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. Use of molecular dynamics (MD) simulations to accurately predict high-temperature thermophysical properties, essential for fuel design, performance modelling, and safety assessment, requires availability of suitable interatomic potentials of the component oxides. Although Cooper-Rushton-Grimes (CRG) potential for NpO2 exists, it cannot predict melting temperature and other high-temperature thermo physical 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), significantly improving over the prediction (3550±50 K) as computed 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, specific heats (Cp) of U1−yNpyO2 and Pu1−yNpyO2 (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 α and Cp of U1−yNpyO2 and Pu1−yNpyO2 (for 0 < y < 0.5) are fitted to the Bathellier equation, enabling prediction of high temperature fuel behaviour in regimes where experimental measurements are lacking. Additionally, the influence of hyper-/hypo-stoichiometry, prevalent in high-temperature MOX fuels, on the thermal properties is also investigated, providing information highly relevant to reactor operations.