Defect-induced strong localization of uranium dicarbide on the graphene surface

Abstract

Defects such as the most stable hexavacancy (V6) distribute widely on neutron-irradiated graphite surfaces, which play a dominant role in immobilizing radioactive products released from nuclear fuels. By performing DFT calculations, we explore the interaction of gaseous uranium dicarbide (UC₂) molecules on a graphene nanosheet with a V6 defect, in order to investigate the behavior of the representative vapor species of uranium carbide fuels in reactor cores. Results suggest that UC₂ can be trapped in the V6 defect with considerable binding energy of >10 eV, with all the six dangling bonds of the V6 defect being saturated by UC₂. Bonding nature analyses also reveal that the U–C interaction lies in the synergistic interplay between electrostatic and covalent interaction with extensive participation of U valence electrons from 5f to 7p orbitals, which further stimulate polarization of semi-core 6p orbitals and their subsequent contributions to the bonding. This strong interaction leads to a favorable binding of UC₂ to the defective graphite surface, which reduces the capability of nuclear graphite to retain harmful fission products by the vacancies being filled with UC₂. These findings highlight substantial chemical reactivity and strong localization of UC₂ on the widespread V6 defects in nuclear graphite, and may provide an important reference in establishing modern nuclear reactor safety at the atomic level.