Evolution of cryogenic structural material development for superconducting coils in fusion reactors

Cryogenic structural materials are an essential element to fulfill the performance of superconducting coils for fusion reactors. In this field, there was no specialized steel and mechanical properties data at 4 K until 30 years ago when the Japan Atomic Energy Agency (JAEA) started the development of the Tokamak fusion reactor. JAEA set a target of mechanical characteristics which could satisfy requirements for the coil structure at 4 K and equipped evaluation facilities at 4 K for tensile tests, fatigue tests and so on. At the same time, JAEA initiated collaboration with the steel industries in order to realize new cryogenic structural materials and carried out mechanical evaluation tests at 4 K on numerous samples which were supplied from the industries. There are two areas: a coil case and a conductor jacket for forced flow conductor. The former requires large forging products and thick hot-rolled plates for the coil case. The latter is influenced by heat treatment at approximately 650°C and more than one-hundred hours to produce Nb3Sn superconducting material at the final stage. This is a unique issue for the jacket. Finally, both the case and the jacket material for construction of the ITER are now being supplied from the Japanese industries through JAEA. In addition, JAEA contributed to the standardization of these testing methods at 4 K in Japanese industrial standards (JIS). Furthermore, JAEA supported the establishment of a design code for the structure of superconducting coil for a fusion facility at the Japan Society of Mechanical Engineers (JSME). This paper describes the more than 30-year history of the development of cryogenic structural material. (author)