Electronic property and effective diffusion coefficient calculation model of hydrogen isotopes in multicomponent steel 2.25Cr1Mo from first-principles calculations

Chrome-molybdenum (2.25Cr1Mo) ferritic steel is one of the primary materials used in heat exchanger pipes in the steam generators of high-temperature gas-cooled reactors (HTGRs). Studying the tritium diffusion mechanism in 2.25Cr1Mo ferritic steel is of vital importance for the prediction and interpretation of tritium activity concentration in the reactor. However, the permeation of hydrogen isotopes in these alloys remains difficult to predict owing to the lack of understanding of the microscopic interactions between hydrogen isotopes and multicomponent alloys. In this study, the electronic and diffusion properties of the hydrogen isotopes in 2.25Cr1Mo alloy were investigated using first-principles calculations. Hydrogen preferentially intercalated into tetrahedral interstitial sites rather than octahedral ones in 2.25Cr1Mo alloy. The Cr and Mo dopants induced strong repulsion between H and the matrix. Interaction between H and 2.25Cr1Mo exhibited a combination of ionic and covalent bonds with electrons transferred from the steel to H atom. Dopants acted as scattering centers with local interaction with H and expanded the preventive effect of H diffusion to the second-nearest tetrahedral interstitials with a high energy barrier. The effective H diffusion coefficient in the alloy was calculated by an approximate model based on the volumetric ratio of the components for the first time. The obtained pre-exponential factor and activation energy agreed well with the experimental results. The current research provides a reliable way to determine the effective diffusion coefficient of hydrogen isotopes in multielement alloys, which can accelerate R&D of tritium-resistant materials in both fusion and fission reactors.