The Effects of Point Defects on Thermal Conductivity in ThO2, PuO2, and UO2.

Uranium, a common element that can be found across the world, can be used in nuclear reactors as UO2 or a mixture of UO2 and PuO2. Plutonium, created in reactors as a by-product, is responsible for more than one third of the energy production coming from nuclear power plants. Thorium is three times more abundant than uranium in the earth’s crust and is an easily exploitable resource in several countries. It is found primarily as fertile Th-232. This isotope of thorium has an enormous potential for breeding fissile U-233 in fast nuclear reactors. Although ensuring efficient thermal transport in these actinide oxides has been considered to be one of the most important agenda items for improving the safety and efficiency in nuclear reactors, the microscopic understanding on the thermal transport phenomenon is very limited. In this research study, the detailed phonon scattering by point defects in ThO2, UO2, and PuO2 are investigated using classical molecular dynamics simulations. Across all fuels, it is observed that oxygen vacancy defects, even at lower defect concentrations such as 0.1%, have a detrimental effect on the phonon scattering, comparable to the effect of large vacancy sites such as uranium and plutonium vacancies in spite of their significantly small sizes. The strong negative impact of small vacancy sites such as oxygen vacancies implies that the major phonon transport mechanism in actinide oxides largely depends on their lattice vibrations and any alteration to their lattice vibration degrades thermal conductivity of actinide oxides significantly