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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.