Endurance of polymeric insulating materials in nuclear power plants and needs for condition monitoring of electric cables

Electric cables in nuclear power plants have to function even after serious events such as a loss-ofcoolant accident (LOCA). In most cables, organic polymers that are very sensitive to radiation are used. The Japanese government carried out an extensive research project to examine the degradation behavior of safety-related cables at three different temperatures with three different dose rates. Its report published recently revealed that some cables would be unable to function after the LOCA test. In this paper, a brief outline of the report is described. Categorization of all the causes of malfunctions occurred in electric apparatus in all the Japanese nuclear power plants for recent about 40 years is also reviewed. As a result, it became clear that not a single trouble had occurred in any cable in any Japanese nuclear power plant by pure insulation degradation, indicating that the risk of serious accidents is very low. Nevertheless, in order secure even higher reliability, Japanese government has been supporting several research projects to develop a reliable monitoring method and to study the physics underlying the aging phenomena to cause degradation of cables in nuclear power plants. Fundamental researches such as those to investigate the degradation mechanisms of polymer materials for cables by various chemical analyses have been carried out mainly in the Japan Atomic Energy Agency (JAEA) as well as in several universities. The dependence of the degradation behavior on the sample thickness is a function of various aging conditions such as the aging temperature and radiation dose rate. Such dependence seems to be caused by the volatilization of stabilizing agents from the polymer. One of the monitoring methods being developed is the broadband impedance spectroscopy (BIS), which measures the changes in frequency spectra of cable impedance and phase angle by applying low voltages. A clear difference found between the spectra obtained before and after thermal and radiation aging indicates a potential ability of this method to monitor the insulation degradation of polymer-insulated cables in nuclear power plants.