Development of Double Pancake Manufacturing Technology for ITER TF Coil: Results of Developments in High-accuracy Manufacturing Technology

The ITER Toroidal Field (TF) coil radial plate (RP) is the main structure of a double pancake (DP), and a TF conductor is inserted into the RP groove and affixed using cover plates (CPs). Since the RP and CPs are used at around 4 K and should sustain huge electromagnetic force, full-austenite stainless steel (SS) is used. Furthermore, high-power laser-beam welding (LBW) is applied for welding during RP assembly and RP-CP welding with the aim of minimizing welding deformation and achieving very tight dimensional tolerances. In addition, cold-drawing is applied in CP fabrication for high production efficiency. Combining full-austenite SS and LBW normally generates welding defects and cold-drawing deteriorates fracture toughness. These technical issues have been overcome by introducing the following technical developments. A 75-mm-thick high-power LBW is obtained without defect by optimizing the chemical composition of the RP base metal. A yield stress (YS) of 900 MPa and fracture toughness (KIC) of 180 MPam0.5 can be achieved for both the base metal and weld joint at 4 K. In addition, the cold-drawing process for straight CP was optimized to achieve the required YS and KIC through process control and intermediate relevant mechanical testing. Furthermore, optimal LBW conditions for wide-gap weld joints, such as 0.5 mm and 0.7 mm, were developed for RP-CP and CP-CP welding, respectively. Applying these techniques to the fabrication process during RP, CP, and CP welding, optimized manufacturing procedures have been successfully developed to achieve the technical requirements. In addition, these fabrication procedures are well rationalized to satisfy schedule requirements in ITER. Accordingly, series production of RP and CP, and CP welding has commenced, and is proceeding. As of May, 2020, 61 RPs and 50 CP welding out of 63 have been successfully completed.