Degradation of Flame-Retardant Cross-Linked Polyethylene Caused by Heat, Gamma-Rays, and Steam

Various kinds of polymers are used for electrical insulation in nuclear power plants. They are subjected to thermal and radioactive stresses, depending on their locations. They are also subjected to high-temperature steam in the event of an accident. Regarding this, sheets of flame-retardant cross-linked polyethylene (FR-XLPE) were subjected to heat, gamma-rays, and steam exposure, and their infrared absorption spectra, stress-strain relations, indenter moduli, and permittivity spectra were measured. As a result, the following findings are clarified. The oxidation of FR-XLPE occurs more actively by the gamma-ray irradiation of 250 kGy at 100 °C and 0.1 kGy/h than that of 800 kGy at 25 °C and 10 kGy/h. This result indicates that the low dose rate at the high temperature enhances the diffusion of oxygen into the sample. On the other hand, oxidation and cross-linking proceed at the high dose rate. Such oxidation and cross-linking make FR-XLPE hardened by the formation of three-dimensional structures. Although the charge transport is significantly promoted by the steam exposure at 220 °C, this promotion is suppressed if cross-linked structures were formed beforehand by the prior irradiation with gamma rays.