Development of an interatomic potential for mixed uranium-americium oxides and application to the determination of the structural and thermodynamic properties of (U,Am)O2 with americium contents below 50%

B Labonne and S Orlat and M Bertolus, JOURNAL OF NUCLEAR MATERIALS, 579, 154390 (2023).

DOI: 10.1016/j.jnucmat.2023.154390

Uranium-americium mixed oxides are envisaged as targets in fast neutron reactors to allow the transmu-tation of americium in view of the reduction of its quantity in nuclear waste. The knowledge of structural and thermodynamic properties of (U,Am)O2 fuels is of crucial importance for fuel manufacturing, as well as for safe reactor operation. Atomic scale modelling has been proved an important complement to ex-perimental characterizations to obtain the necessary data and bring insight into the behaviour of fuel materials over a wide range of compositions and conditions. An important feature of (U,Am)O2 com-pounds observed experimentally by several authors and confirmed by electronic structure calculations for all compositions, is the presence, not only of cations in the ( + IV) oxidation state, but also of the Am3 + and U 5 + species, inducing a complex behaviour as a function of composition. To allow the study of these mixed oxides using classical molecular dynamics, we optimized interaction parameters for the Am3 + and U 5 + cations for the Cooper-Rushton-Grimes interatomic potential, which has yielded excellent results on several pure and mixed actinide oxides. These potential terms were parametrized on a small set of the experimental data available, namely the room temperature lattice parameters, the thermal ex-pansion and the enthalpy increment for several americium contents. The potential obtained was validated against experimental data not included in the parametrization process and then applied to determine var-ious structural and thermodynamic properties of U1-yAmyO2 compounds with 0 <= y <= 0 . 5 as a function of temperature and composition. The results yielded by this parametrization are in excellent agreement with the available experimental data and provide significant insight into the properties of uranium-americium mixed oxides, especially at high temperature. (c) 2023 Elsevier B.V. All rights reserved.

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