Pipe Maintenance Tooling development for the ITER Divertor Remote Handling System

The ITER Divertor's 316L stainless steel cooling water pipe connections are to be remotely maintained by the first-of-a-kind Divertor Remote Handling System (DRHS) and its suite of cutting, welding and inspection tools. The requirement for welding of the highest quality in a radioactive, magnetic and high temperature environment, places significant technical risks on the technologies selected for use. This paper reports the R&D activities undertaken to mitigate these risks and concludes with the key findings and recommendations for the ongoing design. Gas Tungsten Arc Welding (GTAW) trials demonstrated that a code compliant, autogenous butt weld of 3 mm wall thickness is possible, but requires precise process control, novel techniques, and tight tolerances on material composition. The most significant findings were that vertically-offset filler-rings could be employed to positive effect, and a controlled Sulphur content of 100–150 ppm for the stainless-steel pipework was found to be necessary. The galvanic cell technology selected for the welding back-purge oxygen sensor was tested and verified for use in operating environments of 300 Gy/h gamma dose rate, 3mT magnetic field, and 60 °C ambient temperature. Cutting trials developed optimum parameters for lathe-based orbital cutting with regards to the required tool longevity, the vacuum-clean environment and the need to re-weld the joint. Finally, Volumetric Inspection trials investigated the effectiveness of Electro Magnetic Transducer (EMAT) and Phased Array Ultrasonic Testing (PAUT) methods at detecting a range of manufactured surface and volumetric defects in the weld. EMAT was found better suited to the application, but the work highlighted the need for a deeper understanding of the detection needs and their relationship to potential failure modes.