TRITIUM RELEASE FROM JT-60U VACUUM VESSEL FOLLOWING HIGH-POWER HEATED DEUTERIUM OPERATIONS

Tritium concentration measurements have been made of the tritium recovery from the JT-60U after high power neutral beam injected discharges. This work has been done to study tritium release characteristics and to assess the tritium inventory in the vacuum vessel. Cleanup operations by hydrogen divertor discharges and helium glow discharge cleanings were performed to deplete the tritium inside the wall prior to in-vessel maintenances. After the beginning of the divertor discharges the tritium release from walls increases gradually. The release rate depends on the surface temperature of divertor plates and it is enhanced by high-power neutral beam heated divertor discharges combined with high-temperature vessel baking. Helium glow discharges, furthermore, give strong release characteristics compared with divertor discharges in spite of their short duration within similar to 1 hr. After the deuterium discharges and cleanup operations from July 1991 to July 1994, it is estimated that 70 similar to 80% of the total tritium of 39 GBq generated in the plasma is retained inside the graphite first wall.