Frequency Domain Reflectometry NDE for Aging Cables in Nuclear Power Plants

Abstract

<p>Degradation of the cable jacket, electrical insulation, and other cable components of installed cables within nuclear power plants (NPPs) is known to occur as a function of age, temperature, radiation, and other environmental factors. System tests verify cable function under normal loads; however, demonstration of some cable’s ability to perform under exceptional loads associated with design-basis events is essential to assuring plant integrity. The cable’s ability to perform safely over the initial 40-year planned and licensed life has generally been demonstrated and there have been very few age-related cable failures.With greater than 1000 km of power, control, instrumentation, and other cables typically found in an NPP, replacing all the cables would be a severe cost burden. Justification for life extension to 60 and 80 years requires a cable aging management program to justify cable performance under normal operation as well as accident conditions. A variety of tests are available to assess various aspects of electrical and mechanical cable performance, but none of these tests are suitable for all cable configurations nor does any single test confirm all features of interest. One particularly powerful test that is beginning to be used more and more by utilities is frequency domain reflectometry (FDR). FDR is a nondestructive electrical inspection technique used to detect and localize faults in power and communication system conductors along the length of a cable from a single connection point. For the measurement, two conductors in the cable system are treated as a transmission line, which propagates a low-voltage swept-frequency waveform to interrogate the cable length. Note that because the applied signal is low-voltage (<5 volts), the test is completely nondestructive and poses no special safety concerns to operators. An inverse Fourier transform is used to convert the resulting frequency-domain data into a time-domain format, which can determine the physical location of signal reflections if the signal propagation velocity is known. FDR detects discontinuities in the electrical impedance that arise due to cable splices or similar changes along the path of the conductor pair. In addition, FDR has the potential to provide sensitivity to insulation degradation by detecting small changes in capacitance between the cable conductors being examined. Example changes that impact the insulation capacitance include exposure to heat, radiation, water damage, corrosion, or mechanical fatigue. The technique is also sensitive to cable bends, the particular lay of the cable in tray, proximity to other cable, and other factors that bear consideration when interpreting these tests. This paper examines various influences on the FDR approach and compares results of three different instruments capable of producing the FDR to assess accelerated aging cable damage among several NPP representative cables.</p>