The purpose of the non-destructive evaluation (NDE) R&D Roadmap for Cables is to support the Materials Aging and Degradation (MAaD) R&D pathway. A workshop was held to gather subject matter experts to develop the NDE R&D Roadmap for Cables. The focus of the workshop was to identify the technical gaps in detecting aging cables and predicting their remaining life expectancy. The workshop was held in Knoxville, Tennessee, on July 30, 2012, at Analysis and Measurement Services Corporation (AMS) headquarters. The workshop was attended by 30 experts in materials, electrical engineering, and NDE instrumentation development from the U.S. Nuclear Regulatory Commission (NRC), U.S. Department of Energy (DOE) National Laboratories (Oak Ridge National Laboratory, Pacific Northwest National Laboratory, Argonne National Laboratory, and Idaho National Engineering Laboratory), universities, commercial NDE service vendors and cable manufacturers, and the Electric Power Research Institute (EPRI).
The motivation for the R&D roadmap comes from the need to address the aging management of in- containment cables at nuclear power plants (NPPs). The most important criteria for cable performance is its ability to withstand a design basis accident. With nearly 1000 km of power, control, instrumentation, and other cables typically found in an NPP, it would be a significant undertaking to inspect all of the cables. Degradation of the cable jacket, electrical insulation, and other cable components is a key issue that is likely to affect the ability of the currently-installed cables to operate safely and reliably for another 20 to 40 years beyond the initial operating life. The development of one or more NDE techniques and models that could assist in determining the remaining life expectancy of cables or their current degradation state would be of significant interest. The ability to non-destructively determine material and electrical properties of cable jackets and insulation without disturbing the cables or connections is essential.
The major emphasis of the workshop focused on the chemical changes in the material caused by the environment (thermal, radiation, and moisture, its relationship to mechanical, physical, and electrical property changes of dielectric materials used in cable insulation and jackets and the current state-of-the- art in NDE techniques for detecting aging and degradation of cables. The only current technique accepted by industry to measure cable elasticity (the gold standard for determining cable insulation degradation) is the indentation measurement. All other NDE techniques are used to find flaws in the cable and do not provide information to determine the current health or life expectancy.
Currently, there is no single NDE technique that can satisfy all of the requirements needed for making a life expectancy determination, but a wide range of methods have been evaluated for use in NPPs as a part of a continuous evaluation program. The commonly used methods are indentation and visual inspection, but these are only suitable for easily accessible cables. Several NDE methodologies utilizing electrical techniques are in use today for flaw detection but there are none that can predict the life of a cable.
The results from the workshop identified three key areas of importance:
- Determine key indicators of cable aging that correlate with measureable changes in material properties at the macroscopic scale.
- Advance state-of-the-art in current cable NDE methods and develop new and transformational NDE methods to enable in-situ cable condition measurements that can be used to assess remaining life expectancy. The data for these developments would be collected from samples generated in laboratory cable aging experiments as well as field samples.
- Develop models for predicting remaining useful life of cables based on condition indices. The data for these models would come from existing databases, the information generated in topic 1 and 2, and other relevant sources.
In order to make a determination on the appropriate NDE technology or the need for a new technology, the key indicators of cable aging need to be identified so correlations with measureable changes in material properties at the macroscopic scale can be evaluated with the right sensitivity and measurement technique. Depending on the insulating material type, the environmental effects can happen at different degradation rates and mechanisms that change the properties. For example, ethylenepropylene rubber (EPR) can undergo thermal or radiation degradation at different rates depending on temperature or radiation dose rate. However, cross-linked polyethylene has been shown in laboratory experiments to experience an inverse temperature effect that ages the cable faster at lower temperatures with radiation.
With the determination of key indicators, advancing current state-of-the-art cable NDE methods or developing new NDE methods can close the gap with measuring material properties. Non-invasive methods are needed that are capable of measuring in discrete and difficult to access locations. In order to evaluate any new or advanced NDE method, laboratory and field aged material samples for experimentation are required to evaluate the sensitivity for identifying the key indicators to aging and to compare candidate NDE techniques.
With data from advanced NDE methods or new technologies to measure indicators of cable aging, prognostic models are needed for predicting remaining useful life of cables based on their condition and future operating environments. The NDE measurements and corresponding models will provide important information to the Light Water Reactor Sustainability (LWRS) program about the current and future condition of cables in NPPs. The data for these models would come from existing databases and information generated in improving, developing, and evaluating NDE techniques.
A consistent theme from the LWRS NDE workshops was the need for a comprehensive and fully characterized common set of samples for NDE experimentation. The required cable samples would need to be representative of various cable materials, configurations, and environmental conditions that could be characterized by destructive techniques for baseline properties and established as calibration references for NDE techniques. In order to achieve the DOE vision for NDE research on cable aging management, this sample set must be assembled.
The workshop concluded that some emerging NDE techniques show promise for detecting cable flaws and some existing instrumentation may be suitable, but new algorithms and techniques need to be developed for alternative property measurements and characterizing changes in cable aging. Promising emerging techniques include non-linear ultrasonic, Fourier infrared spectroscopy, frequency or time- domain reflectometry, and dielectric based techniques. These techniques may offer a method of nondestructive characterization of cables which can then be correlated to the remaining cable life.