Development of tritium cleanup system for LHD

Energy is vital for humans and we have been consuming a large amount of fos sil fuel especially from the beginning of the industrial revolution. Nowada ys its huge consumption has however come to threaten our life and we have t o prepare nonfossil fuels, for instance solar energy, biomass energy, nucle ar 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 a s 'LHD' hereafter) in 1998. The device will generate a small amount of trit ium, as a fusion product. In order to remove it from the exhaust gas, we ha ve designed a tritium cleanup system based on a new concept. This system is mainly composed of a palladium permeater, a decomposer and hydrogen absorb ing alloys. It may perfectly recover the tritium from exhaust gas without o xidizing it. This system is applicable for the future needs at fusion power plants. In order to remove tritium discharged from fusion experimental fac ilities, 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 abs orbed by molecular sieve (referred to as 'wet system' hereafter). However, in the case of LHD, it is not rational to deliberately convert the discharg ed 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 hy drogen molecules. In accordance with these view points, we have developed a nother type of tritium cleanup system based on a new concept, in which hydr ogen molecules including tritiated ones (HT, DT and T-2) found in the exhau st gas of LHD are directly fixed to hydrogen absorbing alloys. Other impuri ties such as methane and water vapor, parts of which are tritiated, will be decomposed into each elemental form by the decomposition process and hydro gen molecules, including tritiated constituents from the decomposition (ref erred to as 'dry system' hereafter). (C) 2000 Elsevier Science B.V. All rig hts reserved.