著者

百島 則幸

出版者

富山大学水素同位体科学研究センター

雑誌

富山大学水素同位体科学研究センター研究報告 = Annual Report of Hydrogen Isotope Research Center, Toyama University (ISSN:09168486)

巻号頁・発行日

vol.20, pp.1-10, 2000

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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.