Influence of radiation defects on deuterium permeation behavior in tungsten

Understanding the hydrogen isotope (H) permeation behavior in neutron irradiated tungsten (W) is highly desired for the accurate prediction of H behavior in W first walls in a fusion DEMO reactor. Contrary studies have been published where some indicated a relatively higher H permeability in the irradiated W whereas others showed a lower H permeability. In these studies the radiation defects were produced with ions (as surrogates for neutrons) and were concentrated in a shallow layer (<200 nm), their effects could be mixed up with surface effects during H permeation. Herein W specimens damaged to 800 nm by Au-ion implantation are adopted to investigate the effects of radiation defects on deuterium (D) permeation. Microstructural analyses with slow positron annihilation technique and transmission electron microscopy revealed that vacancy-type defects and dislocation loops were produced in the implanted W. Results from D ₂ gas-driven permeation measurements at 700–1000 K showed, surprisingly, radiation defects did not influence the D permeability. The phenomenon was illustrated from a thermodynamic perspective. Radiation-induced dislocations and/or monovacancies, possessing low H coverage fractions, do not affect permeability because their effects on effective diffusivity and solubility can fully cancel each other. Vacancy clusters, having a high fractional H coverage and owing to their strong trapping nature, will not come to effect during the steady-state permeation portion, thus do not contribute to permeability.