chapter 4 — safety systems chapter safety systemscetnar/abwr/chapter4.pdf · chapter 4 — safety...

CHAPTER 4 — SAFETY SYSTEMS

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4ChapterSafety Systems

Overview

The ABWR Safety Systems design incorporates threeredundant and independent divisions of EmergencyCore Cooling Systems (ECCS) and containment heatremoval (Figures 4-1 and 4-2). The RPV has noexternal recirculation loops or large pipe nozzles belowthe top of the core region. This allows for a reducedcapacity ECCS while still keeping the fuel coveredfor the full spectrum of postulated LOCAs evenassuming a single failure. Each of the three divisionswithin the ECCS network has one high pressure andone low pressure inventory makeup system. The highpressure configuration consists of two motor-drivenhigh pressure core flooders (HPCF), each with its ownindependent sparger discharging inside the shroud, andthe steam-driven Reactor Core Isolation Cooling

System (RCIC), which discharges intothe feedwater injection line. The HPCFpumps provide core makeup over theentire range of system operatingpressures. The RCIC System, whichhas been upgraded to a safety system,has the dual function of providing highpressure ECCS flow following apostulated LOCA and reactor coolantinventory control for reactor isolationtransients. The RCIC System, with itssteam turbine-driven power, alsoprovides a diverse makeup sourceduring loss of all Alternating Current(AC) power events. The low pressureECCS for the ABWR utilizes the threeresidual heat removal (RHR) pumps inthe post-LOCA Low PressureFlooding (LPFL) mode and arelabeled LPFL. For small LOCAs thatdo not depressurize the reactor system,if the high pressure makeup isunavailable, an AutomaticDepressurization System (ADS)actuates to vent steam from the reactorthrough the safety/relief valves (SRVs)to the suppression pool, anddepressurizes the reactor vessel toallow the LPFL pumps to provide corecoolant makeup flow.

The RHR System has a dual role ofproviding reactor cooling for normalshutdown and providing core andcontainment cooling following apostulated LOCA. The ABWR RHRSystem has been improved such thatcore and suppression pool cooling are

Figure 4-1. ECCS Divisional Configuration

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ADS

HPCF (C)

RHR (C) RHR (B)

Steam

Feedwater

RHR (A)

HPCF (B)

RCIC

CST


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Figure 4-2. ECCS Schematic

RHR

HPCF

Drywell spray

RCIC

Wetwell spray


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achieved simultaneously since, in the core coolingmode, the flow from the suppression pool passesthrough the RHR heat exchanger and the supportingheat removal systems. Reactor Building Cooling Water(RBCW) and Service Water (SW) are also initiatedupon a LOCA signal.

As a result of these improvements in the ECCSnetwork and the RHR System, there is an increase inthe calculated safety performance margin of theABWR over earlier BWRs. This has been confirmedby a Probabilistic Risk Assessment (PRA) for theABWR, which shows that the ABWR is a calculatedfactor of at least 10 better than BWR/5 and BWR/6 inavoiding possible core damage from degraded events.

In addition to the ECCS, there are several other importantsafety systems in the ABWR, including the Standby GasTreatment System (SGTS), Atmospheric Control System(ACS), Flammability Control System (FCS), andStandby Liquid Control System (SLCS). Finally, thereis the Emergency Diesel Generator (EDG) System,which provides emergency AC power to operate thesafety systems upon loss of offsite power.

Emergency Core CoolingSystems

High Pressure Core Flooder

The primary purpose of the HPCF System (Figure4-3) is to maintain reactor vessel inventory after smallbreaks which do not depressurize the reactor vessel.HPCF systems, which are provided in two divisions,maintain an adequate coolant inventory inside thereactor vessel to limit fuel cladding temperatures inthe event of breaks in the reactor coolant pressureboundary (RCPB). Electrical and mechanicalseparation between the two divisions is assured, inaddition to the physical separation, by placing eachdivision in a different area of the Reactor Building.The HPCF systems are initiated by either high pressure

in the drywell or low water level in thevessel. They operate independently ofall other systems over the entire rangeof system operating pressures.

Both HPCF systems take their primarysuction from the condensate storagepool (CSP) with the suppression pool(SP) as a secondary source. Thesuction source transfers automaticallyupon low level in the CSP or high levelin the SP. The pumps are located belowthe condensate storage and suppressionpools’ normal water level to assure thatnet pump suction head is maintained.The HPCF System pump motors arepowered by emergency dieselgenerators if auxiliary power is notavailable. The systems are also abackup to the RCIC System inresponse to transients.

Reactor Core Isolation Cooling

The primary purpose of the RCICSystem (Figure 4-4) is to providemakeup water to the reactor vesselwhen the vessel is isolated. It is alsopart of the emergency core coolingnetwork. The RCIC System uses asteam-driven turbine-pump unit andoperates automatically in time and withsufficient coolant flow to maintainadequate water level in the reactorvessel for the following events:

• Vessel isolated and maintained athot standby.

• Complete plant shutdown with lossof normal feedwater before thereactor is depressurized to a levelwhere the shutdown coolingsystem can be placed in operation.

• Loss of AC power.


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The RCIC System is contained withinone division and consists of a steam-driven turbine which drives a pumpassembly and the turbine and pumpaccessories. The system also includespiping, valves, and instrumentationnecessary to implement several flowpaths. The RCIC steam supply linebranches off one of the mainsteamlines (leaving the RPV) and goesto the RCIC turbine with drainageprovision to the main condenser. Theturbine exhausts to the suppressionpool with vacuum breakingprotection. Makeup water is suppliedfrom the condensate storage tank(CST) or the suppression pool, withthe preferred source being the CST.RCIC flow is discharged to thefeedwater injection line.

Following a reactor scram, steam generation in thereactor core continues at a reduced rate due to the corefission product decay heat. The turbine condenser andthe feedwater system supply the makeup waterrequired to maintain reactor vessel inventory. In theevent the reactor vessel is isolated and the feedwatersupply is unavailable, relief valves are provided toautomatically maintain vessel pressure withindesirable limits. The water level in the reactor vesseldrops due to continued steam generation by decay heat.Upon reaching a predetermined low level, the RCICSystem is initiated automatically. The turbine-drivenpump supplies water from the suppression pool or fromthe CST to the reactor vessel. The turbine is drivenwith a portion of the decay heat steam from the reactorvessel, and exhausts to the suppression pool.

In the event of a LOCA, the RCIC System is designedto pump water into the vessel from full operatingpressure down to approximately 150 psig. During

Figure 4-3. HPCF Schematic

S

MAIN PUMP

FROMCONDENSATESTORAGETANK

PRIMARY CONTAINMENT

SP

SLC (B)

RPV


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RCIC operation, the wetwell suppression pool acts asthe heat sink for steam generated by reactor decayheat. This results in a rise in pool water temperature.Heat exchangers in the RHR System are used tomaintain pool water temperature within acceptablelimits by cooling the pool water directly.

Automatic Depressurization System

The ADS logic is automatically initiated after a shortdelay if an RPV low water level signal is presentconcurrently with a high drywell pressure signal. TheADS logic is also automatically initiated if only theRPV low water level signal is present. This initiationwill occur after a longer delay to allow the highpressure ECCS a chance to restore the RPV water levelto normal levels and thus avoid the ADS actuation.Both ADS initiation paths require an indication thatat least one of the RHR or HPCF pumps is runningbefore the initiation sequence is complete.

ADS initiation is accomplished byredundant trip channels arranged intwo divisionally separated logics thatcontrol two separate solenoid-operated pneumatic pilots on eachADS SRV. Either pilot can operate theADS valve. These pilots control thepneumatic pressure applied by theaccumulators and the High PressureNitrogen Gas Supply (HPIN) System.The DC power for the logic is obtainedfrom two separate divisions within theSafety System Logic and Control(SSLC). This arrangement makes theADS initiation logic single-failure proof.

For ATWS mitigation, the ADS has anautomatic and manual inhibit of theautomatic ADS initiation to prevent

Figure 4-4. RCIC Schematic

RPV

PRIMARYCONTAINMENT

MAIN STEAMLINE "B"

SUPPRESSIONPOOL

TURBINE

TOFEEDWATERLINE "B"

S

FROMCONDENSATESTORAGE TANK

MAINPUMP

KEEPFILLPUMP


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ADS actuation during an ATWS.Automatic initiation of the ADS isinhibited unless there is a coincidentlow reactor water level signal and anaverage power range monitors(APRMs) downscale signal. There arealso main control room switches forthe manual inhibit of automaticinitiation of the ADS.

The ADS can also be initiatedmanually. On a manual initiationsignal, concurrent with positiveindication of at least one of the RHRor HPCF pumps is running, the ADSfunction is initiated.

Residual Heat Removal

The RHR System removes residualheat during normal plant shutdown,reactor isolation and loss-of-coolantaccident (LOCA). This system, which

consists of three divisions (A, B and C), has sixprincipal functions, each with a specific purpose(Figure 4-5):

Low Pressure Flooder (LPFL) Mode (3 loops): TheLPFL function provides a core cooling water supplyto compensate for water loss beyond the normal controlrange from any cause up to and including the designbasis LOCA. During the LPFL mode, water is initiallypumped from the suppression pool and divertedthrough the minimum flow lines until the injectionvalve in the discharge line is signaled to open on lowreactor pressure. As the injection valve opens on lowreactor pressure, flow to the RPV comes from thesuppression pool, through the RHR heat exchanger,and the injection valve. This creates a flow signal thatcloses the minimum flow line. Loop flow is alwaysthrough the RHR heat exchanger, which performscooling. Safety requirements can be achieved even ifany one loop is failed. Three individual loops areavailable. This mode is initiated automatically by alow water level in the reactor vessel or high pressure

Figure 4-5. Residual Heat Removal System

HX

RBCW

FEEDWATER A

KEEP-FILL PUMP

MAIN PUMP

FROMFPCU

TOFPCU

PRIMARYCONTAINMENT

S/P

S

RPV

DRYWELL SPRAYSPARGER (B&C)

WETWELL SPRAYSPARGER (B&C)

REACTOR BUILDING

EXTERNALCONNECTION

FROM FP

TO FCS(B&C)


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in the drywell (LOCA signal). Manual operationaction may also initiate the LPFL mode. In addition,Divisions B and C supply a small amount of water tothe Flammability Control System (FCS) upon receiptof a LOCA signal.

Suppression Pool Water Cooling Mode (3 loops):This mode cools the suppression pool water duringnormal state below 49°C, which is the maximumallowable temperature in order to retain thetemperature below 97°C just after reactor coolantblowdown in the worst LOCA event. It isautomatically initiated on high suppression pooltemperature, and can also be manually initiated.

Reactor Shutdown Cooling Mode (3 loops): Thismode removes decay heat and sensible heat of theRPV, piping and reactor coolant in cooperation withthe turbine main condenser and feedwater systemoperation after reactor shutdown. It can also cool downthe reactor water below 60°C within 20 hours aftershutdown, and it makes refueling activity andequipment maintenance possible.

Primary Containment Vessel Spray Cooling Mode(2 loops): This mode is manually initiated and spraysthe water from the suppression chamber pool into thedrywell and wetwell in the event of a LOCA. Thissprayed water in the drywell returns to the suppressionchamber through vent pipes after the drywell water levelreaches the vent pipe inlet level. It is mixed with thesprayed water in the wetwell and cooled by the RHRSystem heat exchangers, and then it is sprayed again.

This mode is available in Divisions B and C. Eachsystem can remove released coolant energy during anassumed feedwater line break, decay heat andgenerated heat by fuel cladding-H

2O reaction

accompanied by overheated fuel in conjunction withLPFL mode. It can also prevent the primarycontainment from exceeding its maximum operatingpressure and temperature. About 88% of this systemflow rate is sprayed in the drywell and the remaining12% is sprayed in the wetwell. It can also remove

released iodine in the gas phaseexisting in the primary containment.The heat exchanger is cooled by theReactor Building Closed CoolingSystem (RBCW).

Supplemental Fuel Pool Cooling (3loops): The three RHR loops arecapable of providing supplemental fuelpool cooling. This mode will be usedonly when the reactor is shut down andthe Fuel Pool Cooling System (FPCU)is unable to maintain the fuel poolwater temperature below the requireddesign limits. This is a manuallyinitiated operation.

AC-Independent Water Addition (1loop): The ACIWA mode of RHRLoop C provides a means forintroducing water from Fire Protection(FP) through RHR Loop C piping andvalves directly into either the RPV,drywell spray header, or wetwell sprayheader. The purpose is to prevent coredamage or, if core damage has alreadyoccurred, to terminate meltprogression when AC power is notavailable from either onsite or offsitesources. The ACIWA mode of RHRprovides manual capability to preventcore damage when all ECCS are lost.

Standby GasTreatment System

The Standby Gas Treatment System(SGTS) consists of two 100% capacitydivisions (B&C) which treat and thendischarge either primary or secondarycontainment air to the plant stack(Figure 4-6).


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The SGTS automatically starts, takessuction from the secondary containmentfollowing a LOCA, and maintains anegative pressure of approximately6 mm water in the SecondaryContainment. In addition, the SGTSwill automatically process SecondaryContainment atmosphere duringrefueling operations or PrimaryContainment atmosphere duringpurging operations if a high radiationsignal is received. The system can alsobe manually initiated.

Each filter train consists of a moistureseparator, main electric heater, prefilter,primary HEPA filter, charcoal adsorberand secondary HEPA filter. Thecharcoal adsorber bed is 15 cm thick andremoves more than 99% of elementaliodine or methyl iodide.

Atmospheric ControlSystem

The Atmospheric Control System (ACS) is designedto establish and maintain an inert atmosphere(nitrogen) within the primary containment volume(PCV) (Figure 4-7). An inert atmosphere is maintainedin all operating modes except plant shutdown forrefueling and/or maintenance. The ACS is sized toreduce containment oxygen concentrations fromatmospheric to <3.5% by volume in less than 4 hours.

During plant startup, liquid nitrogen from the storagetanks is vaporized and injected into the wetwell anddrywell regions of the containment. The nitrogen is mixedwith the PCV atmosphere by the Drywell Cooling (DWC)System fans. Once inerting is complete, the ACS providesnitrogen makeup to maintain the required oxygen

Figure 4-6. Standby Gas Treatment System

REACTORBUILDING

SECONDARYCONTAINMENT

PDT

NORTH SIDE

PDT

EAST SIDE

PDT

SOUTH SIDE

PDT

WEST SIDEOUTSIDEATMOSPHERE

MAIN FAN

DIVISION B

MAIN FANFILTER TRAIN

FILTER TRAIN

DIVISION C

TOPLANTSTACKFROM

ACS

FROMSECONDARY

CONTAINMENT

FROMSECONDARY

CONTAINMENT

FROMACS

ADSORBERCOOLING FAN

ADSORBERCOOLING FAN


4-9

concentration and maintain a slightly positive pressurewithin the PCV to preclude air in-leakage from thesecondary containment.

The ACS also has a Containment OverpressureProtection System (COPS) which is designed to relievecontainment pressure against rare severe accidentsequences in which the structural integrity of thecontainment is challenged by overpressure. If thewetwell pressure reaches the setpoint of the innerrupture disk, the rupture disk opens and containmentgases from the wetwell airspace, which have beenscrubbed by the suppression pool, are vented toatmosphere through the plant stack. Once thecontainment pressure has been reduced to a safe leveland normal containment heat removal has beenregained, the two normally open air-operated isolationvalves in the COPS relief path can be manually closedto reestablish the containment boundary. The pressure

Figure 4-7. Atmospheric Control System

setpoint is established to assure thecontainment pressure does not exceedthe Service Level C capability of thecontainment.

FlammabilityControl System

The Flammability Control System(FCS) controls the potential buildup ofa combustible mixture of hydrogen andoxygen inside the PCV which couldbe produced from a design basis metal-water reaction and radiolysis of waterduring LOCA, or from beyond designbasis events. The FCS is comprisedof two redundant thermal hydrogenand oxygen recombiner units locatedin two separate divisions (Figure 4-8).

REACTOR BUILDING HVACPURGE EXHAUST FAN

REACTOR BUILDINGHVAC PURGESUPPLY FAN

INNERRUPTURE

DISK

INERTINGNITROGEN

AA

NITROGENMAKEUP

PROTECTION SYSTEM

BLEED LINE VALVE

TO PLANT STACK

OUTER RUPTURE DISK

WETWELLAIR

SPACE

WETWELLAIR

SPACE

EH

LIQUID NITROGENSTORAGE,VAPORIZED, ANDTEMPERATURECONTROLS

TURBINEBUILDINGNITROGEN

SUPPLYMAKEUPHEATER

NITROGEN SUPPLYSYSTEM

DRYWELL

STANDBY GAS TREATMENTSYSTEM FILTER C

STANDBY GAS TREATMENTSYSTEM FILTER B

VB

CONTAINMENTOVERPRESSURE


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Each unit is designed to maintain theconcentration of oxygen below theflammability limit by recombininghydrogen and oxygen without relyingon purging or release of radioactivematerial to the environment. Theseunits are skid mounted with blowers,electric heaters, water spray coolers,piping, valves and instrumentation.The FCS is initiated manually bymonitoring hydrogen and oxygenlevels in the PCV.

Standby LiquidControl System

The Standby Liquid Control System(SLCS) provides a backup method tobring the nuclear reactor to subcriticalityand to maintain subcriticality as thereactor cools. The system makespossible an orderly and safe shutdown

in the event that not enough control rods can be insertedinto the reactor core to accomplish shutdown in thenormal manner. The SLCS is sized to counteract thepositive reactivity effect of shutting down from ratedpower to cold shutdown condition.

The SLCS automatically initiated or can be manuallyinitiated from the main control room to pump theneutron absorbing solution into the reactor.

The SLCS includes a boron solution tank, a test watertank, two positive displacement pumps, motor-operated injection and pump suction valves, andassociated local piping, valves and controls in theReactor Building outside the Primary Containment.The liquid is piped into the reactor vessel through theHPCF line downstream of the HPCF inboard checkvalve inside the Primary Containment. Figure 4-9illustrates the SLCS configuration.

The boron absorbs thermal neutrons and therebyterminates the nuclear fission chain reaction in the fuel.The specified neutron absorber solution is sodiumpentaborate. At all times, when it is possible to makethe reactor critical, the SLCS will be able to deliverenough sodium pentaborate solution into the reactorto assure reactor shutdown.

Figure 4-8. Flammability Control System

FROMRHR-B

WETWELL

DRYWELL

FROMRHR-C

WETWELL

DRYWELLRPV

RECOMBINERUNIT B

RECOMBINERUNIT C

FROMMWFROM

MW


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Emergency DieselGenerator System

The Emergency Diesel Generator (EDG) System consistsof three diesel engines and their respective combustionair intake system, starting air system, fuel oil system (fromthe day tank to the engine), lubricating oil system, enginejacket cooling water system, engine exhaust system andsilencer, governor system, and generator with itsexcitation and voltage regulation systems.

The three DGs are classified as Class 1E, safety-relatedand supply standby AC power to their respective Class1E Electrical Power Distribution (EPD) Systemdivisions (Divisions I, II, and III). The DieselGenerator (DG) connections to the EPD System areshown on Figure 9-2.

The DGs are sized to supply their loaddemand following a LOCA. The DGair start receiver tanks are sized toprovide five DG starts withoutrecharging their tanks.

A Loss of Preferred Power (LOPP)signal (bus undervoltage) from an EPDSystem medium voltage divisional busautomatically starts its respective DG,and initiates automatic load sheddingand connection of the DG to itsdivisional bus. A DG automaticallyconnects to its respective bus when DGrequired voltage and frequencyconditions are established and requiredmotor loads are tripped. After a DGconnects to its respective bus, the

Figure 4-9. Standby Liquid Control System

POSITIVEDISPLACEMENTPUMPS

TESTTANK

INLETVALVE

OUTLETVALVE

VENT

HEATER

STORAGETANK

PRIMARYCONTAINMENT

HPCF-B

THROTTLEVALVE

PRESSUREBREAKDOWNORIFICE

VENT

RPV


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non-accident loads are automaticallysequenced onto the bus.

LOCA signals from the RHR(Division I) and HPCF (Divisions IIand III) Systems automatically starttheir respective divisional DG. Afterstarting, the DGs remain in a standbymode (i.e., running at required voltageand frequency, but not connected totheir buses), unless a LOPP signalexists. When LOCA and LOPP signalsexist, load shedding occurs andrequired motor loads are tripped, andthe DG automatically connects to itsrespective divisional bus. After a DGconnects to its respective bus, theLOCA loads are automaticallysequenced onto the bus.

A manual start signal from the maincontrol room (MCR) or from the localcontrol station in the DG area starts aDG. After starting, the DG remains ina standby mode, unless a LOPP signalexists.

DGs start, attain required voltage and frequency, andare ready to load in <20 seconds after receiving anautomatic or manual start signal.

When a DG is operating in parallel (test mode) withoffsite power, a loss of the offsite power source usedfor testing or a LOCA signal overrides the test modeby disconnecting the DG from its respective divisionalbus.

The DG units are located in their respective divisionalareas in the Reactor Building. The DG combustionair intakes are located above the maximum flood level.The DG combustion air intakes are separated fromDG exhaust ducts. Class 1E DG unit auxiliary systemsare supplied electrical power from the same Class 1Edivision as the DG unit. Independence is providedbetween Class 1E divisions and also between Class1E divisions and non-Class 1E equipment. Eachdivisional DG with its auxiliary systems, is physicallyseparated from the other divisions.

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