A Computational Analysis of Thorium Dioxide and Th(1-x)UxO2 Systems

Nuclear power generation is an important way to satisfy rising global energy needs without increasing dependence on coal and petroleum. However, conventional nuclear fuels, such as uranium and plutonium dioxides, raise several safety concerns. Many countries have shown a renewed interest in thorium-based fuels as a potentially safer alternative. Thorium dioxide requires small amounts of a neutron source, such as uranium or plutonium, to generate a sustainable fission reaction. Due to the hazards of conducting experimental work on radioactive substances, a robust theoretical understanding of this doped-ThO2 fuel is needed. Using Density Functional Theory (DFT), we have studied the effects of uranium addition on the electronic structures of both the pure thoria bulk structure and three flat surfaces of ThO2 and simulated Scanning Tunneling Microscopy (STM) images of each surface. We have also studied the effect of a uranium adatom on these surfaces. However, we wished to study larger systems than are practical to simulate with DFT, and so we developed a new Th-O Buckingham-type force field that has been optimized to work with a leading UO2 interatomic potential. With this new potential, we have completed a configurational analysis of uranium substitution in thoria supercells of varying sizes and investigated the effects of uranium-doping on the thermophysical properties of the system. We have also modeled ThO2 and U-doped thoria stepped surfaces and grain boundaries, including the segregation energies of a uranium impurity and an oxygen defect in these systems