Structural safety assessment of a tokamak-type fusion facility for a through crack to cause cooling water leakage and plasma disruption

Nakahira, Masataka

A tokamak-type fusion machine is said to have inherent safety associated with plasma shutdown. A small leak of water can terminate the plasma safely and can cause a plasma disruption which will induce electromagnetic(EM) forces in the vacuum vessel (VV). From a radiological safety view point, the VV forms the physical barrier that encloses tritium and activated dust. If the VV can sustain an unstable fracture by EM forces from a through crack to cause the leak, the structural safety will be assured and the inherent safety will be demonstrated. Therefore, a systematic approach to assure the structural safety is developed. A new analytical model to evaluate the through crack and leak is proposed, with verification by experiment. Based on the analyses, the critical crack length to terminate plasma is evaluated as about 2 mm, and the critical crack length for unstable fracture is obtained as about 400 mm. It is therefore concluded that EM forces induced by small leak to terminate plasma will not cause the unstable fracture of VV, and then the inherent safety is demonstrated.