The servicing and maintenance of the miles of I&C, low — and medium-voltage cables in each light water plant has historically been reactive in nature. Such reactive efforts have successfully resolved connector problems, corrected signal-to-noise ratios, and improved grounding and shielding. However, they have done little to identify the condition, age, or remaining useful life of cables, especially the insulation material (AMS Corp., 2010). Not enough research has been completed to identify a useful, practical method, procedure or technique for accurately evaluating the ageing condition of plant wiring or correlating the condition of cables to measurable electrical, mechanical, or chemical properties (AMS Corp., 2010). In 2010 , the U. S. NRC (NRC DG1240) stated that ‘research and experience have shown that no single, nonintrusive, currently available condition monitoring method can be used alone to predict the survivability of electric cables under acci­dent conditions’ (U. S. NRC, 2010b).

Because of the safety-related importance of I&C cables functioning effec­tively on an ongoing basis, efforts to use prognostic techniques to predict residual life in cables continue.

Such techniques attempt to establish relationships between condition indicators and ageing stressors (IAEA, 2011). To predict future perfor­mance, a trendable indicator and a well-defined end point are essential. From them, a trend curve can be used to estimate the time remaining before the end point is reached (U. S. NRC, 2001). Used with appropriate mate­rial ageing models and knowledge of environmental conditions, such trend data can be used to estimate residual cable lifetimes, but only when suffi­cient data has been generated to validate predictive ageing models (IAEA, 2011). Currently, both the NRC and DOE are sponsoring research at AMS, national laboratories such as Sandia National Laboratories (SNL), Oak Ridge National Laboratory (ORNL), Idaho National Laboratory (INL), and elsewhere to address cable aging and cable qualification issues.

In recent years researchers have developed analytical ageing models based on experimental data from cable samples that have been subjected to accelerated ageing. For example, the power law extrapolation model extrapolates radiation ageing data obtained under isothermal conditions at several dose rates. Similarly, the superposition of time-dependent data model combines data from both thermal and radiation ageing to account for both dose rate effects and the synergistic relationship between radiation and thermal ageing. The superposition of end-point dose data model also uses a superposition approach to radiation and thermal ageing data, but can be used in materials where a single dominant degradation mechanism is lacking (AMS Corp., 2011).

There have been recent efforts toward integrated cable residual life analysis systems that combine existing methods to provide cable testing, ageing assessment and cable management as part of a plant-wide cable age­ing assessment program (AMS Corp., 2010) (see Table 6.2). For example,

Analysis and Measurement Services (AMS) Corp. of the United States is developing methods to ‘calibrate’ results from classical testing methods so they can be categorized, evaluated consistently, and if necessary improved. Correlations between measurable parameters and the health and condition of the cable using classical ageing tests such as the elongation at break (EAB) test would be identified. The classical tests would then be integrated with promising new cable testing technologies, such as the wireless AgeAlert™ micro-sensors (AMS Corp., 2010) (see Table 6.3). Testing methods are then categorized according to their capability to show a particular fault, faults or developing cable conditions that indicate degraded performance (see Fig. 6.2). These tests are performed using laboratory and plant-aged cables of the types found in nuclear power plants.

The correlations between the changes measured by the various methods and condition or age of the cable will form the foundation of a database that will be the core of the integrated cable testing and analysis system (AMS Corp., 2010). This database would contain the information to provide default configuration settings for the various devices that could be tested, optimized data acquisition parameters for the equipment under test, control of data acquisition hardware, and the ability to analyze and store the results of the testing. The program for the AMS integrated cable testing practice would incorporate eleven different modules (see Fig. 6.3): user interface; test lead compensation; test data acquisition; data storage; data qualifica­tion; data review; statistical analysis; historical data trending; similar equip­ment data comparison; report generation; default equipment setting (AMS Corp., 2010 ).

The result will be a user-friendly and technically feasible solution for examining low — and medium-voltage plant cables and wiring to determine their ageing condition and residual life (AMS Corp., 2010).

6.2 Testing and analysis techniques in an integrated cable condition monitoring system.