The Background Levels of Atmospheric Tritium and OBT at NIFS Toki Site of Japan

Abstract

Atmospheric tritium concentrations and organically bonding tritium OBT of pine needles at NIFS Toki site had been measured considering environmental safety of future deuterium plasma experiment by large helical device (LHD). Atmospheric tritium concentrations of three chemical forms such as water HTO, hydrogen HT and methane CH₃T had been measured since 2004. Tritium concentrations of HTO, HT and CH₃T were distributed around 2-23 mBq/m³, 6-11 mBq/m³ and 0.5-3 mBq/m³ respectively. The HTO concentration principally depends on humidity in air. Recent decreasing rate of HT concentration to half was estimated 19.4 years. The OBT concentration tends to decrease and the rate to half was almost 9.4 years. Also the OBT measurements were cross checked with burning and liquid scintillation counting method and ³He mass spectrometric method. Both analyzed results showed good agreement. Long term monitoring of environmental tritium would be necessary from view point of safety on future nuclear fusion development.