Analysis of the Leakage Events in the Thermal Shield Cooling Pipes of the ITER Magnet 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.