Hirofumi Homma

Correlation between hydrogen isotope permeation through niobium and bulk oxygen concentration : Possible influence of oxygen on tritium recovery from Pb-Li by vacuum permeator

Abstract Niobium is a potential candidate of tube material in vacuum permeator for tritium recovery from Pb-17Li liquid blanket system. From this viewpoint, the permeation of hydrogen through a Nb membrane was investigated with an ultra-high vacuum apparatus under the conditions relevant to the blanket system where no oxide films could be retained on the membrane surfaces. It was, however, found that the permeation rate sharply decreased with increasing oxygen concentration in the bulk of membrane; at upstream H2 pressure of 1 Pa and membrane temperature of 700°C, for example, the permeation rate at oxygen concentration corresponding to oxygen potential in Pb-17Li (0.054 at%) was evaluated to be 1/5 of the value expected from hydrogen solubility and diffusivity in Nb. Such small permeation rate was ascribed to the presence of oxygen monolayer formed by surface segregation from the bulk. Surface modification by Pd coating was found to give only limited improvement due to degradation in coating effect induced by interdiffusion between Pd and Nb. Methods to improve the high temperature stability of Pd coating was discussed.

Hydrogen Diffusion in Ti-Cr Hydrogen Absorption Alloys by Tritium Radioluminography

Tritium radioluminography has been applied to Ti50Cr50 and Ti40Cr60 alloys with two-phase structure of BCC phase and Laves phase by examining hydrogen distributions and by measuring a hydrogen penetration profiles. Hydrogen diffusion coefficients in these alloys have been successfully determined by analyzing the tritium depth profile to be 3.3×10-12 m2/s and 1.3× 10-12 m2/s, respectively. This value has been suggested that the hydrogen diffusion in the Laves phase is slower than that in the BCC phase.

Observation of Tritium Distribution in Pure Vanadium and V-5 mol%Fe Alloy by Tritium Radioluminography

ABSTRACT Tritium radioluminography was applied to pure vanadium and V-5 mol%Fe alloy to observe the tritium distribution and to evaluate the local tritium concentration in them. It was demonstrated that the tritium distribution at a microscopic area in the specimens was quantitatively and graphically displayed. In the pure vanadium specimen, the local tritium concentration was about three times different depending on the crystal orientation of the grains. The tritium radioactivity of the grains with (001) and (111) orientation are 1 Bq/mm2 and 0.4 Bq/mm2, respectively. These values correspond to the tritium concentration of 15 mol ppb and 6 mol ppb. The difference of the local tritium concentration was attributed to the variety of the morphology of precipitated hydride depending on the crystal orientation of the grains. For the radioactivity recorded in the imaging plate, the component of the X-rays generated from tritium in the specimen was only 2%, i.e., most of the intensity was attributed to the β-rays. In the V-Fe alloy specimen, it was shown that the tritium distribution correlates with iron segregation formed during solidification after the arc melting. The cross sectional observation showed that the local tritium concentration in equilibrium distribution depends on the local iron concentration in the specimen. The local tritium concentration gradually decreases from 115 mol ppb to 70 mol ppb as the iron concentration at the iron segregated region increases from 3 mol% to 4.5 mol%.