First principles investigation of adsorption and diffusion behavior of fission products (Cs, Sr and I) on carbide ceramic (SiC, NbC and ZrC) coatings

In this study, the adsorption and diffusion behaviors of three typical fission products Cs, Sr and I on the (111) surfaces of SiC, NbC and ZrC were studied by the first-principles calculations. Cs atom tended to be adsorbed on the H sites of SiC (111) and ZrC (111) surfaces, while on the T site of NbC (111) surface. H site of SiC (111) surface and F sites of NbC (111) and ZrC (111) surfaces were the most preferable adsorption sites for Sr. I atom was more likely to be adsorbed on the T site, H site and F site of SiC (111) surface, NbC (111) surface and ZrC (111) surface, respectively. All the above adsorption processes were evidenced to be chemisorption of both ionic and covalent features according to electron structure analysis. The smallest diffusion coefficients of Cs and Sr on these three surfaces at 1373 K were 1.155 × 10⁻⁸ m²·s⁻¹ on ZrC (111) surface and 1.544 × 10⁻⁸ m²·s⁻¹ on SiC (111) surface, respectively, indicating that ZrC has the best ability to prevent the surface diffusion of Cs, while SiC was the best for preventing Sr. All three carbide ceramic coating materials could effectively resist the surface diffusion of I since the diffusion coefficients of I on them were all relatively low, with the smallest diffusion coefficient of 1.130 × 10⁻⁸ m²·s⁻¹ on NbC (111) surface.