The Effect of antioxidants on degradation mechanism of cable insulation material for nuclear power plant

Shimada, Akihiko; Kudo, Hisaaki*; Idesaki, Akira; Sugimoto, Masaki; Yoshikawa, Masahito; Tamura, Kiyotoshi; Seguchi, Tadao*

The lifetime of cables for a nuclear power plant has been estimated by accelerated ageing testing. It is necessary for the accurate lifetime estimation of cables to study thermal and radiation degradation mechanisms of cable insulation. EPR (ethylene propylene rubber), XLPE (crosslinked polyethylene), and SiR (silicon rubber) as the typical insulation polymers were selected and the degradation mechanisms were studied by the accelerated thermal and radiation ageing. The tensile properties, the decomposition product content by FT-IR analysis and crosslinking or chain scission by gel fraction measurement were investigated. An antioxidant as a stabilizer formulated in EPR and XLPE has a significant influence on the degradation rate. The antioxidant is much effective for thermal oxidation, but not for radiation oxidation. The content of AX (antioxidant) decreases with radiation decomposition and also with evaporation during thermal ageing. An increase in Arrhenius activation energy over 100 C is caused by decreasing of AX content. The degradation mechanism of SiR is different from EPR and XLPE, that is, crosslinking by oxidation at any ageing. For radiation oxidation, the dose rate determines the oxidation profile in polymer materials. The irradiation at elevated temperature around 100 C is the useful technique in radiation ageing testing to increase the oxidation depth.