Absorption of Hydrogen Isotopes by Vanadium-based Low Activation Materials

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Vanadium alloys are potential candidates of low activation structural materials for fusion reactors. Permeability of hydrogen isotopes including tritium, however, is high in these alloys. It is known that the permeation of hydrogen isotopes though group 5 metals sensitively depends on surface states, but the systematic study has not been carried out for these alloys. From this viewpoint, absorption of H_2 and D_2 by V-Ti and V-Ti-Cr alloys was examined in the present study in combination with surface analysis by X-ray photoelectron spectroscopy to understand the correlation between surface chemical states and the reaction rates of hydrogen isotopes.Sheet type specimens of V-4Ti alloy and V-4Ti-4Cr alloy supplied by the National Institute for Fusion Science were installed in an ultra-high vacuum chamber, and their surfaces were analyzed after heat treatments at 673-1273 K. Enrichment of Ti on the surfaces were clearly observed above 800 K for both alloys. The specimens with different surface Ti concentrations were prepared by respective heat treatments, and hydrogen absorption was examined in a temperature range from 523 to 1023 K. The sticking coefficient α on the surface without Ti enrichment was 10^<-3> -10^<-4>, while the surface segregation of Ti up to [Ti]/[V] 〜0.5 led to the reduction in α by 2-3 orders of magnitude. Namely, the surface segregation of Ti caused the barrier effect against hydrogen permeation. The potential barrier against dissociative adsorption of hydrogen evaluated by ab initio calculations showed that direct influence of Ti is negligibly small. It was concluded that the reduction in α was due to the increase in the surface coverage of impurity oxygen caused by the presence of Ti whose chemical affinity to oxygen is larger that of V. The isotope effect between H_2 and D_2 was small and not observable under the present conditions.