Tritium Release Behavior from JT-60U Vacuum Vessel during Air Exposure Phase and Wall Conditioning Phase

The behavior of tritium release from the vacuum vessel of JT-60U during air exposure phase at controlled water vapor concentration and gas purging in the wall conditioning phase has been investigated. For the air exposure with varying water vapor concentrations of 40ppm, 300ppm, 680ppm and 3400ppm, tritium concentration in the vacuum vessel of JT-60U was measured. At each water vapor concentration, tritium concentration initially increased with time and then became steady finally. The steady tritium concentration increased with water vapor concentration. The total amount of tritium released from the vacuum vessel was 13MBq for 3400ppm. This amount is almost the same as that removed by 5 hours’ H2-GDC, which has been the most effective method for tritium removal from JT-60U. This suggests that tritium in the vacuum vessel of JT-60U can be easily removed by water vapor. Tritium released into exhaust gas during gas purging was also measured for varying gases (H2, He and Ar), at different pressures and temperatures of the vacuum vessel. Tritium concentration of the exhaust gas was about 0.1Bq/cm3 at room temperature and was independent of gas species within the pressure from 0.05 to 0.3 Pa. This result indicates that isotope exchange of tritium with hydrogen molecules was not so active under these purge conditions.