Atomistic Simulations of Point Defect Behavior in Nuclear Fuels

摘要

Nuclear fuel performance modeling relies on accurate representation of material properties as function of fuel chemistry and micro-structure, and changes to these properties induced by, for example, irradiation, temperature variations and chemical evolution during reactor operation. We have used atomistic simulations based on density functional theory (DFT) and semi-empirical force-field representations of inter-atomic interactions to study the behavior of point defects in pristine and irradiated nuclear fuels. The results feed meso-scale models of point defect and microstructure evolution, which provide input to models of fission gas retention/release, swelling and thermal conductivity. The meso-scale models use the MARMOT phase-field code and, at the engineering scale, reduced order models of material properties are implemented in the BISON fuel performance code , which are both based on the MOOSE framework developed at Idaho National Laboratory (1NL). In this summary, we provide brief overviews of three examples of atomistic modeling of point defects in nuclear fuels to support meso- and engineering scale simulations of nuclear fuel performance. First, fission gas diffusion and its relation to gas release in irradiated UO_2 is discussed in the context of vacancies and vacancy clusters interacting with fission gas atoms and giving rise to diffusion. Second, we explore the defect chemistry of UO_2 doped with cations such as chromium, which are added by fuel vendors to improve performance, and we provide an explanation supported by modeling results for the increased grain size observed in chromium-doped UO_2. The mechanism responsible for large grains in chromium doped UO_2 is also investigated for other doping elements . Third, the work on point defects and fission gas diffusion in UO_2 is extended to the U_3Si_2 accident tolerant fuel candidate. All of the studies are performed under the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program.