Annual Reports

Vol.20 (2000)

Vol.20 - 1 Review

Environmental Tritium

Noriyuki Momoshima
Faculty of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555


Abstract
 Environmental tritium was first observed in a helium fraction at a liquid air production facility in Germany in 1949. During the 1950s and early 1960s, huge amounts of artificial tritium were released into the atmosphere by nuclear testing. The environmental tritium level increased to more than 200 times the natural tritium level. Since the signing of a test ban treaty in 1963, the environmental tritium level has decreased, and analysis of recent Japanese rain samples has shown that the environmental tritium level is close to that before the nuclear testing. Tritium released from nuclear bombs into the atmosphere has been used as a global-scale tracer in studies on water mass movement in the ocean, groundwater flow and atmospheric air mass movement. Useful and valuable results have been obtained in those studies. In the atmosphere, tritium exists in three different chemical forms: hydrogen (HT), water vapor (HTO) and hydrocarbons (CH3T). The concentration of HT the highest, followed by those of CH3T and HTO. The most interesting feature of these chemical species is their significantly different specific activities. HT has 106 TU, CH3T has 104 TU and HTO has 10 TU, suggesting that HT and CH3T have been released from nuclear facilities. Vegetation is sensitively responds to a change in environmental HTO level by rapid exchange of water molecules between leaf water and atmospheric water vapor. HTO vapor released into the air slowly contaminates soil water. A nuclear fusion facility is planed to use a large quantity of tritium that is comparable to natural tritium on the earth, indicating the necessity to maintain tritium in a nuclear fusion facility and the necessity to carefully monitor the environmental tritium level.

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Vol.20 - 2 Original

Hydrogen Induced Disproportionation of Zr2Ni -- Temperature Dependence --

M. Hara, Y. Kaneko, K. Watanabe
Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930-8555, Japan


Abstract
 The kinetics of hydrogen induced-disproportionation of Zr2Ni was measured in a hydrogen atmosphere in a temperature range from 723 to 898 K. The disproportionation process of Zr2Ni was found as Zr2Ni + H2 —> ZrH2 + ZrNi. This reaction was completed within 100 s at all temperatures. The Avrami-Erofeev equation was used to analyze the reaction mechanism. It was found that the mechanism could be described by a nucleation and nuclei growth model. The activation energy for the disproportionation was determined to be -3 kJ/mol, which is slightly larger than that of Zr2Co (-15 kJ/mol). The negative temperature dependence was examined by the decrease in the number of product nuclei, owing to the reduction of hydrogen solubility in the reactant phase, with increase in temperature.

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Vol.20 - 3  Original

Analysis of Hydrogen Absorption Kinetics for Pd-Pt Alloys

Y. Jin, M. Hara, K. Watanabe
Hydrogen Isotope Research Center, Toyama University, Toyama 930-8555, JAPAN


Abstract
 Hydrogen absorption kinetics of palladium alloys was studied by using a vacuum microbalance system. The data analysis procedures were shown in detail in the present paper for a spherical sample as an example, where the second order hydrogen adsorption and desorption were assumed to have occurred on the sample surface. The factors that may influence hydrogen absorption curves and kinetic parameters are discussed.

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Vol.20 - 4  Original

Solid State Reaction between Tungsten and Amorphous Carbon Film

Y. Hatano, K. Watanabe
Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930-8555, Japan
M. Takamori, K. Matsuda, S. Ikeno
Department of Material System Engineering and Life Science, Faculty of Engineering, Toyama University, Gofuku 3190, Toyama 930-8555, Japan


Abstract
 Solid state reaction between tungsten and carbon film was examined at 1073K. Carbon film was deposited onto tungsten sheet by vacuum evaporation at room temperature and analyzed by means of X-ray photoelectron spectroscopy. Carbon and tungsten beneath the carbon film were detected, whereas only trace amount of impurities such as oxygen were present. The binding energy of W 4f electrons indicated that tungsten was in a metallic state at the interface. These observations suggested that no impurity layer existed at the interface. Analysis using transmission electron microscopy showed that carbon was in an amorphous state. The specimen was heated at 1073 K in vacuum, and change in crystal structure was analyzed by means of X-ray diffraction. The peak of W2C appeared after an induction period for ca. 1 min, although W2C is not thermodynamically stable at 1073 K. The peak intensity ratio of W2C to metallic tungsten increased in proportion to the square root of time to take the maximum value at 50 min and then decreased with elapse of time. No peak of WC was observed. These results indicate that the nucleation rate of W2C was much faster than that of WC at 1073 K. The decomposition of W2C was ascribed to the reaction between carbon in W2C and H2O in residual gas.

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Vol.20 - 5  Original

Kinetic Analysis of Reactions between Hydrogen-containing Carbon Film and Substrate Metals (Be, Mo, W)

K. Ashida, Y. Hatano, K. Watanabe
Hydrogen Isotope Res. Centr.,
*Faculty of Science, Toyama Univ., Gofuku 3190, Toyama 930-8555, Japan


Abstract
 Kinetic analyses were performed for solid-state reactions between a hydrogen-containing carbon film and substrate metals to form metallic carbide of Me2C (Me=Be, Mo, W). Isothermal and non-isothermal reaction curves could be explained by a random nucleation and two-dimentional crystal growth model. The kinetic equation can be approximated well by -ln(1-x)= kappt2, where x is the extent of reaction and kapp the apparent rate constant. The temperature dependence of the rate constants are given as

kapp(Be)=3.10 exp(-109.[kJ/mol]/RT) [1/sec2],
app(Mo)=231. exp(-161.[kJ/mol]/RT),
app(W) =516. exp(-183.[kJ/mol]/RT),

for Be-C, Mo-C and W-C systems, respectively, where the apparent activation energy is considered to be the sum of those for nucleation and diffusion. Linear relations were found between the apparent activation energy and the melting point of substrate metals as well as the bond strength of Me-C (Me=Be, Mo, W).

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Vol.20 - 6  Original

Examination of Tritium Retention in Boron-Coated Graphite by β-Ray-Induced X-ray Spectrometry

M. Matsuyama, T. Murai, K. Watanabe
Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930-8555, Japan
M. Matsuyama, T. Murai, K. Watanabe
Hydrogen Isotope Research Center, Toyama University
Gofuku 3190, Toyama 930-8555, Japan
K. Tsuzuki, N. Noda
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi 509-5292, Japan


Abstract
 Tritium retention in boron-coated graphite (B/Graphite) irradiated with tritium ions at room temperature and at high temperatures was examined by β-ray-induced X-ray spectrometry (BIXS). The results of analyses by X-ray diffraction and X-ray photoelectron spectroscopy for an unirradiated sample suggested that boron coated on the graphite surface formed an amorphous structure and that it contained impurities such as oxygen and carbon. Both a sharp intense peak and a broad weak peak induced by beta-rays appeared in the observed X-ray spectra. It was seen from the X-ray spectra that most of tritium implanted into the B/Graphite at room temperature was retained on the surface and in subsurface layers. In addition, it was seen that the implanted tritium can be easily removed at a relatively low temperature and that the retention amount decreased to 1/3 by heating at 300°C for 30 min. However, a considerable increase in tritium retention was observed when the irradiation temperature was increased to more than 300°C, indicating adsorption of tritium on the bare surface graphite.

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Vol.20 - 7  Original

Tritium Breeding Materials Data Base for Fusion Reactor Blankets (5) (TiO2-doped Li2TiO3 Solid Breeding Materials)

Y. Futamura
Hydrogen Isotope Research Center, Toyama University, Gofuku 3190, Toyama 930-8555, Japan
H. Kawamura, K. Tsuchiya
Japan Atomic Energy Research Institute, Shinbori 3607, Narita-cho, Oarai-machi Higashi-Ibaragi-gun, Ibaragi 311-1394, Japan


Abstract
 Recently, lithium titanate (Li2TiO3) has attracted the attention of many researchers. Small pebble-like shapes of Li2TiO3 have been selected as the most suitable for the design of a fusion blanket. As the fabrication method of Li2TiO3 pebbles, the wet process is most advantageous from the view point of various factors such as mass production. However, the grain size oh Li2TiO3 pebbles fabricated by the wet process was larger than that of Li2TiO3 pebbles fabricated by the rotating granulation method. Therefore, materials such as TiO2-doped Li2TiO3 that have improved properties such as microcrystal and moisture absorption properties have been developed.

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