High temperature thermal and melting properties of uranium–neptunium and plutonium–neptunium mixed oxides: a MD simulation study

Abstract

Mixed oxides (MOX) of uranium, plutonium, and neptunium are proposed as fast-reactor fuels with the aim to transmute long-lived neptunium into shorter-lived, less radioactive products. The use of molecular dynamics (MD) simulations to accurately predict high-temperature thermophysical properties, essential for fuel design, performance modelling, and safety assessment, requires the availability of suitable interatomic potentials of the component oxides. Although the Cooper–Rushton–Grimes (CRG) potential for NpO₂ exists, it cannot predict the melting temperature and other high-temperature thermophysical properties of NpO₂ accurately. In the present work, a new interatomic potential for NpO₂ is developed by re-optimizing the CRG potential, with the target of accurate prediction of the melting temperature (MT). The re-optimized potential predicts the MT (3025 ± 50 K) of NpO₂ in good agreement with the experimental result (3070 ± 62 K), representing a significant improvement over the predicted value (3550 ± 50 K) in this work using the CRG potential, without significantly affecting the other characteristics of the material at lower temperatures. Other thermal properties, such as thermal expansion coefficients (α), enthalpy increments, and specific heats (C_p) of U_1−yNp_yO₂ and Pu_1−yNp_yO₂ (for 0 < y < 0.5) systems computed from MD simulations using the re-optimized potential across temperatures up to MT, show excellent agreement with experimental results, wherever available. The calculated data for the α and C_p of U_1−yNp_yO₂ and Pu_1−yNp_yO₂ (for 0 < y < 0.5) are fitted to the Bathellier equation, enabling the prediction of high temperature fuel behaviour in regimes where experimental measurements are lacking. Additionally, the influence of hyper/hypostoichiometry, prevalent in high-temperature MOX fuels, on the thermal properties is also investigated, providing information highly relevant to reactor operations.