The Background Levels of Atomospheric Tritium Concentration in the Environment at NIFS Toki Site of Japan

Atmospheric tritium concentrations at the National Institute for Fusion Science (NIFS) Toki site of Japan, where the Large Helical Device (LHD) has been operating, were measured considering future deuterium plasma experiments and environmental safety. The major chemical forms of atmospheric tritium are water (HTO), hydrogen (HT) and methane (CH₃T). Average tritium concentrations of HTO, HT and CH₃T observed from January 2003 to March 2006 were 9.0 mBq/m³, 9.0 mBq/m³ and 2.0 mBq/m³, respectively. To examine about the systematic error of the air sampling device, we cross-checked with the sampling device of Kumamoto University. The values obtained with both devices were almost consistent. The HTO concentration principally depends on humidity in air. The HTO concentration in the collected water and the HT concentration tend to show seasonal variation. The atmospheric tritium levels measured at Toki were consistent comparing with another environmental values measured in Japan. The present atmospheric tritium monitoring would be useful for safety consideration.