First-principles study on the diffusion behavior of Cs and I in Cr coating

Cs and I can migrate through fuel-cladding interfaces and accelerate the cladding corrosion process induced by the fuel-cladding chemical interaction. Cr coating has emerged as an important candidate for mitigating this chemical interaction. In this study, first-principles calculations were employed to investigate the diffusion behavior of Cs and I in the Cr bulk and grain boundaries to reveal the microscopic interaction mitigation mechanisms at the fuel-cladding interface. The interaction between these two fission products and the Cr coating were studied systematically, and the Cs and I temperature-dependent diffusion coefficients in Cr were obtained using Bocquet’s oversized solute-atom model and Le Claire’s nine-frequency model, respectively. The results showed that the Cs and I migration barriers were significantly lower than that of Cr, and the Cs and I diffusion coefficients were more than three orders of magnitude larger than the Cr self-diffusion coefficient within the temperature range of Generation-IV fast reactors (below 1000 K), demonstrating the strong penetration ability of Cs and I. Furthermore, Cs and I are more likely to diffuse along the grain boundary because of the generally low migration barriers, indicating that the grain boundary serves as a fast diffusion channel for Cs and I.