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    Boiler Water Reactor Systems

    Bill Henwood

    Director, Nuclear

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    Boiler Water Reactor Systems

    Insidetheboilingwaterreactor(BWR)vessel,asteamwatermixtureisproducedwhenverypurewater(reactorcoolant)movesupwardthroughthecoreabsorbingheat.

    ThemajordifferenceintheoperationofaBWR

    fromothernuclearsystemsisthesteamvoid

    formationinthecore.

    Thesteamwatermixtureleavesthetopofthe

    coreandentersthetwostagesofmoistureseparation,wherewaterdropletsareremovedbeforethesteamisallowedtoenterthesteam

    line.

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    Boiler Water Reactor Systems

    Thesteamline,inturn,directsthesteam

    tothemainturbinecausingittoturnthe

    turbineandtheattachedelectricalgenerator.

    The

    unused

    steam

    is

    exhausted

    to

    the

    condenserwhereitiscondensedintowater.

    Theresultingwaterispumpedoutofthe

    condenserwithaseriesofpumpsandbacktothereactorvessel.

    Therecirculationpumpsandjetpumpsallow

    theoperatortovarycoolantflowthroughthe

    coreandchangereactorpower.

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    BWR 6 Reactor Vessel

    Thereactorvesselassemblyshown,consistsofthereactorvesselanditsinternalcomponents,includingthecore

    supportstructures,coreshroud,moistureremovalequipment,andjetpumpassemblies.

    Thepurposesofthereactorvesselassemblyareto:

    Housethereactorcore,

    Serveaspartofthereactorcoolantpressureboundary,

    Supportandalignthefuelandcontrolrods,

    Provideaflowpathforcirculationofcoolantpastthefuel,

    Removemoisturefromthesteamexitingthecore,and

    Providearefloodablevolumeforalossofcoolantaccident.

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    BWR 6 Reactor Vessel

    Thereactorvesselisverticallymountedwithinthedrywell

    andconsistsofacylindricalshellwithanintegralrounded

    bottomhead.

    Thetopheadisalsoroundedinshapebutisremovableviathestudandnutarrangementtofacilitaterefuelingoperations.

    Thevesselassemblyissupportedbythevesselsupportskirt

    (20)whichismountedtothereactorvesselsupportpedestal.

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    BWR 6 Reactor Vessel

    Theinternalcomponentsofthereactorvesselaresupportedfromthebottomheadand/orvesselwall.

    Thereactorcoreismadeupoffuelassemblies(15),controlrods(16),andneutronmonitoringinstruments(24).

    Thestructuresurroundingtheactivecoreconsistsofacore

    shroud

    (14),

    core

    plate(17),

    and

    top

    guide

    (12).

    Thecomponentsmakinguptheremainderofthereactorvesselinternalsarethejetpumpassemblies(13),steamseparators(6),steamdryers(3),feedwaterspargers(8),and

    coresprayspargers(11).

    Thejetpumpassembliesarelocatedintheregionbetween

    thecoreshroudandthevesselwall,submergedinwater.

    Thejetpumpassembliesarearrangedintwosemicirculargroupsoften,witheachgroupbeingsuppliedbyaseparate

    recirculationpump.

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    BWR 6 Reactor Vessel

    Theemergencycorecoolingsystems,penetrationsnumber5and9,andthereactorvesseldesignsarecompatibletoensurethatthe

    corecanbeadequatelycooledfollowingalossofreactorcoolant.

    Theworstcaselossofcoolantaccident,withrespecttocore

    cooling,isarecirculationlinebreak(penetrationsnumber18and19).

    Inthisevent,reactorwaterleveldecreasesrapidly,uncovering the

    core.

    However,severalemergencycorecoolingsystemsautomatically

    providemakeupwatertothenuclearcorewithintheshroud,providingcorecooling

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    BWR 6 Fuel Assembly

    Thecontrolcellassemblyisrepresentativeforboilingwaterreactor1through6.

    Eachcontrolcellconsistsofacontrolrod(7)andfourfuelassembliesthatsurroundit.

    Unlikethepressurizedwaterreactorfuelassemblies,theboilingwaterreactorfuelbundleisenclosedinafuel

    channel(6)todirectcoolantupthroughthefuelassembly

    andactasabearingsurfaceforthecontrolrod.

    Inaddition,thefuelchannelprotectsthefuelduringrefuelingoperations.

    Thepowerofthecoreisregulatedbymovementofbottom

    entrycontrolrods.

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    Reactor Water Cleanup System

    Thepurposeofthereactorwatercleanupsystem(RWCU)istomaintainahighreactorwaterqualitybyremovingfissionproducts,corrosionproducts,andothersolubleand

    insolubleimpurities.

    Thereactorwatercleanuppumptakeswater

    fromtherecirculationsystemandthevesselbottomheadandpumpsthewaterthroughheatexchangerstocooltheflow.

    The

    water

    is

    then

    sent

    through

    filter/demineralizersforcleanup.

    Aftercleanup,thewaterisreturnedtothereactorvesselviathefeedwaterpiping.

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    Decay Heat Removal

    Heatisremovedduringnormalpoweroperationbygeneratingsteaminthereactorvesselandthenusingthatsteamtogenerateelectricalenergy.

    Whenthereactorisshutdown,thecorewillstill

    continuetogeneratedecayheat.

    Theheatisremovedbybypassingtheturbineanddumpingthesteamdirectlytothe

    condenser.

    Theshutdowncoolingmodeoftheresidualheatremoval(RHR)systemisusedtocompletethe

    cooldownprocesswhenpressuredecreasesto

    approximately50psig.

    Waterispumpedfromthereactorrecirculationloopthroughaheatexchangerandbacktothereactorviatherecirculationloop.

    Therecirculationloopisusedtolimitthe

    numberofpenetrationsintothereactorvessel.

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    Reactor Core Isolation Cooling

    Thereactorcoreisolationcooling(RCIC)systemprovidesmakeupwatertothereactor

    vessel

    for

    core

    cooling

    when

    the

    main

    steam

    linesareisolatedandthenormalsupplyofwatertothereactorvesselislost.

    TheRCICsystemconsistsofaturbinedrivenpump,piping,andvalvesnecessarytodeliver

    watertothereactorvesselatoperatingconditions.

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    Reactor Core Isolation Cooling

    Theturbineisdrivenbysteamsuppliedbythemainsteamlines.Theturbineexhaustisroutedtothesuppressionpool.

    Theturbinedrivenpumpsuppliesmakeupwaterfromthecondensatestoragetank,

    withanalternatesupplyfromthesuppressionpool,tothereactorvesselviathefeedwaterpiping.

    Thesystemflowrateisapproximatelyequaltothesteamingrate15minutesaftershutdown

    withdesignmaximumdecayheat.

    Initiationofthesystemisaccomplishedautomaticallyonlowwaterlevelinthereactor

    vesselormanuallybytheoperator.

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    Standby Liquid Control System

    The

    standby

    liquid

    control

    system

    injects

    a

    neutronpoison(boron)intothereactorvesseltoshutdownthechainreaction,independentof

    thecontrolrods,andmaintainsthereactorshutdownastheplantiscooledtomaintenancetemperatures.

    Thestandbyliquidcontrolsystemconsistsofaheatedstoragetank,twopositivedisplacement

    pumps,twoexplosivevalves,andthepiping

    necessarytoinjecttheneutronabsorbingsolutionintothereactorvessel.

    Thestandbyliquidcontrolsystemismanually

    initiatedandprovidestheoperatorwitha

    relativelyslowmethodofachievingreactor

    shutdownconditions.

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    Emergency Core Cooling

    Theemergencycorecoolingsystems(ECCS)providecorecoolingunderlossofcoolantaccidentconditionstolimitfuelcladdingdamage.

    Theemergencycorecoolingsystemsconsistoftwohighpressureandtwolowpressuresystems.

    Thehighpressuresystemsarethehighpressurecoolant

    injection(HPCI)systemandtheautomaticdepressurizationsystem(ADS).

    Thelowpressuresystemsarethelowpressurecoolant

    injection(LPCI)modeoftheresidualheatremovalsystem

    andthecorespray(CS)system.

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    Emergency Core Cooling

    Themannerinwhichtheemergencycorecoolingsystems

    operatetoprotectthecoreisafunctionoftherateatwhich

    reactorcoolantinventoryislostfromthebreakinthenuclearsystemprocessbarrier.

    Thehighpressurecoolantinjectionsystemisdesignedto

    operate

    while

    the

    nuclear

    system

    is

    at

    high

    pressure.

    Thecorespraysystemandlowpressurecoolantinjection

    modeoftheresidualheatremovalsystemaredesignedforoperationatlowpressures.

    Ifthebreakinthenuclearsystemprocessbarrierisofsuchasizethatthelossofcoolantexceedsthecapabilityofthehighpressurecoolantinjectionsystem,reactorpressure

    decreasesataratefastenoughforthelowpressure

    emergencycorecoolingsystemstocommencecoolant

    injectionintothereactorvesselintimetocoolthecore.

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    Emergency Core Cooling

    Automaticdepressurizationisprovidedtoautomatically

    reducereactorpressureifabreakhasoccurredandthehigh

    pressurecoolantinjectionsystemisinoperable.

    Rapiddepressurizationofthereactorisdesirabletopermit

    flowfromthelowpressureemergencycorecoolingsystemssothatthetemperatureriseinthecoreislimitedtolessthanregulatoryrequirements.

    If,foragivenbreaksize,thehighpressurecoolantinjectionsystemhasthecapacitytomakeupforallofthecoolant

    loss,flowfromthelowpressureemergencycorecooling

    systemsisnotrequiredforcorecoolingprotectionuntilreactorpressurehasdecreasedbelowapproximately100psig.

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    Emergency Core Cooling

    Theperformanceoftheemergencycorecoolingsystemsas

    an

    integrated

    package

    can

    be

    evaluated

    by

    determining

    whatisleftafterthepostulatedbreakandasinglefailureofoneoftheemergencycorecooingsystems.

    Therem

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