Expanded Materials Degradation Assessment (EMDA): Aging of Cables andCable Systems (NUREG/CR-7153, Volume 5)
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Manuscript Completed: October 2013
Date Published: October 2014
Prepared by Expert Panel:
Robert Bernstein, Sandia National Laboratory; Sue Burnay,
John Knott Associates; Clifford Doutt, U.S. Nuclear Regulatory
Commission; Ken Gillen, Sandia National Laboratory-Retired;
Robert Konnik, Marmon Innovation and Technology Group;
Sheila Ray, U.S. Nuclear Regulatory Commission;
Kevin Simmons, Pacific Northwest National Laboratory;
Gary Toman, Electric Power Research Institute;
Gregory Von White II, Sandia National Laboratory
On behalf of:
Oak Ridge National Laboratory
Managed by UT-Battelle, LLC
J. T. Busby, DOE-NE LWRS EMDA Lead.
P. G. Oberson and C. E. Carpenter, NRC Project Managers
M. Srinivasan, NRC Technical Monitor
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
In NUREG/CR-6923, "Expert Panel Report on Proactive Materials Degradation Assessment," referred to as the PMDA report, NRC conducted a comprehensive evaluation of potential aging-related degradation modes for core internal components, as well as primary, secondary, and some tertiary piping systems, considering operation up to 40 years. This document has been a very valuable resource, supporting NRC staff evaluations of licensees' aging management programs and allowing for prioritization of research needs.
This report describes an expanded materials degradation assessment (EMDA), which significantly broadens the scope of the PMDA report. The analytical timeframe is expanded to 80 years to encompass a potential second 20-year license-renewal operating-period, beyond the initial 40-year licensing term and a first 20-year license renewal. Further, a broader range of structures, systems, and components (SSCs) was evaluated, including core internals, piping systems, the reactor pressure vessel (RPV), electrical cables, and concrete and civil structures. The EMDA uses the approach of the phenomena identification and ranking table (PIRT), wherein an expert panel is convened to rank potential degradation scenarios according to their judgment of susceptibility and current state of knowledge. The PIRT approach used in the PMDA and EMDA has provided the following benefits:
- Captured the status of current knowledge base and updated PMDA information,
- Identified gaps in knowledge for a SSC or material that need future research,
- Identified potential new forms of degradation, and
- Identified and prioritized research needs.
As part of the EMDA activity, four separate expert panels were assembled to assess four main component groups, each of which is the subject of a volume of this report.
- Core internals and piping systems (i.e., materials examined in the PMDA report) – Volume 2
- Reactor pressure vessel steels (RPV) – Volume 3
- Concrete civil structures – Volume 4
- Electrical power and instrumentation and control (I&C) cabling and insulation – Volume 5
This volume summarizes the results of an assessment of the aging and degradation of cable and cable insulation by an expert-panel. The main objective of the work was to evaluate these cable systems in NPPs where, based on specific operating environments, degradation is likely to occur, or may have occurred; to define relevant aging and degradation modes and mechanisms; and, to perform systematic assessment of the effects of these aging related degradation mechanisms during continued operation up to 80 years. The approach utilized by each expert panel was based on the Phenomena Identification and Ranking Technique (PIRT) process to identify safety-relevant phenomena and assess their importance as well as identify and prioritize research needs. Additional objectives of this effort are to determine the degradation mechanisms known for cable systems (cables, wires, insulation, terminations, and splices) specifically listing the current knowledge on aging degradation of cable and cable systems and the confidence level of this knowledge.
The panelists used the PIRT process to prioritize the different material/environmental concerns and the PIRT scores are shown in Appendix A. There are several notable trends. First, the panelists were in agreement as to the present levels of knowledge and overall aging related susceptibility of cable insulation materials, as demonstrated by the uniformity of the knowledge and susceptibility scores. Further, there were very few material/mode combinations where susceptibility was ranked above "3" with the generic susceptibility increasing with increasing severity in environment conditions. The knowledge ranking was either 2 or 3 for all materials, environments, and conditions considered. This is likely a reflection on the 40 years of accumulated information on generic aging although this may not extend to specific plant locations/conditions, as noted above.
The panelists found that the main area of uncertainty for extending NPP life beyond 60 years relates to the pre-aging carried out during the equipment qualification (EQ) process and whether it can adequately predict aging over that time scale. However, most concerns are based on the premise that cables will be exposed to the operating and design basis environments (temperature, radiation, humidity, chemical spray, and other environmental factors) that were used in the EQ process. The current understanding, based on general opinion and utility experience, is that most cables are exposed to environments that are considerably less severe than the design environment. Actual environmental conditions should be quantified by measurement and analysis so that the temperatures and dose rates to which different types of cable are exposed are quantified over their qualified life. That information would clarify the necessity and priorities for addressing the concerns raised in Chap. 5.
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