Impact of surface phenomena in metals on hydrogen isotope permeation

The ion and gas driven deuterium permeation behavior in niobium (Nb) was investigated. The GDP (gas driven permeation) rate (flux) was measured to be proportional to the D2 gas pressure and to increase with the temperature, whereas the IDP (ion driven permeation) rate (flux) was 0.2–0.3 times the incident ion flux and was almost independent of the temperature. Based on the diffusion-recombination limited hydrogen transport model, the “phenomenological” recombination rate coefficient, kR, was calculated for both upstream-side and downstream-side from the IDP experiment and the obtained result agreed well with the GDP result. Furthermore, in-situ surface analysis by Auger electron spectroscopy (AES) revealed that the presence of carbon suppressed the GDP rate to a larger extent than that of oxygen. These newly obtained results on Nb enable comparison of the hydrogen transport characteristics of various metals which are of interest for fusion applications in terms of the so-called “recycling constant”, D/2kR (D is the diffusivity). It is shown that Nb, together with vanadium, has larger values of D/2kR.