Theoretically, (Pu, Am) MOX also poses challenges due to the incompletely occupied 5f orbitals, coexistence of itinerancy and localization of 5f electrons, 1 variable oxidation/valence states from charge transfer and near-degeneracy of Am/Pu 5f, 6d and 7s orbitals 6 and Pu/Am-O chemical bonding. 1 In analogy to (Pu, U) MOX, 10 the oxidation/valence states, and 5f configuration of Pu/Am ions have a significant effect on the thermal conductivity and metal-atom diffusion (thus the creep, grain growth, sintering, and swelling) in (Pu, Am) MOX.…”

Evolving Defect Chemistry of (Pu,Am)O2±x

“…Am builds up relatively quickly due to the short half-life of 241 Pu (14.4 years), with Am concentrations peaking after approximately 70 years, at which point, it too begins to decay faster than it is produced . Incorporation of Am is predicted to alter the defect chemistry of PuO 2 ; in a density functional theory (DFT) investigation on Pu–Am mixed oxide surfaces, Chen et al report that the presence of Am promotes the formation of O vacancies that increase the favorability of molecular adsorption of water on PuO 2 surfaces while reducing the favorability of dissociative water adsorption. The consequence of this could be an increased likelihood of chemical reactions including the aforementioned pressurization …”