Theoretical Investigation Of Electronic Structures For Adsorption Between Uranium Carbide Molecules And Graphene

Uranium carbides are considered to be one of the most ideal candidates for nuclear fuelmaterials in the next generation nuclear reactors. Gaseous uranium carbide molecules could bereleased from uranium carbide fuels during reactor operation, and subsequently be in direct contactwith graphite, which is popularly used as neutron moderator, reflector and barrier of radioactiveproducts in reactor cores. Understanding and even predicting the interaction between theseradioactive uranium carbides and graphite is significant for the safety control and design of Gen-IVnuclear energy systems. Since uranium dicarbide (UC2) was found to be the most abundant vaporspecies of uranium carbides, herein, using density functional theory calculations, we investigate theinteraction between typical UC2molecules and simplified models of different graphite surfaces(i.e.,graphene), in order to predict possible behavior and influence of gaseous uranium carbides ongraphite in nuclear reactors at atomic level.First of all, we study the interactions between two typical UC2isomers (linear CUC andsymmetric triangular structures) and pristine graphene. Results reveal strong chemisorption betweenthe UC2molecules and the pristine graphene, which is found different from the conventional weakintermolecular interaction. Interestingly, although the CUC structure can induce doublesp3-hybridization at the graphene surface, the most stable adsorption structure is formed by thetriangular UC2adsorbed at the hollow site of the graphene (binding energy:2.27eV). Furtherbonding analysis indicates that U5f orbitals of the triangular UC2are more active than that of CUC,providing a larger effective bonding area in the adsorption system. These results obtained from thepristine structure may be viewed as a lower limit case for the adsorbability of nuclear graphite. Itcan be thus predicted that graphite would effectively adsorb and retain gaseous UC2molecules aspossible radioactive products, without being significantly damaged to the structural integrity.However, defects of the nuclear graphite are inevitable and widespread under neutronirradiation, which also play a dominant role in immobilizing radioactive products released from nuclear fuels. And hexavacancy (V6) was identified as the most stable and abundant vacancy defectdistributed on heavily neutron-irradiated graphite surfaces. Therefore, to get more insight into theinteraction between uranium carbide molecules and nuclear graphite, we explore the adsorptivebehavior of the symmetric triangular UC2molecule (the most stable UC2structure theoretically) ona graphene nanosheet with a V6defect. Our calculations suggest that UC2can be localized in theV6defect with considerable binding energy of>10eV, which is owing to formation of twopentagon carbon rings and two U-C bonds, with all the six dangling bonds of the V6defect beingsaturated by UC2. Bonding nature analyses also reveal that the U-C interaction lies in the synergisticinterplay between electrostatic and covalent interaction with extensive participation of U valenceelectrons from5f to7p orbitals, which further stimulate polarization of semi-core6p orbitals andtheir subsequent contributions to the bonding (about20%). This strong interaction leads to afavorable binding of UC2to the defective graphite surfaces, which reduces the capability of nucleargraphite to retain harmful fission products by the vacancies being filled with UC2.The above results of interactions between typical uranium carbide molecules and graphene atatomic level, are helpful for facilitating progress of electronic structures investigation for complexsystems containing actinides, and also provide theoretical reference to design of modern nuclearenergy systems.