Fatigue assessment of the conductor jacket for the Next European Torus toroidal field and poloidal field coils

The Next European Torus (NET) superconducting toroidal field (TF) coils and the poloidal field (PF) coils, which are used to confine plasma, operate at cryogenic temperatures utilizing liquid helium as coolant. The jacket which is used to surround the coils contains the helium and provides structural strength. Candidate materials for the jackets are 316 LN stainless steel and a nickel-base alloy IN 908. It is essential that cracks introduced during fabrication by welding do not propagate by fatigue and cause a leak during the coilʹs operational lifetime. Two defect assessment procedures BS PD6493 and R6 are applied to determine tolerable crack sizes for two extreme cases of thumbnail and circumferential flaws. It is shown, for the present materials, that leak before break can be demonstrated for some thumbnail flaws only. It is also demonstrated that the NET design limit of 2 × 105 cycles to failure can be achieved with an adequate margin of safety for thumbnail defects with current non-destructive inspection capabilities.