Degradation of electrical insulating properties of polyethylene by combination of γ-irradiation and thermal aging

It is important to clarify the equivalence of the accelerated degradation test for and the deterioration process under complex environment such as heat and radiation of the insulating materials used for the cables and wires employed in nuclear power plants, especially for the safety and protection systems. Specifically, it is intensively required to clarify the difference of degrading behavior between the sequential and simultaneous exposure to heat and radiation and their multiplied effect, and to establish the method of equivalent accelerated deterioration. In this paper, the examination of the effect of test sequence of heat and radiation on polyethylene, to which anti-oxidant or radiation protection agent was not added. The authors examined complex deterioration by means of electrical insulation and investigated the results using infrared absorption and electron spin resonance. The electrical conductivity and dielectric characteristics were examined after γ-ray irradiation at room temperature, in the air, at dose rate 6.26 x 104 rad/h, up to 10 Mard, and heat treatment in the air at 90 deg C. The measurement was conducted in vacuum of about 1 Pa to remove the moisture effect as far as possible. Conductivity increased when only γ-ray was applied, γ-ray was applied after heat test or both were applied at the same time. The dielectric loss tangent measured at room temperature becomes larger when γ-ray was applied before heat test or both were applied at the same time. These are discussed in the paper. (Wakatsuki, Y)