Abstract Body

To investigate neutron irradiation effects on practical superconductors which must be recognized to design and construct D-T fusion reactors, irradiation tests were caried out in BR2 in Belgium and JRR-3 in Japan. A 15.5 T superconducting magnet and a variable temperature insert in a radiation control area at Oarai center in Tohoku University were used to study T_C, B_C2 and I_C of the irradiated samples. In case of Nb₃Sn wires, a parameter, I_C/I_C0, once exceeds 1.0 and decreases drastically to below 1.0, when the I_C/I_C0 is plotted against the neutron fluence of over 0.1 MeV. The main mechanism of the property change would be “displacement effect” by fast neutrons. On the other hand, a GdBCO tape showed poor superconductivity after small amount of neutron irradiation of 8.20x10²⁰ n/m² (> 0.1 MeV) in JRR-3. The irradiated sample was investigated carefully with a Ge detector 159 days after irradiation. As the results, two sharp peaks of gamma ray were observed at 41 keV and 102 keV and the production of ¹⁵³Gd of which half decay period is about 240 days was confirmed. It reveals that the nuclear transmutation of Gd degrades its superconductivity of GdBCO tape along with the displacement effect. Because a lot of thermal neutrons were captured, it is considered that a large amount of ¹⁵³Gd was produced, which was then transmuted into Eu. The superconductivity of YBCO tape did not change when irradiated by the same neutron fluence as the GdBCO tape.

Acknowledgment

The authors would like to express sincere thanks to Mr. Masanori Yamazaki and Prof. Takeshi Toyama at Tohoku University for their encouragement and contributions. The research samples of Nb₃Sn wires were supplied by companies in Japan, China and Korea. ReBCO tapes were provided by Shanghai Superconductor Technology Co., Ltd. The authors would like to express their gratitude to them.

This work was performed at the International Research Centre for Nuclear Materials Science in the Institute for Materials Research, Tohoku University.

Keywords: Neutron irradiation, Nb₃Sn, GdBCO, Nuclear transmutation