Mitigation of Helium Leakage Risks Within the Cryostat of ITER Including Diagnosing the Occurrence of Stress Corrosion Cracking Within the Thermal Shield Cryogenic Helium Pipes

The insulating vacuum within the ITER Cryostat has a prime purpose to minimize the heat load on the superconducting coil system by gas thermal conductivity. These coils are shielded from thermal radiation by an 80 K helium-cooled silver-coated thermal shield (TS) consisting of
$\sim$
500 panels. Leakage within a cryogenic circuit inside the cryostat is considered to be the prime risk to the ITER project’s mission, as leakage could be disabling to the operation of the magnet systems, be extremely challenging to localize and may be inaccessible for repair. To date, four leaks have been detected and localized within in-cryostat components delivered to ITER. Specifically, three of these leaks have been diagnosed within the TS cooling circuits as having been caused by chlorine-induced stress corrosion cracking (SCC). The technique of high-resolution X-ray computed tomography followed by precision cutting for the generation of macros and micros has been applied to successfully characterize small vacuum leaks for the first time. The techniques were further extended to nonleaking cooling pipes and the onset of SCC was observed in many places, indicating the high probability of the development of many thousands of leaks over time. Such analysis has led to the full replacement/repair program for the TSs before they become overly embedded into ITER. This article summarizes how the helium leakage risk is mitigated and details the detection of extensive SCC on the ITER TSs.