FATIGUE ASSESSMENT OF THE CONDUCTOR JACKET FOR THE NEXT-EUROPEAN-TORUS TOROIDAL FIELD AND POLOIDAL FIELD-COILS

The Next European Torus (NET) superconducting toroidal held (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 316LN 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 coil's operational lifetime. T wo defect assessment procedures BS PD6493 and R6 are applied to determ ine tolerable crack sizes for two extreme cases of thumbnail and circu mferential flaws. It is shown, for the present materials, that leak be fore break can be demonstrated for some thumbnail flaws only. It is al so demonstrated that the NET design limit of 2 x 10(5) cycles to failu re can be achieved with an adequate margin of safety for thumbnail def ects with current non-destructive inspection capabilities.