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

A Chakraborty and PS Ghosh and N Choudhury and A Arya, JOURNAL OF NUCLEAR MATERIALS, 617, 156152 (2025).

DOI: 10.1016/j.jnucmat.2025.156152

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)A(y)mO(2) and Pu1-yAmyO2 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 U1-yAmyO2 MOX. For linear thermal expansion, the results obtained by using new interatomic potential in U1-yAmyO2 with 0 <= y <= 0.3125 and Pu1-yAmyO2 are in excellent agreement with those obtained from the experimental studies. The peaks in the thermal expansion coefficient (alpha) and specific heat (C-p) versus temperature plots originated due to the Bredig transition in U1-yAmyO2 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 and values of U1-yAmyO2 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.

Return to Publications page