Radiation resistance of insulation varnish

Recently, new polymer insulation materials including polyimide group have been developed and utilized. These materials are excellent in their radiation resistance in addition to heat resistance, but usually the test has been carried out by the irradiation of gamma ray or electron beam at high dose rate. Actually, polymer materials are used under the condition of low dose rate for long time, and the deterioration is mainly caused by radiation oxidation, showing the different behavior from that by the irradiation at high dose rate. In this study, the irradiation in pressurized oxygen atmosphere was carried out on various insulation varnishes including polyimide group, simulating the irradiation at low dose rate, and the deterioration of the varnishes due to irradiation was evaluated mainly by the electrical properties and gel fraction. For comparison, also the irradiation at high dose rate in the air and the test combining the effects of heat and hot water besides irradiation were carried out. The specimens were enamel wires and thin varnish films. Co-60 gamma ray was used, and the dose rate was 1 Mrad/h in the air, 0.5 Mrad/h in 7 kg/cm2G oxygen and 0.1 Mrad/h in 30 kg/cm2G oxygen. Dose rate effect was hardly observed in polyimide varnish, but in other varnishes, the electrical properties remarkably lowered by the irradiation in oxygen. (Kako, I.)