Completion of the first ITER toroidal field coil structure

本論文は、日本が100%調達責任を有するITERのトロイダルコイル構造物の第1号機の完成を報告するものである。主な技術的な課題は、(i)極低温(4K)でも高い延性を持つ新規材料の開発、(ii)部分溶け込み溶接の適用、(iii)溶接変形対応、(iv)オーステナイトステンレス鋼溶接部の減衰効果を加味した超音波探傷試験法の開発、(v)巨大で複雑なD形状構造物の封止溶接開先の0.5mmオーダー公差での開先合わせ、などである。これらの各技術課題を解決し、ITER　TFコイル構造物第1号機は成功裏に完成することができた

Measurement of Decay Time Constant of Shielding Current in ITER-TF Joint Samples

Joint sample tests have been carried out as a qualification test for ITER Toroidal Field (TF) coils. The joint sample comprises two short TF conductors that have "twin-box" joint terminals at both ends. The lower joint is a testing part that is a full size joint of the TF coils. Hall probes are attached on the lower joint box at around the center of the external field coil of the test facility. The magnetic field induced by shielding currents in the joint can be estimated from the difference between the measured magnetic field strength and the magnetic field generated by the external field coil. The magnetic field by the shielding currents during shut-off of the external field coil from -1.0 T is evaluated for six samples. The decay time constants of the shielding currents are gradually elongated with decrease of the shielding currents in all the samples. In comparison with simulation results, it is considered that the main shielding current flows in superconducting cables in the two conductors with crossing the jointed plane and that the joint resistance is decreased at low total current

Evaluation of ITER TF Coil Joint Performance

To evaluate the ITER TF joint performance, the joint test sample, which consists of two short TF conductors and has full size joint, shall be tested using NIFS test facility under the condition of current of 68 kA and external field of 2 T. For high accuracy, the issue of voltage difference between cable and jacket had been anticipated in the evaluation of joint resistance. If a voltage difference exist between them, it is difficult to measure real joint resistance using voltage taps on the jacket. Therefore, the author first calculated the position where voltage of cable and jacket become equipotential and then decided the voltage tap position where the influence of voltage drop could be avoided. Thus, a high accuracy measurement of joint resistance could be achieved and the joint resistance was accurately evaluated as around 1 n Ω , which is well below the ITER requirement of 3 n Ω

Test of ITER-TF Joint Samples With NIFS Test Facilities

Qualification tests of the ITER toroidal field (TF) conductor joints have been carried out by testing joint samples with test facilities in the National Institute for Fusion Science, NINS, Toki, Japan. The joint sample consists of two short TF conductors with the length of 1535 mm, which is restricted by the test facility with 9-T split coils and 100-kA current leads. The sample current is supplied from a dc 75-kA power supply. Each conductor has two joint boxes at both terminals. The lower joint is a testing part that is a full-size joint of the TF coil. The joint resistance of the lower joint is estimated from the increase of the average voltage drop among the six taps on the conductor against the currents. Five joint samples were tested until 2016, and all the samples satisfied the requirement of the joint resistance at less than 3 nΩ. The method of the measurement and the results are summarized, and the voltage distribution among the voltage taps is discussed

Results of All ITER TF Full-Size Joint Sample Tests in Japan

Nine toroidal field (TF) coils have been developed in Japan for the international thermonuclear experimental reactor (ITER). The joint resistance of TF coil should satisfy the requirement of smaller than 3 nano-ohm at 2 T of external magnetic field and 68 kA of transport current. Full-size joint sample (FSJS) tests were performed for joint development and for TF coil manufacture, as part of the process control. 11 FSJS tests are conducted in total. FSJS tests were conducted with assistance from a test faculty in the National Institute for Fusion Science as reported in a previous paper. All FSJS tests successfully satisfied the requirement of resistance less than 3 nΩ at 2 T. Additionally, the TF coil joints are subjected to cyclic electromagnetic force and warm-up/cool-down during the ITER operation. The authors investigated the joint performance for the abovementioned influence. The results showed no degradation in the joint resistance. Thus, the TF joint developed in Japan was qualified successfully