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The ITER magnet system contains 50 GJ of energy in 3 main coil sets and feeders. Lesson learned from arc incidents in JT-60SA and CS module 3 cold test show the consequences of insulation failure. There are three major insulation challenges in ITER magnets: bulk insulation in the coil, ground insulation and high voltage wiring. The most issues are found at the vacuum interface with Paschen breakdown in poor vacuum leading to arcs from ground insulation and the risk of high voltage wire insulation cracking. High voltage arcing can potentially be a safety concern in ITER due to arcing to the vacuum vessel. The mitigation focuses in 3 main areas: Prediction: Simulation of arc development within the cryostat to assess potential damage and if necessary implementation of extra protection; Prevention: Minimisation of weak areas, notably high voltage (HV) wire exits from ground insulation. As a result of failure root cause analysis, chemical corrosion between coil resin and wire insulation is a concern being investigated, with if necessary quick “universal” solutions to repair or reinforce predicted weak areas. Vacuum monitoring in the cryostat is being improved to identify potential Paschen conditions in advance; Early Detection: Implementation of regular intermediate global electrical tests under controlled Paschen conditions at low temperature to detect breakdown with high sensitivity of leakage currents (∼10 µA) which may in turn require room temperature water cooled busbars to be easily isolated. At least initially, in-cryostat cameras or other spark detection methods are needed to localise failures and repair methods need to be pre-qualified.

The early plasma ramp-up on ITER will be performed under a limiter configuration. In this early plasma phase, the possible misalignments between the plasma configuration and the ITER first wall panels (FWP) in the inboard regions can lead to an increase of the power flux density acting on the FWPs. This situation is considered as a safety risk for the machine: a significant increase of the flux density may exceed the design limits of the FWPs and damage the inboard modules. Therefore, in order to avoid an excessive increase of the power flux density on the FWPs, one of the main goals of the tokamak assembly strategy is to achieve the most symmetric magnetic field structure and the best blanket alignment. The symmetric toroidal field (TF) coils structure is key for achieving the symmetric magnetic field. In the ideal case, the assembly gap dimensions in the wedging interfaces of every two adjacent TF coils are designed as an uniform value, i.e., 2 mm. However, in reality the assembly gap tolerances can be generated during the manufacturing and assembly process of the TF coils, which can lead to the asymmetric TF coils structure configuration after energization. The purpose of this paper is to investigate and discuss the consequent displacement distortions of the TF coils by simulating the non-constant assembly gap distributions in a 360° mechanical FE model of the ITER TF system.

The ITER Thermal Shields (TS) consist of actively cooled stainless steel panels. Their role is to minimise the radiation heat load from warm components, such as the Vacuum Vessel (VV) and the cryostat, hence contributing to insulating the magnet system operating at 4.5 K. The panels, cooled by 304L stainless steel pipes stitch welded to 10 mm to 20 mm thick 304LN plates, were coated by a low emissivity silver layer following welding of the pipes and bending operations. Pressurised He gas flows in the pipes at a temperature of 80 K and a pressure of 1.8 MPa at the inlet. These components have been manufactured and delivered to IO and their assembly had been started. Three leaks were detected in Helium leak tests during site acceptance test of the TS. A failure analysis was carried out based on advanced non-destructive and destructive examination techniques, including X-ray microtomography and Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM), confirming crack initiations of different severities, in particular presence of multi-branched “lightning bolt” transgranular cracks through the thickness of the pipes and of corrosion residues featuring high chlorine content. The root cause of the leakages has been univocally associated to Stress Corrosion Cracking (SCC), initiated by halide residues from the silver coating process in combination with the stress induced by the attachment, by welding, of the pipes to the plates. The paper summarises the results of these investigations and the lessons learned, the remedial actions implemented and the planned repair/replacement solutions.