Thermal and diffusional properties of uranium-americium and plutonium-americium mixed oxides

Abstract

The knowledge of thermo-physical and transport properties of mixed oxide fuels containing minor actinides such as Americium (Am) and Neptunium (Np) is of paramount importance in the front end of the nuclear fuel cycle. Because of the radiotoxic nature of these materials, a priory evaluation of these properties using theoretical and computational means is immensely useful. In the present work, various thermal and diffusional properties, like, thermal expansion, specific heats, thermal conductivity, and oxygen vacancy migration barriers of U${}_{1-y}$Am_yO₂ and Pu${}_{1-y}$Am_yO₂ mixed oxides (MOX) are determined by extensive molecular dynamics (MD) simulations. Unlike most of the previous MD simulation studies, the present work explicitly considers the effect of different oxidation states, such as U⁺⁴, U⁺⁵ and Am⁺³, by using a new interatomic potential to evaluate the thermal properties of U${}_{1-y}$Am_yO₂ MOX. For linear thermal expansion, the results obtained by using new interatomic potential in U${}_{1-y}$Am_yO₂ with 0 ≤ y ≤ 0.3125 and Pu${}_{1-y}$Am_yO₂ are in excellent agreement with those obtained from the experimental studies. The peaks in the thermal expansion coefficient (α) and specific heat ($C_p$) versus temperature plots originated due to the Bredig transition in U${}_{1-y}$Am_yO₂ MOX shifts towards lower temperature as y increases up to 0.3125. This is qualitatively consistent with an almost linear decrease of the melting points with the increase in y as observed in experiments. The MD calculated α and $C_p$ values of U${}_{1-y}$Am_yO₂ MOX (for $0<y<0.3125$) are fitted to the Bathellier equation which can be used to determine high temperature behavior of the MOX fuel where no experimental data is available. The impact on the thermal conductivity (TC) for the introduction of AmO₂ into UO₂ is much stronger than that of the introduction of the same into PuO₂. Depending on the temperature, reduction in TC is in the range of 27.0–2.5% when 12.5 atom% AmO₂ is added in UO₂ as compared to 13.5-2.3% reduction in case of 12.5 atom% AmO₂ incorporated PuO₂. Based on the migration energies and vacancy-solute interaction energy, the concentration dependence of the mean square deviation in MOX is explained. A schematic of high oxygen diffusivity regions in MOX is also identified.