Information Notice 1992-16, Loss of Flow from the Residual Heat Removal Pump During Refueling Cavity Draindown
UNITED STATESNUCLEAR REGULATORY COMMISSION'OFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555February 25, 1992NRC INFORMATION NOTICE 92-16: LOSS OF FLOW FROM THE RESIDUAL HEAT REMOVALPUMP DURING REFUELING CAVITY DRAINDOWN
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 a recent event involving the loss of flow from theresidual heat removal pump during refueling cavity draindown. It is expectedthat recipients will review the information for applicability to theirfacilities and consider actions, as appropriate, to avoid similar problems.However, suggestions contained in this information notice are not NRCrequirements; therefore, no specific action or written response is required.
Description of Circumstances
On October 26, 1991, the Vogtle Electric Generating Plant, Unit 1, was in Mode 6(Refueling) with the reactor vessel head removed. The Georgia Power Company(the licensee) had reloaded the core and reinstalled the upper tnternals. Thelicensee was using the 1B residual heat removal (RHR) pump to provide shutdowncooling and the 1A RHR pump to drain the refueling cavity by taking suction fromone of the reactor coolant system (RCS) hot legs'and discharging to the refuelingwater storage tank (RWST). The RCS temperature was approximately 870F. Thewater level in the refueling cavity was at 210 feet 4 inches. Operationspersonnel were preparing to lower the level to 192 feet, 2 feet below the reactorvessel head flange, to allow the reactor vessel head to be reinstalled. Themid-loop elevation of the RCS for Unit 1 is 187 feet. An assistant plantoperator (APO) in the Unit 1 containment was directed to establish a watch at atygon tube to monitor the RCS level during draindown and mid-loop operations.During the outage, the licensee had installeda permanent sight glass in theUnit 1 containment for monitoring the RCS level. This new sight glass hadneither been tested nor aligned for the operators to use. The APO assumed thatthe new sight glass was operable and established communications with thecontrol room at the permanent sight glass, rather than at the tygon tube, tomonitor the draindown. The licensee then started the draindown.When the day shift ended, a night shift plant equipment operator (PEO) relievedthe day shift APO who was monitoring the permanent sight glass. The PEOdiscovered that the valves for the permanent sight glass were not alignedcorrectly. The PEO informed the control room and the operators stopped the9202190317 ?D
- a f 00V; qt IN 92-16February 25, 1992 draindown while the problem was investigated. The PEO and APO then filledand vented the sight glass without using a procedure. In their attempt toplace the permanent sight glass in service, the upper isolation valve, whichwas not readily visible, was not opened as required.The licensee resumed the cavity draindown and, approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> later,received a control room annunciator which indicated a high level, 192 feet6 inches, in the reactor vessel. The control room operator observed that thecontrol room level indicator was at the top of scale (100 percent) and tappedon the indicator, causing it to drop to a reading of 60 percent (190 feet 9inches). The licensee again stopped the draindown. The PEO monitoring thesight glass level reported that reactor vessel water level appeared to be evenwith the reactor vessel head flange (194 feet), which agreed with the levelindicated by the permanent sight glass and the temporary tygon tube. Thelicensee assumed that the control room level indicator was inaccurate andcontinued the draindown, believing that it had three reliable indications ofthe RCS level, i.e., visual vessel water level, the permanent sight glass, andthe temporary tygon tube.When the level in the RCS reached approximately 193 feet, as indicated by thesight glass, a control room operator observed discharge pressure, flow, andmotor current oscillations for the 1B RHR pump, indicating that the coolant wasforming a vortex on the suction side of the pump or that the pump was cavitating.The operators closed the discharge valve for the lB RHR pump, thus putting the1B RHR pump on the miniflow line. Although the electrical current reading forthe motor of the 1B RHR pump became more stable, the discharge pressure remainedlow.The licensee again stopped the draindown by shutting down the 1A RHR pump andrealigning its suction to the refueling water storage tank (RWST) to refillthe refueling cavity. Shortly after beginning to refill the RCS, the licenseenoted that the discharge pressure of the 1B RHR pump began to improve. Whenthe flow of the 1B RHR pump reached approximately 2600 gallons per minute, thelicensee again observed indications of vortex formation or cavitation. Thelicensee reduced the flow from the 1B RHR pump to 1800 gallons per minute andfound that the pump operated satisfactorily with no indication of vortexformation or cavitation. The licensee used the 1A RHR pump to refill therefueling cavity from the RWST and stopped refilling when the sight glassindicated a level of 194 feet 10 inches. The licensee increased the flow fromthe 1B RHR pump to approximately 3000 gallons per minute and found that thepump operated satisfactorily with no further indication of vortex formation orcavitation.When operators performed a walkdown inspection of the tygon tube and the sightglass level indicators, they found the upper isolation valve for the sightglass closed with a tag on it which indicated that the new sight glass had notbeen released for use. The licensee later determined that a similar tag hadalso been installed on the lower isolation valve but apparently had fallen offthe valv IN 92-16February 25, 1992 The licensee also discovered that a high efficiency particulate absorber(HEPA) filter unit was connected, by means of a flexible duct, to the openingfrom which a pressurizer safety valve had been removed to provide a vent pathfor all level instrumentation. The licensee found that the HEPA unit wasrunning and the flexible duct was collapsed, apparently caused by the vacuumcreated by the running HEPA filter unit and the RCS draindown. This resultedin an inadequate vent path from the pressurizer. (LER 50-424/91-09 and NRCInspection Report 50-424,425/91-30)DiscussionFalse high RCS level indications led to the RCS level being inadvertentlylowered to the point at which the coolant formed a vortex in the RHR pumpsuction line. The false high level indications were caused by an inadequatevent path from the pressurizer and by the closed upper isolation valve for thesight glass. When conditions in the pressurizer changed, it affected all ofthe reactor vessel level instruments, because their reference legs connected tothe pressurizer. The system installed at Vogtle did not meet the intent of twoindependent continuous water level indications as discussed in Generic Letter88-17, "Loss of Decay Heat Removal."Procedures for the initial RCS draindown during refueling operations providedsufficient steps to ensure that the level instrumentation was installed properlyand the vent paths were adequate. However, the procedures for the subsequentdraindowns did not include sufficient steps to reverify these actions. Adminis-trative controls were inadequate in addressing the reviews and documentsrequired for attaching HEPA filter units to plant equipment. In this case,the HEPA filter unit was installed without a temporary modification or a workorder, and consequently the control room was not aware of the installation.During the event, the 1B RHR pump was not available to provide recirculationshutdown cooling for approximately 16 minutes. Core temperature as indicatedat the RHR pump discharge increased from approximately 870F to 1070F. Therewas no radiological release to the environment. The licensee reviewed availabledata further and found that the coolant on the suction side of the 1B RHR pumphad formed a vortex but the pump did not cavitate.Air may have begun entering the 1A RHR pump shortly before the pump's dischargevalve was closed. This resulted in a slightly reduced discharge pressure andflow. The coolant in the RCS reached the lowest level, 186 to 187 feet, whenthe discharge valve for the 1A RHR pump's heat exchanger was closed. After theevent, the licensee performed an inservice test on the 1A and 1B RHR pumps andfound that the performance of neither pump was degrade IN 92-16February 25, 992This information notice requilyou have any questions about lof the technical contacts lis-Reactor Regulation (NRR) projiTechnical contacts: Doug Starres no specific action or written response. Ifthe information in this notice, please contact oneted below or the appropriate Office of Nuclearect manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationI-kov -Region II(40i) 554-9901Pierce Skinner, Region II(404) 331-6299
Attachment:
Cowm~-List of Recently Issued NRC Information NoticesPa.'~. A x 9- AttachmentIN 92-16February 25, 1992 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICESInformation Date ofNotice No. Subject Issuance Issued to92-1592-1492-02,Supp. 192-1392-12Failure of Primary SystemCompression FittingUranium Oxide Fires at FuelCycle FacilitiesRelap5/Mod3 Computer CodeError Associated with theConservation of EnergyEquationInadequate Control OverVehicular Traffic atNuclear Power Plant SitesEffects of Cable LeakageCurrents on InstrumentSettings and IndicationsSoil and Water Contamina-tion at Fuel Cycle Facil-itiesBrachytherapy IncidentsInvolving Iridium-192 WireUsed in EndobronchialTreatmentsOverloading and SubsequentLock Out of ElectricalBuses During AccidentConditions02/24/9202/21/9202/18/9202/18/9202/10/9202/05/9201/31/9201/30/92All holders of OLs or CPsfor nuclear power reactors.All fuel cycle and uraniumfuel research and developmelicensees.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 uranium fuel fabrica-tion and conversion facil-ities.All Nuclear Regulatory Com-mission (NRC) licenseesauthorized to useiridium-192 for brachy-therapy; manufacturers anddistributors of iridium-192wire for use in brachy-therapy.All holders of OLs or CPsfor nuclear power reactors.92-1192-1092-09OL = Operating LicenseCP = Construction Permit