Following the incident on 9th March 2021 involving Equilibrium Field Coil No 1(EF1), the joints of the EF1 feeders were found to have been damaged during the coil energization test. Experts in Japan and in Europe have jointly investigated the root cause of the incident. The measures needed to recover have been identified and a revised schedule for the integrated commissioning has been prepared.
It was determined that the primary reason for the damage to the joints was insufficient insulation at the point where a quench detection (QD) cable emerged from the ground insulation around a joint. A short circuit occurred due to this insufficient insulation and the arc formed damaged the shells of the EF1 terminal joints. This in turn allowed a helium leak to the cryostat.
The insulation of the joints will be reinforced so as to ensure enough insulation capability. Moreover, other joints with a similar structure will be repaired as well in order to prevent recurrence elsewhere. Furthermore, other joints without a similar structure will also be reinforced in view of thorough prevention of recurrence. The repair on site will be carried out under the thorough quality management. After the repair work, the high voltage holding capability will be tested under locally-simulated worst-case conditions to confirm a sufficient insulation capability. To complete the repair and the test, the process to be used on site will be thoroughly qualified and it will be shared thoroughly among the workers.
Then in around February 2022, vacuum evacuation of the vacuum vessel and the cryostat will start and cooling down of the super-conducting coils will follow to resume the suspended integrated commissioning of JT-60SA. The experience and knowledge obtained from this incident is being widely disseminated to support the consolidation of knowledge necessary for ITER, DEMO and any future superconducting tokamak.