Nuclear qualification of PVC insulated cables

Cables insulated with eleven different PVC compounds representing fire-retardant, non-fire retardant, 60, 75 and 90/spl deg/C temperature ratings were evaluated. The specimens were subjected to accelerated thermal, radiation, and sequential radiation and thermal exposure simulating 15, 20, 30 and 40 yr in-service environments in Ontario Power Generation Nuclear Plants. The radiation level was limited to 12 to 30 Mrad. Two sets of samples were evaluated. The first contained 15 cm insulation specimens in tubular form with the conductors removed. The material performance of the insulations was assessed using the 15 cm specimens during the various stages of aging exposures by conventional elongation measurement. The second set was made from 4.3 m single or twisted pair of cable samples (unjacketed) wound on mandrels. The cable samples were sequentially irradiated and thermally aged and subjected to an additional 5 Mrad accident radiation exposure, followed by a simulated loss of coolant accident (LOCA). Similarly thermal only aged specimens with 2 Mrad of background radiation representing powerhouse environment were subjected to a main steam line break (MSLB) steam test. During the steam tests, samples were energized and their insulation resistance (IR) values were continuously monitored using an on-line setup and periodically with a megohm meter at 500 V/sub dc/. The results from the insulation 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 elongation values were not a good indicator of electrical performance of PVC insulated cables subjected to radiation and thermal environment. With remaining elongation values ranging from 50 to 150% absolute, four out of eleven compounds performed poorly electrically during the steam test. All thermal only aged specimens functioned satisfactorily. In addition, two additional compounds rated for 60/spl deg/C applications were investigated and functioned unsatisfactorily despite high elongation values. It was found that specific additives were responsible for poor electrical behavior. During radiation exposure ionic salts were formed as a result of break down of PVC and of some plasticizers. Upon thermal aging or later in the steam test these ionic compounds migrated to the conductor surface. Then, under high temperature and pressure steam conditions, the salts were dissolved and created an electrical leakage path. Similarly, in thermal only aged specimens, breakdown products of specific plasticizers caused poor electrical behavior.