Applicability of LBB concept to tokamak-type fusion machine

Nakahira, Masataka

A tokamak-type fusion machine has been characterized as having inherent plasma shutdown safety. An extremely small leakage of cooling water will cause a plasma disruption. This plasma disruption will induce electromagnetic forces (EM forces) acting in the vacuum vessel (VV) which forms the physical barrier enclosing tritium and activated dust. If the VV has the possibility of sustaining an unstable fracture from a penetrating crack caused by EM forces, the structural safety will be assured and the inherent safety will be demonstrated. This paper analytically assures the Leak-Before-Break (LBB) concept as applied to the VV and is based on experimental leak rate data of a through crack having a very small opening. Based on the analysis, the critical crack length to terminate plasma is evaluated as about 2 mm. On the other hand, 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.