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The degradation behavior of SiR for the cable insulation by accelerated thermal and radiation ageing was studied and the degradation mechanism was proposed. The degradation was observed by the change of tensile properties, the distribution of crosslinking, and the change of weight. The chemical reaction under the both ageing in oxidation conditions was crosslinking and the oxidation mechanism was found to be the same between thermal and radiation ageing. The yield of crosslinking was proportional to the ageing time and also the dose. The effect of irradiation temperature on oxidation was accelerated with an increase of temperature above around 120 0 C, which might be due to the specific radiation chemical reactions. Therefore, the degradation by simultaneous ageing at higher temperatures above 155 0 C was much higher than that for sequential ageing, such as irradiation followed by thermal ageing or thermal ageing followed irradiation. At a high temperature, the degradation by thermal ageing under vacuum (without oxidation) was more progressed than that for the ageing in air (with oxidation). The reason was assumed to be the thermal decomposition of crosslinks between SiR molecules formed by the chemical crosslinking agent. The hardness (Shore hardness) reflected well the degradation of the SiR material for any ageing conditions.

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