Impact of small-molecule adsorbates on the morphology of PuO₂ nanoparticles from first-principles modelling

The safe management of legacy civil plutonium stockpiles is among the most difficult challenges facing the nuclear industry. While geological disposal facilities (GDFs) are seen as the best solution, anticipating the evolution of waste forms over the lifetime of the GDF forms a critical part of the safety case. In the typical storage form of PuO₂ powders, the chemical reactivity of Pu is determined by the exposed crystal facets and surface speciation, which are a complex function of temperature and the partial pressures of oxygen and small-molecule adsorbates. In this work, we use a first-principles modelling approach to develop a predictive thermodynamic model for the impact of the ubiquitous environmental contamininats H₂O, CO₂ and H₂O₂ on the equilibrium particle morphpology and surface speciation of stoichiometric and oxygen-deficient PuO₂. We find that the presence of multiple adsorbates can lead to both synergistic and antagonistic interactions, with significant impacts on the energeticallyaccessible nanoparticle morphologies and the exposure of the major {100}, {110} and {111} facets. Our model provides important reference data for the impact of environmental conditions on the surface chemistry of PuO₂, and can be systematically improved to account for other variables including additional adsorbates, solvation, and surface coverage.