Cryogenic structural materials of the ITER Toroidal Field Coil structure

The International Thermonuclear Experimental Reactor (ITER) is a large-scale experimental facility that aims to demonstrate the engineering and physical feasibility of fusion energy. The ITER Toroidal Field coils (TFCs) is one of the coils in the ITER superconducting magnet system, which have a welded D-shape with a height of 16.5m, a width of 9m and a weight of about 300 tons, are among the most important components in the ITER superconducting magnet system. All the structural materials used in the TFC cases are full austenitic stainless steels and are classified into four grades depending on stress distribution in the TFC case. The requirements are that yield strength at 4K are larger than 1000MPa, 900MPa, 700MPa, 500MPa and that fracture toughness at 4K is larger than 180MPam0.5. For thirty years, QST has developed new cryogenic structural materials for the next Fusion Experimental Reactor in collaboration with Japanese steel companies. The targets for mechanical properties were set as yield strength >1200MPa and fracture toughness > 200MPam0.5. From five candidate materials, finally two high manganese stainless steels have been successfully developed. These two materials became commercially available based on a demonstration of their productivity and weldability of materials, and evaluations of 4 K mechanical properties of trial products including welded parts. For the highest grade of ITER TFC case, JJ1was selected from two developed materials taking into account of matching of thermal contraction to stainless steel, 316LN. For the remaining three material grades, QST made industrial specifications by controlling nitrogen contents of 316LN corresponding to stress requirements based on the results of material manufacturing trials and mechanical tests at 4K. QST has standardized these specifications of 316LN and JJ1 as a material section of "Codes for Fusion Facilities - Rules on Superconducting Magnet Structure (2008)" issued by the Japan Society of Mechanical Engineers in October 2008. As the final evaluation prior to starting actual material manufacturing, QST manufactured actual shape materials and confirmed satisfactory results of mechanical properties at 4K. After this qualification, manufacturing of structural materials for actual TFC cases started from in August 2012. This paper describes the history of the development of cryogenic structural materials and the status of TFC case manufacturing.NEA International Workshop on Structural Materials for Innovative Nuclear System