CHAPTER UCLEAR BOILER 3 SYSTEMScetnar/ABWR/Chapter3.pdf · systems are described in Chapter 5,...

CHAPTER 3 — NUCLEAR BOILER SYSTEMS

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3ChapterNuclear Boiler System

OverviewThe Nuclear Boiler System (NBS) produces steamfrom the nuclear fission process, and directs this steamto the main turbine. The NBS is comprised of (1) thereactor vessel, which serves as a housing for thenuclear fuel and associated component, (2) therecirculation system, (3) the control rod drive system,(4) the main steam system and (5) the reactor buildingportion of the feedwater system. Other supportingsystems are described in Chapter 5, AuxiliarySystems.

Reactor Vessel andInternalsThe reactor vessel houses the reactor core, which isthe heat source for steam generation. The vesselcontains this heat, produces the steam within itsboundaries, and serves as one of the fission productbarriers during normal operation. The ABWR reactorassembly is shown in Figure 3-1. For this size reactor,the diameter of the ABWR reactor pressure vessel(RPV) is increased but the height is decreasedcompared to earlier product lines. The increaseddiameter has resulted in increased wall thickness. TheRPV is approximately 21m in height and 7.4m indiameter.

The most significant differences between the ABWRRPV and earlier product lines are as follows:

• Inward vessel flange design.• Steam nozzle with flow restrictor.• Double feedwater nozzle thermal sleeve.

• Conical vessel support skirt.• Relatively flat bottom head.• Elimination of nozzles below

the core.• Reactor internal pump

penetrations.• Use of forged shell rings at and

below core elevation.

The RPV design is based on provenBWR technology. A noteworthyfeature is the lack of any large nozzlesbelow the elevation of the top of thecore. This RPV nozzle configurationprecludes any large pipe ruptures at orbelow the elevation of the core. It is akey factor in the ability of ABWRsafety systems to keep the corecompletely and continuously floodedfor the entire spectrum of design basisloss-of-coolant accidents (LOCAs).

The vessel contains the core supportstructure that extends to the top of thecore. The presence of a large volume ofsteam and water results in two veryimportant and beneficial characteristics.It provides a large reserve of water abovethe core, which translates directly into amuch longer period of time beingavailable before core uncovery can occuras a result of feed flow interruption or aLOCA. Consequently, this gives anextended period of time during whichautomatic systems or plant operatorscan reestablish reactor inventorycontrol using any normal, non-safety-related system capable of injectingwater into the reactor. Timely initiation


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of these systems is designed topreclude initiation of the emergencysafety equipment. This easilycontrolled response to loss of normalfeedwater is a significant operationalbenefit. In addition, the larger RPVvolume leads to a reduction in theABWR pressurization rate that wouldoccur after a rapid isolation of thereactor from the normal heat sink.

The following sections provide furtherdescriptions of the unique features ofthe ABWR RPV and internals.

Vessel Flange and Closure Head (1)

To minimize the number of main closure bolts, theABWR RPV has an inside type flange. This is differentfrom the earlier product lines, which had outside typevessel flanges. The inside type vessel flange allows ahemispherical main closure with a radius less than thevessel radius. Also, this helps minimize the weight ofthe main closure. The vessel closure seal consists oftwo concentric O-rings which perform withoutdetectable leakage at all operating conditions,including hydrostatic testing.

Figure 3-1. ABWR Reactor Assembly

9

1

22

2

213

16

9

20

4

10

15

19

13

18

25

24

12

14

5

6

23

27

17

7

8

26

11

1 - Vessel flange and closure head2 - Steam outlet flow restrictor3 - Feedwater nozzle4 - Vessel support skirt5 - Vessel bottom head6 - RIP penetrations7 - Forged shell rings8 - Core shroud9 - Core plate10 - Top guide11 - Fuel supports12 - Control rod drive housings13 - Control rod guide tubes14 - In-core housing15 - In-core instrument guide tubes16 - Feedwater sparger17 - High pressure core flooder (HPCF) sparger18 - HPCF coupling19 - Low pressure flooder (LPFL)20 - Shutdown cooling outlet21 - Shroud head and steam separator assembly22 - Steam dryer assembly23 - Reactor internal pumps (RIP)24 - Fine motion control rod drives25 - Local power range monitor26 - Fuel assemblies27 - Control rods


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Reactor Internal PumpPenetrations and Weld (6)The most significant differencebetween the ABWR and earlier BWRproduct lines is the elimination of allmajor pipe connections below the coreby incorporating internal recirculationpumps in the reactor. The RIP motorcasings are welded to the vessel bottomhead by the design shown inFigure 3-3.

Vertical restraints are provided to preventthe motor casing or the motor cover fromblowing out in the unlikely event of afailure of the weld between the RPVand the motor casing or a failure of themotor cover bolts. In addition, if therestraints should fail, the pump impelleris designed to backseat on the stretch tubethat keeps the pump diffuser in place andprevent significant leakage through thefailed part. More information about theRIP can be found under “RecirculationSystem” later in this chapter.

Steam Nozzle with Flow Restrictor (2)

The ABWR RPV has flow restricting venturi locatedin the steam outlet nozzles. Besides providing an outletfor steam from the RPV, the steam outlet nozzles willprovide for (1) steamline break detection by measuringsteam flow to signal a trip for the main steam isolationvalves, (2) steam flow measurement for input to thefeedwater control system, and (3) a flow-chokingdevice to limit blowdown and associated loads on theRPV and internals in the event of a postulated mainsteam line break. Calculations show that the pressuredrop in the nozzle is within the requirements of thesteady-state performance specification.

Feedwater Nozzle Thermal Sleeve (3)

The feedwater nozzles utilize double thermal sleeveswelded to the nozzles. The double thermal sleeveprotects the vessel nozzle inner blend radius from theeffects of high frequency thermal cycling. A schematicof the feedwater nozzle is shown in Figure 3-2.

Vessel Support Skirt (4)

The vessel support skirt has a conical geometry andis attached to the lower vessel cylindrical shell course.The support skirt attachment (knuckle) is an integralpart of the vessel shell ring. Locating the conicalsupport skirt on the lower shell ring provides:

• Needed space for the reactor internal pump (RIP)heat exchangers.

• Access for ISI of the bottom head weld.

Reactor Vessel Bottom Head (5)The bottom head consists of a spherical bottom cap,made from a single forging, extending to encompassthe control rod drive (CRD) penetrations and a conicaltransition section to the toroidal knuckle between thehead and vessel cylinder. With a bottom head thicknessof approximately 250 mm, the bottom head meets theASME allowables for the specified design loads. Themain advantage of using a single forging for thebottom head is that it eliminates all RPV welds withinthe CRD pattern, thus reducing future in-serviceinspection (ISI) requirements.

Figure 3-2. ABWR Reactor Pressure VesselFeedwater Nozzle


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Use of Forged Shell Rings (7)

The ABWR RPV utilizes low alloyforged shell rings, per ASME SA-508,Class 3, adjacent to and below the corebelt line region. The flanges and largenozzles are also per ASME SA-508,Class 3. The shell rings above the corebeltline region and the main closure aremade from low alloy steel plate perASME A-533, Type B, Class 1. Therequired Reference Nil Ductility, RT

NDT,

of the vessel material is -20°C. Figure3-4 shows one of the RPV forged shellrings during fabrication.

Shroud (8)

The shroud is a stainless steelcylindrical assembly that provides apartition to separate the upward flowof coolant through the core from thedownward recirculation flow. Thevolume enclosed by the shroud ischaracterized by upper and lower

regions. The upper portion of the shroud surroundsthe active fuel and forms the longest section of theshroud. This section is bounded at the bottom by thecore plate. The lower shroud, surrounding part of thelower plenum, is welded to the RPV shroud support.The shroud provides lateral support for the core bysupporting the core plate and top guide.

Core Plate (9)The core plate consists of a circular plate with roundopenings. The core plate provides lateral support andguidance for the control rod guide tubes, in-core fluxmonitor guide tubes, peripheral fuel supports, andstartup neutron sources. The last two items are alsosupported vertically by the core plate. The entireassembly is bolted to a support ledge in the shroud.The core plate also forms a partition within the shroud,which causes the recirculation flow to pass into theorificed fuel support and through the fuel assemblies.

Top Guide (10)The top guide consists of a grid that gives lateralsupport of the top of the fuel assemblies, a cylindersupporting core flooder spargers, and a top flange forattaching the shroud head. Each opening provideslateral support and guidance for four fuel assemblies

Figure 3-3. Reactor Internal Pump MotorCasing Including Weld to RPV

Figure 3-4. RPV Forged Steel Ring


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or, in the case of peripheral fuel, one, two or threefuel assemblies. Holes are provided in the bottom ofthe support intersections to anchor the in-core fluxmonitors and startup neutron sources. The top guideis bolted to the top of the shroud.

Fuel Supports (11)The fuel supports are of two basic types; namely,peripheral fuel supports and orificed fuel supports. Theperipheral fuel supports are located at the outer edgeof the active core and are not adjacent to control rods.Each peripheral fuel support sustains one fuelassembly and contains an orifice designed to assureproper coolant flow to the peripheral fuel assembly.Each orificed fuel support sustains four fuel assembliesvertically upward and horizontally and is providedwith orifices to assure proper coolant flow distributionto each fuel bundle. The orificed fuel support sits onthe top of the control rod guide tube, which carriesthe weight of the fuel rods down to the bottom of theRPV. The control rods pass through cruciformopenings in the center of the orificed fuel support.

Control Rod Drive Housing (12)

The control rod drive housing provides extension ofthe RPV for installation of the control rod drive, andthe attachment of the CRD line. It also supports theweight of a control rod, control rod drive, control rodguide tube, orificed fuel support and four fuelassemblies.

Control Rod Guide Tubes (13)The control rod guide tubes extend from the top ofthe control rod drive housings up through holes in thecore plate. Each guide tube is designed as the guidefor the lower end of a control rod and as the supportfor an orificed fuel support. This locates the four fuelassemblies surrounding the control rod drive housing,which, in turn, transmits the weight of the guide tube,fuel support, and fuel assemblies to the reactor vesselbottom head. The control rod guide tube also containsholes, near the top of the control rod guide tube andbelow the core plate, for coolant flow to the orificed

fuel supports. In addition, the guidetube provides a connection to theFMCRD to restrain an hypotheticalejection of the FMCRD.

In-Core Housing (14)The in-core housings provideextensions of the RPV at the bottomhead for the installation of various in-core flux monitoring sensorassemblies, which are components ofthe Neutron Monitoring System. It alsosupports the weight of an in-core fluxmonitoring sensor assembly, in-coreguide tube and part of the in-core guidetube stabilizer assembly.

In-Core Guide Tubes andStabilizers (15)

The in-core guide tubes extend fromthe top of the in-core housing to thetop of the core plate. They provide thein-core instrumentation withprotection from flow of water in thebottom head plenum, and guidance forinsertion and withdrawal from the core.The in-core guide tube stabilizersprovide lateral support and rigidity tothe in-core guide tubes.

Feedwater Spargers (16)

The feedwater spargers are attached tobrackets on the vessel wall and delivermakeup water to the reactor during plantstartup, power generation and plantshutdown modes of operation. Nozzlesin the spargers provide uniformdistribution of feedwater flow within thedowncomer flow passage.


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High Pressure Core FlooderSparger Assembly (17)

The high pressure core flooder(HPCF) spargers inside the cylinder ofa top guide are arranged to provideemergency coolant injection over theupper end of the core. The spargershave the function of a standby liquidcontrol solution injection. The HPCFspargers are connected to the HPCFnozzles by means of an HPCFcoupling (18).

Low Pressure Flooder Spargers (19)The two flooding spargers that areattached to the vessel wall deliver flowat low pressure from the RHR Systemand distribute it in the upper plenumabove the shroud head of the reactor.Flow is delivered in either of twomodes: (1) for the flooding of thereactor in the event of an abnormaldrop in water level, or (2) in thecirculation of cooling water to removeresidual and core decay heat from thereactor during shutdown.

Shutdown Cooling Nozzles (20)

Suction for the RHR System in theshutdown cooling mode is provided bythree shutdown cooling nozzles.

Shroud Head and SteamSeparator Assembly (21)The steam separator assembly consistsof a slightly domed base on top ofwhich is welded an array of standpipeswith a three-stage steam separatorlocated at the top of each standpipe.The steam separator assembly rests onthe top flange of the core shroud andforms the cover of the core dischargeplenum region. The seal between the

separator assembly and core shroud flanges is metal-to-metal contact and does not require a gasket or otherreplacement sealing devices. The separator assemblyis bolted to the core shroud flange, by long holddownbolts which, for ease of removal, extend above theseparators. During installation, the separator base isaligned on the core shroud flange with guide rods andfinally positioned with locating pins. The objective ofthe long-bolt design is to provide direct access to thebolts during reactor refueling operations withminimum-depth underwater tool manipulation duringthe removal and installation of the assemblies. It isnot necessary to engage threads in mating up theshroud head. A tee-bolt engages in the top guide andits nut is tightened to only nominal torque. Finalloading is established through differential expansionof the bolt and compression sleeve. The fixed axialflow type steam separators have no moving parts andare made of stainless steel. In each separator, the steam-water mixture rising through the standpipe impingeson vanes which give the mixture a spin to establish avortex wherein the centrifugal forces separate the waterfrom the steam in each of three stages. Steam leavesthe separator at the top and passes into the wet steamplenum below the dryer (Figure 3-5). The separatedwater exits from the lower end of each stage of theseparator and enters the pool that surrounds thestandpipes to join the downcomer annulus flow.

Steam Dryer Assembly (22)The steam dryer assembly consists of multiple banksof dryer units mounted on a common structure whichis removable from the RPV as an integral unit. Theassembly includes the dryer banks, dryer supply anddischarge ducting, drain collecting trough, drain ducts,and a skirt which forms a water seal extending belowthe separator reference zero elevation. Steam from theseparators flows upward and outward through thedrying vanes (Figure 3-6). These vanes are attachedto a top and bottom supporting member forming arigid, integral unit. Moisture is removed and carriedby a system of troughs and drains to the poolsurrounding the separators and then into therecirculation downcomer annulus between the coreshroud and reactor vessel wall. Upward and radialmovement of the dryer assembly under the action of


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RecirculationSystemThe functions of the ReactorRecirculation System (RCIR) are to:

• Provide forced circulation ofreactor coolant for energy transferfrom fuel to the cooling fluid and,as a result, generate a largeramount of steam.

• Control the reactor power bychanging the recirculation flow;the flow is controlled by the use ofadjustable speed pumps.blowdown and seismic loads is limited by support

brackets on the vessel shell and holddown brackets insidethe main closure. The assembly is arranged for removalfrom the vessel as an integral unit on a routine basis.

Figure 3-5. Schematic of Steam Flow through Separator

DRYER STEAM

RETURNINGWATER

WATER LEVELOPERATING RANGE

SKIRT

WETSTEAM STANDPIPECORE

DISCHARGEPLENUM

TURNINGVANES

Figure 3-6. Schematic of Steam FlowThrough Dryer

A A

DRYERLIFTINGBAR (4)

DRYERSKIRT

STEAMWATER

FLOW

DRAIN CHANNELS(AT EACH DRYER

SECTION)

STEAM WATER FLOWFROM SEPARATORS

VERTICAL GUTTER STRIPOR "MOISTURE HOOK"

STEAM DRYER ASSEMBLY

CROSS SECTION A-A


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The RCIR System provides forcedcirculation of reactor water through thecore, removing the heat produced bythe fuel. The reactor water is made upof water removed from the two-phasereactor coolant (core flow) in themoisture separators and steam dryersand the incoming feedwater flow. TheRCIR System uses an arrangement often reactor internal pumps (RIPs). Thepumps are mounted internally in thereactor vessel to provide the motiveforce for core flow. The RIPs functioncollectively to force the reactor coolantthrough the lower plenum of thereactor and upward through openingsin the fuel support castings, throughthe fuel bundles, steam separators, anddown the annulus to be mixed withfeedwater and recirculated through thecore. Figure 3-7 shows the RIPs andthe pumped flow path.

Recirculation flow rate is variable overa range from natural circulation flowof 20% to above the rated flowrequired to achieve rated core power.In fact, the RCIR design can producerated core flow rate at 100% reactorpower with nine of its ten pumpsoperating. The flow control range allowsautomatic regulation of reactor poweroutput between ~70 to 100% withoutcontrol rod movement. Core flow (RCIRpumping capacity) is regulated by theRecirculation Flow Control System(RFC). The RFC System providesconditioned control and logic signals,which regulate the RIP speed, which, inturn, regulates the pump flow. Becausethe core flow affects reactor power andfuel thermal margins, the RCIR Systemis also used to mitigate the effects oftransient, upset and emergency modesof reactor operation.

Three RCIR subsystems are used in conjunction withthe reactor internal pumps:

• Recirculation Motor Cooling Subsystem(RMC Subsystem).

• Recirculation Motor Purge Subsystem(RMP Subsystem).

• Recirculation Motor Inflatable Shaft SealSubsystem (RMISS).

Figure 3-7. Recirculation Flow Schematic


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Recirculation Motor CoolingSubsystem (RMC)

Each RIP has its own external heat exchanger (Figure3-8). Each RIP motor casing and the RIP heatexchanger is connected via stainless steel piping. Theheat exchanger is a typical shell-tube type with U-tubes supported by baffles. The hot water coming fromthe motor enters from the upper end of the heatexchanger shell side and leaves from the lower end ofthe shell side and returns back to the motor. Theconnecting piping is welded to the RIP motor casingand also to the heat exchanger shell to prevent anyleakage during the plant operation.

Recirculation Motor PurgeSubsystem (RMP)

The RMP Subsystem provides a source of cleancontrol rod drive (CRD) water that flows up theannulus between the stretch tube and the shaft andprevents the intrusion of reactor water with itsassociated contamination into the motor. The purgesystem normally operates continuously even when theRIPs are tripped or the reactor is shutdown for therefueling and/or maintenance.

Recirculation Motor Inflatable Shaft SealSubsystem (RMISS)

During normal operation, the purpose of this system isto prevent any leakage of reactor water (escaping fromthe primary seal) during plant outages and to assist inmaintenance or inspection of motors. During themaintenance of the RIPs, a portable pump is used topressurize the seal using water from the Makeup WaterSystem (MUW). The seal is made of elastomeric materialand seals tightly between the pumpshaft and the pumpmotor casing. The pump pressurizes the seal andmaintains it at shutoff head conditions.

Reactor Internal PumpsThe vessel-mounted RIPs simplify the RPV byeliminating all large nozzles below the core, significantlyreducing piping in-service inspection (ISI) and personnelexposure, and allowing for a compact containment designdue to elimination of the external recirculation systempiping. Use of the RIP feature allows for the

optimization of the Emergency CoreCooling System (ECCS) and assuresno core uncovery for postulated pipebreaks. One of the goals for the ABWRis to reduce calculated core damagefrequency by an order of magnituderelative to GE’s previously designedBWR operating plants. One of themost important design featurescontributing to this goal was theadoption of RIPs in place of externallypumped reactor recirculation system/pumps.

The internal pumps are an improvedversion of a European designed RIPthat is in operation in many Europeannuclear power plants. About 9 millionpump hours of successful operatingexperience has been accumulated, withsome pumps having been in servicesince the mid-1970’s.

The general design details of the RIP,motor, and heat exchanger are shownin Table 3-1:

Number of Pumps: 10

Type of Pump: Vertical shaft,

single stage,

mixed flow

Rated Flow: 7700 m3/hr/

pump

Rated Head: 40m

Rated Pump Speed: 1500 rpm

Overall Height

(Impeller & Motor): 3m

Overall Weight: 5000 kg

Motor Type: 3-Phase, Wet

Induction Motor

Rated Output Power: 830 kW

Rated Voltage: ~ 3300V

Heat Exchanger Type: Shell & Tubes

Hx Cooling Capacity: 1.15 kcal/hr

Table 3-1. Key RIP Parameters


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Reactor Internal PumpComponent Description

There are 10 RIPs arrangedcircumferentially between the shroudand the RPV near the RPV bottomhead. Figure 3-9 shows a cross-sectionof the RIP used in the ABWR, and keycomponents are described below.

Diffuser: The RIP has the impellerand diffuser inside the RPV. Thediffuser is installed in the pump deckand sealed by a piston ringarrangement. The RIP diffuser isremovable. The diffuser is retained onthe RPV nozzle by the stretch tube.

Stretch Tube: The stretch tube is essentially a longhollow bolt which passes through the RPV nozzlepenetration from the diffuser to the top of the RIPmotor casing where it is held in tension by a large nut.The stretch tube is preloaded by use of a stud tensionersimilar to that used for the main closure studs of theRPV. The pump shaft passes through the center of thestretch tube and motor rotor. The pump shaft key fitsin a slot in the motor rotor tube.

Impeller and Pump Shaft: The impeller and thepump shaft are connected by the impeller bolt.The pump shaft passes through the stretch tube, rotorand is connected to the thrust bearing disk at its lowerend. The motor rotor keyway slot fits with the key on thepump shaft and transmits motor torque.

Figure 3-8. RCIR Subsystems

PUMPMOTORCASING

RMCSUBSYSTEM

RBCW

REACTOR VESSEL

RMP SUBSYSTEM

RMISS SUBSYSTEM

ARD

PUMPDECK

SHROUD

HEATEXCHANGER

RIP MOTORCOOLING PIPING

RPV

RIPs

A

B

C

D

EF

G

H

J

K

MOTORCOVER


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Radial Bearings: The motor upper radial bearing isbelow the secondary seal. This bearing design has beentested and proven to eliminate bearing instability dueto half-speed rotation.

The lower radial bearing is located below the motorrotor and the stator. The lower radial bearing is similarto the upper radial bearing.

Thrust Bearing: The thrust bearingis of an offset tilting pad configuration.The rotating portion of the thrustbearing is integral with the coolingwater auxiliary impeller, whichcirculates water through the motor andbearing to provide cooling andcleaning via the RMP Subsystem.

Anti-Reverse Rotation Device:Below the cooling water auxiliaryimpeller is the Anti-Reverse RotationDevice (ARD). This is a cam clutcharrangement that prevents the RIPfrom rotating in the reverse directionwhen one RIP is tripped while theothers are running (which can resultin backflow through the tripped RIP).The purpose of the ARD is to preventreverse rotation of the pump shaft andminimize the backflow through an idle/tripped RIP.

Motor (Stator and Rotor): The RIPmotor is a 3-phase, 4-pole wetinduction motor. The cooling waterflows upward through the windings ofthe stator and the rotor. The motorstator is attached to the motor cover.

Terminal Box: The electrical terminalbox is bolted to the motor cover. Themotor winding cable penetrations passthrough the motor cover coolant pressureboundary and are connected to the powersupply leads at this location. Each motoris driven by its own variable frequencypower supply known as the AdjustableSpeed Drive (ASD).

Speed and Vibration Sensors: Two-pump speed sensors and two-motorcasing vibration sensors are on eachRIP motor casing. There is anadditional sensor on each RIP to detectrubbing of internal parts of the pump.

Figure 3-9. Cross-Section of RIP

RPV

IMPELLER

PURGE WATERINLET

SECONDARY SEALPRESSURIZATIONWATER INLET

COOLING WATEROUTLET

MOTOR CASING

PUMP SHAFT

MOTOR ROTOR

COUPLING STUD

THRUST DISKAUXILIARY IMPELLLER

TERMINALBOX

ARD

CABLECONNECTOR

MOTOR COVER

AUXILIARY COVERSPEED SENSOR

COOLINGWATERINLET

THRUSTBEARINGPADS

LOWER JOURNALBEARING

STATOR

UPPER JOURNALBEARING

SECONDARYSEAL

STRETCH TUBENUT

STRETCH TUBE

DIFFUSERWEAR RING

PISTON RING

DIFFUSER


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RIP Operation

Whenever the RIP motor is started, itis controlled to reach its minimumspeed. Similarly, one by one, the othernine RIPs are started and brought tothe minimum speed level. From thiscondition, the speed of all 10 RIPs canbe increased individually when in theindividual speed control mode, or as agroup when in the automatic gangedmode of operation, with the gangedmode being the normally preferredmode after all 10 RIPs have beenstarted. The RFC System controls thespeed of the RIPs, as described earlierin this section. A change in RIP speedconditions will vary the core flow inthe reactor, which, in turn, will changethe reactor power during normal powerrange operation.

The RFC operational modes alsoinclude the core flow control mode andthe automatic load-following mode.The core flow mode controls the speedof the RIPs selected for gang speedoperation to maintain the steady-statecore flow equal to the core flowdemand signal. For the automatic load-following mode, the RFC Systemcontrols the speed of those RIPsselected for gang speed operation toreduce the load demand error signal(from the turbine control system) to zero.

Individual RIP speed control operationmode and the ganged speed mode ofoperation provide significantflexibility during normal plantoperation. If, for any reason, one RIPdevelops a problem, then either speedcan be reduced to eliminate theproblem or that RIP can be tripped, ifnecessary, without affecting thecontinued operation of other RIPs.

During normal plant rated power operation (in eitherthe core flow control mode or the automatic load-following mode), if one RIP is lowered in speed ortripped, then the speed of the remaining nine RIPs isincreased by the control system to maintain thedemanded core flow; thus, steady-state plant outputpower remains unaffected.

RIP Power SupplyThe RIP motor is driven by a solid-state variable-frequency power supply known as the Adjustable SpeedDrive (ASD). The ASD is a proven product with wideindustrial applications as well as experience in theEuropean nuclear plants. The ABWR application uses~ 3000V for the output voltage rating. The ASD powersupply provides extremely low maintenance, highreliability, and excellent RIP speed maneuverability.

Each RIP is driven by its dedicated ASD. Six RIPASDs receive power from constant speed Motor-Generator (M-G) sets and the other four directly frommedium voltage buses. A representative simplifiedpower distribution one-line diagram is shown in Figure3-10. Each M-G set provides power to three associatedRIP ASDs. The other four RIP ASDs are divided intotwo sets receiving power directly from two separatemain buses.

The assignment of the power distribution to individualRIP ASDs is chosen to balance the azimuthal distributionwithin the vessel (e.g., when a M-G set trips or onemedium voltage bus is lost). The M-G sets have inertialflywheels to provide continued operation of theassociated RIPs during either the momentary or completeloss of incoming power. After complete loss of the mainbus power, continued operation of these RIPs for at least3 seconds is provided via the M-G sets.

Control Rod Drive SystemThe Control Rod Drive (CRD) System controlschanges in core reactivity during power operation bymovement and positioning of the neutron absorbingcontrol rods within the core in fine increments in


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response to control signals from the Rod Control andInformation System (RCIS). The CRD Systemprovides rapid control rod insertion in response tomanual or automatic signals from the ReactorProtection System (RPS). Figure 3-11 shows the basicsystem configuration and scope.

When scram is initiated by the RPS, the CRD Systeminserts the negative reactivity necessary to shut downthe reactor. Each control rod is normally controlled byan electric motor unit. When a scram signal is received,high-pressure water stored in nitrogen chargedaccumulators forces the control rods into the core. Thus,the hydraulic scram action is backed up by an electricallyenergized insertion of the control rods.

The CRD System consists of three major elements:

• Electro-hydraulic fine motion control rod drive(FMCRD) mechanisms.

• Hydraulic control unit (HCU)assemblies.

• Control Rod Drive HydraulicSystem (CRDHS).

The FMCRDs provide electric-motor-driven positioning for normal insertionand withdrawal of the control rods andhydraulic-powered rapid control rodinsertion for abnormal operatingconditions. Simultaneous with scram, theFMCRDs also provide electric-motor-driven run-in of control rods as a path torod insertion that is diverse from thehydraulic-powered. The hydraulic powerrequired for scram is provided by highpressure water stored in the individualHCUs. An HCU can scram twoFMCRDs. It also provides the flow pathfor purge water to the associated drivesduring normal operation. The CRDHSsupplies pressurized water for chargingthe HCU scram accumulators andpurging to the FMCRDs.

Fine Motion Control Rod Drives

The ABWR FMCRDs are distin-guished from the locking piston CRDs,which are in operation in all currentGE plants, in that the control bladesare moved electrically during normaloperation. This feature permits smallpower changes, improved startup time,and improved power maneuvering.The FMCRD, as with current drives,is inserted into the core hydraulicallyduring emergency shutdown. Becausethe FMCRD has the additionalelectrical motor, it drives the controlblade into the core even if the primaryhydraulic system fails to do so, thusproviding an additional level ofprotection against ATWS events. TheFMCRD design is an improved version

Figure 3-10. RIP Power Supply Diagram

MEDIUM VOLTAGEBUS NO. 1



MEDIUM VOLTAGEBUS NO.2

DIS-CONNECT

SWITCH

RECTIFIER

DC LINK

GTOINVERTER

OUTPUTTRANS-

FORMERRIPB

RIPE

RIPH

RIPA

RIPF

RIPD

RIPJ

RIPC

RIPG

RIPK

M

G

M

G


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of similar drives that have been inoperation in European BWRs since1972.

Figure 3-12 shows a cross-section ofthe FMCRD as used in the ABWR.The FMCRD consists of four majorsubassemblies: the drive, spool piece,brake and motor/synchros. The spoolpiece and motor may be removedwithout disturbing the drive, allowingmaintenance with low personnelexposure.

The drive consists of the outer tube,hollow piston, guide tube, buffer,labyrinth seal, ball check valve,spindle adaptor and splined spindleadaptor back seat.

The coupling is a bayonet-typeconfiguration which, when coupledwith the mating coupling on thecontrol rod blade, precludes separationof the blade and the hollow piston.

The hollow piston is a long hollowtube with a piston head at the lowerend. The hollow piston is driven intothe reactor during scram by thepressure differential that is producedby the high scram flow. The labyrinthseal, which is contained inside thebuffer, at the top end of the outer tuberestricts the flow from the drive to thereactor, thereby maximizing thepressure drop which enhances scramperformance. Additionally, it allowsthe purge flow during normaloperation to preclude entrance ofreactor water and associated crud intothe drive. The piston head containslatches that latch into notches in thedrive guide tube after scram. Thescram buffering action is provided byan assembly of belleville washers in

the buffer and is supplemented by hydraulic dampingas the buffer assembly parts come together.

The outer tube performs several functions, one ofwhich is to absorb the scram pressure, preventing itsapplication to the CRD housing, which is part of thereactor core pressure boundary (RCPB). The outer tubetop end is a bayonet connection similar to thatemployed on the hollow piston which couples with asimilar bayonet connection on the control rod guidetube, sandwiching the CRD housing end cap betweenthe two. The outer tube lower end is a flange whichbolts to the CRD housing flange. The bolts allow thedrive to remain in place when the motor and spoolpiece are removed. The combination of the positivecoupling of the control rod guide tube and the driveand the flange on the lower end of the outer tube forma positive means of preventing ejection of the FMCRD/control rod for any postulated housing break.Protection against the postulated failure of the housingto stub tube weld is provided by the same features,with the shootout load being transferred to the coreplate by the flange at the top end of the control rod

Figure 3-11. CRD System Schematic

SEPARA-TIONSIGNAL

POSITIONSIGNAL

CRD

CRD MOTOR

SCRAM ANDPURGE

WATER TOREACTOR

POSITIONSIGNAL

CRD

CRD MOTOR

SCRAM AND

WATER TOREACTOR

SCRAMVALVE

TESTCONNECTION

TO OTHERHCUS

TO OTHERHCUS

SCRAMACCUMULATOR

SCRAMVALVE

SCRAMPILOTVALVE

EXHAUST

TO ALLOTHERSCRAMPILOT

VAVLES

INSTRUMENTAIR SUPPLY

EXHAUST EXHAUST

AIR HEADER DUMPVALVES

CSPCONDENSATE/FEEDWATER

CHARGING LINE

PURGEWATERLINE

HYDRAULIC CONTROL UNIT (HCU)(TYPICAL)

EXHAUST

PURGE

N2SUPPLY

PURGEFLOW

CONTROLVALVES

DRIVEWATERFILTERS

FILTER

FILTER

SUCTIONFILTERS

FILTER

FILTER

CRDPUMPS


3-15

guide tube. These internal CRD blowout supportfeatures allow the elimination of the external supportstructure of beams, hanger rods, grids and support barsused to prevent rod ejection as in previous GE BWRproduct lines.

The latches are designed so that with only one beingengaged it is sufficient to hold the control rod in placeunder all loading, including the ejection load causedby a scram line break.

In normal operation, the hollow pistonrests on the ball nut and is raised andlowered by translation of the ball nutalong the ball screw resulting fromrotation of the ball spindle. The latchesare held in a retracted position by theball nut. During scram, the hollowpiston is lifted off the ball nut by thehydraulic pressure.

The spindle adaptor and splinedspindle adaptor back seat provide aback seat type anti-withdrawal latchfeature, which consists of a gear thatautomatically engages whenever thespool piece is lowered. This preventsthe ball spindle from rotating andwithdrawing the rod.

The spool piece contains a redundantpacking type shaft seal to precludeleakage from the drive. The spool piecealso contains radial and thrust bearingsfor the stub shaft that is also part of thespool piece. The stub shaft transmitstorque from the motor to the ball screwvia a slip coupling. The spool piece alsocontains the weighing platform.

The weighing device is a spring-loadedplatform with two magnets located onit. In normal service, weight of thehollow piston and control rod istransferred to the weighing device. If,during withdrawal, the weight of therod or hollow piston is removed fromthe device, then the device will moveupwards and trigger two external reedswitches. The two external reedswitches are called separation switchesand, if either is opened, withdrawalmotion is inhibited. There are twoseparation switch probes which aredirectly opposite each other. Eachprobe contains one switch.

Figure 3-12. Fine Motion Control Rod Drive Cross-Section

SPOOL PIECE

POSITION INDICATORPROBE (PIP)

FULL-IN MECHANISM

BAYONET COUPLING TYPEINTERNAL CRD BLOWOUTSUPPORT (TO CONTROL RODGUIDE TUBE BASE COUPLING)

BUFFER

SCRAM POSITIONSENSING MAGNET

BACK SEAT

SEAL HOUSING

MOTOR

SYNCHRO SIGNALGENERATOR

SEPARATION PROBE

SEPARATIONSENSING SPRING

SEPARATIONSENSING MAGNET

BALL CHECKVALVE

SCRAM LINEINLET

GUIDE TUBE

BALL SCREW

LABYRINTH SEAL

OUTER TUBE

BAYONET COUPLING TYPECRD SPUD(TO CONTROL RODSOCKET COUPLING)

BALL NUT

MIDDLE FLANGE

HOLLOW PISTON

FMCRD HOUSING

LEAK-OFF PIPING

BRAKE


3-16

The spool piece is bolted to the CRDhousing by bolts which pass throughthe outer tube flange. As mentionedabove, the outer tube is also bolted tothe CRD housing. The double boltingarrangement, combined with the backseat type lock feature discussed above,allows spool piece servicing withoutdisturbing the drive.

The motor bolts to the spool piecethrough a motor bracket. The motor isa stepping motor similar to that usedin robotic and precision positioningapplications. The use of the steppingmotor is the major change from theconfiguration used in Europe. TheEuropean drives used a gear motorand, because of this, had less precisepositioning capability than the ABWRFMCRD. In addition, the gear motorrequired increased maintenancecompared to the stepping motor. Thestepping motor design is based onmotors that have had successfulexperience in industrial applications.

There are two synchro-type positionindicators located below the steppingmotor. The synchros provide acontinuous readout of the rod positionduring normal operation and are drivenby gears from the motor shaft.

The brake is mounted between the motorand the synchros. The brake serves torestrain the rod against withdrawal in theunlikely event that the scram line breaks.The brake is redundant with the ballcheck valve in mitigating the scram linebreak. It should be noted at this pointthat the check valve on the FMCRD hasno function other than to mitigate thescram line break and to limit leakageduring drive replacement. Table 3-2provides some key FMCRD parameters.

The balance of the FMCRD System includes the scramposition probes which are mounted on the outside ofthe CRD housing. The scram probe provides a positionsignal at 10%, 40% and 60% insertion, as well ascontinuous full-in. The continuous full-in signalprevents the loss of position indication that wouldotherwise occur while the hollow piston is held by thescram latches at the top latched position.

The probes use reed switches similar to the LockingPiston Control Rod Drive (LPCRD), as do theseparation switch probes that are mounted on the sideof the spool piece. The separation probes andassociated circuits and equipment are consideredimportant to safety and are therefore categorized asClass 1E.

In addition to the FMCRD and probes, other items inthe system include the power supply to motor (alsoknown as the inverter controller), the HydraulicControl Unit (HCU), scram piping, wiring and theCRD pump and its associated equipment.

Power to the FMCRD is provided by a solid state,thyristor-driven power supply. The power supply isbased on proven products with successful experiencein industrial applications. The power supply is avariable voltage and frequency device which starts themotor at low speed, accelerates it to the normal runspeed and then slows it when approaching the specifiedposition. The power supply interfaces with the RodControl and Information System (RCIS). The power

FMCRD

Step Size 18.3 mm

MovementSpeed 30 mm/sec

Scramtime 60% 1.7 sec

Table 3-2. Key FMCRD Parameters


3-17

supply includes the capability to move the individualdrive, while the RCIS provides the logic and controlfor overall control rod motion.

The DC power for the brake, which is an energize-to-release model, is supplied by an inverter which isintegrated with the motor power supply cabinets.

Hydraulic Control Units

The HCU consists of a gas bottle and accumulatorwhich are mounted on a frame. The HCU also includesthe scram and scram pilot valves. In an ABWR, thereis one HCU for every two FMCRDs, rather than theone HCU per CRD as in past GE plants. The use ofthe paired arrangement allows savings in space andmaintenance without sacrificing reliability or safety.The two FMCRDs on a given HCU are widelyseparated in the core so that there is no additional lossof shutdown margin if an HCU fails.

Control Rod Drive Hydraulic System

The ABWR Control Rod Drive Hydraulic System(CRDHS) supplies clean, demineralized water, whichis regulated and distributed to provide charging of theHCU scram accumulators and purge water flow to theFMCRDs. The CRDHS is also the source ofpressurized water for purging the RIP and the ReactorWater Cleanup (RWCU) System pump.

The CRD pump is basically the same as that used inBWR/6 plants (i.e., a multi-stage centrifugal pump).The filtration system is basically also the same as thatused on BWR/6.

Main Steam SystemThe purpose of the Main Steam System (MS) is todirect steam flow from the RPV steam outlet nozzlesto the main turbine. A main steamline flow restrictoris provided in each steam outlet nozzle. It is designed tolimit the flow rate in the event of a postulated steamlinebreak. The system also incorporates provisions forrelief of over-pressure conditions in the RPV.

In the ABWR design, four 28-inchsteamlines transport steam from thesteam outlet nozzles on the RPVthrough Reinforced ConcreteContainment Vessel (RCCV)penetrations and then through thesteam tunnel to the turbine. Main steamisolation valves (MSIVs) are installedin each steamline inboard and outboardof the RCCV penetrations. Eighteensafety/relief valves (SRVs) areinstalled on horizontal steamlineheaders, and the discharge from eachSRV is routed through the associatedSRV discharge line to quencherslocated in the suppression pool. Of the18 SRVs, 8 provide the AutomaticDepressurization System (ADS)function during an accident condition.Figure 3-13 is a simplified pipingdiagram of the MS, and Figure 3-14illustrates the three-dimensionalconfiguration of the piping, MSIVsand SRVs.

The MS is composed of severalcomponents and subsystems inaddition to the above, which arenecessary for proper operation of thereactor under various operating,shutdown and accident conditions.Some of these subsystems include:main steam bypass/drain subsystem,SRV and ADS, reactor head ventsubsystem, and system instrumentation.

Main Steam Isolation ValvesTwo MSIVs are welded in a horizontalrun of each of the four main steampipes. The MSIVs are designed toisolate primary containment uponreceiving an automatic or manualclosure signal, thus limiting the lossof coolant and the release of radioactivematerials from the nuclear system.


3-18

Figure 3-13. Main Steam System

Each MSIV is a Y-pattern, globe valveand is powered by both pneumaticpressure and compressed spring force(Figure 3-15). The main disk assemblyis attached to the lower end of the stem.Normal steam flow tends to close thevalve and the pressure is over the disk.The bottom end of the valve stem or astem disk attached to the stem closesa small pressure balancing hole in themain disk assembly. When the hole isopen, it acts as an opening to relievedifferential pressure forces on the maindisk assembly. Valve stem travel issufficient to provide flow areas past thewide open main disk assembly greaterthan the seat port area. The main diskassembly travels approximately 90%of the valve stem travel to close themain seat port area; approximately the

last 10% of the valve stem travel closes the pilot seat.The air cylinder actuator can open the main diskassembly with a maximum differential pressure of 1.38MPaG (200 psig) across the isolation valve in adirection that tends to hold the valve close. The Y-pattern valve permits the inlet and outlet passages tobe streamlined; this minimizes pressure drop duringnormal steam flow and helps prevent debris buildupon the valve seat.

Attached to the upper end of the stem is an air cylinderthat opens and closes the valve and a hydraulic dashpotthat controls its speed. The speed is adjusted byhydraulic control valves in the hydraulic return linesbypassing the dashpot piston.

The valve is designed to close quickly when nitrogenor air is admitted to the upper piston compartment toisolate the MS in the event of a LOCA, or other eventsrequiring containment or system isolation to limit therelease of reactor coolant. The MSIVs can be test

TURBINEBUILDING

MAINCONDENSER

CONTAINMENT

MAINTURBINE

SEISMICGUIDE

FROMSTEAMLINESB, C & D


DRAIN LINE

DRAIN LINE


INBOARD MSIV

DRAIN LINE

WETWELL

QUENCHER

SUPPRESSION POOL

VACUUMBREAKER(TYPICAL)

SRV(TYPICAL)

MAIN STEAMLINE ARPV

RADWASTESUMP

OUTBOARDMSIV

AV

AV AV


3-19

Figure 3-14. Main Steam System Schematic

Main steam

SRV

Feedwater


3-20

closed one at a time at a slow closingspeed by admitting nitrogen or air toboth the upper and lower pistoncompartments. This is to ensure thatthe slow valve closure does notproduce a transient disturbance largeenough to cause a reactor scram.

When all the MSIVs are closed, thecombined leakage through the MSIVsfor all four steamlines is monitored towithin the offsite radiation dose releaselimit.

Nitrogen is used for the inboard MSIVoperation because of the inerteddrywell environment where theinboard MSIVs are located.Instrument air is used for the outboardMSIV operation.

A separate pneumatic accumulator isprovided and located close to eachMSIV and supplies pressure as backupoperating gas to assist in valve closurein the event of a failure of pneumaticsupply pressure to the valve actuator.

Safety/Relief Valves

The SRV is a dual function, direct-acting valve and is classified as safety-related (Figure 3-16). The SRV isconsidered as part of the RCPBbecause the inlet side of the valve isconnected to the steamline prior to theinboard MSIV. The SRV logic andsolenoids are also classified andqualified as Class 1E per the IEEEStandards. This classification is alsoapplied to the ADS function and otherassociated systems.

Figure 3-15. Main Steam Isolation Valve

The SRV capacity is sized to maintain primary systempressure below the ASME Code design limits. Figure3-17 illustrates the SRV and MSIV configuration.

The SRVs are located on the main steamlines betweenthe RPV and the inboard MSIV. These valves providethree main protection functions:

• Overpressure Safety Operation: The valvesfunction as spring-loaded safety valves and open toprevent RCPB overpressurization. The valves are self-actuated by inlet steam pressure.

• Overpressure Relief Operation: The valves areopened using a pneumatic actuator upon receiptof an automatic or manually initiated signal at thesolenoid valve located on the pneumatic actuatorassembly. This action pulls the lifting mechanismof the main disk, thereby opening the valve to allowinlet steam to discharge through the SRV. The SRV

PNEUMATICCONTROL

PANEL

HYDRAULICDASHPOT

PNEUMATICCYLINDER

EXTERNAL SPRINGS

COVER

VALVE BODY DRAIN

DISK ANDPISTON

ASSEMBLY

LIMIT SWITCH

YOKE ROD

STEM

POPPETPILOT POPPET

PILOTPOPPETSPRING

ACCUMULATOR

AIR SUPPLY

FLOW


3-21

pneumatic operator is so arranged that, if itmalfunctions, it does not prevent the SRV fromopening when steam inlet pressure reaches thespring lift setpoint.

Eight of the 18 SRVs are designatedfor the ADS function, and are equippedwith three separate solenoid valvespowered by 125 VDC. The other non-ADS SRVs are each equipped with onesolenoid valve powered by 125 VDC.

The SRVs are divided into five setpointgroups to relieve the RPV pressure inaccordance with the RPV overpressureprotection evaluation.

A separate pneumatic accumulator isprovided for each SRV function and islocated close to each SRV to supplypressure for the purpose of valveactuation. SRVs that are designated forADS function are provided with oneadditional accumulator for each valve.

The SRVs can be operated individuallyin the power-actuated mode by remotemanual switches located in the maincontrol room. They are provided withposition sensors which providepositive indication of SRV disk/stemposition.

Each SRV has its own discharge linewith two vacuum breakers. The SRVdischarge lines are sized so that thecritical flow conditions occur throughthe valve. This prevents the conditionsin the discharge lines of waterhammerand pressure instability. The SRVdischarge lines terminate at thequenchers located below the surface ofthe suppression pool (SP).

Figure 3-16. Safety/Relief Valve

��

yyyyyyyyyyyyyy

LIFTINGMECHANISM

SPINDLE

BONNET

PACKINGGLAND

VENT

DISCHARGE

ADJUSTINGRING

DISC

NOZZLEINLET

LEVER

SOLENOID ANDAIR CONTROLVALVE ASSEMBLY

PISTONLINER

PISTON

BODY

SET POINT SPRING(BELLEVILLE WASHERS)(COMPRESSION)

PISTON-TYPEPNEUMATICACTUATOR

• Automatic Depressurization System (ADS)Operation: The ADS valves open automaticallyor manually in the power actuated mode whenrequired during a LOCA. The ADS designatedSRVs open automatically as part of the EmergencyCore Cooling System (ECCS) as required tomitigate a LOCA when it becomes necessary toreduce RCPB pressure to admit low pressureECCS coolant flow to the reactor.


3-22

Figure 3-17. MSIV and SRV Configuration

Figure 3-18. Feedwater System (Nuclear Island)

Feedwater System(Nuclear Island)Two 22-inch feedwater lines transportfeedwater from the feedwater pipes inthe steam tunnel through RCCVpenetrations to horizontal headers inthe upper drywell which have three12-inch riser lines that connect tonozzles on the RPV (Figure 3-18).Isolation check valves are installedupstream and downstream of theRCCV penetrations, and manualmaintenance gate valve are installedin the 22-inch lines upstream of thehorizontal headers. Also shown in thefigure are the interconnections fromthe RCIC, RHR and RWCU Systems.

MAIN STEAMLINES

OUTBOARD MSIV

INBOARD MSIV

DRYWELL CONTAINMENTWALL

MSL

MSL

MSL

MSL

7C

6C *

5C

4C*

3C

6D

5D*

4D

3D*

6A

5A*

4A

3A*

7B

6B *

5B

4B*

3B

ADS DESIGNATED SRV*

RPVMS RPV MS

C D BA

RPV STEAM OUTLETNOZZLE/RESTRICTOR(TYPICAL)

REACTORBUILDING

CONTROLBUILDING

TURBINEBUILDING

MAIN TURBINE

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

SRV

REACTORPRESSURE

VESSEL

REACTORPRESSURE

VESSEL

CONTAINMENTWALL

FROMRWCU

FROMCRD

FROMRCIC

FROMRHR-A

TURBINE BUILDING

REACTOR BUILDING

CONTROL BUILDINGSEISMIC GUIDE

FROM FW-TI

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