Hydrogen isotope plasma-driven permeation through RAFM steel: isotope exchange and helium irradiation effect

Abstract

Hydrogen isotope exchange in China low activation martensitic (CLAM) steel was investigated by plasma-driven permeation (PDP) measurements in the temperature range of ∼670–920 K. It was found that as H was introduced into D plasma, the permeation flux of HD increased while that of D2 decreased. The total D permeation flux, however, remained almost unchanged. Such a result allowed the analysis of isotope replacement and showed that the reduction of D2 is balanced by the rise of HD. In addition, the formation of HD was found to be closely related to the H and D content in plasma. Hydrogen isotopic effects on H and D plasma dissociation/ionization rate and on incident flux were evaluated by measuring plasma parameters including electron temperature and electron density using a triple Langmuir probe. The correlation between isotope effects on ionization rate in the plasma and isotope effects in the permeation flux was discussed. He plasma pre-irradiation and seeding during D PDP were also conducted to investigate the He effects on D diffusion. The total D permeation flux was found to be reduced by He seeding. Data analysis of Langmuir probe showed that ionization rate of D plasma was influenced by He seeding. Besides, under simultaneous D + He irradiation, He atoms were preferentially trapped at the defects resulting in a shallower D concentration gradient that described the observed decrease in D permeation rate. He plasma pre-irradiation was found to reduce D permeation flux with a much slower breakthrough to reach steady-state. The amount of D trapped at He-induced defects was derived quantitatively from the transient permeation curves. It is believed that He bubbles formed at the near surface act as trapping sites of D atoms, leading to a less effective diffusion coefficient and an enhanced retention in the material.