NRC Generic Letter 1987-12
e ---i$J .14 #UNITED STATESNUCLEAR REGULATORY COMMISSIONWASHINGTON. D. C. 205July 9, 1987ALL LICENSEES OF OPERATING PWRS AND HOLDERS OF CONSTRUCTION'PERMITS FOR PWRSGentlemen:
SUBJECT: LOSS OF RESIDUAL HEAT REMOVAL (RHR) WHILE THE REACTOR COOLANTSYSTEM (RCS) IS PARTIALLY FILLED (GENERIC LETTER 87-12 )Pursuant to 10 CFR 50.54(f), the NRC is requesting'information to assess safe,operation of pressurized-water reactors (PWRs) when the reactor coolant system(RCS) water levelis below the top of the reactor vessel (RY). The principalconcerns are (1) whether the RHR system meets the licensing basis of theplant, such as General Design Criterion 34 (10 CFR'Part 50, Appendix A) andTechnical Specifications (TS), in this condition; (2) whether there is a'resultant unanalyzed event that may have an impact upon safety; and (3)whether any threat to safety that warrants further NRC attention exists inthis condition.Our concerns regarding this issue have'increased over the past several years,and lessons learned from the April 10, 1987 Diablo Canyon loss-of-RHR eventrequire an assessment of operations and planned operations at all PWRfacilities toensure that these plants meet the licensing basis. Study of theDiablo Canyon event has led to identification of unanalyzed conditions thatare of significance to safety. Although Diablo Canyon never came close to coredamage, and could have withstood the loss-of-RHR condition for more than a daywith no operator action, slightly different conditions could have led to anaccident involving core damage within several hours. One unanalyzed conditioninvolves boiling within the RCS in the presence of air, leading to RCSpressurization with the potential for ejecting RCS water via cold-leg openings,such as could exist during repair to a reactor coolant pump (RCP) or to a loopisolation valve. The lost water would no longer be available to cool the core,and if makeup water were unavailable, the core could be damaged in asignificantly decreased time. The pressurization could also affect thecapability to provide makeup water to the core. Other unanalyzed situationsare also possible, and occurred at Diablo Canyon (e.g., boiling in the core).The seriousness of this situation is exacerbated by the practice of conductingoperations with the equipment hatch removed, and by the lack of procedures thataddress prompt containment isolation should the need arise.Loss of RHR and related topics are not a new concern to the NRC staff. Thistopic has been addressed in numerous communications with the licensee. Yet,these events continue to occur at a rate of several per year. This conditionneeds to be fully considered in order to ensure compliance with the licensingbasis. Therefore, we request that you provide the NRC with a description of theoperation of your plant during the approach to a partially filled RCS conditionand during operation with a partially filled RCS to ensure that you meet thelicensing basis. Your description is to include the following:1OO 112m> toSoC a 2(1) A detailed description of the circumstances and conditions under whichyour plant would be entered into and brought through a draindown processand operated with the RCS partially filled, including any interlocks thatcould cause a disturbance to the system. Examples of the type ofinformation required are the time between full-power operation andreaching a partially filled condition (used to determine decay heatloads); requirements for minimum steam generator (SG) levels; changes inthe status of equipment for maintenance and testing and coordination ofsuch operations while the RCS is partially filled; restrictions regardingtesting, operations, and maintenance that could perturb the nuclear steamsupply system (NSSS); ability of the RCS to withstand pressurization ifthe reactor vessel head and steam generator manway are in place;requirements pertaining to isolation of containment; the time required toreplace the equipment hatch should replacement be necessary; andrequirements pertinent to reestablishing the integrity of the RCS pressureboundary.(2) A detailed description of the instrumentation and alarms provided to theoperators for controlling thermal and hydraulic aspects of the NSSS duringoperation with the RCS partially filled. You should describe temporaryconnections, piping, and instrumentation used for this RCS condition andthe quality control process to ensure proper functioning of suchconnections, piping', and instrumentation, including assurance that they donot contribute to loss of RCS inventory or otherwise lead to.;perturbationof the NSSS while the RCS is partially filled. You should also provide adescription of your ability to monitor RCS pressure, temperature, andlevel after the RHR function may be lost.(3) Identification of all pumps that can be used to control NSSS inventory.Include: (a) pumps you require be operable or capable of operation(include information about'such pumps that may be temporarily removedfrom service for testing or maintenance); (b) other pumps not included initem a (above); and (c) an evaluation of items a and b (above) withrespect to applicable TS requirements.(4) A description of the containment closure condition you require for theconduct of operations while the RCS is partially filled. Examples ofareas of consideration are the equipment hatch, personnel hatches,containment purge valves, SG secondary-side condition upstream of theisolation valves (including the valves), piping penetrations, andelectrical penetrations.(5) Reference to and a summary description of procedures in the control roomof your plant which describe operation while the RCS is partially filled.Your response should include the analytic basis you used for proceduresdevelopment. We are particularly interested in your treatment ofdraindown to the condition where the RCS is partially filled, treatmentof minor .variations from expected behavior such as caused by airentrainment and de-entrainment, treatment of boiling in the core with andwithout RCS pressure boundary integrity, calculations of approximate time0 VI- 3 -from loss of RHR to core damage, level differences in the RCS and theeffect upon instrumentation indications, treatment of air in the RCS/RHRsystem, Including the impact of air upon NSSS and instrumentationresponse, and treatment of vortexing at the connection of the RHR suction1ine(s) to the RCS.Explain.how your analytic basis supports the following as pertaining toyour facility: (a) procedural guidance pertinent to timing ofoperations, required instrumentation, cautions, and critical parameters.;(b) operations control and communications requirements regarding Aoperations that may perturb the NSSS, including restrictions upon testing,.maintenance, and coordination of operations that could upset thecondition of the NSSS; and (c) response to loss of RHR, includingregaining control of RCS heat removal, operations involving the NSSS ifRHR cannot be restored, control of effluent from the containment if-containment was not in an isolated condition at the time of loss of RHR,and operations to provide containment isolation if containment was notisolated at the time of loss of RHR (guidance pertinent to timing ofoperations, cautions and warnings, critical parameters, and notificationsis to be clearly described).(6) A brief description of training provided to operators and other affectedpersonnel that is specific to the issue of operation while the RCS ispartially filled. We are particularly interested in such areas asmaintenance personnel training regarding avoidance of perturbing the NSSSand response to loss of decay heat removal while the RCS is partiallyfilled.(7) Identification of additional resources provided to the operators while theRCS is partially filled, 'such as assignment of additional personnel withspecialized knowledge involving the phenomena and instrumentation.(8) Comparison of the requirements implemented while the RCS is partiallyfilled and requirements used in other Mode 5 operations. Somerequirements and procedures followed while the RCS is partially filledmay not appear in the other modes. An example of such differences isoperation with a reduced RHR flow rate to minimize the likelihood ofvortexing and air ingestion.(9) As a result of your consideration of these issues, you may have madechanges to your current program related to these issues. If such changeshave strengthened your ability to operate safely during a partially filledsituation, describe those changes and tell when.they were made or arescheduled to be made.Enclosure 1 contains insight which experience indicates should be wellunderstood before commencing operation with a partially filled RCS. Yourresponse to this 50.54(f) letter request should encompass the topics containedin Enclosure 1. Additional information is contained in the NRC AugmentedInspection Team report, NUREG-1269, "Loss of Residual Heat Removal System,Diablo Canyon Unit 2, April 10, 1987." A copy of NUREG-1269 is enclose Your response addressing items 1 through 9 (above) is to be signed under oath oraffirmation, as specified in 10 CFR 50.54(f), and will be used to determinewhether your license should be modified, suspended, or revoked. We requestyour response within 60 days of receipt of this letter. This information isrequired pursuant to 10 CFR 50.54(f) to assess conformance of PWRs with theirlicensing basis and to determine whether additional NRC action is necessary.Our review of information you submit is not subject to fees under the provisionof 10 CFR 170. If you choose to provide a portion of your response inassociation with your owners group, such action is acceptable.This request for information was approved by the Office of Management andBudget under clearance number 3150-0011 which expires December 31, 1989.Comments on burden and duplication may be directed to the Office of Managementand Budget, Reports Management Room 3208, New Executive Office Building,Washington D.C. 20503.
Sincerely,Frank J. Miraglia 1r£Associate Director 'or Projects uOffice of Nuclear Reactor RegulationU.S. Nuclear Regulatory Commission
Enclosures:
As stated ENCLOSURE 1INFORMATION PERTINENT TO LOSS OF RESIDUAL HEAT REMOVAL SYSTEMSWHILE THE RCS IS PARTIALLY FILLEDMany maintenance and test activities conducted during an outage requirelowering the water level in the reactor coolant system (RCS) to below the topof the reactor vessel. (RV) or (as is done many times) to the centerlineelevation of the RV nozzles. This operating regime is sometimes known as"mid-loop" operation. It.places unusual demands on plant equipment andoperators because of narrow control margins and limitations associated withequipment, instrumentation, procedures, training, and the ability to-isolatecontainment. Difficulty in controlling the plant while in this condition oftenleads to. loss of the residual heat removal (RHR) system (Table 1).Although this issue has been the topic of many communications and investigations,such events continue to occur at a rate of several per year.Recent knowledge has provided additional insight into these events. Althoughthe full implications of this knowledge remain to be realized, our preliminaryassessments have clearly established real and potential inadequacies'associated with operation while the RCS is partially filled. These include:not understanding the nuclear steam supply system (NSSS) response to loss ofRHR, inadequate instrumentation, lack of analyses addressing the issue, lackof applicable procedures and training, and failure to adequately address thesafety impact of loss of decay heat removal capability.The following items are applicable to these conclusions:(1) Plants enter an unanalyzed condition if boiling occurs following loss ofRHR. For example:(a) Unexpected RCS pressurization can occur.No pressurization would occur with a water/steam-filled RCS withwater on the steam generator (SG) secondary side, because RCS steam
-2 -would condense in the SG tubes and the condensate would return to theRV. Air in the RCS can block the flow of steam through passages,such as the entrance portion of SG tubes, so that steam cannot reachcool surfaces. Failure to condense the steam causes pressurizationin the RCS until the air compresses enough for steam to reach cooledtube surfaces. This pressurization occurred during the April 10,1987 event at Diablo Canyon since the RCS contained air. Pressurereached 7 to 10 psig, and would have continued to increase if RHR hadnot been restored. The operators began to terminate the event byallowing water to flow from the refueling water storage tank (RWST)into the RCS. Increasing pressure would have eliminated this option,and would have jeopardized options involving pumps with suction linesaligned (in part) to the RCS.(b) Water that ordinarily would be available to cool the core might beforced out of the RV, thereby reducing the time between loss of RHRand initiation of core damage.This is a potential concern whenever-there is an opening in the col'dleg, such as may exist for repair of reactor coolant pumps (RCPs) orloop isolation valves. Upper vessel/hot-leg pressurization couldforce the RV water level down with the displaced water lost throughthe cold-leg opening. A corresponding decrease in levet would occurin the SG side of the crossover pipes between the SGs and the RCPs.This occurrence could be particularly serious if the cold-legopening were large or if makeup water flow to the RCS were small, asfrom a charging pump. Cold-leg injection with'elevated pressure inthe upper vessel may not provide water to the core.(2) RCS water level instrumentation may provide inaccurate information.There are many facets to this issue. Instrumentation may be indicating r ?I1-3 -a level that differs from level at the RHR suctioninstrument may be in use that has no indication orroom,.and design and installation deficiencies mayobserved the following:line, a temporaryalarms in the controlexist. We have(a) Connections to the RCS actually provide a water level indication up-stream of the RCP.location. This water.level is higher than thewater level at the RHR suction connection because of flow from theinjection to the suction locations and because of entering watermomentum, which increases level on the RCP side of the cold-leginjection location.Ingestion of air at the RHR suction connection will result intransporting air into the cold legs; this can potentially increasepressure in the air space in the cold legs relative to the hot legs.Level instrumentation may respond to such a pressure change asthough RCS level were changing. In addition, such a pressurizationwould move cold-leg water into the hot legs and upper RV (or thereverse if a depressurization occurs).(b), Use of long lengths of small-diameter tubing which can lengthen.instrument* response time and cause perturbations such as RCS pressure changesto appear as level changes; installation with tubing elevation changeswhich can trap air bubbles or water droplets, and installation whichmakes it possible for tubing to be kinked or constricted.(c) Some installations provide no indication in the control room, yetlevel is important to safety. Some provide one indication. Othersprovide diversity via different instrumentation, but.do not provideindependence because they share common connection (d) Tygon tube installations faintly marked at 1-foot intervals thathave no provision for holding the tube in place.(e) Instrumentation in which critical inspections were not performedafter the installation.(f) Instrumentation in which no provisions were made to ensure a singlephase in connection tubing or that tubing was not plugged.(g) Use of instrumentation without performing an evaluation of indicatedRCS level behavior and instrument response.(3) Vortexing and air ingestion from the RCS into the RHR suction line arenot always understood, nor is NSSS response understood for thiscondition.(a) On-April 10, 1987, Diablo Canyon operators reduced indicated RCSlevel to plant elevation 106' 6" immediately after steam generatortubes drained, and indications of erratic RHR pump current wereobserved. Restoring the RCS level to 106' 10" was reported to haveeliminated the problem. RHR operation was terminated a few hourslater at an indicated level of 107' 4" because the operators observederratic RHR pump current indications. The licensee later reportedthat vortexing initiated under those conditions at 107W 5-112", andwas fully developed at 107' 3-1/22". Procedures in place at the timeof the event indicated the minimum allowable level to be 107' 0" (thehot- and cold-leg centerline elevation) or 107' 3".Cb) Additional phenomena appear to occur under air ingestion conditions.These include:
-5-0 RHR pumps at Diablo Canyon were reported to handle severalpercent air with no discernible flow or pump current changefrom that of single-phase operation.o A postulate is that air in the RHR/reactor coolant system canmigrate or redistribute, and thus cause level changes which areat variance with those one would expect. This.is a possibleexplanation for observed behavior in which lowering the RCS waterlevel is followed by a level increase. Water in the RHRappears to be replaced by air. Similarly, an increase in RCSwater level that is followed by a decreasing level.may be dueto voids in the RHR system being replaced by RCS water.Failure to understand such behavior leads operators to mistrustlevel instrumentation and to perform operational errors.(c) Operators typically will start another RHR pump if the operatingpump'is lost. Experience and an understanding of the phenomenaclearly show that loss of the second pump should be expected. Thecause of loss of the first pump should be well understood andnormally should'be'corrected before attempting to run another RHRpump. --(d) Typical operation while the RCS is partially filled provides a highRHR flow rate, which may be required by TS, .but which maybeunnecessary under the unique conditions associated with thepartially filled RCS. Air ingestion problems are less at low flowrates.(4) Only limited instrumentation may be available to the operator while theRCS is partially fille (a) Level indication is many times available only in containment via aTygon tube. Some plants provide one or more level indications inthe control room, and additionally provide level alarms.(b) Typically, RHR system temperature indication is the only temperatureprovided to the operators. Loss of RHR leaves the operator with noRCS temperature indication. This can result in a TS violation, asoccurred at Diablo Canyon on April 10 when the plant entered Mode 4,unknown to the operators, with the containment equipment hatchremoved. It also resulted in failure to recognize the seriousness ofthe heatup rate, or that boiling had initiated.(c) RHR pump motor current and flow rate may not be alarmed and scalesmay not be suitable for operation with a partially filled RCS.(d) RHR suction and discharge pressures may not be alarmed and scalesmay not be suitable for operation with a partially filled RCS.(5) Cicensees typically conduct operations while the RCS is partially filled,the containment equipment hatch has been removed, and operations are inprogress which impact the ability to isolate containment. Planning,procedures, and training do not address containment closure in response toloss of RHR or core damage events. This is inconsistent with thesensitivity associated with partially filled RCS operation and the historyof loss of RHR under this operating condition.(6) Licensees typically conduct test and maintenance operations that canperturb the RCS and RHR system while in a partially filled RCScondition. The sensitivity of the operation and the historical recordindicate this is not pruden r ---1 p F%, *: -; 7S w .Table 137 LOSS-OF-DHR* EVENTS ATTRIBUTED TO INADEQUATERCS LEVELDocket344334366272334344369339338369339PlantDateTrojanBeaver ValleMillstone 2Salem 1 -Beaver Valle.TrojanMcGuire 1North Anna 2North Anna 1fMcGulre 1North Anna 205/21/7703/25/78280328370344316368295339413327296361382327323Surry 1Sequoyah 2McGuire 2TrojanDC Cook 2ANO-2Zion 1North Anna 2Catawba 1Sequoyah 1Zion 2San Onofre 2Waterford 3Sequoyah 1Diablo Canyo04/17/78y 1 09/04/7803/04/7906/30/79y 1 01/17/8004/08/8004/11/8003/05/8106/26/8103/02/8207/30/8210/19/8210/20/8204/05/8305/03/8305/20/8205/17/8308/06/8312/31/83.01/09/8405/04/8405/21/8408/29/8409/14/8410/16/8404/22/8510/09/8512/14/8503/26/8607/14/8601/28/87n 2 04/10/87Duration55 min.10 min.10 min.Unknown60 min.Unknown:34 min.Unknown35 mMn.70 min.54 min.75 min.-:50 min.46 min.36 mmn.-33min.UnknownUnknown.8 mn.-26 min.-t60 min.Unknown77 min.43 min..62 min.; 40 min.25 min.35 mdb1 .,t45 min. 7120 min.81 min.43 min.75 min.49 min.221 min.90 min.85 min.HeatupUnknownUnknownUnknownUnknown145-175°F150-208°FUnknownUnknownNone101-108°F102-168°F140-150°F105-130°FUnknownUnknownUnknownUnknownUnknown'UnknownUnknownUnknownUnknown103-195°FUnknownUnknown105-201°F-'Unkhownsi-t~-e140`-2050°F-;3 11110-17°FUnknown*140.1750F<10F114-210°F138-175°F95-115°F100-220°F* Decay heat removal V--L .U 1 ;TLIST OF RECENTLY ISSUED GENERIC KsTERqto4.-..--.I.L.ttm NO, subjectDats m4IsasuanceIssued ToIL *7-;20.54 #1 LETTER RN. LOSS OF; U SDUAL HEAT REMOVAL (RHRDURINO MID-LOOP 0PERATION3L 37-11 RELAxATION IN ARBITRARYINTERMEDIATE PIPE RUPTUREREOUIREMENTS07/09T37 ALL LICENSEESOF OPERATINGPWRS ANDHOLDERS OFCONSTRUCTIONPERMITS FORPWRS04/23/37 ALL OPERATINICONSTRUCTIONPERMITHOLDERS, ANDAPPLICANTS FORCONSTRUCTIONPERMITS3L R7-tO IMPLEMENTATION OF 10 CFr 0b/12/8773 .3, REQUIREMENTS FOR FSI-CRIMINAL HISTORY CHECKS3L C7-09 SECTIONS ;.o AND 4.0 OF THE 04V04/97STANpARD TECHNICALSPECIFICATIONS ON THEAPPLICASILITY OF LCO ANDSURVEILLANCE REQUIREMENTS3L 97-oa IMPLEMENTATION OF 10 CFR 73.53 03/1197MISCELLANEOUS AMENDMENTS ANDSEARCH REQUIREMENTSGL 87-07 INFORMATION TRANSMITTAL OF 03/19/87FINAL RULEMAKING FOR REVISIONSTQ OPERATOq LICNSItN%- OCFRS3AND CONFORMING AMENOMENT1oL 97-04 TgSTIN3 OF PRESSUAR tSOLATIO4 03/13197VALVESALL POWERREACTORLICENSEESALL LIGHTWATER REACTORLICENSEES ANDAPPLICANTSALL POWERREACTORLICENSEESALL FACILITYLICENSEESALL OPERATINGREACTOROL $7-05 REQUEST FOR ADDITIONALINFORMATION-ASSESSMENT OFLICENSEE MEASURES Td MITIGATEAND/OR IDENTIFY POTENTIALDEGRADATION MKI03/12/37. LICENSEES OFOR-' XAPPLICANTSFOROL.S, ANDHOLDERS OFCP'S FOR DWRMARK ICONITAINMENTSSL 87-04 TEMPORARY EXEMPTION FROM 03/04/97PROVISIONS OF THE FBI CRIMINALHISTORY RULE FOR TErPORARYWORKERSUNITED STATESNUWLEAR REGUATORY COMMISSIONWASHINGTON. D.C. 2555OFFICIAL BYSINESSPENALTY FOR PRIVATE USE. IIALL POWERREACTORLICENCESFIRST CLASS MAILPOSTAGE & FIES PAIDUSINiRCI WASH. D.C. IPERMIT No. N./ ?" .9#