Advanced ultrasonic examination of heavy–gauge high strength studs for the ITER toroidal field gravity supports

The ITER Tokamak will include the largest superconducting magnet system ever built, with 50 GJ of stored energy. A series of structural elements guarantee its integrity under extreme load and temperature conditions. Key structural components to support the superconducting magnet coils are the Toroidal Field Gravity Supports (TFGS), which not only must sustain the deadweight of the magnetic system (11000 tonnes), but also withstand large electromagnetic forces during operation and accelerations induced by possible seismic events. Each of the 18 TFGS is secured to the cryostat's base by a set of 26 heavy – gauge (M60 to M85) studs. The selected alloy for these fasteners is the high-strength precipitation–hardening Ni-base superalloy UNS N07718, featuring a suitable combination of strength and fracture toughness. An essential characteristic of these studs is internal soundness, which must be guaranteed thanks to a 100 % volumetric non–destructive inspection by ultrasonic testing (UT). Due to the very stringent reliability requirements imposed on these components, the UT procedures gathered in industrial standards for this size of products proved to be insufficient to guarantee the absence of internal flaws of detrimental size. Thus, a special procedure was developed and tailored to inspect every single stud installed in the TFGS. This paper describes the implemented UT procedure to assure the internal soundness of the studs, with a focus on control parameters, calibration procedures, and acceptance criteria. Additionally, the results of UT testing obtained on a selection of studs using such procedure is presented, moreover cross-checked by other volumetric NDE techniques such as computed microtomography to confirm the nature and size of imperfections.