Condition assesment of cable insulation systems in operating nuclear power plants

Four crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) insulated cables were subjected to accelerated thermal, radiation, and sequential radiation and thermal exposure to simulate 20, 30, and 40 yr in-service environments in Ontario Hydro Nuclear Plants. Two sets of samples were evaluated. The first set contained 15 cm cable specimens in tubular form with the conductors removed, while the second set was made up of 4.3 m cable samples wound on mandrels, The cable samples were sequentially irradiated and thermally aged and subjected to an additional accident radiation exposure followed by a simulated loss of coolant accident (LOCA)/main steam line break (MSLB) steam test. During the steam test, samples were energized and their electrical characteristics continuously monitored. The material performance of the insulation was assessed using the 15 cm specimens during the various stages of aging exposure by conventional elongation measurement and by the use of micro-specimens with differential scanning calorimeter (DSC), oxidation induction time (OIT) and infrared carbonyl absorption techniques. The results from these samples were compared with the electrical performance of the cable samples wound on mandrels that had been subjected to the LOCA/MSLB steam test. The test results indicate that with the use of micro-specimens meaningful changes were obtained until the elongation values of the samples were reduced to 50% absolute. Electrical test data obtained for XLPE and EPR during the steam tests indicate that 50% absolute elongation values provide sufficient margin to withstand additional accident radiation, and to function electrically during design basis events.