The Japan Atomic Energy Agency has developed a JK2LB conduit for the Nb3Sn conductor of the ITER central solenoid(CS). Mechanical requirements for the CS conductor conduit are a 0.2% yield strength of more than 900 MPa and a fracture toughness KIC(J) of more than 130 MPa√m after a compaction and aging heat treatment (650°C, 240 hours). In a previous work, it was shown that an aged JK2LB conduit has high strength and sufficient fracture toughness enough to satisfy the requirements. As the next step, work was performed to determine the specifications of the JK2LB conduit taking into account of cold work, including compaction and winding, and to simplify its fabrication process. To simulate the cold work effect with cold work of 10% and aging, mechanical tests were performed at 4.2 K on laboratory-scale (20-30 kg) ingot samples at 4.2 K. It was found that the sum of carbon and nitrogen content should be in the range from 0.11% to 0.18% to achieve the ITER mechanical requirements. To obtain a grain-sized of conduit as well as that of a small ingot sample, the appropriate solution heat-treatment temperature and holding time were studied. In order to simplify the billet production process, we confirmed the internal metallurgical qualities of a JK2LB-cast ingot. Since significant segregation was not observed, it was possible to exclude the electroslag remelting process. Based on the above achievements, full-size JK2LB conduits that satisfy the ITER mechanical requirements were fabricated.
