Information Notice 2006-17, Recent Operating Experience of Service Water Systems Due to External Conditions
| ML061510292 | |
| Person / Time | |
|---|---|
| Issue date: | 07/31/2006 |
| From: | Ho Nieh NRC/NRR/ADRA/DPR |
| To: | |
| C. Vernon Hodge, NRR/DIRS/IOEB | |
| References | |
| IN-06-017 | |
| Download: ML061510292 (6) | |
July 31, 2006
NRC INFORMATION NOTICE 2006-17: RECENT OPERATING EXPERIENCE OFSERVICE WATER SYSTEMS DUE TO EXTERNAL CONDITIONS
ADDRESSEES
All holders of operating licenses for nuclear power reactors, except those who havepermanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to informaddressees of operating experience within the past few years affecting the operability of theservice water system at several nuclear power plant The NRC expects that recipients will review the information for applicability to their facilities and consider actions, as appropriate, toavoid similar problem However, suggestions contained in this IN are not NRC requirements;therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
During 2004 through 2005, 15 events occurred related to blockages in service water systems. These events were primarily self-revealin The various blocking agents included silt, sand, small rocks, grass or weeds, frazil ice, and small aquatic fauna, such as fis All these eventswere of low safety significance but illustrate the susceptibility of the safety-significant service water syste For instance, in September 2005, NRC inspectors identified a condition at FortCalhoun that allowed small rocks to regularly enter the raw water system, contribute to trippingof a pump and strainer motors, and interfere with traveling screen operation (NRC InspectionReport 50-285/2005-11, Agencywide Documents Access and Management System (ADAMS)
Accession No. ML052920543). In June 2005, NRC inspectors found a portion of a servicewater accumulator outlet line at Salem to be nearly full of silt (NRC InspectionReport 50-272/2005-03, ADAMS Accession No. ML052090344). Salem - Hope Creek Nuclear Power PlantsOn December 2, 2004, crude oil was found leaking from a ship (Athos I) on the Delaware Riverupstream of the Salem and Hope Creek Generating Station To mitigate the potential for oilintrusion into the cooling water systems, the licensee placed booms around the intakestructures at both station The booms are effective at controlling oil that is at or near the surface; however, the effectiveness of the booms was lessened because the spilled oil was"heavy" crude and was suspended at varying depths in the rive On December 3, 2004, the licensee commenced shutdown of both Salem units due to the conditions on the rive There were no issues associated with the shutdown Hope Creek was already shut down for a refueling outag The licensee restarted both Salem units after review of heat exchanger performance and monitoring of the oil spill. Cooper Nuclear StationOn November 20, 2004, the service water system was clogged with sediment, resulting in anunexpected pressure drop in both loops of service water, high differential pressure alarms on both strainers, and isolation of the nonessential service water load Both trains exceeded the differential pressure operability limit of 15 psi Backwash automatically initiated andsuccessfully cleaned the Loop A strainer, but the analogous action for Loop B did not succeed in cleaning the straine Operators opened the strainer bypass valve to restore service water flow and subsequently cleaned both strainers.On October 20, 2005, while preparing for online maintenance of the service water system,operators started a fourth service water pump and received high differential pressure alarms on both Loops A and The automatic backwash did not sufficiently decrease the differentialpressure, and operators bypassed the straine Following these actions, the service water system header pressures returned to norma During the event, operators declared both loopsof service water inoperabl Both loops exceeded the strainer differential pressure structural integrity limit of 15 psi The high differential pressure across the strainers was the result of debris (small rocks and sediment) introduced by the start of the fourth pum With both loopsof service water inoperable, operators declared both emergency diesel generators inoperabl In 2005, the NRC Region IV office organized a special inspection based on the repetitive natureof this type of event (NRC Inspection Report 50-298/2005-15, ADAMS Accession No. ML061160027).Watts Bar Nuclear PlantOn November 22, 2004, while performing a manual valve exercising procedure, the licenseeidentified that a centrifugal charging pump backup cooling line from the essential raw coolingwater system was completely blocked with sil Approximately 2.5 gallons of muddy pastepassed through the 1-inch drain valve before the valve became blocke The line had to be cleared mechanicall This line is significant in that this is the only high head pump with a backup source of cooling water (NRC Inspection Report 50-390, 391/2004-05, ADAMSAccession No. ML050280344).
DISCUSSION
Cooper Nuclear StationIn both events, for a few minutes service water flow was lost to the nonessential header andgreatly reduced to the essential header In each case, the successful Loop A automatic backwash precluded the need for a manual scram, which would have been required if the loss of turbine equipment cooling water had been prolonge In each event, the Loop B filteringfunction was overwhelmed by the inrush of sedimen The Loop B automatic backwash function failed due to the lack of downstream pressure, which provides the motive force for the backwashing operatio The licensee believes that the contributing external factor was the lowlevel of the Missouri River, the source of the service water syste Both of these events occurred during autumn, following the navigation seaso A weir wall is installed in the river in front of the intake structur The low river level caused an increased portion of the water that flows into the intake structure to go around (rather than over) the weir wall and jet into the service water ba This circuitous flow entrained more sand due to the high flow and deposited it in the intake structure near the service water pump intakes in the low-flow areas.At the time of the October 2005 event, the licensee had not completed its actions to modify thesetpoint for automatic backwash of the strainer, alter the strainer intermittent backwashfrequency, modify the strainer differential pressure alarm setpoint, and implement weir-wall and traveling-screen modifications.NRC inspectors noted that the licensee had not performed certain actions committed to in itsresponse to NRC Generic Letter (GL) 89-13, "Service Water System Problems AffectingSafety-Related Equipment," specifically to periodically monitor silt levels and to periodically examine the intake structure basin for silt, debris, and deterioration (including corrosion), using divers or by dewatering the intake structure ba At the time of the event, the licensee had not examined the intake structure bay to assess its conditio Watts Bar Nuclear PlantThe licensee generated 13 problem evaluation reports from early 2002 through late 2005 forblockages identified in raw cooling water line The licensee identified silt accumulation in portions of systems providing raw cooling water for both essential and nonessential purposesand for high pressure water for fire protectio These accumulations were identified in both stagnant and active cooling water lines, typically in system low points and in piping with lowwater velocit In 1999 and 2002, clam accumulations resulted from missed biocide treatment The licensee implemented periodic ultrasonic testing and flushing to identify and minimize blockages due to silt and clam accumulation The initial frequency of ultrasonic testing was every 6 months, later shortened to every 3 month However, the licensee determined that thisprogram did not cover all susceptible lines and components.The centrifugal charging pump backup cooling line was not included in the ultrasonic testingmonitoring progra In 2000, a maintenance rule panel review left the flushing frequency for this line at 18 months, not recognizing the consequences of silt accumulatio This conclusion was consistent with the general site perception that silt accumulation was not a significantproble The blockage was found by means of an 18-month manual valve tes Most other lines were being flushed or tested every 3 month This issue resulted in a White finding in the NRC's Significance Determination Process. Raw water systems draw from a section of the Tennessee River downstream of the Watts Barda The suspended solids count in the river water increases after periods of heavy rains upstrea The suspended solids are transported into the affected systems where they settle atpoints with low fluid velocitie The licensee's corrective actions for the violation included increasing the frequency of ultrasonictesting, developing higher velocity flush procedures, and modifying systems to improve flushing. Lessons learned included the following observations:*Silt accumulation in smaller diameter lines may not flush as readily as in larger diameterlines.*Silt accumulates in stagnant lines off the main headers.
- Lines with a vertical drop off the main headers are more susceptible to silt accumulationthan lines with horizontal legs off the main headers.RELEVANT GENERIC COMMUNICATIONS NRC Generic Letter 89-13, "Service Water System Problems Affecting Safety-RelatedEquipment"NRC GL 89-13 lists the following five recommendations for licensees:*Significantly reduce the incidence of flow blockage problems resulting from biofouling.*Conduct a test program to verify the heat transfer capability of all safety-related heatexchangers cooled by service water, including initial and periodic retesting.*Ensure by a routine inspection and maintenance program for open-cycle service watersystem piping and components that corrosion, erosion, protective coating failure, silting,and biofouling cannot degrade the performance of the safety-related systems suppliedby service water.*Confirm that the service water system will perform its intended function in accordancewith the licensing basis for the plant.*Confirm that maintenance practices, operating and emergency procedures, and trainingthat involves the service water system are adequate for ensuring that safety-relatedequipment cooled by the service water system will function as intended and thatoperators of this equipment will perform effectively.NRC Information Notice 2004-07: "Plugging of Safety Injection Pump Lubrication Oil Coolerswith Lakeweed" NRC IN 2004-07 also discusses operating experience related to service water systemsusceptibilities due to external events.CONCLUSION The above events involve instances in which sediment and debris has blocked flow in one ormore service water line A number of the events described above involved the failure to take adequate and timely corrective actions that could have prevented the event from occurring. Often there were multiple previous occurrences that could have alerted licensees to take more aggressive or broader corrective action IN 2006-17