Information Notice 2007-06, Potential Common Cause Vulnerabilities in Essential Service Water Systems
| ML062840608 | |
| Person / Time | |
|---|---|
| Site: | Point Beach, South Texas  |
| Issue date: | 02/09/2007 |
| From: | Case M J NRC/NRR/ADRA/DPR |
| To: | |
| Richard Laura, NRR/DIRS/IOEB | |
| References | |
| IN-07-006 | |
| Download: ML062840608 (6) | |
UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001February 9, 2007NRC INFORMATION NOTICE 2007-06:
POTENTIAL COMMON CAUSE
VULNERABILITIES IN ESSENTIAL SERVICE
WATER SYSTEMS
ADDRESSEES
All holders of operating licensees for nuclear power reactors, except those who havepermanently ceased operations and have certified that fuel has been permanently removedfrom the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alertaddressees of the importance of maintaining essential service water (ESW) systems in amanner that precludes the development of potential common cause failure vulnerabilities due topiping or heat exchanger degradation. It is expected that recipients will review the informationfor applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this IN are not NRC requirements; therefore, no specificaction or written response is required.
DESCRIPTION OF CIRCUMSTANCES
There have been two events at foreign operating reactors in which external corrosion of pipinglocated in vaults has caused a catastrophic loss of one train of the ESW system. These twoevents also had common cause failure aspects that were identified as part of the extent-of-condition reviews. The first foreign event occurred at a pressurized-water reactor (PWR)designed by Westinghouse, that began commercial operation in 1988. On August 25, 2004,while operating at 100 percent power, an ESW system manhole pipe broke circumferentiallyprompting a plant shutdown from the total loss of the B train of the ESW system. The eventwas of great concern for plant safety because it could have led to a common cause failure in asystem that is the ultimate heat sink for most safety loads. The ESW pipes are buried and usemanholes at certain intervals to allow for inspection and maintenance. The ESW piping is aBONNA design that uses a steel pipe lined with cement on both sides to prevent corrosion. The manhole filled with surface water and corroded the exposed carbon steel manhole pipingneck. The break resulted from external corrosion due to improper installation of externalcement pipe coating. The licensee extent-of-condition review found corrosion at two otherESW manhole necks that also required significant repair. The second foreign event occurred at a PWR, designed by Framatome, that began commercialoperation in 1988. On December 11, 1998, while operating at 75 percent power, an ESW piperuptured and flooded the piping gallery with sea water. Since the B train of the ESW systemwas lost, the operators shutdown the plant. The pipe is composed of prestressed, inner and outer concrete pipes with embedded strings and steel pipe. A visual inspection of the failedpipe revealed a 3.3 foot long by 2.8 inches wide through-wall rupture. The prestressed wireand steel cylinder were found to be severely corroded. The pipe is located in an undergroundgallery that has very humid conditions. The licensee determined that condensation got inside asmall crack on the surface of the pipe causing corrosion of the steel wire. The licensee extent-of-condition review identified 16 other pipe spools that were also replaced. There also have been two events at domestic operating reactor plants that experiencedsignificant internal degradation of critical ESW components that had the potential for commoncause failures. On June 27, 2005, at the South Texas Project, Unit 2, a three gallons/hourthrough-wall leak was identified in Train 2A, essential cooling water (ECW) system pipingflange. The pipe is 30 inches in diameter, composed of an aluminum-bronze, and is locatedimmediately downstream of an ECW return throttle (butterfly) valve. Subsequent licenseeinvestigation found cavitation pitting, circumferential pipe cracking, and pipe-to-end flange weldseparation in the pipe. On August 15, 2005, as part of their extent-of-condition review, thelicensee inspected Train 2B of the ECW system and determined that a similar downstreamflange had an approximately 30 inches-long by three inches-wide segment of the aluminum-bronze pipe that broke free. Train 2B of the ECW system was declared inoperable for thesame reasons as ECW Train 2A and was reported to the NRC as a common-causeinoperability of independent trains (Licensee Event Report No. 499/2005004, AgencywideDocuments Access and Management System (ADAMS) Accession No. ML052630031). Thelicensee determined the cause of the damage was cavitation impingement from the heavilythrottled butterfly valve. The root cause was the failure to incorporate requisite inspectionactivities for ECW piping into station programs.On March 20, 2005, at Point Beach Nuclear Plant, Unit 1, the G-01 emergency diesel generator(EDG) became inoperable due to a through-wall leak in the endbell of the EDG cooling systemheat exchanger (Inspection Report 50-266/2005-010, ADAMS Accession No. ML053000237).Licensee inspections identified significant damage to the endbell due to microbiologicallyinduced corrosion (MIC). The licensee maintenance rule evaluation determined that the poorcondition of the endbells from pitting caused by MIC was known for nearly ten years, but timelyand effective corrective actions were not taken. Also, on April 22, 2005, a through-wall servicewater (SW) leak occurred on the G-02 EDG heat exchanger alternate SW supply line due toMIC.
BACKGROUND
The ESW system (or its equivalent) for U. S. commercial reactor plants is the assured, safety-related means of transferring sensible and decay heat from the reactor coolant system to theultimate heat sink. The ESW system is also relied upon for other critical safety functions, suchas providing cooling water for most of the essential, safety-related equipment used formitigating plant accident and transient conditions, reactor coolant pump seal cooling, spent fuelpool cooling, and for dissipating sensible and reactor decay heat during shutdown conditions. Also, on PWRs, the safety related auxiliary feedwater systems typically rely on the ESW systemas an emergency makeup water source for feeding the steam generators. Plant-specificprobabilistic risk assessments (PRAs) have shown that the loss of the ESW system may be asignificant contributor to the potential for a core damage accident.
DISCUSSION
The first foreign event involved the failure to properly protect the exterior piping surfaces in thevicinity of weld neck flanges that had been installed to allow personnel access for inspecting theinside ESW system piping surfaces. The unprotected weld neck joints were subject tocorrosion. All of these areas were also subject to routine wetting of the unprotected exteriorsurface of weld neck areas. In addition, NRC staff concluded that a fundamental shortcomingassociated with the first foreign event was the utility's failure to adequately trend and takeappropriate corrective action for a known degraded condition. Domestic operating reactorplants are subject to Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix B,Criterion XVI, "Corrective Action," which requires that prompt and effective corrective actions betaken to address significant conditions adverse to quality. The second foreign event also involved the failure to protect the exterior piping surface of theESW system. One of the lessons learned was to implement an inspection program thatinspects for both internal and external corrosion. The licensee also painted the outside of theESW pipes to minimize the potential for corrosion.Domestically, a potentially similar problem was noted during an NRC inspection at the PointBeach facility. Like the first foreign event, the licensee for Point Beach did not take appropriatecorrective action to resolve a known degraded condition until MIC resulted in an actual failure. Also, like the first foreign event, MIC attack of the EDG endbells represented a common causefailure mechanism that could render all of the EDGs inoperable if not properly monitored andresolved.ESW systems for U. S. nuclear power plants are generally unique from one plant to another. However, domestic ESW systems typically include piping sections that are buried and notreadily accessible for inspection. Buried sections of piping can be subject to periodic wettingfrom storms or local flooding conditions. Exterior protective coatings may also not be fully intactdue to improper installation, age degradation, or maintenance practices. It is also possible forsome ESW piping sections to be located in vaults or pipe chases that are subject to periodicflooding and/or high humidity that can closely mimic the foreign events. Also, at South Texas Project Unit 2, ESW system degradation led to a through-wall leak thatultimately became an actual common cause failure vulnerability. Similar cavitation damage wasfound in two trains of the ESW system at Unit 2. Proper application of the recommendations ofNRC Generic Letter (GL) 89-13, "Service Water System Problems Affecting Safety-RelatedEquipment," and 10 CFR Part 50, Appendix B, Criterion XVI, "Corrective Action," likely wouldhave prevented this event.RELEVANT GENERIC COMMUNICATIONSNRC GL 89-13, "Service Water System Problems Affecting Safety-Related Equipment," datedJuly 18, 1989, requested specific licensee actions to resolve SW system problems. Inparticular, this GL recommended that licensees ensure by a routine inspection andmaintenance program for open-cycle SW system piping and components that corrosion,erosion protective coating failure, silting, and biofouling cannot degrade the performance ofsafety related systems supplied by service water.NRC GL 90-05, "Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2, and 3 Piping," dated June 15, 1990, provides guidance for Code Class 3 piping. Two specificflaw evaluation approaches, as discussed in Enclosure 1 to the GL, should be considered,namely, the "through-wall flaw" and the "wall thinning" approaches.NRC IN 92-49, "Recent Loss Or Severe Degradation Of Service Water Systems," dated July 2,1992, alerted licensees to several service water related events, including one event thatresulted in the loss of the ultimate heat sink for a short period of time.NRC IN 94-79, "Microbiologically Influenced Corrosion Of Emergency Diesel Generator ServiceWater Piping," dated November 23, 1994, alerted licensees that stagnant or intermittent-flowconditions, as in the case of emergency diesel service water supply headers, are conducive tothe growth of microorganisms that can accelerate corrosion rates. NUREG-1275, Volume 3, "Operating Experience Feedback Report - Service Water SystemFailures and Degradations in Light Water Reactors," (November 1988) summarizes anddiscusses SW system events from 1980 to early 1987.NUREG-1461, "Regulatory Analysis for the Resolution of Generic Issue 153: Loss of EssentialService Water in LWRs," (August 1993) provides a review of industry experience and plant-specific PRAs and insights related to ESW system vulnerabilities.
CONTACT
This information notice requires no specific action or written response. Please direct anyquestions about this matter to the technical contacts listed below./TQuay for MCase/
Michael J. Case, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor RegulationTechnical Contacts:
Richard A. Laura, NRR/DIRS
301-415-1837 E-mail: ral1@nrc.gov
James E. Tatum, NRR/DSS
301-415-2805 E-mail: jet1@nrc.gov Note: NRC generic communications may be found on the NRC public web site,http://www.nrc.gov under Electronic Reading Room/Document Collections.
CONTACT
This information notice requires no specific action or written response. Please direct anyquestions about this matter to the technical contacts listed below./TQuay for MCase/
Michael J. Case, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor RegulationTechnical Contacts:
Richard A. Laura, NRR/DIRS
301-415-1837 E-mail: ral1@nrc.gov
James E. Tatum, NRR/DSS
301-415-2805 E-mail: jet1@nrc.gov Note: NRC generic communications may be found on the NRC public web site,http://www.nrc.gov under Electronic Reading Room/Document Collections.ADAMS Accession Number: ML062840608OFFICE
DIRS:IOEB
Tech.Editor
BC:DSS:SBPB
TL:DIRS:IOEB
PGCB:DPRNAME
RLaura
CBladey
JSegala
MKing
CHawes CMHDATE
10/18/2006
10/18/2006
10/23/2006
02/01/2007
10/27/2006OFFICE
PGCB:DPR
IP
BC:ADRA:DPR
D:DPRNAME
QNguyen
JDunnLee
CJackson
MJCaseDATE
02/02/2007
01/22/2007
02/082007
02/09/2007OFFICIAL RECORD COPY