Systematic study of the structural, energetic and elastic properties of U_1-AmO_2- compounds using empirical...

Uranium-americium mixed oxides are envisaged as fuels or blankets for the transmutation of americium in fast neutron reactors, which requires an accurate knowledge of their properties. Here, the Cooper-Rushton-Grimes (CRG) potential terms related to Am⁴⁺ are reoptimized to lower the very high melting temperature of AmO₂ predicted by the potential (≈3350 K) while keeping other properties unchanged. The resulting terms are used alongside U⁴⁺ parameters as well as previously optimized terms for the Am³⁺ and U⁵⁺ oxidation states in combination with molecular dynamics to perform a systematic study of the structural, thermodynamic and elastic properties of hypostoichiometric uranium-americium mixed oxides U_1-AmO_2- with 0 ≤ y ≤ 1 and 0 ≤ x ≤ 0.1 over a wide temperature range (250-4500 K). The properties calculated include the lattice parameter and related properties (density, thermal expansion coefficient), the enthalpy increment and related properties (specific heat capacity, enthalpy of fusion), the melting temperature and the elastic constants. Our study shows a peculiar evolution of properties with the americium content with a change in the evolution for 50% americium due to the various oxidation states of the cations present in these compounds. Results presented in this paper provide significant insight into the properties of uranium-americium mixed oxides for all compositions, in particular for high americium contents, non-stoichiometric oxides and at high temperature and complement the few experimental data available for these materials.