Cable insulation polymers are among the more susceptible materials to age-related degradation within a nuclear power plant. This is recognized by both regulators and utilities, so all plants have developed cable aging management programs to detect damage before critical component failure in compliance with regulatory guidelines. Although a wide range of tools is available to evaluate cables and cable systems, cable aging management programs vary in how condition monitoring and nondestructive examination is conducted as utilities search for the most reliable and cost-effective ways to assess cable system condition. Frequency domain reflectometry (FDR) is emerging as one valuable tool to locate and assess damaged portions of a cable system with minimal cost and only requires access in most cases to one of the cable terminal ends. To appreciate FDR capabilities, a number of laboratory studies have been performed where samples are aged in small ovens or are mechanically damaged in a controlled fashion and the response is noted. Other studies have reported field experience where an FDR has identified a potential cable damage that was subsequently confirmed by an alternate local inspection. Isolating the specific insulation material damage aspect and all relevant variables associate with the indication however is difficult and parametrically changing these cable conditions to examine a range of variables is expensive. A model-based approach facilitates controlled variation of the cable system parameters to assess their influence on the FDR response and thereby allow practitioners to better interpret the FDR measurements.
This work describes development of a finite element physics-based model of a cable segment that feeds into an LRC cascaded circuit model of the entire cable system. One or more segments of the cable system model have altered component coefficients (mostly changed capacitance) to represent the degree of damage and the location of the damage in the system. This circuit model is then subject to a simulated FDR examination. The model is verified using a limited number of laboratory controlled experimental cases and by comparing it to a commercial simulator suitable for simulation of some cable configurations. Then the model is used to examine a broader range of parameters including: defect length, defect profile, degree of degradation, number and location of defects, and addition of impedance-matched extensions to minimize the end-shadow effect.