NRC Generic Letter 1992-04

NRC Generic Letter 1992-004 - Resolution of Issues Related to Reactor Vessel Water Level Instrumentation in BWRs Pursuant to 10 CFR 50.54(F)
	ML031130402
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, Washington Public Power Supply System, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Clinch River
Issue date:	08/19/1992
From:	Partlow J G; Office of Nuclear Reactor Regulation
To:	;
References
	GL-92-004, NUDOCS 9208190057
	Download: ML031130402 (11)
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UNITED STATESNUCLEAR REGULATORY COMMISSIONWASHINGTON, D. C. 20555August 19, 1992TO: ALL BOILING WATER REACTOR (BWR) LICENSEES OFOPERATING REACTORS

SUBJECT: RESOLUTION OF THE ISSUES RELATED TO REACTOR VESSELWATER LEVEL INSTRUMENTATION IN BWRs PURSUANT TO10 CFR 50.54(F) (GENERIC LETTER NO. 92-04)

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing thisgeneric letter to request information regarding the adequacy ofand corrective actions for Boiling Water Reactor (BWR) waterlevel instrumentation with respect to the effects ofnoncondensible gases on system operation.Background and Safety ConsiderationsAs discussed in NRC Information Notice No. 92-54 "LevelInstrumentation Inaccuracies Caused by Rapid Depressurization,"the staff is concerned that noncondensible gases may becomedissolved in the reference leg of BWR water level instrumentationand can lead to a false high level indication after a rapiddepressurization event. The dissolved gases which accumulateover time during normal operation can rapidly come out ofsolution during depressurization and displace water from thereference leg. A reduced reference leg level will result in afalse high level indication. This is important to safety becausewater level signals are used for actuating automatic safetysystems and for guidance to operators during and after an event.On July 29, 1992, the NRC staff held a public meeting with theRegulatory Response Group (RRG) of the Boiling Water ReactorOwners Group (BWROG) to discuss the effect of inaccuracies in thereactor vessel level instrumentation system in BWRs. During themeeting, theBWROG and its consultant, General Electric Company(GE), presented the results of analyses assessing the safetyimplications of the postulated error in level indication. Theanalyses consisted of two basic parts: (1) an assessment of themechanism and potential magnitude of errors in the levelinstruments and (2) a review of the relevant licensing basistransients and accidents to determine the effect of this error onplant response, including post-accident operator actions.The BWROG analyses indicated that significant errors in levelindication can occur as a result of degassing the instrumentreference leg if noncondensible gas is dissolved in the referenceleg and if the reactor abruptly depressurizes below 450 psig.(9208190057 Z4r pli-4 /- t Generic Letter 92-04 -2 -August 19, 1992The NRC staff reviewed the BWROG analyses and selected designbasis accident scenarios which lead to a lowering of the reactorvessel water level and has concluded that automatic safetysystems will be actuated at pressures well above 450 psig, evenfor postulated worst-case noncondensible gas concentrations inthe reference legs. Therefore, the NRC is confident that allemergency cooling systems will initiate as they were designed todo. In addition, the BWROG discussed diverse signals which wouldalso initiate ECCS for reactor water level lowering events. TheNRC staff reviewed the backup systems and concluded that the ECCSwould be initiated by diverse signals as analyzed by the BWROG.After ECCS actuation, reactor water level indication is used bythe operators for long term actions (i.e., maintaining adequatereactor water level and ensuring adequate core cooling).Operators would not utilize only reactor vessel level indicationsto determine accident mitigation actions but would also utilizeother indications such as containment pressure, temperature, andhumidity to determine accident mitigation strategies.Additionally, events characterized by gradual depressurizationwould lead to a reduced error in the indicated level. There aretwo or four reference leg columns in each plant, depending onplant design. The amount of noncondensible gases dissolved ineach depends primarily upon system leakage and geometry. Becauseof this, a common mode, common magnitude level indication erroris unlikely. Operators would therefore see a mismatch inindicated level alerting them to a level indication problem.Finally, emergency procedure guidelines (EPGs) state that whenreactor vessel water level is indeterminate, operators shouldflood the reactor vessel using at least one pump guided by theunaffected diverse instrumentation (i.e., high containmentpressure indication). Reactor operators are trained to deal withthese situations should they occur.Upon reviewing the information provided by the BWROG and thestaff's assessment, the staff concluded that interim plantoperation is acceptable. The bases for the staff's conclusionare as follows: 1) the level instrumentation is expected toinitiate safety systems prior to a significant depressurizationof the reactor; 2) emergency procedures which are currently inplace in conjunction with operator training are expected toresult in adequate operator actions; and 3) an abruptdepressurization event resulting in a common mode, commonmagnitude level indication error is unlikely.For longer term operation however, the staff considers potentialwater level instrumentation inaccuracies an important issuebecause level indication has safety and control functions in all Generic Letter 92-04-3 -August 19, 1992modes of BWR operation. Furthermore, since the analyses providedare of a generic nature and the magnitude of possible errorsdepends strongly upon plant-specific factors such as systemleakage and geometry, it is important that the analyses bereviewed promptly by all individual licensees.Basis for Compliance DeterminationThe level errors that could result from the effects ofnoncondensible gas may prevent the level instrumentation systemsin BWRs from satisfying the following regulations:(1) General Design Criterion (GDC) 13, "Instrumentation andcontrol," which requires that "Instrumentation shallbe provided to monitor variables and systems over theiranticipated ranges for normal operation, foranticipated operational occurrences, and for accidentconditions as appropriate to assure adequate safety."Existing instrumentation may not accurately monitorreactor vessel water level under accident conditions.(2) GDC 21, "Protection system reliability and testability,"which requires that "The protection system shall bedesigned for high functional reliability...commensuratewith the safety function to be performed." Theinstrumentation may not be reliable under rapiddepressurization conditions.(3) GDC 22, "Protection system independence," which requiresthat "The protection system shall be designed to assurethat the effects of natural phenomena, and of normaloperating, maintenance, testing, and postulatedaccident conditions...do not result in loss of theprotection function." The natural phenomena ofdegassi'ng may cause a loss of the reactor vessel waterlevel indication function following a rapiddepressurization.(4) Section 50.55a(h) of Title 10 of the Code of FederalRegulations (10 CFR 50.55a(h)), which requiresthat protection systems, for those plants withconstruction permits issued after January 1, 1971,shall meet the requirements stated in editions of theInstitute of Electrical and Electronics EngineersStandard "Criteria for Protection Systems for NuclearPower Generating Stations" (IEEE-279). Section 4.20 ofIEEE-279 requires that "The protection system shall bedesigned to provide the operator with accurate, Generic Letter 92-04-4 -August 19, 1992complete, and timely information pertinent to its ownstatus and to generating station safety." The waterlevel instrumentation for the reactor vessel may not beaccurate after a rapid depressurization event.Requested Actions1. In light of potential errors resulting from the effects ofnoncondensible gas, each licensee should determine:a. The impact of potential level indication errors onautomatic safety system response during all licensingbasis transients and accidents;b. The impact of potential level indication errors onoperator's short and long term actions during and afterall licensing basis accidents and transients;c. The impact of potential level indication errors onoperator actions prescribed in emergency operatingprocedures or other affected procedures not covered in(b).2. Based upon the results of (1), above, each licensee should-notify the NRC of short term actions taken, such as:a. Periodic monitoring of level instrumentation systemleakage; and,b. Implementation of procedures and operator training toassure that potential level errors will not result inimproper operator actions.3. Each licensee should provide its plans and schedule forcorrective actions, including any proposed hardwaremodifications necessary to ensure the level instrumentationsystem design is of high functional reliability for longterm operation. Since this instrumentation plays animportant role in plant safety and is required for bothnormal and accident conditions, the staff recommends thateach utility implement its longer term actions to assure alevel instrumentation system of high functional reliabilityat the first opportunity but prior to starting up after thenext refueling outage commencing 3 months after the date ofthis lette Generic Letter 92-04-5. -August 19, 1992Required Information --Because of the importance of plant-specific aspects of this issueand the potential magnitude of the-errors, the staff requires,-pursuant to 10 CFR 50.54.(f) and Section 182 of the Atomic EnergyAct, that you provide a response to this letter by September 27,1992.Merely committing to evaluate the safety significance as part ofthe individual plant examination (IPE) program is not an -acceptable alternative to the actions rdescribed herein; since thelicensee should resolve this issue as a matter of compliance.

Backfit Discussion

In accordance with NRC procedures, the actions requested hereinare considered a backfit to assure that facilities are incompliance with existing regulatory requirements discussed above.Thus, a backfit analysis is not-required by 10 CFR 50.109(a)(4)(i),and the staff performed a-documented evaluation as discussed in10 CFR 50.109(a) (6) .The-documented evaluation is provided inthe preceding discussions.Burden InformationThis request is covered by Office of Management and BudgetClearance Number 3150-0011, which expires May. 31, 1994. Theestimated average number of burden hours is 200 person hours foreach licensee response, including the time required to assess thequestions, search data sources, gather and analyze the data, andprepare the required response. These estimated average burdenhours pertain only to the identified response-related matters anddo not include the time for actual implementation of therequested actions. Comments on the accuracy of this estimate andsuggestions to reduce the burden may be directed to Ronald Minsk,Office of Information and Regulatory Affairs (3150-0011), NEOB-3019, Office of Management and Budget, Washington, D.C. 20503 andto the U.S. Nuclear Regulatory Commission, Information andRecords Management Branch, Division of Information SupportServices, Office of Information and Resources Management,Washington, D.C. 20555.Although no specific request or requirement is intended, thefollowing information would be helpful to the NRC in evaluatingthe cost of complying with this generic letter:(1) the licensee staff time and costs to perform requestedinspections, corrective actions, and associated testing;(2) the licensee staff's time and costs to prepare the requestedreports and documentation; Generic Letter 92- 04 -6 -August 19t 1992(3) the additional short-term costs incurred as a result of theinspection findings such as the costs of the correctiveactions or the costs of down time; and(4) an estimate of the additional long-term costs which will beincurred in the future as a result of implementingcommitments such as the estimated costs of conducting futureinspections or increased maintenance.Please address your response to this generic letter to the U.S.Nuclear Reguiatory Commission, Attn: Document Control Desk,Washington, D.C. 20555 pursuant to 10 CFR 50.4(a) of the NRC'sregulations.

Sincerely,James G. PartlowAdsociate Director for ProjectsOffice of Nuclear Reactor Regulation