Information Notice 1994-36, Undetected Accumulation of Gas in Reactor Coolant System

Undetected Accumulation of Gas in Reactor Coolant System

ML031060539
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:	05/24/1994
Revision:	0
From:	Grimes B K Office of Nuclear Reactor Regulation
To:
References
-nr IN-94-036, NUDOCS 9405190137
Download: ML031060539 (10)

UNITED STATES P/-3YNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555May 24, 1994NRC INFORMATION NOTICE 94-36: UNDETECTED ACCUMULATION OF GAS IN REACTORCOOLANT SYSTEM

Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this informationnotice to alert addressees to problems that could result from unrecognizedevolution and accumulation of gas in reactor coolant system high points. Itis expected that recipients will review this information for applicability totheir facilities and consider actions, as appropriate, to avoid similarproblems. However, suggestions contained in this information notice are notNRC requirements; therefore, no specific action or written response isrequired.

Description of Circumstances

Sequoyah Unit I had been shut down in March 1993. In early September 1993,after the unit had been refueled, the reactor vessel was reassembled and thereactor coolant system was degassed, filled, and vented. The reactor coolantsystem was then depressurized to atmospheric pressure. One pressurizer power-operated relief valve (PORV) was open and the reactor coolant system averagetemperature was being maintained at approximately 49C [120'F]. Nitrogencover gas was being supplied to the volume control tank at approximately239 kPa [20 psig] and reactor coolant was being circulated by the chargingsystem and the residual heat removal (RHR) system. The reactor vessel levelindication system (RVLIS) was functional for a significant portion of theevent. Before this event, instrument maintenance stickers were placed on theRVLIS indicators for other maintenance work. Although the RVLIS informationwas available, the operators were not monitoring this information because itwas not procedurally required to be used in shutdown modes of operation.Reactor coolant system inventory was being monitored using pressurizer coldcalibration level indications.On December 17, 1993, the operators began to pressurize the containment to191 kPa [13 psig] in order to perform a containment integrated leak rate test.As containment pressure increased, the operators noticed a decrease inpressurizer water level and, over a period of time, added approximately 31,400liters [8,300 gallons] of water to maintain the pressurizer level. Licenseepersonnel evaluated this situation and recognized that gas was accumulating inthe reactor coolant system. However, they failed to recognize the magnitude9405190137 po\ fit JE ote qq-f3tn 9t1S EsoI'I'

..~ -IN 94-36May 24, 1994 or cause of the problem. Three days later, as the containment was beingdepressurized upon completion of the containment leak test, the operatorsnoted that approximately the same amount of water had to be removed from thereactor coolant system to maintain the level. On December 21, 1993, on thebasis of operators' observations of the changing coolant system Inventoryduring the containment leak test, the reactor vessel head was vented. It wassubsequently vented several times to maintain appropriate reactor vesselinventory based on RYLIS information. The reactor vessel gas voidingconditions went undetected from early September 1993, when the reactor coolantsystem was verified to be filled, until December 21, 1993, when the reactorhead was vented.The gas evolution resulted from the temperature in the volume control tankbeing much lower than that normally expected. (This lower temperature was dueto unusually low component cooling water temperatures and to a maintenanceproblem with a cooling water valve that resulted in the reduction of the heatsink temperature in the letdown heat exchanger.) The lower temperaturesincreased the solubility of gas in the volume control tank water so that moregas was dissolved in the water. This gas evolved when the water wastransferred to the reactor coolant system by the charging system and heated upin the reactor vessel.Further details concerning this event are in NRC Inspection Report No. 50-327;50-328/94-04.DiscussionThe licensee completed a final evaluation in January 1994 and determined thatduring this event the reactor vessel water level had decreased to slightlybelow the top of the hot leg, and that the steam generator tubes were nearlyempty. The reactor coolant system inventory was being monitored solely on thebasis of pressurizer level, which was not indicative of the reactor vessel andsteam generator levels. Calculations by the licensee indicated that anequilibrium had been reached during the event so that the water level wasapproximately 1.6 meters [5.25 feet] above the top of the core. Anyadditional gas evolved in the reactor vessel would be expected to be ventedthrough the hot leg and surge line to the pressurizer and out through the openpressurizer PORV.The reactor water level at which equilibrium was established at Sequoyah wassufficiently high so as not to interfere with reactor coolant flow through theRHR cooling system. However, at other plants, the equilibrium level, which isrelated to pressurizer surge line geometry and RHR suction line location,might be such that a similar event could interfere with shutdown cooling.Another potential safety concern is related to the capability of the steamgenerators to transfer heat from the reactor coolant to secondary coolant. Inthe equilibrium condition at Sequoyah, the steam generator tubes were almostempty during at least a portion of the duration of this event. During this IN 94-36--JMay 24, 1994 period, the steam generators were assumed to be available as an alternatemeans for shutdown cooling. It is not clear that cooling through the steamgenerators could have been established, if needed, especially because theplant operators were unaware that the tubes were empty.Several factors involving licensee performance contributed to the delay in theidentification and evaluation of this event. They included (1) lack ofconsideration on the part of licensee personnel with respect to compression ofgas in the system caused by containment pressure transmitted to the reactorcoolant system water through the open pressurizer PORV; (2) misunderstandingby the operators about the operability of RVLIS; (3) failure to stop thecontainment leak test when water level changes were first noted so that thesituation could have been adequately evaluated; (4) an apparent lack ofthorough evaluation of previously published information on similar events.The event discussed in this information notice highlights the potential forgas evolution and accumulation in the reactor coolant system in locations andquantities that may not be evident to operators. Changes in temperature andpressure can have significant effects on the solubility of gas in water,especially at or near atmospheric pressure conditions. During cold shutdown,this phenomenon can permit significant quantities of cover gas to be dissolvedin lower-temperature, higher-pressure volumes such as the volume control tankand to evolve into a large gas bubble in the reactor vessel (a lower-pressure,higher-temperature area). This phenomenon can occur without a sudden changein pressurizer level, which is the indicator normally used by the operators tomonitor water inventory. Use of available instrumentation in shutdown modesto monitor reactor vessel water level can provide detection of unexpectedsystem conditions when reduced vessel inventory is not planned as well as inreduced inventory evolutions.On April 12, 1994, a similar gas accumulation event was identified at theSalem Generating Station (Unit 1) shortly after it entered Mode 5 (coldshutdown) operation. Although this occurrence did not involve amounts of gasaccumulation as large as in the Sequoyah event, many similarities existedregarding both the process of gas formation and the lack of operator awarenessof the abnormal condition. In both events, reactor vessel level informationwas or could have been made available for prompt identification of theproblem.Related Generic Communications* NRC Information Notice 93-12, "Off-Gassing in Auxiliary FeedwaterSystem Raw Water Sources," discusses similar solubilityconsiderations related to air pockets in piping at the McGuireNuclear Station.* NRC Information Notice 87-46, "Undetected Loss of ReactorCoolant," discusses undetected loss of reactor coolant inventoryat North Anna Unit 1 resulting from inadequate use of availableindications and failure to perform mass inventory balance \ > <_IN 94-36May 24, 1994 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Of ice ofNuclear Reactor Regulation (NRR) project manager.Brian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical contacts: R. Benedict, NRR(301) 504-1157S. M. Shaeffer, RII(615) 842-8001

Attachment:

List of Recently Issued NRC Information Notices IAt,.thmentIN 94-36May 24, 1994 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICESInformation Date ofNotice No. Subject Issuance Issued to91-81,Supp. 194-3594-3494-3393-53,Supp. 194-3294-3190-68,Supp. 194-30Switchyard Problems thatContribute to Loss ofOffsite PowerNIOSH Respirator UserNotices, "InadvertentSeparation of the Mask-Mounted Regulator (MMR)from the Facepiece on theMine Safety Appliances (MSA)Company MMR Self-ContainedBreathing Apparatus (SCBA)and Status UpdateThermo-Lag 330-660Flexi-Blanket AmpacityDerating ConcernsCapacitor Failures inWestinghouse Eagle 21Plant Protection SystemsEffect of HurricaneAndrew on Turkey PointNuclear GeneratingStation and Lessons LearnedRevised Seismic HazardEstimatesPotential Failure ofWilco, Lexan-Type HN-4-LFire Hose NozzlesStress Corrosion Crackingof Reactor Coolant PumpBoltsLeaking Shutdown CoolingIsolation Valves atCooper Nuclear Station05/19/9405/16/9405/13/9405/09/9404/29/9404/29/9404/14/9404/14/9404/12/94All holders of OLs or CPsfor nuclear power reactors.All holders of OLs or CPsfor nuclear power reactors,and all licensed fuelfacilities.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.All holders of OLs or CPsfor nuclear power reactors.All holders of 01 or CPsfor pressurized waterreactors.All holders of OLs or CPsfor nuclear power reactors.OL -Operating LicenseCP -Construction Permit IN 94-xxMay xx, 1994 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Brian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical contacts:R. Benedict, NRR(301) 504-1157S. M. Shaeffer, R11(615) 842-8001

Attachment:

List of Recently Issued NRC Information Notices*See Dnrvinim- concurrence---- wBw _. _ .-1 _._OFFICE *OEAB:DORS *DRP:RII *OGCB:DORS *SC/DRP:RII *TECH EDNAME RBenedict SMShaeffer PCWen MLesser MFMejaclDATE 05/04/L94 05/05/94 05/05/94 05/05/94 05/05/94[*AC/SRXB:DSSA *AD/DSSA *C/OEAB:DORS *AC/OGCB:DORS D/DORSTECollins MVirgilio AChaffee AJKugler BKGrimes05/06/94 05/09/94 05/09/94 05.12/94 / /94DOCUMENT NAME:SLQGAbIN.KV1NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.informed of the development of this informationreviewed the draft information notice.Stone) have beennotice and they havePeter Wen, 5/10/94 IN 94-xxMay xx, 1994 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactthe technical contact listed below or the appropriate Office of NuclearReactor Regulation (NRR) project manager.Brian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical Contacts:R. Benedict, NRR(301) 504-1157S. M. Shaeffer, RI!(615) 842-8001

Attachment:

List of Recently Issued NRC Information Notices*See previous concurrenceOFFICE *OEAB:DORS *DRP:RII *OGCB:DORS *SC/DRP:RII *TECH EDNAME RBenedict SMShaeffer PCWen MLesser MFMejacDATE 05/04/94 05/05/94 05/05/94 05/05/94 05/05/94*AC/SRXB:DSSA *AD/DSSA *C/OEAB:DORS *AC/OGCB:DORS D/DORSTECollins 8EXI MVirgilio I AChaffee AJKugler BKGrimes05/06/94 1 05/09/94 05/09/94 05/12/94 / /94nnlflMFNT IAMr. eCnveCTit niti.-- -- I ia11 I EAdIL* .JLA(Un4.1 n .* r ANOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.informed of the development of this informationreviewed the draft information notice.Stone) have beennotice and they havePeter Wen, 5/10/94 IN 94-xxMay xx, 1994 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactthe technical contact listed below or the appropriate Office of NuclearReactor Regulation (NRR) project manager.Brian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical Contacts:R. Benedict, NRR(301) 504-1157S. M. Shaeffer, R11(615) 842-8001

Attachment:

List of Recently Issued NRC Information NoticesSee previous concurrenceOFFICE l *OEAB:DORS *DRP:RII *OGCB:DORS l *SC/DRP:RII *TECH EDNAME } RBenedict SMShaeffer PCWen MLesser MFMejacDATE 1 05/04/94 1_05/05/94 1 05/05/94 _ 05/05/94 1 05/05/94*AC/SRXB:DSSA *AD/DSSA j *C/OEAB:DORS AC/OGCB:DORS D/DORSTECollins MVirgilio AChaffee AJKugler i(: BKGrimes05/06/94 = 05/09/94 05/09/94 5/1V94 / /94DOCUMENT NAME:SEQGASIN.RV1NOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.informed of the development of this informationreviewed the draft information notice.Stone) have beennotice and they havePeter Wen, 5/10/94 IN 94-xxMay xx, 1994 This information notice requires no specific action or writttten response.you have any questions about the information in this notice, please contactthe technical contact listed below or the appropriate Office of NuclearReactor Regulation (NRR) project manager.IfBrian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical Contacts:R. Benedict, NRR(301) 504-1157S. M. Shaeffer, RII(615) 842-8001

Attachment:

List of Recently Issued NRC Information NoticesOFFICE IOEABDOR5 DRP:RIIpcyJ OGCB_ _ _ SC/DRP:RI TECH EDNAME Benedict 'MShaeffer PCWen LesserDATE </A/94 5 /5/94 5 / 94/94 5//94 5 / 5/94AC/SRXB:DSSA , AD/DSSA j C/OEAB:DORS AC/OGCB:DORS D/DORSTEColli MVirgil ' A2 fe AJKugler BKGrimes_____94 /$\/94 /947/94 94 / /94UULUMtN I NAML:x 01 I J//qbtbAbI1 N.KV1jp'A4Mi: THE R& FR FoY(AH (-. LAMRPE) Azb 6MPA (j. )ROeia^ SoF'E Deospwt oF Ttts um. waIce ARD'NW ME .F) T"E JPV O D0e bWf ^ .&4-vhiw S/o/oA IN 94-36May 24, 1994 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.orig /s/'d by BKGrimesBrian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical contacts:R. Benedict, NRR(301) 504-1157S. M. Shaeffer, R11(615) 842-8001

Attachment:

List of Recently Issued NRC Information Notices*See Previous concurrenceE OFFICE *OEAB:DORS *DRP:RII *OGCB:DORS *SC/DRP:RII *TECH EDNAME RBenedict SMShaeffer PCWen MLesser MFMejacDATE j05/04/94 05/05/94 05/05/94 05/05/94 05/05/94l*AC/SRXB:DSSA *AD/DSSA 5*C/OEAB:DORS *AC/OGCB:DO RS*TECollins MVirgilio AChaffee AJKugler Iv _s05/06/94 05/09/94 i 05/09/94 05/12/4 05//f/94UuUMtrII NAM t:vq-io. INNOTE: The PMs for Sequoyah (D. LaBarge) and Salem (J.informed of the development of this informationreviewed the draft information notice.Stone) have beennotice and they havePeter Wen, 5/10/94