Kuniaki Watanabe, Kan Ashida, Masao Matsuyama

Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930, Japan

Kan Ashida, Kuniaki Watanabe

Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930, Japan

Abstract

    The diffusion constants of tritium and/or its isotope for graphite so far reported scatter considerably in the Arrhenius plot diagram. This, however, has not been seriously considered, despite that the understanding of the scattering is indispensable to estimate the tritium inventory and permeation in/through the graphite first walls of D-T burning experimental devices. We show in the present paper that there is regularity in the scattering: namely, the presence of a linear relation between the logarithms of the pre-exponential factors and the activation energies (the compensation effect). The temperature dependence of the diffusion constants and the compensation effect were analyzed by assuming the presence of three distinct diffusion channels in graphite: channels along the a- and c-axis in a graphite grain and along the grain boundary. Computational simulations could reproduce fairly well the temperature dependence of the experimental data and the compensation effect by assuming proper effective pre-exponential factors for the respective channels, where the activation energies for the respective channels were chosen from the observed values. Namely, the scattering of the diffusion constants and the compensation effect can be interpreted as a consequence of the multi-diffusion channels.

Kuniaki Watanabe, Kan Ashida, Masao Matsuyama, Hitoshi Miyake

Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930, Japan

Abstract

    Attention has been to several Li-compounds which are expected to be tritium breeding materials in thermonuclear fusion reactors. Diffusion constants of tritium for them reported so far by many investigators, however, differ considerably from each other, even for a given material. We looked at the diffusion constants of tritium for Li₂O, LiX (X=F, Cl, Br, I), Li-Pb, and LiF-MF2 (M=Be, Pb) and found regularity on the scatters of the diffusion constants. Namely, there was a linear relation as In(D₀) =αE_d + β, where D₀ and E_d are the pre-exponential factors and activation energies for the diffusion constants of hydrogen in relevant materials. It is a similar phenomenon known as the “compensation effect” in chemical reactions. In the light of this concept, the scatters of the diffusion constants for a given system could be interpreted as a consequence of the difference in diffusion channels and/or in free energy change for tritium capture in relevant materials.

Hitoshi Miyake, Kan Ashida, Masao Matsuyama, Kuniaki Watanabe, Mitsuru Sakurai

Abstract

    To examine tritium contamination/decontamination characteristics of turbomolecular pumps, which are regarded as potential candidates of vacuum and/or circulation pumps in large scale tritium handling systems, we measured the ad/absorption of tritium on/in the component materials of a turbomolecular pump (TG600M, Osaka Vacuum).

    It was observed that the amount of adsorption was in the order of [MoS₂ coated metals] > [organic materials] > [metals], and that of absorption was [organic materials] > [MoS₂ coated metals] > [metals]. The overall tritium inventory in this pump was found to be at a rather low level, about 2.5 GBq, after being exposed to the tritium gas amounting to about 2×10⁵ Pa･sec. The tritium ad/absorbed on/in the materials could be removed easily with vacuum heating above 200℃, but below this temperature it was hard to remove most of ad/absorbed tritium. From the view point of safety and environmental aspects, however, it is required to develop suitable decontamination techniques and contamination free materials.

Kuniaki Watanabe^a, Kouri Tanaka^a, Masao Matsuyama^a, Kiyoshi Hasegawa^b

^a Hydrogen Isotope Research Center ^b Faculty of Engineering, Toyama University, Gofuku 3190, Toyama 930, Japan

Abstract

    Reversible tritium getters are indispensable for the safe handling of a large amount of tritium in the fuel processing for thermonuclear fusion reactors. In Zr-Ni metallic compounds, we looked for the reversible getters showing the heat of deuteride formation around -25 kcal/mol (D₂) which will be appropriate to the storage-supply-recovery of tritium.
    Among five metallic compounds used in this study, we found that the first plateau for ZrNi and Zr₉Ni₁₁ showed ΔH⁰ = -25 kcal/mol (D₂). The second plateau for ZrNi gave ΔH⁰ = -19.2 kcal/mol (D₂). This suggests that ZrNi is also applied as the source of tritium supply near one atmosphere at relatively low temperature (260℃). They were withstanding pulverization and nonflammable against air exposure. In addition, ZrNiD_2.3 was reactivated quite easily with vacuum heat at 300℃ even after air exposure. Those properties of ZrNi and/or Zr₉Ni₁₁, are quite promising as the reversible tritium getter in the fuel processing for thermonuclear fusion reactors.

Masao Matsuyama, Kuniaki Watanabe

Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930, Japan

Abstract

    Small sized ionization chambers are attractive as a continuous tritium flow monitor in the fuel processing systems. We made a small ionization chamber and examined some basic properties of it such as saturation-current characteristics, concentration and/or pressure dependence of the ionization current, and impurity gas effects. A small ionization chamber of 1.9 cm³ in volume showed satisfactory saturation-current characteristics above 20 V, and also gave a good linear relationship between ionization current and tritium concentration in a wide range from 1×10^-6 to 3×10^-1 Ci/cm³ under condition of 1 atm. The pressure dependence of the ionization current was analyzed through a simple two-dimensional model taking account of the energy consumption of β particles in the chamber. Addition of inert gas caused the ionization current to increase. This phenomenon was attributed to the change in the stopping power and W value with the mixing up of different gases.

Masao Matsuyama, Kan Ashida, Toyosaburo Takeuchi

Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930, Japan

Abstract

    The reduction behavior of powdered Fe-Ni oxides in a hydrogen atmosphere at 380℃ was studied by means of radioactive tracer of ⁶³Ni. Change in the radioactivity of the surface gives information on mutual diffusion of Fe and Ni between surface and bulk during the reduction process.
    The radioactivity of ⁶³Ni in the surface layer of the alloy oxides increased with the progress of reduction in the initial stages and then drastically decreased, suggesting that Ni is covered with metallic Fe at the final stage of reduction. X-Ray diffraction showed that unreduced Fe-Ni oxides were composed of a mixture of Fe₂O₃ (hematite) and NiO, and the reduced alloys were composed of homogeneous Fe-Ni and Fe or Ni. Fe₂O₃ was changed to Fe₃O₄ (magnetite) before metallic Ni appeared, and metallic Ni appeared earlier than Fe-Ni. Metallic Fe appeared after Fe-Ni, and no FeO was detected throughout the reduction.
    The following reactions were proposed to be involved in the reduction mechanism; FeO + Fe₂O₃→ Fe₃O₄ and NiO + Fe + Fe₂O₃→ Ni + Fe₃O₄. It was also concluded that an Fe-rich surface is formed by the belated diffusion of Fe to metallic Ni, which is produced earlier than metallic Fe.