High-temperature thermal and melting properties of AmO2-x and (Pu,Am)O2-x mixed oxides.

The present study uses comprehensive molecular dynamics (MD) simulations utilizing newly developed interatomic potential models for the Am3+-O2- system to investigate the thermophysical properties and phase stability of AmO2-x and Pu1-yAmyO2-x mixed oxides (MOX) in the temperature range 300-3000 K. The new potential successfully captures the 7-coordinated Am3+ structure of hexagonal A-Am2O3, showing excellent agreement with high-temperature X-ray diffraction measured lattice parameters and demonstrating superior sensitivity to stoichiometry changes (x) in the cubic fluorite C-AmO2-x compared to existing potential models for the prediction of structural properties. The MD simulations reveal that, while AmO2 undergoes a gradual, higher-order diffuse Bredig transition, the hexagonal A-Am2O3 sesquioxide exhibits a cooperative, first-order-like structural transition at 2300 K, a divergence strongly supported by the calculated specific heat capacity and enthalpy trends. In the Pu1-yAmyO2-x MOX, high-temperature transition energetics are governed by the oxygen-to-metal (O/M) ratio, where severe hypo-stoichiometry reduces thermal sensitivity and elevates the threshold for anion sublattice disordering. Pure oxides exhibit predicted melting temperatures of 2775 ± 12.5 K (A-Am2O3), 2500 ± 12.5 K (C-Am2O3), and 2800 ± 12.5 K (AmO2), reproducing experimental phase diagrams. In the (Pu,Am)O2-x MOX, substituting AmO2-x into the PuO2 lattice forces a charge-compensating Am4+ → Am3+ reduction. The resulting ionic size mismatch expands the lattice, induces non-ideal solution mixing, and systematically depresses both the solidus and liquidus boundaries. Quantitative coherence between microscopic hop-counting and macroscopic fitting validates the random-walk framework, demonstrating that geometry-dependent oxygen pathways in trigonal Am2O3 and fluorite AmO2 converge on a shared, highly isotropic macroscopic diffusion behaviour, despite extensive high-temperature anion disorder. These insights provide a robust predictive framework for evaluating the thermal performance and limits of advanced americium-bearing oxide nuclear fuels.