@misc{oai:repo.qst.go.jp:00073285,
 author = {井口, 将秀 and 櫻井, 武尊 and 中平, 昌隆 and 小泉, 徳潔 and 中嶋, 秀夫 and Iguchi, Masahide and Sakurai, Takeru and Nakahira, Masataka and Koizumi, Norikiyo and Nakajima, Hideo},
 month = {Nov},
 note = {National Institutes for Quantum and Radiological Science and Technology (QST) (formerly, Japan Atomic Energy Research Institute (JAERI) up to 2006.9, and Japan Atomic Energy Agency (JAEA) from 2006.10 to March 2016.3) successfully developed new cryogenic materials as a result of development activities over 30-years. Two of them have been produced in industrial scale for magnet structures of the International Thermonuclear Experimental Reactor (ITER), which is a large-scale fusion experimental facility in order to demonstrate the engineering and physical feasibility of fusion energy. 
QST started the design of Tokomak fusion reactor in early 1980s and recognized necessity of new cryogenic materials to realize the fusion reactor because conventional steels did not have enough strength and toughness to fulfill performance of superconducting coil for fusion reactor. Therefore, QST settled a target of 4K mechanical characteristics, yield strength ≥ 1,200MPa and fracture toughness KIc≥200 MPa√m. The target was named as “JAERI-Box” by US after it was presented in Kobe ICMC in 1984, and it was recognized by worldwide cryogenic society as shown in Fig. 1. QST also adopted a unique development procedure in collaboration with Japanese steel companies that QST performed evaluation of all materials at 4K and steel companies supplied samples of candidate materials for 4K test at QST. As results of the numerous tests of samples 5 new cryogenic materials, JJ1, JK2, JN1, JN2, and JKA1, were developed as shown in Fig. 1. 
QST also performed industrialization of these materials for application to the ITER magnet structures and selected two materials JJ1 and JK2 as structural materials for Toroidal Field Coil (TFC) case of ITER. All the structural materials used in the TFC Structures 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 1000 MPa, 900 MPa, 700 MPa, 500 MPa and that fracture toughness at 4K is larger than 180 MPa√m. JJ1 is used as a highest grade material of TFC case taking into account of matching of thermal contraction to stainless steel. 
On the other hand, JK2LB, which was improved from JK2 so as to keep enough ductility after aging for Nb3Sn reaction (for example, 650°C x 200h), is used to a jacket material of Central Solenoid (CS) of ITER. In addition, QST contributed to the standardization of material testing methods at 4K in JIS (Japanese industrial standards), and to establish a design code for structure of superconducting coil for fusion facility at JSME (Japan Society of Mechanical Engineer) to use JJ1 and 316LN in TFC case of ITER.
This paper describes history over 30-years on the development of cryogenic structural material and application to ITER magnets., 1st International Cryogenic Materials Conference in Asia and Cryogenics and Superconductivity Society of Japan 50th Anniversary Conference},
 title = {Development of Cryogenic Structural Materials for ITER Magnet Structures in Japan},
 year = {2016}
}