Improvement of inspection and maintenance procedures of polymeric cable insulation used in nuclear power plants in Japan

In order to develop a reliable monitoring method to evaluate the degradation caused in polymeric cable insulation, research was conducted to examine whether we can locate the position of a degraded portion in a cable by measuring the magnitude and phase angle of the cable's impedance as a function of frequency from 1 to 110 MHz and by carrying out inverse fast Fourier transform (IFFT) analyses on the spectra. The experimental results conducted at early stages indicated that the location would be possible if the degradation given to the cable was very severe. This means that we have to increase the sensitivity. One of the easiest ways is to increase the number of points analyzed by inverse IFFT. Tests were conducted for a 25-m flame-retardant ethylene-propylene rubber (FR-EPR) insulated cable, part of which had been irradiated by y-rays to a total dose of 4 MGy at a dose rate of 1 kGy/h for 4000 hours at 100 °C. The peak seen at the aged portion was much clearer in the case of 8721 data than in the case of 801 data. Therefore, the effect of the increase of data acquisition points on the location ability is obvious. A further study has revealed that the increase of the 801 actually measured data to 8721 data for IFFT analysis by numerical interpolation shows a very similar effect in locating the aged portion without increasing the measurement time. The sensitivity of the method is discussed by taking a 25-m FR-EPR insulated cable as an example. The results indicate that the method can detect the degradation clearly if the cable was irradiated to a fairly high total dose. Therefore, further improvement in sensitivity is necessary before the application of the method in nuclear power plants as a highly reliable cable insulation monitoring method. It has also become clear that the method is superior to the time domain reflectometry. In a research project supported by the Japanese Government, activation energies were estimated for various kinds of cable insulating materials used in nuclear power plants in Japan by conducting thermal aging tests in a temperature range lower than that often used in conventional tests. In addition, aging characteristics under combined thermal and radiation environment were also obtained for the materials. From the results, a proper method to superpose data acquired at different temperatures with different dose rates was developed based on a procedure proposed in IEC 1244-2-1996. Moreover another recent research activity, carried out aiming at supporting the inspection and maintenance procedures is also explained in relation to the degradation mechanisms of polymeric insulating materials under thermal and irradiation stress.