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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.

Theremainingemergencycorecoolingsystemsandcomponentsmustmeetthe10CFRrequirementsoverthe

entirespectrumofbreaklocationsandsizes.

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High Pressure EmergencyCore Cooling System

Thehighpressurecoolantinjection(HPCI)systemisanindependentemergencycore

coolingsystemrequiringnoauxiliaryacpower,

plantairsystems,orexternalcoolingwatersystemstoperformitspurposeofproviding

makeupwatertothereactorvesselforcore

coolingundersmallandintermediatesizelossofcoolantaccidents.

Thehighpressurecoolantinjectionsystemcan

supplymakeupwatertothereactorvesselfrom

aboveratedreactorpressuretoareactor

pressurebelowthatatwhichthelowpressureemergencycorecoolingsystemscaninject.

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High Pressure EmergencyCore Cooling System

Theautomaticdepressurizationsystem

(ADS)consistsofredundantlogicscapableofopeningselectedsafetyreliefvalves,when

required,toprovidereactordepressurizationforeventsinvolvingsmallorintermediate

sizelossofcoolantaccidentsifthehighpressurecoolantinjectionsystemisnot

availableorcannotrecoverreactorvessel

water

level.

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Low Pressure EmergencyCore Cooling System

Thelowpressureemergencycorecoolingsystemsconsistoftwoseparateand

independentsystems,thecorespraysystem

andthelowpressurecoolantinjection(LPCI)

mode

of

the

residual

heat

removal

system.

Thecorespraysystemconsistsoftwoseparateandindependentpumpingloops,eachcapable

ofpumpingwaterfromthesuppressionpool

intothereactorvessel.

Corecoolingisaccomplishedbysprayingwater

ontopofthefuelassemblies

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Low Pressure EmergencyCore Cooling System

Thelowpressurecoolantinjectionmodeoftheresidualheatremovalsystemprovidesmakeup

watertothereactorvesselforcorecoolingunderlossofcoolantaccidentconditions.

Theresidualheatremovalsystemisamultipurposesystemwithseveraloperationalmodes,eachutilizingthesamemajorpieces

ofequipment.

Thelowpressurecoolantinjectionmodeisthe

dominantmodeandnormalvalvelineupconfigurationoftheresidualheatremovalsystem.

The

low

pressure

coolant

injection

mode

operatesautomaticallytorestoreand,ifnecessary,maintainthereactorvesselcoolant

inventorytoprecludefuel claddingtemperaturesinexcessof2200F.

Duringlowpressurecoolantinjection

operation,theresidualheatremovalpumpstakewaterfromthesuppressionpoolanddischargetothereactorvessel.

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BoilingWaterReactorContainments

MarkI MarkII MarkIII

Theprimarycontainmentpackageprovidedforaparticularproductlineisdependentuponthevintage

oftheplantandthecostbenefitanalysisperformedpriortotheplantbeingbuilt.

Duringtheevolutionoftheboilingwaterreactors,threemajortypesofcontainmentswerebuilt.

The

major

containment

designs

are

the

Mark

I

,Mark

II

,

and

the

Mark

III.

UnliketheMarkIII,thatconsistsofaprimarycontainmentandadrywell,theMarkIandMarkIIdesignsconsistofa

drywellandawetwell(suppressionpool).

Allthreecontainmentdesignsusetheprincipleofpressuresuppressionforlossofcoolantaccidents.

Theprimarycontainmentisdesignedtocondensesteamandtocontainfissionproductsreleasedfromalossofcoolant

accidentsothatoffsiteradiationdosesspecifiedin10CFR100 arenotexceededandtoprovideaheatsinkandwater

sourceforcertainsafetyrelatedequipment.

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Mark I Containment - GE

TheMarkIcontainmentdesignconsistsofseveralmajor

components,manyofwhichcanbeseenonthisslide

Thesemajorcomponentsinclude:

Thedrywell,whichsurroundsthereactorvesselandrecirculationloops,

Asuppressionchamber,whichstoresalargebodyofwater(suppressionpool),

Aninterconnectingventnetworkbetweenthedrywelland

the

suppression

chamber,

and

Thesecondarycontainment,whichsurroundstheprimary

containment(drywellandsuppressionpool)andhousesthespentfuelpoolandemergencycorecoolingsystems.

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Mark II Containment - GE

TheMarkIIprimarycontainmentconsistsofasteeldomeheadandeitheraposttensionedconcretewall orreinforcedconcrete

wallstandingonabasematofreinforcedconcrete.

Theinnersurfaceofthecontainmentislinedwithasteelplatethatactsasaleaktightmembrane.

Thecontainmentwallalsoservesasasupportforthefloorslabsof

thereactorbuilding(secondarycontainment)andtherefuelingpools.

TheMarkIIdesignisanoverunderconfiguration.

Thedrywell,intheformofafrustumofaconeoratruncatedcone,islocateddirectlyabovethesuppressionpool.

Thesuppressionchamberiscylindricalandseparatedfromthedrywellbyareinforcedconcreteslab.

Thedrywellistoppedbyanellipticalsteeldomecalledadrywell

head.

Thedrywellinertedatmosphereisventedintothesuppressionchamberthroughasseriesofdowncomerpipespenetratingandsupportedbythedrywellfloor.

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Mark III Containment - GE

TheMarkIIIprimarycontainmentconsistsof

severalmajorcomponents,manyofwhichcanbeseenhere.

Thedrywell(13)isacylindrical,reinforcedconcretestructurewitharemovablehead.

Thedrywellisdesignedtowithstandand

confinesteamgeneratedduringapiperupture

insidethecontainmentandtochannelthe

releasedsteamintothesuppressionpool(10)viatheweirwall(11)andthehorizontalvents(12).

Thesuppressionpoolcontainsalargevolumeofwaterforrapidlycondensingsteamdirectedto

it.

Aleaktight,cylindrical,steelcontainmentvessel

(2)surroundthedrywellandthesuppressionpooltopreventgaseousandparticulatefission

productsfromescapingtotheenvironmentfollowingapipebreakinsidecontainment.