2, Idaho National Laboratory, Idaho Falls, Idaho, United States
3, Westinghouse Electric Sweden AB, Vasteras, , Sweden
The U3Si2 compound is being developed as an advanced accident tolerant nuclear fuel, which benefits from high thermal conductivity compared to the UO2 fuel currently used in most light water reactors. U3Si2 also has higher fissile density than UO2, giving economic benefits, while an elevated oxidation rate upon cladding breach due to an accident scenario or wear may be a drawback. In order to model the fuel performance of U3Si2, material properties such as the diffusion rate of point defects and fission gas atoms must be determined. In this study, the thermodynamic and kinetic properties of U and Si point defects (vacancies and interstitials) as well as Xe atoms interacting with point defects are investigated by means of density functional theory (DFT) calculations. The nudged elastic band (NEB) method is employed to calculate migration barriers. The Hubbard U methodology (DFT+U) is used to describe the properties of U 5f electrons, which has been shown to be necessary in order to recover the experimentally observed U3Si2 crystal structure (P4/mbm) as the ground state. The same processes have been investigated using a U-Si empirical potential, which also gives us access to defect entropies and attempt frequencies for migration. A few entropies will be validated against DFT calculations. The DFT and empirical potential results are combined to predict diffusivities. Both U and Si self-diffusion and Xe diffusion are anisotropic as a consequence of the tetragonal crystal structure of U3Si2. Self-diffusion of U and Si in U3Si2 is faster than U self-diffusion in UO2. Interstitial diffusion of U is very fast in U3Si2 and it is on par with O diffusion in UO2. Xe diffusion is also faster in U3Si2 than in UO2. Implication of the predicted diffusivities on fuel behavior will be discussed.