Study on irradiation effect of insulating materials for fusion superconducting magnets -interlaminar shear strength at liquid helium temperature-

Abstract

Insulating materials for superconducting magnets of fusion reactors are not only exposed to electromagnetic forces at cryogenic temperatures, but also to neutrons and γ-rays which are secondary radiation generated by nuclear fusion reactions. Hence, insulating materials are required to keep high radiation resistance, high mechanical strength and high insulation performance even at cryogenic temperatures. To evaluate the physical properties of insulating materials at cryogenic temperatures, most of the previous studies have been conducted in liquid nitrogen. However, it has been pointed out that the molecular motion freezing between liquid nitrogen temperature and liquid helium temperature may cause changes in mechanical properties. In this study, the experiments were conducted in liquid helium, which is closer to the actual environment of ITER, to evaluate the temperature dependence of irradiation effects. Interlaminar shear strength (ILSS) was measured at room temperature (RT), liquid nitrogen temperature (LNT), and liquid helium temperature (LHeT) after γ-ray irradiation of glass reinforced plastics (GFRP) made of epoxy resin. As a result, in the unirradiated samples, ILSS was maximum at LNT, whereas in the samples irradiated at 5 and 10 MGy, ILSS was maximum at LHeT. It was considered that the change in molecular structure caused by γ-ray irradiation led to stress relaxation phenomena even at LHeT in the irradiated samples.