By William E Berger Jr., Senior Vice President, Fauske & Associates, LLC
When designing systems whose failure can result in unacceptable losses, reliability becomes an important consideration. Reliability is implemented through redundant design and strict performance requirements for the components used in these systems. This however may not be enough as witnessed by the beyond design basis events at Fukushima, whose consequences could not be protected against through redundancy. There will be many lessons learned from this tragic event of which one will be a closer examination of potential common cause failures. It is these mechanisms which defeat the intent of a redundant system design, one such mechanism is degradation due to aging. Even though we analyze and test under accelerated aging conditions, not all elements related to aging can be addressed in the laboratory. Programs are needed which investigate and analyze components in situ to ensure degradation mechanisms are held in check. There are required functional tests performed on a periodic basis which are intended to ensure proper performance as well as early detection of potential modes of failure. However not all components fall into such periodic testing and one such critical component is that of cables.
Cables are the lifeline for plant operation and part of each redundant system required to protect and shut down a nuclear power plant. The loss of redundant cables at a time when they are needed may compromise plant safety. Cables are routed through a cross section of environmental conditions and give the appearance of satisfactory performance primarily because they have no moving parts and thus failure is not easily detected. The difficulty with cables is that in general there is no visual detection of incipient failure and no such diagnostic method currently exists.
The NRC issued in 1999 the GALL Report, “Generic Aging Lessons Learned Report” as part of license renewal effort. This report, identified as NUREG 1801, allowed cables which were part of an Equipment Qualification methodology to be analyzed for extended life, this extension process did not require any field testing. Again the premise is that aging tests were adequate to consider all potential mechanisms for cable aging. Cables in today’s nuclear stations are subject to many mechanisms which result in some form of degradation. These include tight bends during installation which over years produce mechanical stresses. Cables are routed close to heat sources which cause high localized temperatures even though average ambient values are well within qualification specifications. There may also be heating as a result of I2R due to overloads. These all need to be considered in a life assessment. Cables subjected to these mechanisms may perform correctly under normal operating conditions but may then fail when subjected to accident type environments.
There is a new effort underway to adequately address some of the issues. The NEA, Nuclear Energy Agency, which is a specialized agency under OECD, Organization for Economic Co-Operation and Development, has formed a group called CADAK, Cable Aging Data and Knowledge. Their first meeting was held November 21-23, 2011. Their goals are to establish the technical basis for assessing the qualified life of electrical cables in light of the uncertainties identified following the initial qualification testing; estimate the remaining qualified lifetime of cables and available margins; and update and benchmark data of various cable condition monitoring techniques used in nuclear power plants adopted by various countries. The NRC is participating and plans to use information gained under this program to improve cable condition monitoring. The CADAK project is expected to continue through 2014.
Prior to 2014 it would be prudent for utilities to consider some form of a cable aging program. To develop a cable program it is necessary to focus on a reduced set of key cables to be monitored. Next a method for field testing needs to be defined and finally test results recorded and trended. The intent is not to monitor all cables within a plant but to look at those which are important and potentially subjected to environmental stresses. The list of cables for such a program is typically developed by selecting cables in known harsh environments followed by plant walk downs to try and uncover areas where unexpected conditions may occur. Numerous techniques are available for field testing including Tan δ, partial discharge and a variety of time and frequency domain reflectometry. Trending and analyzing cable performance and testing will necessitate some form of a data base.
The NRC issued in June of 2010 a draft regulatory guide, DG-1240, “Condition Monitoring Program for Electric Cables used in Nuclear Power Plants”. That draft regulatory guide establishes a foundation for a program and will more than likely be revised or issued formally based on the CADAK study.
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