Reasons for Resistivity Increase in FR-EPDM Insulation of Cables Aged in Nuclear Power Plants

Two low-voltage cables, insulated with flame-retardant ethylene-propylene-diene rubber (FR-EPDM), were removed after being laid in nuclear power plants for many years. We measured the leakage currents flowing through their insulators and found that the resistivity of cable insulation is higher in the two harvested cables than in a similar but new and unused FR-EPDM one. The reason for this result is discussed in this paper based on a comparison of the leakage currents between new and gamma-ray irradiated FR-EPDM sheets. The discussion is continued by referring to our previous papers on mechanical properties and infrared absorption spectra of similar new and gamma-ray irradiated FR-EPDM sheets. As a result, it has been clarified that additives such as pigments contained in the cable insulators are likely to act as a source of charge carriers. In addition, the cable insulators became hardened by the long-term aging in an environment of about 60 C-?, which made the ionic conduction difficult and the insulation resistivity higher.