Diffusion, permeation and solubility of hydrogen, deuterium and tritium in crystalline tungsten: First principles DFT simulations

Abstract

Tungsten (W) has shown promise to be the most favored plasma facing material in nuclear fusion with deuterium (D) and tritium (T) as fuel. In order to identify a better material, atomistic understanding of the behavior of D/T with W is highly desirable. Here, we report the mechanistic pathways of diffusion of H, D and T in bcc W employing density functional theory (DFT) using PBE and PBE-D3 functional and harmonic transition state theory. The activation diffusion barrier was determined using nudge elastic band techniques. The zero point energy (ZPE) was incorporated by phonon calculations to include the isotope effects. The surface adsorption of H, D and T is predicted to be exothermic, whereas surface to sub-surface and bulk absorption is endothermic but the vacancy induced absorption in bulk W was predicted to be exothermic. H, D and T atoms were observed to be diffused preferably from tetrahedral site to the nearest tetrahedral site. The calculated diffusion coefficients, rate constants, permeability constants and solubility are found to be higher for H compared to its heavier isotopes D and T. The trends of diffusion, permeation and solubility for H, D and T are well in agreement with the experimental results. The diffusion values predicted for H isotopes in bcc W from PBE-D3 method is quite close to the experimental values compared to PBE method. But, the calculated and experimental ratio of diffusivity, permeability and solubility of any two H isotopes follow the similar trend and no considerable change is noticed between PBE and PBE-D3 methods. Further, the calculated diffusion coefficients are shown to be one order of magnitude lower than Cr and two orders than Fe and thus might be the basis for considering W as plasma facing materials. The present study might be useful in modeling the behavior of tritium in W metal for which data is either scattered or least available.