Diffusion of hydrogen isotopes in 3C-SiC in HTR-PM: A first-principles study

Silicon carbide (SiC) is the main diffusion barrier of tri-isotropic particles in high-temperature gas-cooled reactor pebble-bed modules (HTR-PMs). When analyzing the source term of tritium in a HTR-PM primary circuit, it is essential to determine the amount of tritium released, which requires the diffusion coefficient of tritium in SiC. In the present work, the diffusion behavior of hydrogen in 3C-SiC is investigated using density functional theory-based first-principles calculations, which can be used to estimate the diffusivity of tritium while ignoring the neutron and mass effects. Different charge states of hydrogen (i.e., H⁰, H⁻ and H⁺) and all possible low-energy configurations and diffusion paths are also considered in the calculations. The results indicate that the charge state of hydrogen changes from negative to positive with the increase in Fermi energy. In addition, the most stable position of H⁻ in SiC is a tetrahedral site surrounded by four Si atoms. Furthermore, minimum diffusion barriers of 0.45, 0.46 and 1.52 eV are determined for H⁰, H⁻ and H⁺, respectively. The calculated diffusion coefficients from the present work agree well with those computed in the literature. In addition, the experimental results are closer to the negative hydrogen values computed in this study, indicating that the most likely charge state of hydrogen is negative. Our calculations provide a good reference for nuclear safety evaluation in HTR-PMs using the diffusivity of hydrogen in SiC.