Information Notice 1991-64, Site Area Emergency Resulting From a Loss of Non-Class 1E Uninterruptible Power Supplies

Site Area Emergency Resulting From a Loss of Non-Class 1E Uninterruptible Power Supplies

ML031190491
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	10/09/1991
Revision:	0
From:	Rossi C E Office of Nuclear Reactor Regulation
To:
References
IN-91-064, NUDOCS 9110030270
Download: ML031190491 (10)

UNITED STATESNUCLEAR REGULATORY-COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555October 9, 1991NRC INFORMATION NOTICE 91-64: SITE AREA EMERGENCY RESULTING FROM A LOSS OFNON-CLASS 1E UNINTERRUPTIBLE POWER SUPPLIES

Addressees

All holders of operating licenses or construction permits for nuclear powerreactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information noticeto alert addressees to problems resulting from the common mode failure ofuninterruptible power supplies used in nonsafety-related applications at Unit 2of the Nine Mile Point Nuclear Station. It is expected that recipients willreview the information for applicability to their facilities and consideractions, as appropriate, to avoid similar problems. However, suggestionscontained in this information notice are not NRC requirements; therefore, nospecific action or written response is required.

Description of Circumstances

On August 13, 1991, Nine Mile Point.Unit 2 was operating at full power when afault occurred on phase B of the main transformer which caused the generator,turbine, and reactor to trip. Station loads were transferred automaticallyfrom the normal station service transformer, which receives power from thegenerator, to the reserve station transformer, which-receives power from thegrid. During the transient which lasted for about 12 cycles, voltage on thestation's phase B buses decreased to approximately .50 percent of normal valuebefore returning to normal.The voltage transient resulted in loss of the power output from five of eightnonsafety-related uninterruptible power supplies. 'Loss of these power suppliescaused the loss of control room annunciators,.indication of control rod posi-tions, the core thermal limits computer, the process computer, the safetyparameter display system computer, the feedwater control system, some instru-mentation for balance-of-plant systems, some instrument recorders, the plantradio and paging systems, and some of the lighting for the plant. Some of theinstrument recorders that were lost failed "as is." For.example, the averagepower range monitors continued to indicate 100% after the reactor tripped.Nevertheless, control room operators were able to verify from average powerrange meters and local power range lights mounted on back panels'and from otherindications that the reactor actually had tripped and.was shut down...91100302 IN 91-64October 9, 1991 Because of the loss of control room annunciators and the reactor transientresulting from automatic tripping of the generator, turbine, and reactor,Niagara Mohawk Power Corporation (the licensee) declared a site area emergencyin conformance with the emergency plan. The operators shut down the plant inaccordance with emergency operating procedures. Thirty-four minutes after thetrip, plant personnel had restored power output from the five nonsafety-relateduninterruptible power supplies using the alternate maintenance power sources.Thirteen hours after the trip, the reactor was in the cold shutdown condition,and an hour later, the licensee ended the site area emergency.As shown in Attachment 1, each uninterruptible power supply cabinet receives3-phase 600-Vac power from the plant's electrical distribution system. Thissystem is connected to either the generator or the grid. Inside the cabinet,the 600-Vac input power passes through an ac-to-dc converter to obtain approxi-mately 140-Vdc power which then passes through an inverter to generate quality3-phase 208/120-Vac power that is within specified limits for voltage ampli-tude, frequency, and phase. The output from the cabinet is distributed tocritical loads, many of which are sensitive to the voltage irregularities thatare sometimes present with normal ac power. To prevent the interruption ofpower to these loads when the normal supply of power is lost, a 125-Vdc batteryis connected to the uninterruptible power supply between the ac-to-dc converterand the dc-to-ac inverter.The uninterruptible power supply cabinet also receives three-phase208/120-Vac maintenance power. This power is provided through a voltageregulator and is used to supply critical loads when the supply of uninter-ruptible power from the inverter is not available. Within the cabinet, acontrol logic system selects either inverter or maintenance power for distribu-tion to the critical loads.Prior to and at the time of the event, the control logic power supply normallyreceived power from phase B of the maintenance power source. If maintenancepower was not available, the input for the control logic power supply automati-cally transferred to the output of the inverter. However, on August 13, 1991,the control logic tripped the uninterruptible power supply in response to thetransient on phase B of the maintenance power source. The licensee hadexpected that the control logic power supply would transfer to the output ofthe inverter and, when inplant electrical distribution system voltages returnedto normal, would transfer back to the maintenance source. Had this happened,the uninterruptible power supply would have continued'to supply the criticalloads.If the control logic power supply had been wired to receive its normal powerfrom the inverter and backup power from the maintenance source, the controllogic would have had a continuous source of power and the loss of power outputfrom the uninterruptible power supplies would not have occurred. The controllogic power supplies for the three nonsafety-related and the two safety-relateduninterruptible power supplies that were not lost during the event normallyreceive dc power for control logic through a dc-to-dc converte IN 91-64October 9, 1991e te 5 Six battery packs, each consisting of 3 small rechargeable lead-acid cellsith nInterruptible.Rower.kupply, provide power to the control logic andWen~fdr pio i &itfi'io t~ir vowerrfs2Isaqisl'bW1

• If the.ibattery packsn .1 tfb 1A i rtWIt 46Glid hWteprbVided sufficiente to NeltdJ &fiebei~rsiti td:h&Viepreveivtedilos df theuninterruptible power supplies. After thi9Evhtt hfeoltccnSeerfoundithatstheelectrolyte in the lead-acid cells was dry.Although the prev nt ve zintenahcg:section in the vendor's manual does notaddress the bdittNe 0 aSot b Pecti bn of the manual states that theystiyl be rb la 4 Si* ry r4i rg .2Tbvendor had installed the battery packsz03 r 2 rtie licensee determined that the batterypac k1s9gb6iI29e' e d q because of the environmental condi-tions within the cabinets.The licensee obtained the unintaiript~ bEt.iber ideas es:ffomEx-de-slettronicsin 1981. In 1985, the licensee obtained)-a" eieV-- Wn'ual for theuninterruptible power supplies from Exide Electronics which recognized theimportance of reversing the connectios o$foWatTWndbackup power for thecontrol logic power supplies. New unitsb~pl-dtk(,Exide Electronics did havethe connections reversed. However, Exide Electronics did not specificallyadvise the licensee of this change. The licensee had obtained theuninterruptible power supplies as commercial grade products. :>-t;mti1AOt'-' miD IG 5 98 xrqqu2 9 , i- i4J .1The licensee has modifi f 4Mb p'WdI' zuppliesttOlpravide normalinput power to the control logic power supplies from the output of the inverterand alternate power from the maintenance source. The licensee has alsoreplaced the battery packs for the control logic and has established an annualreplacement schedule. The licensee intends to evaluate the performance of theswitching circuitry for supplying input power to the control logic undervarious conditions and to process appropriate changes to the vendor's manual.DiscussionThe transformer fault caused the generator, turbine, and reactor trips. Thetransformer fault also led to a momentary degradation of voltage on the plant'selectrical distribution system and, in turn, the common mode failure of thefive uninterruptible power supplies. Because of the loss of control rodposition indicators due to loss of the uninterruptible power supplies, theoperators could not Immediately verify that all control rods had fully insertedinto the reactor core. This resulted in some difficulty in implementing theemergency procedures. However, the reactor was in a safe condition at alltimes, and operators were able to monitor all safety parameters, except forcontrol rod positions, throughout the event. The two safety-related uninter-ruptible power supplies were not lost. Had a design basis accident occurred inconjunction with this event, power would have been available for the instrumen-tation and control components in the required safety system IN 91-64October 9, 1991 This information notice requires no specific action or writtin response. Ifyou have any questions about the information In this notice, please contact oneof the technical contacts listed below or the appropriate Office-of NuclearReactor Regulation project manager.ars E. Rossi, 1rector-Division of Operational Eve ts AgssssmentOffice of Nuclear.Reactor RegulationTechnical contacts: Jack E. Rosenthal, AEOD(301) 492-4440Roger Woodruff, NRR.(301) 49Z-1152

Attachments:

1. Unlnterruptible Power Supply Before Design Change2. List of Recently Issued NRC Information Notices I=-- 125-VDCBATTERY(i,UN INTERRUPTIBLE POWER SUPPLY BEFORE DESIGN CHANGE(D 0 AD 0v-,.&s a.o T mO-Atoo?

Attachment 2IN 91-64October 9, 1991 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICESInformation Date ofNotice No. Subject Issuance Issued to91-6391-6291-6191-6091-5991-5891-57Naturalat FortNuclearGas HazardsSt. VrainGenerating StationDiesel Engine DamageCaused by HydraulicLockup Resulting fromFluid Leakage IntoCylindersPreliminary Results ofValidation Testing ofMotor-Operated ValveDiagnostic EquipmentFalse Alarms of AlarmRatemeters Because ofRadiofrequency Inter-ferenceProblems with AccessAuthorization ProgramsDependency of OffsetDisc Butterfly Valve'sOperation on Orientationwith Respect to FlowOperational Experienceon Bus TransfersPotential RadioactiveLeakage to Tank Ventedto Atmosphere10/03/9109/30/9109/30/9109/24/9109/23/9109/20/9109/19/9109/19/91Allforholders of OLs or CPsnuclear power reactorsAll holders of OLs or CPsfor nuclear power reactorsAll holders of OLs or CPsfor nuclear power reactorsand motor-operated valve(MOV) diagnostic equipmentvendors identified herein.All Nuclear Regulatory Comimission (NRC) licenseesauthorized to use sealedsources for industrialradiography.All holders of OLs or CPsfor nuclear power reactors.All holders of OLs or CPsfor nuclear power reactors.All holders of OLs or CPsfor nuclear power reactors.All holders of OLs or CPsfor nuclear power reactors.91-56OL = Operating LicenseCP = Construction Permit IN 91-64October 9, 1991 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contact oneof the technical contacts listed below or the appropriate Office of NuclearReactor Regulation project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts:Jack E. Rosenthal, AEOD(301) 492-4440Roger(301)Woodruff, NRR492-1152

Attachments:

1. Uninterruptible Power Supply Before Design Change2. List of Recently Issued NRC Information NoticesDocument Name:IN 91-64*SEE PREVIOUS CONCURRENCES:*OEAB *ADM/RPB *SC:OEABRWWoodruff JOMain RLDennig9/12/91 9/12/91 9/17/91*BC:OEABAEChaffee9/17/91*TL:IITJERosenthal9/24/91*PM:PD11DSBrinkman9/25/91*BC:SICBSNewberry9/25/91*BC:OGCBCHBerlinger9/27/910IRo \W IN 91-XXSeptember xx, 1991 condition at all times, and operators were able to monitor all safetyparameters, except for control roa positions, throughout the event. The twosafety-related uninterruptible power supplies were not lost. Had a loss ofcoolant accident occurred in conjunction with this event, power would havebeen available for the instrumentation and control components in theemergency core cooling system.This information notice requires no specific action or written response. Ifyou have any questionrabout the information in this notice, please contact Y'one of the cal conta low or the appropria project manager .Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical n tact5Jack E. Rosenthal, AEOD(301) 492-4440I1Roger Woodruff, NRR(301) 492-1152

Attachments:

1. Uninterruptible Power Supply Before2. List of Recently Issued InformationK#Design ChangeNotices sy_CONCURRENCE:*OEABRWWoodruff9/12/919gt51*ADM/RPBJDrain9/12/91*SC0OEABRLDennig9/17/91*BC: OEABAE~haffee9/17/91IT I9?IAA791DSB9rn99/ S791B: OCEC4ID: DOEACHBerlinger CERossi91-1191 9/ /91* SEE PREVIOUS CONCURRENCES IN 91-XXSeptember xx, 991lighting. However, the two safety-related uninterruptible power supplieswere not lost. Had a loss of coolant accident occurred in conjunction withthis event, power would have been available for the instrumentation andcontrol components in the emergency core cooling system.This information notice requires no specific action or written response. Ifyou have any question about the information in this notice, please contactone of the technical contacts listed below or the appropriate project managerin the Office of Nuclear Reactor Regulation.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor Regulation

Technical Contact:

Jack E. Rosenthal, AEOD(301) 492-4440Roger Woodruff, NRR(301) 492-1152

Attachments:

1. Uninterruptible Power Supply Before Design Change2. List of Recently Issued Information NoticesCONCURECE:*OEABRWWoodruff9/12/91*ADM/RfBJI)Main9/12/91&6ig9/r1/91D:DOEACERossi9/ /91AIEChaffee9/I1/91TL: IITJERosenthal9/ /91PM:PD11DSBrinkman9/ /91BC:SICBSNewberry9/ /91BC:OGCBCHBerlinger9/ /91* SEE PREVIOUS CONCURRENCES aIN 91-XXSeptember xx, 1991 one of the technical contacts listed below or the appropriate project managerin the Office of Nuclear Reactor Regulation.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor Regulation

Technical Contact:

Jack E. Rosenthal, AEOD(301) 492-4440Roger Woodruff, NRR(301) 492-1152

Attachment:

1. List of Recently Issued Information NoticesCONCURRECE:OEAB thWSRWWoodruff9/q,/91TECH EDITOR?fla;i £9A9/11/91SC: OEABRLDennig9/ /91BC:OEABAEChaffee9/ /91TL: ITJERosenthal9/ /91FM:PD11DSBrinkman9/ /91BC:OGCBCHBerlinger9/ /91D:DOEACERossi9/ /91