Developments of Nondestructive Test Method of Jacket Section for ITER Poloidal Field Coils

The ITER magnet system is one of the largest and most complex superconducting installations ever undertaken. Therefore, all subcomponents have very demanding requirements, including the conductors which are of cable-in-conduit conductor type. Based on the structural analysis and fatigue life requirements of the coil systems, the maximum acceptable defect sizes inside the jacket material were defined. This paper presents the nondestructive test (NDT) method developed for the poloidal field (PF) coil conductor jackets made of 316L stainless steel. The difficulty in the development of the PF jacket NDT arises from the circular-in-square shape, i.e., extruded bars of a square cross section with an inner circular bore. Furthermore, in addition to volumetric defects, the inspection procedure has to detect planar defects oriented longitudinal (i.e., along the jacket) and transversal on the surfaces and inside the jacket material. After a series of experiments and optimization, the phased array ultrasonic test (PAUT) combined with eddy current testing (ECT) was developed for the PF jacket inspection. The outer surface and volume are inspected by PAUT consisting of 11 separate runs per jacket surface and using four different probes. The inner bore surface up to a depth of 2-3 mm is inspected by multifrequency ECT. The characteristics and limits of all inspections runs are discussed, particularly for the most difficult one with detecting transversal planar defects.