Toshiki Kabutomori

Application of hydrogen absorbing alloys to medical and rehabilitation equipment

As power sources for rehabilitation equipment, electric, hydraulic, and pneumatic actuators have been used. However a more human-sized and higher powered actuator that can reduce the equipment size is desired. A new metal hydride (MH) actuator that uses the reversible reaction between the heat energy and mechanical energy of a hydrogen absorbing alloy has recently attracted much attention. The MH actuator is characterized by its small size, low weight, noiseless operation and a compliance similar to that of the human elbow joint. Therefore, the MH actuator has the characteristic of being light and easy to use and so is suitable for use in medical and rehabilitation applications. Some lifting devices using this actuator have already been developed and are being used for the care of the aged and disabled. The characteristics of the MH actuator are presented and then some applications are introduced in this paper. It is our opinion that in our aging society the MH actuator will play an important role in the development of medical and rehabilitation equipment.

Stability of ZrCo and ZrNi to Heat Cycles in Hydrogen Atmosphere

ABSTRACTThe stability of ZrNi and ZrCo to heat cycles in hydrogen atmosphere was studied through changes in absorption-desorption characteristics and in crystallo-graphic structures. ZrCo easily lost its absorption-desorption capacity of hydrogen below 30 heat cycles between room temperature and a given temperature in a range of 400 ∼600 °C. X-ray diffraction analysis showed that ZrCoH3 initially formed decomposed to ZrH2+ ZrCo2. On the other hand, ZrNi was more durable than ZrCo to the similar heat cycles. But, it was found that the absorption-desorption characteristics was degraded by heat cycles over 500. The X-ray analysis showed that ZrNi also disproportionated to ZrH2 and ZrNi3. The difference in the stabilities between the two materials appears to be due to the difference in crystallographic nature upon formation of the respective hydrides.

Development of a tritium cleanup system for a large helical device using nonvolatile getter materials

Abstract A tritium cleanup system has been conceptually developed for the large helical device (LHD) at the National Institute for Fusion Science. The system is a processing device employed to remove tritium from exhaust gas. In the exhaust gas discharged from the LHD in normal operation, the major part of tritium constituents should be in a form of hydrogen molecules because the fuel used in plasma experiments with the LHD is hydrogen molecules. From this viewpoint, we have designed a tritium cleanup system, which is characterized by tritium being removed and stored in a form of hydrogen molecules with less impurities, like oxygen and carbon, and its decomposition and the separation processes are introduced to convert various tritiated compounds into a form of hydrogen molecules of high purity. Besides these, there is another aspect in that getter materials are applied in both decomposition of tritiated compounds and storage of hydrogen molecules containing tritium. The system design is composed of three essential component parts: a hydrogen separator, a hydrogen absorbing vessel, and a decomposition process vessel. The hydrogen separator and the decomposition process vessel make a process loop repeat to remove hydrogen into a form of hydrogen molecules with less impurities. It is important that “less impurities” means having a less bad influence on hydrogen-absorbing materials used in the storage vessel. We think that the hydrogen separator will be manufactured by employing a palladium hydrogen purifier system, which is available in the marketplace, and the hydrogen storage vessel will also be manufactured by using hydrogen-absorbing alloys like titanium. Thus, the serious problem imposed on us is how to realize the decomposition process vessel. To develop the decomposition process vessel, we thought nonvolatile getter materials were promising and carried out performance tests of methane decomposition by the nonvolatile getter materials, where methane was used because it is hardly decomposed and there is little data for a flowing-gas system. The tritium cleanup system that was designed is presented. Also, a methane decomposition curve with ZrNi alloys used as one of the typical nonvolatile getter materials is shown, and the probability of the realization of the decomposition process vessel is examined.

Development of tritium cleanup system for LHD

Abstract Energy is vital for humans and we have been consuming a large amount of fossil fuel especially from the beginning of the industrial revolution. Nowadays its huge consumption has however come to threaten our life and we have to prepare nonfossil fuels, for instance solar energy, biomass energy, nuclear energy and so on. Fusion energy is an unlimited resource and one of the strongest candidates of the future energy source. At the National Institute for Fusion Science (referred to as ‘NIFS’ hereafter), we have constructed a new fusion experimental device called large helical device (referred to as ‘LHD’ hereafter) in 1998. The device will generate a small amount of tritium, as a fusion product. In order to remove it from the exhaust gas, we have designed a tritium cleanup system based on a new concept. This system is mainly composed of a palladium permeater, a decomposer and hydrogen absorbing alloys. It may perfectly recover the tritium from exhaust gas without oxidizing it. This system is applicable for the future needs at fusion power plants. In order to remove tritium discharged from fusion experimental facilities, it is usual to employ a system by which tritiated constituents, in various chemical forms, are entirely converted to a form of water vapor by catalytic oxidation. The water vapor containing tritiated form is then absorbed by molecular sieve (referred to as ‘wet system’ hereafter). However, in the case of LHD, it is not rational to deliberately convert the discharged tritium into the water vapor, because the tritium discharged from LHD is almost in a form of hydrogen molecules. Moreover, the tritium in the form of water vapor affects the human body 18 000 times stronger than that of hydrogen molecules. In accordance with these view points, we have developed another type of tritium cleanup system based on a new concept, in which hydrogen molecules including tritiated ones (HT, DT and T2) found in the exhaust gas of LHD are directly fixed to hydrogen absorbing alloys. Other impurities such as methane and water vapor, parts of which are tritiated, will be decomposed into each elemental form by the decomposition process and hydrogen molecules, including tritiated constituents from the decomposition (referred to as ‘dry system’ hereafter).

Computer simulation of the processing of tritium in exhaust gas from the LHD

Abstract A new type of tritium cleanup system that differs from conventional systems in its application of a decomposition-processing vessel has been investigated by means of computer simulation. In the decomposition-processing vessel, tritiated compounds are decomposed so that it is possible to extract the tritium in the form of hydrogen molecules while other elements such as carbon are fixed within the vessel. In our computer simulation of the removal of tritium, a single run of processing was assumed to handle 1 N m3 of a gaseous mixture that contained 1 GBq tritium and was 90% hydrogen, 5% methane and 5% helium. As a result, three pressure curves, one for hydrogen, a ‘both-sum’ (hydrogen+methane) curve and a ‘total-sum’ (hydrogen+methane+helium) curve, showed strong and marked changes in trend at almost the same elapsed time. This time corresponded to the completion of a single cycle of processing. The changes in trend suggest that the three curves had been dominated in the first cycle by the hydrogen initially contained in the exhaust gas and that, after one cycle, they had come to be dominated by the remaining methane and by the hydrogen generated by methane decomposition. It took about 8.7 h for the concentration of tritium in the gas being processed to fall below the relevant limit.

Methane decomposition reaction on a ZrNi alloy

ABSTRACT A method of decomposing hydrogen compounds was developed by employing a zirconium nickel (ZrNi) alloy. This method enables all tritium compounds (HTO, CH3T, C2H5T, etc.) in an exhaust gas to be decomposed into their respective elements, and the tritium itself to be removed in the form of hydrogen gas (HT). The method was developed through a series of experiments using methane. Using previous study results, a chemical reaction equation of methane decomposition on a ZrNi alloy is proposed and discussed. To ascertain the mechanism of methane decomposition on a ZrNi alloy, alloy samples were examined based on X-ray diffraction spectra and SEM electronographies before, during, and after the experiments. It was found that, as the decomposition time elapsed, peaks attributed to a pure ZrNi alloy gradually disappeared and new ones appeared in the X-ray spectra. The new peaks were attributed to the presence of ZrC, pure Ni, and a simple carbon substance. This indicates that the Zr in a carbon-bound alloy results in ZrC generation that releases Ni metal, and part of the C generated from the methane decomposition remains as a simple, as-grown substance. From these results, the decomposition reaction of methane using a ZrNi alloy can be represented by an equation involving the alpha value. The equation shows that one ZrNi molecule decomposes (1+ α) molecules of methane and generates 2(1+α) molecules of hydrogen. The alpha value was estimated based on the volume of decomposed methane and the weight of the ZrNi alloy used in the experiments. It is known that the alpha value is strongly dependent on the experimental conditions and can be used as an index to evaluate the decomposition condition.