CANDU® 6 Technical Summary

Prepared by: CANDU 6 Program TeamReactor Development Business UnitJune 2005

iii

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1CANDU Nuclear Generating Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Station Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Nuclear Steam Supply System – Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Reactor Assembly and Fuel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Fuel Handling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Moderator and Auxiliary Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Heat Transport System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Heat Transport Auxiliary Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Reactor Regulating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Computer Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Reactivity Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Feedwater and Main Steam System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

CANDU 6 Nuclear Power Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Turbine Generator System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Electric Power System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Electric Power System Station Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Station Instrumentation and Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Station Instrumentation and Control/Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Station Instrumentation and Control/Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Site and Plant Arrangement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Safety SystemsShutdown Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Emergency Core Cooling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48Containment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Heavy Water Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Fuel Cycle Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

The systems described in this brochure are those of a typical CANDU 6 generating station;however, they are essentially the same as those of other CANDU stations in most respects.

Contents

1

CANDU nuclear power stations have consistently proven to be competitive with other types ofnuclear power plants, while offering unique advantages to their operators. Some of the design features and unique characteristics of the CANDU reactor are:

• a reactor core comprising several hundred small diameter fuel channels rather than one huge pressure vessel

• heavy water (D2O) for moderator and coolant

• separate low pressure moderator and high pressure fuel cooling systems

• on-power refuelling

• reactivity devices that are located in the cool low pressure moderator, and not subjected to hightemperatures or pressures

• natural uranium fuel or other low fissile content fuel

• reduced consequences from accidental reactivity fluctuations — excess reactivity available fromthe fuel is small and the relatively long lifetime of prompt neutrons in the reactor precludes rapidchanges in power levels

• two fully capable safety shutdown systems, independent from each other and the reactor regulating system.

This Technical Summary provides an overview of the CANDU 6 nuclear power system. AllCANDU 6 power plants are fundamentally the same, although there are differences in detail: theselargely result from different site conditions, and from improvements made in the newer designs.

The evolution of CANDU 6 is illustrated in the inside back cover.

Introduction

2

Major Systems

Pictorial symbols representing major systems are used throughout this publication to indicaterelationships between major systems and theirsub-systems.

Systems that are not featured on this page are described later in the document.

CANDU Nuclear Generating Station

Nuclear SteamSupply System

NuclearFuel

Powerto Grid

TurbineGenerator System

Station Instrumentationand Control

ElectricPower System

3

Steam

SteamGenerator

Pump/MotorAssembly

To FuelChannels

From FuelChannels

Calandria

Moderator

Fuel

Fuelling Machine

Feedwater

FeedwaterPumpAssembly

H

FuelChannels

T

Nuclear Steam Supply System

A CANDU 6 nuclear steam supply system’spower production process starts like that ofany other nuclear steam supply system, withcontrolled fission in the reactor core. However,unlike other reactors, the CANDU 6 is fuelledwith natural uranium fuel that is distributedamong 380 fuel channels. Each six-meter-longfuel channel contains 12 fuel bundles. The fuelchannels are housed in a horizontal cylindricaltank (called a calandria) that contains cool heavy water (D2O) moderator at low pressure. Fuelling machines connect to each fuel channelas necessary to provide on-power refuelling; this eliminates the need for refuelling outages.

The on-power refuelling system can also be used to remove a defective fuel bundle in theunlikely event that a fuel defect develops.CANDU 6 reactors have systems to identify and locate defective fuel.

Pressurized heavy water (D2O) coolant is circulated through the fuel channels and steamgenerators in a closed circuit. The fission heatproduced in the fuel is transferred to heavywater coolant flowing through the fuel channels.The coolant carries the heat to steam generators,where it is transferred to light water to producesteam. The steam is used to drive the turbinegenerator to produce electricity.

Station Flow Diagram

4

HighPressureTurbine

Low Pressure Turbines

Generator

SwitchyardCoolingWater

Power to Grid

Condenser

F

Turbine

Light Water Steam

Light Water Liquid

Generator

Pump/Motor Assembly

Fuelling Machine

Heavy Water Moderator

Fuel Bundle

Heat Transport System Heavy Water; Inlet

Heat Transport System Heavy Water; Outlet

5

Turbine Generator System

The turbine generator system comprises steamturbines directly coupled to an alternating current electrical generator operating at synchronous speed.

• The steam turbine is a tandem compound unit,generally consisting of a double flow, highpressure turbine and three double flow, lowpressure turbines, which exhaust to a high vacuum condenser for maximum thermal efficiency. The condenser may be cooled bysea, lake or river water, or by atmosphericcooling towers.

• The generator is a high efficiency hydrogen-cooled machine arranged to supply alternatingcurrent at medium voltage to the electricpower system.

Electric Power System

The electric power system comprises a mainpower output transformer, unit and servicetransformers, and a switchyard. This systemsteps up (increases) the generator output voltageto match the electric utility’s grid requirementsfor transmission to the load centres and alsosupplies the power needed to operate all of thestation services.

The main switchyard portion of the electricpower system permits switching of outputsbetween transmission lines and comprisesautomatic switching mechanisms, and lightningand grounding protection to shield the equipment against electrical surges and faults.

Reactor

The reactor comprises a stainless steel horizontalcylinder (called the calandria), closed at eachend by end shields, which support the horizontalfuel channels that span the calandria, and pro-vide personnel shielding. The calandria ishoused in and supported by a light water-filled,steel lined concrete structure (the reactor vault)which provides thermal shielding. The calandriacontains heavy water (D2O) moderator at lowtemperature and pressure, reactivity controlmechanisms and several hundred fuel channels.

Fuel Handling System

The fuel handling system refuels the reactorwith new fuel bundles without interruption ofnormal reactor operation; it is designed to oper-ate at all reactor power levels. The system alsoprovides for the secure handling and temporarystorage of new and irradiated fuel.

Heat Transport System

The heat transport system circulates pressurizedheavy water coolant (D2O) through the reactorfuel channels to remove heat produced by fissionin the uranium fuel. The heat is carried by thereactor coolant to the steam generators, where it is transferred to light water to produce steam.The coolant leaving the steam generators isreturned to the inlet of the fuel channels.

Moderator System

Neutrons produced by nuclear fission are moder-ated (slowed) by the D2O in the calandria. Themoderator D2O is circulated through systemsthat cool and purify it, and control the concen-trations of soluble neutron absorbers used foradjusting the reactivity.

Feedwater and Steam Generator System

The steam generators transfer heat from theheavy water reactor coolant to light water (H2O)to form steam, which drives the turbine genera-tor. The low pressure steam exhausted by thelow pressure turbine is condensed in the cond-ensers by a flow of condenser cooling water. The feedwater system processes condensedsteam from the condensers and returns it to the steam generators via pumps and a series of heaters.

Reactor Regulating System

This system controls reactor power within specific limits and ensures that station loaddemands are met. It also monitors and controlspower distribution within the reactor core, tooptimize fuel bundle and fuel channel powerwithin their design specifications.

Safety Systems

Four special safety systems (shutdown systemnumber 1, shutdown system number 2, the emer-gency core cooling system and containment sys-tem) are provided to minimize and mitigate theimpact of any postulated failure in the principalnuclear steam plant systems. Safety support systems provide services as required (electricpower, cooling water and compressed air) to the special safety systems. (See Safety Systemson page 46.)

6

Nuclear Steam Supply System – Overview

7

Heavy Water Oulet

Heavy Water Inlet

Steam Generator

Fuel BundleFuel Bundle

FuelHandlingSystem

Heat Transport System

Spent Fuel Bay

Steam to Turbine

Reactor

Heat Transport System

Feedwater

Reactivity Control Devices

Reactor Assembly

The CANDU 6 reactor assembly, shown in the figure opposite, includes the fuel channelscontained in and supported by a horizontalcylindrical tank known as the calandria. Thecalandria is closed and supported by end shieldsat each end. Each end shield comprises an innerand an outer tubesheet joined by lattice tubes ateach fuel channel location and a peripheral shell.The inner space of the end shields are filledwith steel balls and water, and are watercooled. The fuel channels, supported by theend shields, are located on a square latticepitch. The calandria is filled with heavy watermoderator at low temperature and pressure.The calandria is located in a steel lined, waterfilled concrete vault.

Horizontal and vertical reactivity measurementand control devices are located between rows and columns of fuel channels, and areperpendicular to the fuel channels.

The fuel channels are also shown in the figureopposite, with additional detail provided in theaccompanying figure. Each fuel channel locatesand supports 12 fuel bundles in the reactor core.The fuel channel assembly includes a zirconium-niobium alloy pressure tube, a zirconium calandriatube, stainless steel endfittings at each end, andfour spacers which maintain separation of thepressure tube and the calandria tube. Eachpressure tube is thermally insulated from thecool, low pressure moderator, by the CO2 filledgas annulus formed between the pressure tubeand the concentric calandria tube.

Each end fitting incorporates a feeder connectionthrough which heavy water coolant enters/leaves the fuel channel. Pressurized heavy watercoolant flows around and through the fuel bundles in the fuel channel and removes the heat generated in the fuel by nuclear fission.Coolant flow through adjacent channels in thereactor is in opposite directions.

During on-power refuelling, the fuellingmachines gain access to the fuel channel byremoving the closure plug and shield plug fromboth end fittings of the channel to be refuelled.

Fuel

The CANDU 6 fuel bundle consists of 37 elements, arranged in circular rings as shown inthe photo opposite. Each element consists ofnatural uranium in the form of cylindrical pelletsof sintered uranium dioxide contained in azircaloy 4 sheath closed at each end by an endcap. The 37 elements are held together by endplates at each end to form the fuel bundle. Therequired separation of the fuel elements ismaintained by spacers brazed to the fuel elementsat the transverse mid-plane. The outer fuel elements have bearing pads brazed to the outer surface to support the fuel bundle in thepressure tube.

8

Reactor Assembly and Fuel

Reactor face (during construction)

9

Fuel Channel

End Shield

Tubesheet

Spacers Gas Annulus

Fuel Bundles

End Fitting

Shield Plug Pressure Tube

Heavy Water Moderator

Feeder Pipe Calandria Tube

Channel Closure

Positioning Assembly

Liner Tube

Lattice Tube

Calandria

Pressure TubeCalandria Tube

Fuel Bundle Annulus SpacerAnnulus Gas Heavy Water CoolantHeavy Water Moderator

Calandria assembly schematic

Fuel channel arrangement

37 Element Fuel Bundle

The fuel handling system:

• provides facilities for the storage and handling of new fuel

• refuels the reactor remotely while it is operating at any level of power

• transfers the irradiated fuel remotely from the reactor to the storage bay.

Fuel Changing

The fuel changing operation is based on thecombined use of two remotely controlledfuelling machines, one operating on each end ofa fuel channel. New fuel bundles, from onefuelling machine, are inserted into a fuel channelin the same direction as the coolant flow and thedisplaced irradiated fuel bundles are receivedinto the second fuelling machine at the other end of the fuel channel. Typically, either four oreight of the 12 fuel bundles in a fuel channel arereplaced during a refuelling operation. For aCANDU 6 reactor, about 10 fuel channels perweek are refuelled.

Either machine can load or receive fuel. Thedirection of loading depends upon the directionof coolant flow in the fuel channel being fuelled,which alternates from channel to channel.

The fuelling machines receive new fuel while connected to the new fuel port and discharge irradiated fuel while connected to the discharge port.

The entire operation is directed from the controlroom through a pre-programmed computerizedsystem. The control system provides a printedlog of all operations and permits manual intervention by the operator, if required.

Fuel Transfer

New fuel is received in the new fuel storageroom in the service building. This room accom-modates six months’ fuel inventory and can storetemporarily all the fuel required for the initialfuel loading.

When required, the fuel bundles are transferredto the new fuel transfer room in the reactorbuilding. The fuel bundles are identified andloaded manually into the magazines of the two new fuel ports. Transfer of the new fuelbundles into the fuelling machines is remotelycontrolled.

Irradiated fuel received in the discharge portfrom the fuelling machine is transferred into anelevator which lowers it into a water filled dis-charge bay. The irradiated fuel is then conveyedunder water through a transfer canal into areception bay, where it is loaded onto storagetrays or baskets and passed into the storage bay.

10

Fuel Handling System

On-Power Refuelling

Calandria

Fuelling Machine

Fuelling Machine

Fuelling machine in operating position at face of reactor.

11

The discharge and transfer operations areremotely controlled by station staff. Operationsin the storage bays are carried out under water,using special tools aided by cranes and hoists.Defective fuel is inserted into cans under waterto limit the spread of contamination beforetransfer to the defective fuel bay.

The storage volume of the bays has sufficientcapacity for a minimum of 7 years’ accumulationof irradiated fuel.

Neither new or nor irradiated CANDU fuel can achieve criticality in air or light water,regardless of the storage configuration.

Calandria

New Fuel Storage

New Fuel

Discharge Bay

Transfer Canal Reception

Bay

Canned Defective Fuel

Irradiated Fuel Storage Bay

Irradiated Fuel

Fuel Channels

Reactor Building

Fuelling Machine

Moderator System

About four per cent of reactor thermal powerappears in the moderator. The largest portion ofthis heat is from gamma radiation; additionalheat is generated by moderation (slowing down)of the fast neutrons produced by fission in thefuel, and a small amount of heat is transferred to the moderator from the hot pressure tubes.

The system includes two 100 per cent capacitypumps, two 50 per cent flow capacity heatexchangers cooled by recirculated cooling water and a number of control and check valves.Connections are provided for the purification,liquid poison addition, heavy water (D2O) collection, supply and sampling systems.

The moderator pump motors are connected tothe medium voltage Class III power supply. Inaddition, each pump has a pony motor, capableof driving the pump at 25 per cent speed, con-nected to the Class II power supply. In the eventof a loss of Class IV power (see page 34), thepower to the main motors is lost until the dieselgenerators can supply Class III power. The cool-ing water supply to the heat exchangers is alsore-established after three minutes at a reducedflow rate following a total failure of Class IVpower. The rate of heat removal is sufficient tolimit the increase of moderator temperature inthe calandria to an acceptable value during afailure of Class IV power and subsequent reactor shutdown.

The heavy water in the calandria functions as a heat sink in the unlikely event of a loss ofcoolant accident in the heat transport systemcoincident with a failure of emergency core cooling.

Cover Gas System

Helium is used as the cover gas for the modera-tor system because it is chemically inert and isnot activated by neutron irradiation. Radiolysisof the heavy water moderator in the calandriaresults in production of deuterium and oxygengases. The cover gas system prevents accumula-tion of these gases by catalytically recombiningthem to form heavy water. The deuterium andoxygen concentrations are maintained well

below levels at which an explosion hazard would exist.

The system includes two compressors and tworecombination units which form a circuit for thecirculation of cover gas through the calandriarelief ducts. Normally one compressor and bothrecombination units operate, with the other compressor on standby.

Purification System

The moderator purification system:

• maintains the purity of D2O, thereby minimiz-ing radiolysis which can cause excessive pro-duction of deuterium in the cover gas.

• minimizes corrosion of components, byremoving impurities present in the D2O andby controlling the pD.

• under operator command reduces the concen-tration of the soluble poisons, boron andgadolinium, in response to reactivity demands.

• removes the soluble poison, gadolinium, after shutdown system number 2 has operated.

D2O Sampling System

This system allows samples to be taken from the:

• main moderator system

• moderator D2O collection system

• moderator purification system

• D2O cleanup system.

Laboratory tests may be performed on the samples to determine:

• pD (pH)

• conductivity

• chlorides concentration

• isotopic purity

• boron and gadolinium concentration

• tritium content

• fluorides content

• organics content.

12

Moderator and Auxiliary Systems

Operation of the Moderator System

The series/parallel arrangement of the systemlines and valves permits the output from eitherpump to be cooled by both of the heat exchang-ers and assures an acceptable level of moderatorcooling when either of the two pumps is isolatedfor maintenance. Reactor power must be reducedto about 60 per cent if one moderator heatexchanger is isolated.

The primary functions of the system are to:

• provide moderator cooling

• control the level of heavy water in the calandria

• maintain the calandria inlet temperature at approximately 70°C.

Purification System

D2O CollectionModeratorCirculationPump

D2O Sampling

Cover Gas System

Recombination Units

Compressors

HeatExchanger

HeatExchanger

Head Tank

RCW

Reactor

Valve Normally Open

Check Valve

To D2O CleanupSupply

From D2O

Moderator

Ion Exchange Columns Filter

14

System Operation

The heat transport system (HTS) circulates pres-surized D2O coolant through the fuel channels to remove the heat produced by fission in thenuclear fuel. The coolant transports the heat tosteam generators, where it is transferred to lightwater to produce steam to drive the turbine. Two parallel HTS coolant loops are provided in CANDU 6. The heat from half of the 380 fuelchannels in the reactor core is removed by eachloop. Each loop has one inlet and one outletheader at each end of the reactor core. D2Ois fed to each of the fuel channels throughindividual feeder pipes

from the inlet headers and is returned from eachchannel through individual feeder pipes to theoutlet headers. Each heat transport system loopis arranged in a ‘Figure of 8’, with the coolantmaking two passes, in opposite directions,through the core during each complete circuit,and the pumps in each loop operating inseries. The coolant flow in adjacent fuel channels is in opposite directions. The HTSpiping is fabricated from corrosion resistantcarbon steel.

The pressure in the heat transport system iscontrolled by a pressurizer connected to the outletheaders at one end of the reactor. Valves provideisolation between the two loops and the pressurizerin the event of a loss-of-coolant accident.

Key Features

• The steam generators consist of an inverted U-tube bundle within a cylindrical shell. Heavywater coolant passes through the U-tubes. Thesteam generators include an integral preheateron the secondary side of the U-tube outlet sec-tion, and integral steam separating equipmentin the steam drum above the U-tube bundle.

• The heat transport pumps are vertical, cen-trifugal motor driven pumps with a single suction and double discharge.

• Cooling of the reactor fuel, in the event ofelectrical power supply interruption, is main-tained by the rotational momentum of the heattransport pumps during reactor power run-down, and by natural convection flow after the pumps have stopped.

• No chemicals are added to the heat transportsystem for the purpose of reactivity control.

• Carbon steel piping, which is ductile and rela-tively easy to fabricate and to inspect is usedin the heat transport system.

• Radiation exposure to personnel is lowbecause of the low fuel defect rate, and is minimized by designing for maintenance,application of stringent material specifications,controlling the reactor coolant chemistry andby providing radiation shielding.

Heat Transport System

Manway

Secondary Cyclone Separators

Primary Cyclone Separators

U-Bend

Tube Bundle

Shroud Cone

Steam Outlet Nozzle

Shroud

Grid Tube Support Plate

Tube Bundle Cold leg

Perheater Section

Feedwater Inlet Nozzle

Tubesheet

Tube Bundle Hot Leg

D O InletD O Outlet 22

Steam Generator

15

Page 15

Pressurizer

Steam Generator

Steam Generator

HTS Loop 1

Calandria

HTS Loop 2

Inlet Header

Outlet Header

Inlet Header

Outlet Header

Heat Transport System Pump Assembly

Fuel Channel

Heat Transport System Pump Assembly

Heavy water, Steam

Heat Transport System Heavy Water: Outlet

Heat Transport System Heavy Water: Inlet

680/633

675/638

678/638

648/600

710/665

715/668

728/668

542/515

904/840

540/516

915/860

936/881

ElectricalOutput(MW)

Gross/Net

Numberof

FuelChannels

Elementsin FuelBundle

Numberof

Loops

Outletheader

Pressure(MPa)

MaximumChannel

Flow(kg/s)

OutletHeaderQuality

(%)

Total Operating MotorRating(kW)

Area (m2)per

SteamGenerator

IntegralPreheater

SteamPressure

(MPa)

Heat Transport System Conditions Heat Transport Pumps Steam Generators

CANDU 6 Operating stations or under construction

380

380

380

380

380

380

380

390

480

390

480

480

37

37

37

37

37

37

37

28

37

28

37

37

2

2

2

2

2

2

2

2

1

2

1

2

10.0

10.0

10.0

10.0

10.0

10.0

10.0

8.7

9.1

8.7

9.1

10.0

24

24

24

24

24

24

24

23

24

23

24

25

4

4

4

4

4

4

4

-

<1

-

<1

2

4

4

4

4

4

4

4

16

4

16

4

4

4

4

4

4

4

4

4

12

4

12

4

4

6700

6700

6700

6700

6700

6700

6700

1420

8200

1420

8200

9600

3200

3200

3200

2800

3200

3200

3200

1850

2400

1850

2400

4900

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

Yes

No

Yes

4.7

4.7

4.7

4.7

4.7

4.7

4.7

4.1

4.4

4.1

4.7

5.1

Point Lepreau,

Gentilly 2

Wolsong 1

Embalse

Cernavoda 1, 2

Wolsong 2, 3, 4

Qinshan 1, 2

Other CANDU operating stations

Pickering A 4 Units

Bruce A 4 Units

Pickering B 4 Units

Bruce B 4 Units

Darlington 4 Units

A comparison ofprincipal CANDU

Heat TransportSystem Parameters

Pressure and Inventory Control System

The heat transport pressure and inventory con-trol system consists of a pressurizer, D2O feed-pumps, feed and bleed valves and a D2O storagetank. This system provides:

• pressure and inventory control for each heat transport system loop

• overpressure protection

• a controlled degassing flow.

Heavy water in the pressurizer is heated electri-cally to pressurize the vapour space above theliquid. The volume of the vapour space isdesigned to cushion pressure transients, withoutallowing excessively high or low pressures in the heat transport system.

The pressurizer also accommodates the changein volume of the reactor coolant in the heattransport system from zero power to full power.This permits the reactor power to be increased or decreased rapidly, without imposing a severedemand on the D2O feed and bleed componentsof the system.

When the reactor is at power, pressure iscontrolled by the pressurizer; heat is added to thepressurizer via the electric heaters to increasepressure, and heat is removed from the pressur-izer via D2O steam bleed to reduce pressure.The coolant inventory is adjusted by the feedand bleed circuit. Pressure can also be controlledby the feed and bleed circuit with the pressurizerisolated at low reactor power and when thereactor is shut down. This feed and bleed circuit isdesigned to accommodate the changes in coolantvolume that take place during heat-up and cool-down.

D2O Collection System

• collects leakage from mechanical components

• receives D2O sampling flow

• receives D2O drained from equipment prior tomaintenance.

The collected D2O is pumped from the collec-tion tank to the storage tanks of the pressure andinventory control system for re-use in the heat

transport system. However, if the isotopic purityof the collection tank contents is low, the D2Ocan be pumped to drums for upgrading.

Shutdown Cooling System

The shutdown cooling system is capable of:

• cooling the heat transport system from 177°Cdown to 54°C, and holding the system at thattemperature indefinitely

• providing core cooling during maintenancework on the steam generators and heat trans-port pumps when the heat transport system is drained down to the level of the headers

• of being put into operation with the heat transport system at full temperature and pressure.

The shutdown cooling system consists of twoindependent circuits, one located at each end ofthe reactor. Each circuit consists of a pump anda heat exchanger, connected between the inletand outlet headers of both heat transport systemloops. The system is normally full of D2O andisolated from the heat transport system by poweroperated valves.

The shutdown cooling pumps are sized so thatno boiling can occur in any of the fuel channels.For normal cool-down, steam from the steamgenerators bypasses the turbine and flows intothe turbine condenser to reduce the heat transport system temperature to 177°C inapproximately 30 minutes.

For cool-down from 177°C to 77°C, the isolat-ing valves at the reactor headers are opened andall heat transport pumps are kept running. Theheat transport pumps force a portion of the totalcore flow through the shutdown cooling heatexchangers, where it is cooled by recirculatedcooling water flowing around the heat exchanger coils.

At 77°C, the heat transport pumps are shut downand the shutdown cooling system pumps arestarted. The system is then cooled to 54°C. D2Ocan be drained down to the level just above thereactor headers, if required for maintenance ofthe steam generators or pumps.

16

Heat Transport Auxiliary Systems

Purification SystemThe heat transport purification system:

• limits the accumulation of corrosion productsin the coolant by removing soluble and insoluble impurities

• removes accumulations of fine solids follow-ing their sudden release due to chemical,hydraulic or temperature transients

• maintains the pD(pH) of D2O at the required value.

Flow is taken from one reactor inlet header ofeach heat transport loop, passed through an

interchanger, cooler, filter and ion exchange column before being returned through the inter-changer to a pump inlet in each circuit. The heattransport pump provides the flow through thepurification system. The interchanger-coolercombination minimizes the heat loss in the D2O purification cycle.

Isolating valves in the purification system inletand outlet lines are provided for maintenance.The valves also allow draining of the heat trans-port system coolant to just above the elevation ofthe headers without the need to drain the purifi-cation system. These valves close automaticallyin the event of loss-of-coolant accident.

Steam Generator

Outlet Header

Inlet Header

Inlet Header

Outlet Header

Bleed Valve

Feed Valve

Shutdown Cooling

Shutdown CoolingValve Normally Open

Valve Normally Closed

Pump

Pressure and Inventory Control

D2O Purification

D2O Collection

To D2OManagement

Heat Transport System Pump

Assembly

Fuel Channel

Storage

Heat Exchanger

Tank

Pres

suri

zer

Deg

asse

rCo

nden

ser

FilterInterchanger

Ion

Exch

ange

Colu

mns

The fundamental design requirement of the reactor regulating system (RRS) is to control the reactor power at a specified level and, whenrequired, to manoeuvre the reactor power levelbetween set limits at specific rates.

The reactor regulating system combines thereactor’s neutron flux and thermal power mea-surements, reactivity control devices, and a setof computer programs to perform three mainfunctions:

• monitor and control total reactor power to satisfy station load demands

• monitor and control reactor flux shape

• monitor important plant parameters and reducereactor power at an appropriate rate if anyparameter is outside specific limits.

Control

Reactor Regulating System action is controlledby digital computer programs which process theinputs from various sensing devices and activatethe appropriate reactivity control devices.

All measurement and control devices are locatedperpendicular to and between rows or columnsof fuel channels, in the low pressure moderator.

Computer Programs

The principal computer programs employed provide the following:

• reactor power measurement and calibration

• the demand power routine

• reactivity control and flux shaping

• setback routine

• stepback routine

• flux mapping routine.

Instrumentation

The principal instrumentation utilized for reactor regulation includes:

• ion chamber system

• self-powered, in-core, flux detector system

• thermal power instrumentation.

The nuclear instrumentation systems aredesigned to measure reactor neutron flux, over the full operating range of the reactor.These measurements are required as inputs tothe reactor regulating system and the safety sys-tems. The instrumentation for the safety systemsis independent of that utilized by the reactor regulating system.

Reactivity Control Devices

Short-term global and spatial reactivity control is provided by:

• light water zone control absorbers

• mechanical control absorbers

• adjusters

• soluble poison* addition and removal to the moderator.

The zone control system operates to maintain a specified amount of reactivity in the reactor,this amount being determined by the specifiedreactor power setpoint. If the zone control sys-tem is unable to do this, the program in the reac-tor regulating system calls on other reactivitycontrol devices. Adjusters are removed from thecore for positive reactivity shim. Negative reac-tivity is provided by the mechanical controlabsorbers or by the automatic addition of poisonto the moderator.

Stepback/Setback Routines

In addition to controlling reactor power to aspecified setpoint, the reactor regulating systemmonitors a number of important plant parame-ters. If any of these parameters is outside specif-ic limits, reactor power is reduced. This powerreduction may be fast (stepback), or slow (set-back), depending on the possible consequencesof the particular parameter excursion. The powerreduction/shutdown functions provided by thereactor regulating systems are completely sepa-rate and independent of the two special safetyshutdown systems (see safety systems section on page 46).

18

Reactor Regulating System

* A neutron absorbing substance

19

Reliability

The reliability of the reactor regulating system is of paramount importance and is achieved by:

• direct digital control from dual redundant control computers

• self-checking and automatic transfer to the standby computer on fault detection

• control programs that are independent of each other

• duplicated control programs

• duplicated and triplicated inputs

• hardware interlocks that limit the amount and rate of change of positive reactivity devices.

Computer Programs

InstrumentationReactivity

Control Devices

Reactor Power Measurements

For the reactor regulating system to control totalreactor power and to maintain the proper powerdistribution in the reactor, power measurementsin the reactor core are required.

In the high power range, zonal reactor powerestimates based on platinum flux detectors areadjusted by comparison with thermal and vana-dium flux detector measurements. In the lowpower range, total reactor power is determinedbased on measurements from uncompensatedion chambers with logarithmic amplifiers.

The Demand Power Routine

The demand power routine serves three functions:

• selects the mode of operation of the plant

• calculates the reactor power setpoint

• calculates an effective power error that is usedas the driving signal for the reactivity controldevices.

Reactivity Control and Flux Shaping

Long-term reactivity control in CANDU 6 is provided by the on-power refuelling system.Depleted fuel bundles are removed and new fuelbundles added to the core in a manner that main-tains a constant long-term reactivity distributionthroughout the core. The functions of short-term reactivity control and flux shaping areperformed by the light water zone controlabsorbers, adjusters, mechanical controlabsorbers, and moderator poison control.

The primary method of short-term reactivitycontrol is by varying the levels of light water in the liquid zone control system water compartments.

Setback Routine

The setback routine monitors a number of plantparameters and reduces reactor power gradually,in a ramp fashion, if any parameter exceedsspecified operating limits. The rate at whichreactor power is reduced and the level at whichthe setback is terminated are determined by theparticular parameter.

Stepback Routine

A situation which could possibly result in dam-age is indicated when certain plant variables are outside their specified ranges. The stepbackroutine checks the values of these variables and if necessary, disengages the clutches of the mechanical control absorbers. This allowsthe absorbers to drop into the core, to produce a rapid decrease in power.

Flux Mapping Routine

The flux mapping routine uses measurementsfrom vanadium detectors to calculate the spatialdistribution of reactor power. Flux mappingserves the following functions:

• guards against locally over-rating the fuel

• helps to calibrate the zone power detectors to properly reflect the spatial flux distribution

• provides information for optimizing poweroutput and fuel management.

20

Reactor Regulating System Computer Programs

21

(from steam generator pressure control or manual input)

Reactor Power

Thermal Power Flux Power

Flux Distribution

Mechanical Control Absorber Clutches

Adjusters

Shutoff Rods (Withdrawal Only)

Mechanical Control Absorber Drive

Light Water Zone Control Absorbers

Steam Generators

Steam to Turbine

Reactivity Devices Control

Demand Power Routine

Reactor Power Setpoint

Flux Mapping

Power Measurement and Calibration

Setback

Vanadium

Reactivity Devices Hardware InterlocksReactor

Flux

Det

ecto

rs

Ion Chambers Platinum

Parameters

Stepback

Parameters

The instrumentation systems provide the operat-ing personnel with measurements of the reactor-neutron flux and with other core information.These systems also provide the necessary inputsfor reactor regulation during start-up, shutdown,steady power and power manoeuvring condi-tions. Proportional counters, uncompensated ionchambers, and self-powered in-core flux detec-tors are used to provide continuous measurementof the reactor power from spontaneous fissionlevel to 150 per cent full power (approximately14 decades). A minimum overlap of one decadeis provided between successive ranges of instrumentation.

Start-Up Instrumentation

This temporary instrumentation is required during the initial reactor start-up to monitor the neutron flux over the range from the sponta-neous fission flux level to the sensitivity level ofthe permanent ion chambers. After start-up, thisinstrumentation is removed and is not requiredfor subsequent start-ups, unless a prolongedshutdown, more than 30 days, occurs. In thiscase, the residual flux, due mainly to photoneu-tron production, decays beyond the sensitivity ofthe ion chambers. Two sets of triplicated fissionchambers are used. One set covers the very lowflux range (10-14 to 10-10 of full power). The sec-ond set covers the flux range from 10-11 to 10-6

of full power and overlaps the ranges of both the first set and the permanent ion chamberinstrumentation.

Ion Chamber System

Three ion chambers are employed in the reactorregulating system, for measuring neutron flux inthe range from 10-7 to 15 per cent of full power.Compensation is not required, since adequatediscrimination against gamma rays is achievedby employing appropriate materials in the detec-tor and by gamma shielding in the constructionof the ion chamber housings. These ion cham-bers are located in housings at one side of thecore. In addition to one ion chamber for thereactor regulating system, each housing alsocontains an ion chamber and shutter for shut-down system number 1. Each of the three chan-nels consists of an ion chamber and amplifierunit. The solid state amplifiers upgrade the ionchamber outputs to suitable input signal levelsfor processing in the control computers. Threesimilar ion chambers, mounted on the other sideof the core, provide inputs to shutdown systemnumber 2.

Self-powered In-core Flux Detectors

In the high power range (above 15 per centpower), self powered in-core flux detectors provide the required spatial flux information not available from the ion chambers.

Two types of in-core detectors are used in thereactor. One type uses platinum as the sensitiveemitter material, while the other uses vanadium.The sheaths of both types are made of Inconel.The platinum detectors are fast-acting, sensitiveto both neutrons and gamma rays, and becauseof their prompt response to flux changes areused in the reactor regulating system and in thetwo shutdown systems. The vanadium detectorsare sensitive to neutrons, but because of a rela-tively slow response to flux changes, are usedonly in the flux mapping system.

The in-core flux detectors of the regulating system and of shutdown system number 1 are mounted vertically in the core, while thoseof shutdown system number 2 are mounted horizontally in the core.

22

Reactor Regulating System Instrumentation

23

(from steam generator pressure control or manual input)

Reactor Power

Thermal Power Flux Power

Flux Distribution

Mechanical Control Absorber Clutches

Adjusters

Shutoff Rods (Withdrawal Only)

Mechanical Control Absorber Drive

Light Water Zone Control Absorbers

Steam Generators

Steam to Turbine

Reactivity Devices Control

Demand Power Routine

Reactor Power Setpoint

Flux Mapping

Power Measurement and Calibration

Setback

Vanadium

Reactivity Devices Hardware InterlocksReactor

Flux

Det

ecto

rs

Ion Chambers Platinum

Parameters

Stepback

Parameters

24

Light Water Zone Control Absorbers

Light water (H2O) is a neutron absorber (poison)in the heavy water cooled and moderatedCANDU 6 reactor. The liquid zone control system takes advantage of this fact to provideshort-term global and spatial reactivity control in the CANDU 6 reactor core.

The liquid zone control system in the reactorconsists of six tubular, vertical zone controlunits that span the core. For flux control thezone control units are located such that 14zone control compartments are formed and are distributed through the core. Each zone controlunit can comprise either of two or three zonecontrol compartments. Flux (power) in eachzone is controlled by the addition or removalof light water from the liquid zone controlcompartment in that zone.

Mechanical Control Absorbers

Four mechanical control absorbers, mountedabove the reactor, can be driven in or out of thecore at variable speeds, or dropped by gravityinto the core, between columns of fuel channels,by releasing a clutch. These absorbers are nor-mally parked out of the core; they are driven in to supplement the negative reactivity from the light water zone control absorbers, or aredropped to effect a fast reduction in reactorpower (stepback). When inserted, the mechanicalcontrol absorbers also help to prevent the reactorfrom going critical when the shutoff rods ofshutdown system 1 are withdrawn, and are inter-locked, in this inserted position, until the shut-down system number 1 is energized and available.

Adjusters

Adjusters are cylindrical neutron absorbing rods.A CANDU 6 reactor typically has 21 verticallymounted adjuster rods, normally fully insertedbetween columns of fuel channels for fluxshaping purposes.

Removal of adjusters from the core providespositive reactivity to compensate for xenonbuildup following large power reductions, or inthe event that the on-power refuelling system isunavailable. The adjusters are capable of beingdriven in and out of the reactor core at variablespeed to provide reactivity control. The adjustersare normally driven in banks, the largest bankcontaining five rods.

Adjusters are usually fabricated from stainlesssteel. In some CANDU plants the adjusters aremade from cobalt, and are used to producecobalt 60 for medical and industrial purposes.

Poison Addition and Removal

A reactivity balance can be maintained by theaddition of soluble poison to the moderator.Boron is used to compensate for an excess ofreactivity when fresh fuel is introduced into thereactor. Gadolinium is added when the xenonload is significantly less than equilibrium (ashappens after prolonged shutdowns).

An ion exchange system removes the poisonsfrom the moderator. Addition and removal ofpoison is normally controlled by the operator.However, the reactor regulating system can also add gadolinium, in special circumstances.

Reactor Regulating System Reactivity Control Devices

25

(from steam generator pressure control or manual input)

Reactor Power

Thermal Power Flux Power

Flux Distribution

Mechanical Control Absorber Clutches

Adjusters

Shutoff Rods (Withdrawal Only)

Mechanical Control Absorber Drive

Light Water Zone Control Absorbers

Steam Generators

Steam to Turbine

Reactivity Devices Control

Demand Power Routine

Reactor Power Setpoint

Flux Mapping

Power Measurement and Calibration

Setback

Vanadium

Reactivity Devices Hardware InterlocksReactor

Flux

Det

ecto

rs

Ion Chambers Platinum

Parameters

Stepback

Parameters

Hardware Interlocks

The reactivity mechanisms are subject to a num-ber of interlocks, external to the control comput-ers, which limit the consequences of erroneousoperation of these mechanisms.

When the reactor is in a tripped state (i.e., shut-down system 1 and/or shutdown system 2 insert-ed) these interlocks prevent withdrawal of theadjusters and mechanical control absorbers.Poison removal from the moderator is also inhibited to prevent increases in reactivity.

The interlocks remain active, preventing reactorstartup, until shutoff rods are fully withdrawnand available for reactor shutdown. There arefurther interlocks to prevent more that a limitednumber of high worth adjusters from being with-drawn at the same time. This limits the rate ofpositive reactivity insertion.

With the exception of the light water zone controlers, which are controlled only from the computer, the reactivity control units canalso be manually controlled from the controlroom panels.

Feedwater System

Feedwater from the regenerative feedwater heat-ing system is supplied separately to each steamgenerator. The feedwater is pumped into thesteam generators by three 50 per cent capacitymulti-stage feedwater pumps with the flow rateto each steam generator regulated by feedwatercontrol valves. A check valve in the feedwaterline to each steam generator is provided to pre-vent backflow in the unlikely event of feedwaterpipe failure. An auxiliary feedwater pump is pro-vided that can supply four per cent of full powerfeedwater requirements during shutdown condi-tions, or if the main feedwater pumps become unavailable.

The chemistry of the feedwater to the steamgenerators is precisely controlled by demineral-ization, deaeration, oxygen scavenging and pHcontrol. A blowdown system is provided foreach steam generator that allows impurities collected in the steam generators to be removedin order to prevent their accumulation and possible long-term corrosive effects.

Steam Generators and Main Steam Systems

Reactor coolant (heavy water) flows throughsmall tubes, arranged in an inverted, vertical, U-tube bundle, within each of the four steamgenerators and transfers heat to the re-circulatedwater outside the tubes, producing steam.Moisture is removed from the steam by steamseparating equipment located in the drum (uppersection) of the steam generator. The steam thenflows via four separate steam mains, through thereactor building wall, to the turbine where theyconnect to the turbine steam chest.

The steam pressure is normally controlled by the turbine governor valves that admit steam to the high pressure stage of the turbine. If theturbine is unavailable, up to 70 per cent of fullpower steam flow can bypass the turbine and go directly to the condenser. During this opera-tion, pressure is controlled by the turbine by-pass valves. Auxiliary bypass valves are alsoprovided to permit up to 10 per cent of fullpower steam flow during low power operation.

Steam pressure can be controlled by dischargingsteam directly to the atmosphere via four atmos-pheric steam discharge valves which have acombined capacity of 10 per cent of full powersteam flow. These valves are used primarily forcontrol during warm-up or cool-down of the heat transport system.

Overpressure protection for the steam system isprovided by four safety relief valves connectedto each steam main.

26

Feedwater and Main Steam System

A steam generator in shipment.

27

Condenser Steam Discharge Valves (NC)

Isolation Valves (NO)

Turbine Stop Valves (NO)

Check Valve (NO)

SteamGenerator

From Feedwater Control Valve Stations

Calandria

ToTurbine

To AuxiliarySystem

Bypass toCondenser

Feedwater

SteamSteam

Generator

FromFeedwaterHeatingSystem

Flow Measurement

NO - Normally OpenNC - Normally Closed

Steam Safety Relief Valve (NC)

Main Steam Isolation Valves (NO)

Atmospheric Steam Discharge Valves (NC)

Feedwater

Steam

FeedwaterLines

28

CANDU 6 Nuclear Power Plant

1. Diesel room2. Water treatment plant *3. Crane hall4. Turbine building5. Turbine building crane6. Generator7. Condenser8. Battery room9. Boiler feed water tanks10. Deaerator storage tank11. Deaerator12. Reactor building13. Dousing tank14. Dousing water supply pipes15. Dousing water valves16. Dousing water spray nozzles17. Steam pipes18. Steam generators19. Pressurizer20. Crane21. Heat transport pumps22. Bleed condenser23. Bleed cooler24. Hatch25. Reactor vault26. Pressure relief pipes27. Reactivity mechanism deck28. Reactivity mechanism guide

tubes29. Calandria30. Poison injection nozzles31. Poison tanks32. Ion chambers33. Fuel channel assemblies34. End shield35. Headers36. Feeder pipes37. Fuelling machine bridge38. Bridge support column

39. Fuelling machine40. Catenary41. Fuel channel end fittings42. Steam generator support

column43. Feeder pipe insulation cabinet44. Fuelling machine vault door45. End shield cooling46. Fuelling machine track47. Moderator inlet pipe48. New fuel handling machine49. New fuel port50. Fuelling machine service ports51. Rehearsal facility52. Spent fuel port53. Spent fuel elevator54. Entrance to spent fuel area55. Airlock56. Crane57. Spent fuel shipping area58. Spent fuel handling area59. Spent fuel bay gantry60. Spent fuel bay61. Spent fuel transfer baskets62. Spent fuel transfer trolley63. Spent fuel storage baskets64. Fuelling machine maintenance

area65. Decontamination room66. New fuel storage67. Tool crib68. Vapour recovery equipment69. Office70. Control room *71. Control equipment room72. Computer room

* Some items have been movedfor clarity.

Key to Diagram

Technical DataReactorType PHWRThermal output (PHTS) 2064 MW(th)Coolant flow rate (PHTS) 7.7 Mg/sDesign temperature (RIH) 279° CDesign pressure (RIH) 12.9 MPa(g)Operating temperature (RIH) 266° COperating pressure (RIH) 11.75 MPa(abs)Design temperature (ROH) 316° CDesign pressure (ROH) 10.7 MPa(g)Operating temperature (ROH) 310° COperating pressure (ROH) 10.0 MPa(abs)

Fuel ChannelsPressure tube inside diameter (cold, unpressurized) 103.38 mmCore Length (between calandria tubesheets) 5.94 mNumber of pressure tubes 380Coolant flow (nominal) 24 kg/sEst. pressure drop – 12 bundles 838 kPa

FuelLength of bundle 495.3 mmOutside dia. of bundle (over bearing pads) 102.4 mm

Weight of bundle (nominal) 23.7 kgWeight of uranium per bundle (nominal) 19.2 kgSheath outside dia. (cold) 13.1 mmSheath thickness (average) 0.4 mmSheath material Zircaloy - 4Elements per bundle 37Fuel material Natural UO2Fuel bundles in core 4560Fuel bundles per channel 12

Heat Transport SystemNumber of loops 2Primary coolant D2OReactor inlet temperature 266° CReactor outlet temperature 310° C

Reactivity Control UnitsPower control 14 light water compartments

21 stainless steel rods4 cadmium rods

Safety shutdown 28 cadmium rods (vertical)6 gadolinium nitrate injection tubes (horizontal)

61

5859

49

48

32

4746

45

41

34 30

29

28 31

24

2322

27

21

1820

16

15

19

17

14

13

12

25

26

40

44

43

38

42

36

35

39

37

33

50 51 52 5354

62

63

60

29

Materials (out of core) Stainless steel(in core) zircaloy/stainless steel/cadmium

Steam GeneratorsType, number of units Vertical U-tube, 4Steam flow for 4 steam generators 1033.0 kg/sSteam pressure at full power 4.7 MPa(abs)Steam temperature at full power 260° CMaximum moisture 0.25%Feedwater temperature 187° C

Reactor Coolant PumpsNumber 4Motor/type AC vertical, TEWAC inductionRated capacity 2228 l/sRated head 215.0 m

ContainmentType Prestressed cylindrical concreteInside diameter 41.46 mHeight Above Grade 46.02 mTotal Inside Containment 65,500 m3

TurbineSingle shaft tandem compound steam turbine directly coupled to 828 MVAgenerator. Steam turbine consists of one double flow high pressure cylinder,two external moisture separator/reheaters and three double flow low pressure cylinders.

GeneratorRated 828 MVA at 0.9 power factor and 414 kPa(g) hydrogen pressure. 1800 rpm with terminal voltage of 22,000 volts, 60 Hz.

CondenserSingle tube sheet shells. Each shell is connected to the three LP turbineexhausts.

Two 100% main condensate extraction pumps and one auxiliary condensateextraction pump. Three 50% main steam generator feedwater pumps andone auxiliary steam generator feedwater pump.

The system consists of a turbine generator unit,and associated condensing and feedwater heat-ing systems.

Steam produced in the steam generators entersthe high pressure turbine and its water contentincreases as it expands through this high pres-sure stage. On leaving this stage, the steam passes through separators where the water isremoved; it then passes through reheaters whereit is heated by live steam taken directly from thesteam mains. The reheated steam then passesthrough the low pressure turbines, into the condenser where it condenses to water which is then returned to the steam generators via thefeedwater heating system.

Steam Turbine

The steam turbine is a tandem compound unit,directly coupled to an electrical generator by asingle shaft. It comprises one double flow, highpressure cylinder followed by external moistureseparators, live steam reheaters and three doubleflow, low pressure cylinders. The turbine isdesigned to operate with saturated inlet steam.The turbine system has main steam stop valves,governor valves, reheat intercept and emergencystop valves. All of these valves close automati-cally in the event of a turbine protection system trip.

Generator

The generator is a three-phase, four-polemachine. The generator typically operates at1800 r.p.m. to serve 60 cycle electrical systems,and at 1500 r.p.m. to serve 50 cycle systems.

The associated equipment consists of a solidstate automatic voltage regulator controlling athyristor convertor which supplies the generatorfield via a field circuit breaker, generator sliprings and brush gear.

The main power output from the generator to thestep-up transformer is by means of a forced aircooled, isolated phase bus duct, with tapoffs tothe unit service transformer, excitation trans-former and potential transformer cubicle.

Condensing System

The turbine condenser consists of three separateshells. Each shell is connected to one of thethree low pressure turbine exhausts. Steam fromthe turbine flows into the shell where it is con-densed by flowing over a tube bundle assemblythrough which cooling water is pumped. Thecondenser cooling water typically consists of a once-through circuit, utilizing water from anocean, lake or river. The condensed steam col-lects in a tank in the bottom of the condensercalled the “hot well”. A vacuum system is pro-vided to remove air and other non-condensablegases from the condenser shells. The condenseris designed to accept turbine bypass steam topermit the reactor power to be reduced from 100 per cent power to 70 per cent if the turbineis unavailable. The bypass can accept 100 percent steam flow for a few minutes, and 70 percent of full power steam flow continuously.

30

Turbine Generator System

Turbine generators at Point Lepreau nuclear generating station.

31

Feedwater Heating System

On its return to the steam generators, condensatefrom the turbine condenser is pumped throughthe feedwater heating system. First it passesthrough three low pressure feedwater heaterunits, each of which contains two heaters fed byindependent regenerative lines. (This permitsmaintenance work to be carried out on theheaters with only a small effect on the turbine

generator output.) Two of the heater units incor-porate drain cooling sections and the third a sep-arate drain cooling stage. Next, the feedwaterenters a deaerator where dissolved oxygen isremoved. From the deaerator the feedwater ispumped to the steam generators through twohigh pressure feedwater heaters each incorporat-ing drain cooling sections.

To Feedwater System

From Steam Generators

Condenser Steam

Discharge Valves

(CSDV's)

High Pressure

Turbine

Bypass Steam

To Steam Generators

High Pressure Heaters

Auxiliary Pump Deaerator

TankFeed Pumps (3)

Low Pressure Heaters

Condensate Extraction

Pumps

Low Pressure Turbines

Condenser

Condenser Hot Well

Steam Separator

Steam Reheater

Valve Normally Open

Valve Normally Closed

Pump

Generator

Cooling Water

Condensate

Feedwater Heating System

Main Power Output

Turbine Generator

The turbine generator system (described on page 30) consists of a turbine generator unit andassociated condensing and feedwater systems.The power transmitted from the generator terminals to the main output transformer and the unit service transformer is at the generatornominal operating voltage.

Main Transformer

The main transformer steps up the generator out-put voltage to the same level as the switchyardtransmission voltage. The transformer is rated tomeet the generator output requirements and siteenvironment. It is equipped with all standardaccessories and the necessary protective equipment.

Switchyard

The switchyard, located near the turbine hall,contains the automatic switching mechanism,including the breakers and disconnects, which isthe interface between the station and the powergrid transmission lines. There are at least twoincoming lines which are synchronized undernormal conditions. However, the switchyardelectrical equipment allows transmission of full station power through any one of the incoming lines.

Unit Service Transformer

During normal station operation the station ser-vices power is supplied by both the unit servicetransformer and the system service transformer.However, either transformer can provide thetotal service load in the event of a failure of onesupply. The transformer is fed from the outputsystem of the turbine generator.

System Service Transformer

The system service transformer is similar to theunit service transformer.

It supplies half of the plant services powerrequirements under normal operating conditionsand is able to provide the total service load when necessary. This transformer is fed from theswitchyard and supplies all plant loads duringthe start-up of the plant, or when the turbinegenerator is unavailable. It is located outdoorsand feeds the station services located in the electrical equipment rooms.

Both the unit service and station service trans-formers are designed to automatically maintainvoltage in the station services.

Electric Power System

Qinshan Phase III, Units 1 and 2

33

Main Output Transformer

Station Services

Generator

Unit Service

Transformer

Station Service Transformer

Power Grid

Switchyard

Power Classification

The station services power supplies are classi-fied in order of their levels of reliability require-ment. The reliability requirement of these powersupplies is divided into four classes that rangefrom uninterruptible power to that which can be interrupted with limited and acceptable consequences.

Class IV Power Supply

Power to auxiliaries and equipment that can tolerate long duration interruptions withoutendangering personnel or station equipment is obtained from Class IV power supply. This class of power supply comprises:

• Two primary medium voltage (MV) buses,each connected to the secondary windings ofthe system service and unit service transform-ers in such a way that only one bus is suppliedfrom each transformer.

• Two MV buses supplied from the secondarywindings of two transformers on the primaryMV buses. These buses supply the main heattransport pumps, feed pumps, circulatingwater pumps, extractor pumps and chillers.

Complete loss of Class IV power will initiate a reactor shutdown.

Class III Power Supply

Alternating current (AC) supplies to auxiliariesthat are necessary for the safe shutdown of thereactor and turbine are obtained from the ClassIII power supply with a standby diesel generatorback-up. These auxiliaries can tolerate shortinterruptions in their power supplies. This class of power supply comprises:

• Two medium voltage buses supplied from thesecondary windings of the two transformerson the Class IV primary MV buses. Thesebuses supply power to the pumps in the service water system, emergency core coolingsystem, moderator circulation system, shut-down cooling system, heat transport systemfeed lines, steam generator auxiliary feed line, and the air compressors and chillers.

• A number of low voltage buses.

Class II Power Supply

Uninterruptible alternating current (AC) suppliesfor essential auxiliaries are obtained from theClass II power supply, which comprises:

• Two low voltage AC three phase buses whichsupply critical motor loads and emergencylighting. These buses are each suppliedthrough an inverter from a Class III bus via a rectifier in parallel with a battery.

• Three low voltage AC single phase buses which supply AC instrument loads and the sta-tion computers. These buses are fed throughan inverter from Class I buses, which are fedfrom Class III buses via rectifiers in parallelwith batteries.

In the event of an inverter failure, power is sup-plied directly to the applicable low voltage busand through a voltage regulator to the applicableinstrument bus. If a disruption or loss of ClassIII power occurs, the battery in the applicablecircuit will provide the necessary power withoutinterruption.

Class I Power Supply

Uninterruptible direct current (DC) supplies for essential auxiliaries are obtained from the Class 1 power supply, which comprises:

• Three independent DC instrument buses, eachsupplying power to the control logic circuitsand one channel of the triplicated reactor safe-ty circuits. These buses are each supplied froma Class III bus via a rectifier in parallel with a battery.

• Three DC power buses which provide powerfor DC motors, switchgear operation and forthe Class II AC buses via inverters. These DCbuses are supplied from Class III buses via a rectifier in parallel with batteries.

Automatic Transfer System

To ensure continuity of supply, in the event of a failure of either the unit or system power, anautomatic transfer system is incorporated on the station service buses.

Transfer of load from one service transformer to the other is accomplished by:

34

Electric Power System Station Services

• A manually initiated transfer of power undernormal operating conditions, or an automati-cally initiated transfer for mechanical trips on the turbine.

• A fast open transfer of power, supplied auto-matically to both load groups of the class IVpower supply system, when power from onetransformer is interrupted. This fast transferensures that the voltage and phase differencebetween the incoming supply and the residualon the motors has no time to increase to a levelthat would cause excessive inrush currents.

• A residual voltage transfer, comprising auto-matic closure of the alternate breaker after theresidual voltage has decayed by approximately70 per cent. This scheme is time delayed, mayrequire load shedding and could result in reac-tor power cut-back. It is provided as a back-up to the above transfers.

Station Battery Banks

The station battery banks are all on continuouscharge from the Class III power supply and inthe event of a Class III power disruption willprovide power to their connected buses.

Standby Generators

Standby power for the Class III loads is suppliedby two (or more) diesel generator sets, housed inseparate rooms with fire resistant walls. Eachdiesel generator can supply the total safe shut-down load of the unit. The Class III shutdownloads are duplicated, one complete system beingfed from each diesel generator. In the event offailure of Class IV power, the two diesel genera-tors will start automatically.

The generators can be up to speed and ready toaccept load in less than two minutes. The totalinterruption time is limited to three minutes. Eachgenerator automatically energizes half of the shut-down load through a load sequencing scheme.There is no automatic electrical tie between thetwo generators, nor is there a requirement forthem to be synchronized. In the event of one generator failing to start, the total load will besupplied from the other generator.

Emergency Power Supply System

The Emergency Power Supply System can provide all shutdown electrical loads that areessential for safety.

This system and its buildings are seismicallyqualified to be operational after an earthquake.The system provides a backup for one group of safety systems (shutdown system number 2,emergency water supply, secondary control area)if normal electric supplies become unavailableor the main control room becomes uninhabit-able. The system comprises two diesel generat-ing sets, housed in separate fire resistant rooms,which are self-contained and completely inde-pendent of the station’s normal services. There is adequate redundancy provided in both thegenerating distribution equipment and the loads.

35

36

Electric Power System Station Services

Primary MV Buses

AutomaticTransfer Switch

MV Buses

S

Turbine ServicesTurbine Services

TReactor

Transformer

R

UnitService

Transformer

Low Voltage Buses L

Rectifiers

B

MotorsM

SafetyCircuits

SafetyCircuits Control

LogicControlLogic

Motors

Motors M

Batteries

Inverters

InstrumentationComputers

IInstrumentation

C

37

StationServiceTransformer

Turbine Services

MechanicalTransferSwitches

StandbyGenerator

Reactor

Turbine Services

U

Low Voltage Buses

RegulatorsRectifiersR

Batteries

MMotors Control

Logic

SafetyCircuits

S

Motors

B

InstrumentationComputers

I

Class IClass IIClass IIIClass IV

Digital computers are used for station control,alarm annunciation, graphic data display anddata logging. The system consists of two inde-pendent digital computers (DCCX and DCCY),each capable of station control.

Both computers run continuously, with programsin both machines switched on, but only the con-trolling computer’s outputs are connected to thestation equipment. In the event that the control-ling computer fails, the control of the station is automatically transfer to the “hot” standbycomputer.

Individual control programs use multiple inputsto ensure that erroneous inputs do not produceincorrect output signals. This is achieved byrejecting:

• analog input values that are outside the expected signal range

• individual readings that differ significantlyfrom their median, average or other reference.

A spare computer is provided as a source ofspare parts for the station computers. It is alsoused for:

• program assembling and checkout

• operator and maintainer training

• diagnosing faults in equipment removed from the station computers.

Alarm Annunciation

Alarm messages are presented on coloured display monitors (cathode ray tubes) which arecentrally located above the station main controlpanels. Two line printers, one for each computer, provide chronological records of all alarm conditions.

Operator Communication StationsThese computerized stations replace much of theconventional panel instrumentation in the controlroom. A number of man-machine communicationstations, each essentially comprising a keyboardand colour CRT monitor, are located on the maincontrol room panels. The displays provided onthe monitors include:

• graphic trends

• bar charts

• status displays

• pictorial displays

• historical trends.

Copies can be obtained from the line printers, of any display monitor the operator wishes to record.

Automatic Transfer

A fault in any essential part of one computerresults in automatic transfer of control to theother computer. If both computers fail, the station is automatically shut down.

38

Station Instrumentation and Control

39

Automatic Transfer

Station Inputs

Station Control Outputs

Digital Computer DCCY

• Reactor Control • Heat Transport System Control • Steam Generator Secondary Side Control • Turbine Control • Alarm Annunciation • Alphanumeric Graphics Displays • Fueling Machine Control.

Digital Computer DCCX

• Reactor Control • Heat Transport System Control

• Steam Generator Secondary Side Control • Turbine Control

• Alarm Annunciation • Alphanumeric Graphics Displays

• Turbine Runup • Fuel Channel Temperature Monitoring.

Operator Communication Stations

Alarm Annunciation

Station instrumentation performs a number of monitoring, control and display functions.Nuclear instrumentation is provided to allowautomatic control of reactor power and fluxshape and to monitor local core behaviour.Conventional instrumentation provides signalsfor control and display of other plant variables.

The plant is automated to a level that requires a minimum of operator action for all phases of station operation. All major control loopsemploy the two computers as direct digital controlers. Conventional analog control instrumentation is used on smaller local loops.

The instrumentation required for the operationof the safety systems incorporates triplicatedinformation channels that provide the systemwith a redundancy that ensures that single component failures will not cause spurious operation. The safety systems operate indepen-dently of the dual computers to avoid crosslinked faults.

Control Room

The latest CANDU 6 control room is illustratedon the opposite page. The control room featuresan array of panels at the perimeter with twolarge central display screens, and the operationsconsole. Information is provided on the panelsand at the operations console to allow the stationto be safely controlled and monitored.

The instrumentation and controls on the panelsare grouped on a system basis, with a separatepanel allocated to each major system. Colouredcathode ray tube (CRT) displays and advancedannunciation systems provide uncluttered con-trol room panel layouts and excellent monitoringcapabilities. The operator can call up informa-tion displays on the panel CRTs, the operatingcontrol console CRTs, and central displayscreens in a variety of alphanumeric and graphicformats via keyboards. All display annunciationmessages are colour coded to facilitate systemidentification and the priority of the alarm.

Conventional display and annunciation instru-mentation is provided for all safety related systems and to permit the station to be safelymonitored in the event of dual computer failure,which automatically shuts down the reactor.

If for any reason the control room has to beevacuated, the station can be shut down andmonitored from a remotely located secondarycontrol area.

40

Station Instrumentation and Control/Instrumentation

41

Qinshan Phase III Unit 1, Control Room

Storage, Filing Cabinets,Shelving within

Surface for document andflowsheet layout

Surface for document and flowsheet layout

PDS LAN:Home for Critical Safety Parameterand Safety System Monitoring

PDS LAN:Access to all process monitoringand setpoint screens

Trackballs• Cursor and selection control for

direct VDU display interaction• Left or right hand mounting

(operator adjustable)

Rotating CarouselBookshelf for importantprocedures (EOPs, etc.)

Additional Console features Controls for:• Alarm Silence/Acknowledge/Reset• RRS Hold Power and Setback• SDS1 Reactor Trip

Telephone and PagingHeadset Communication Jacks

Control Room LAN• Enhanced Annunciation Interrogation• Work Management Access• Electronic Procedures• Equipment Status Monitoring

Soft Function Keypads• Display Selection• Digital Entries• One per VDU Display• 2nd from right switchable for

Overview Display Control• Flat touchscreen technology

Main Operators’ Console

42

Digital computers are used to perform all thecontrol and monitoring functions of the station.The system is designed to:

• handle both normal and abnormal situations

• be capable of automatically controlling theunit at startup and at any preselected powerlevel within the normal loading range

• be capable of automatically shutting down the unit if unsafe conditions arise

• be tolerant of instrumentation failures.

Control Programs

The functions of the overall station control sys-tem are performed by control programs loadedin each of the two unit computers. The majorcontrol function programs are described below,but there are also programs for:

• Heat Transport System Control

• Moderator Temperature Control

• Turbine Runup and Monitoring

• Fuel Handling System Control.

Reactor Regulation

This program adjusts the reactivity controldevices to maintain reactor power equal to its desired setpoint.

Steam Generator Pressure

This program controls steam generator pressureto a constant setpoint, by changing the reactorpower setpoint (normal mode), or by adjustingthe station loads (alternate mode).

Steam Generator Level

This program controls the feedwater valves inorder to maintain the water level in the steamgenerators at a reactor power dependent level setpoint.

Heat Transport System Pressure

This program controls the pressurizer steambleed valves and heaters to maintain heat transport system pressure at a fixed setpoint.

Control ModesRReeaaccttoorr ffoolllloowwiinngg ttuurrbbiinneeIn this mode of operation the turbine-generatorload is set by the operator: the steam generatorpressure control program requests variations in reactor power to maintain steam generator pressure constant. This control mode is termed“reactor follows turbine” or “reactor follows station loads”.

TTuurrbbiinnee ffoolllloowwiinngg rreeaaccttoorrIn this control mode, “turbine follows reactor”,the station loads are made to follow the reactoroutput. This is achieved by the steam generatorpressure control program adjusting the plantloads to maintain a constant steam generatorpressure. This mode is used at low reactor powerlevels, during startup or shutdown, when thesteam generator pressure is insensitive to reactorpower. It is also used in some upset conditionswhen it may not be desirable to manoeuvre reactor power.

Unit Power Regulation

This program manoeuvres the unit power, byadjusting the turbine load setpoint, to maintainthe generator output at the level demanded bythe local operator, or by a generation control signal from a remote control centre.

Station Instrumentation and Control/Control

43

Valve Normally Closed

Flow Measurement

Valve Normally Open

Steam FlowSteam

Pressure

Steam Generator

Feedwater Valves

Reactivity Devices

Ion Chambers

Reactor Regulation

Steam Generator Level Control

Electrical Output Setpoint (normal mode)

Feedwater Flow

Turbine Electrical Output

Turning Gear

Generator

Exciter

Shaft Speed

Steam Generator Pressure Control

Unit Power Regulator

Reactor Power Setpoint

Demanded Reactor Power (alternate mode)

Flux Detectors

Reactor

Neutron Flux and Rate of Change

Steam Generator Level

Atmospheric Steam Discharge Valves

Electro-hydraulic �Controler

Condenser Steam Discharge Valves

Governor Valves

Typical Site Layout

General site requirements for a nuclear power plant:

• Land must be provided for an exclusion areaaround the plant. A perimeter of 914 metersfrom all reactors, on the landward side, wasprovided for CANDU plants put into operationbefore 1990: for some newer plants this dis-tance, known as the exclusion radius, has been reduced to less than 500 meters.

• The site should be located so as to be easilyincorporated into the utility’s electrical grid system.

• A relatively flat shelf of sedimentary rock a few metres above high water level is an ideal site, as both construction costs and site preparation time are reduced.

• Suitable land access by rail and/or road totransport heavy equipment. If site is on a navigable water body, water access may beused to transport the heaviest equipment.

Buildings and Structures

RReeaaccttoorr BBuuiillddiinnggThe reactor building houses the nuclear reactorand auxiliaries, primary heat transport system,fuel handling equipment, and instrumentation.

The reactor building’s major structural components are:

• pre-stressed concrete containment structure

• internal reinforced concrete structures

• reinforced concrete calandria vault.

The containment structure is separated from theinternal structural systems. This provides flexi-bility in over-all building construction and nointer-dependence between the containment walland other structures.

SSeerrvviiccee BBuuiillddiinnggThe service building houses nuclear facilitieswhich can be located outside of the reactorbuilding. More general service facilities, e.g.,equipment maintenance shops and laboratoryfacilities, are located in the common area of this building.

The layout of the rooms within the service build-ing provides for safety and efficiency of plantoperation in terms of traffic patterns, radiationzoning and the routing of services between thebuildings of the proposed unit. The irradiatedfuel storage facility is also located in the service building.

TTuurrbbiinnee BBuuiillddiinnggThe turbine building consists of a turbine hall,auxiliaries bay and two single story annexes.Space is provided in the auxiliaries bay for theelectrical power distribution equipment. Watertreatment plant and diesel generators are locatedat grade level in the annexes.

Overhead traveling cranes are provided in theturbine hall for erection and maintenance of theturbogenerator and some of its auxiliaries. Theturbine building has a reinforced concrete sub-structure, steel framed superstructure, steel rooftrusses and insulated metal walls and roof.

44

Site and Plant Arrangement

Point Lepreau CANDU 6 Nuclear Generating Station.

45

PPuummpphhoouusseeThe pumphouse consists of a reinforced concretesubstructure containing the condenser coolingwater pumps, raw service water pumps, firepumps, screens, and racks and screen washpumps. A steel framed superstructure provideshousing for the pump motors. Roof hatches are provided for installation and maintenance of the pumps.

���@@@���ÀÀÀ���@@@���ÀÀÀ���@@@���ÀÀÀ���@@@���ÀÀÀ���@@@���ÀÀÀ

����������Exclusion Radius (Site dependent)

Exclusion Radius (Site dependent)

Access Road

Shore Line

Turbine BuildingService Building

Exclusion Boundary

Switchyard

Upgrader TowerReactor Building

Pumphouse

46

Overall RequirementsLike most metals, fuel sheaths weaken at veryhigh temperatures. Fuel sheath integrity is there-fore at risk if a component failure causes thecooling of the fuel to be reduced relative to the power it produces.

If such a failure occurs, the reactor process systems can often stop its course or moderate its effects. Backing these up are special safetysystems. They are independent of the processsystems and of each other both functionally andphysically, and are not used in the day-to-dayoperation of the plant. They can, if needed, shutdown the reactor (shutdown systems), refill thereactor fuel channels with coolant and removeresidual or “decay” heat from the fuel (emer-gency core cooling system) and prevent releaseto the environment of radioactivity which mayescape from the reactor (containment systems).

Supporting these special safety systems are sys-tems that provide alternate sources of electricalpower (emergency power supply system) andcooling water (emergency water supply system).

Shutdown SystemsThere are two ‘full capability’ reactor shutdownsystems, each able of shutting down the reactorduring any postulated accident condition.

The two shutdown systems are functionally andphysically independent of each other; and fromthe reactor regulating system.

• Functional independence is provided by utilizingdifferent shutdown principles: solid shutoff rodsfor System number 1, direct liquid poison injec-tion into the moderator for System number 2.

• Physical independence of the shutdown sys-tems is achieved by positioning the shutoffunits vertically through the top of the reactorand the poison injection tubes horizontallythrough the sides of the reactor.

Post-shutdown Safety SystemsThe emergency core cooling system removesdecay (post shutdown) heat produced in the fuel in the event that there is a break in the heattransport system; this serves to prevent radio-activity from being released from the fuel.

The containment system confines any activityreleased from the fuel and heat transport systemto the reactor building during an accident. Alongwith the other special safety systems, it ensuresthat the dose limits for accidents, set by theCanadian regulatory agency, the Atomic EnergyControl Board, and the local regulatory authorityare not exceeded.

Safety Support Systems

These systems may be used for normal stationoperation and are also used to support the operation of the safety systems.

Systems Grouping

To provide defence against low probability incidents such as local fires or missiles (turbineblades, aircraft strikes etc.), the station safety,post shutdown, and safety support systems areseparated into two groups that are functionallyand physically independent of each other. Eachgroup is designed to provide the following functions:

• shut down the reactor

• to contain radioactivity in the process systemsby assuring that decay heat is removed, or ifthe process systems are not intact, to preventits release to the public

• supply the necessary information for post-accident monitoring.

The systems which provide these safety functions are:

• shutdown system number 1 or shutdown system number 2, to shut down the reactor

• the normal process systems, including normalelectric power and service water systems; the emergency power supply and emergencywater supply systems; and the emergency core cooling system, to remove decay heat

• the containment systems, to accommodate any accidental energy release and to containthe radioactivity that may be present in such a release

• the main control room or the secondary control area, for post accident monitoring.

Safety Systems

Shutdown System Number 1

Shutdown system number 1 is the primarymethod of quickly shutting down the reactorwhen certain parameters enter an unacceptablerange. This shutdown system employs a logicsystem, which is independent of those utilizedby shutdown system number 2 and the reactorregulating system, which senses the requirementfor reactor trip and de-energizes the direct cur-rent clutches to release the absorber elementportion of the shutoff units, allowing them todrop between columns of fuel channels, into the moderator. Each shutdown rod is equip-ped with a spring that provides an initial acceleration.

The design philosophy is based on triplicatingthe measurement of each variable, and initiatingprotection action when any two of the three tripchannels is tripped by any variable or combina-tion of variables.

Typical variables (trip parameters) that can initiate a reactor trip through shutdown system number 1 are:

• high neutron power

• low gross coolant flow

• high heat transport pressure

• high rate log neutron power

• high reactor building pressure

• low steam generator level

• low pressurizer level

• high moderator temperature

Shutdown System Number 2

An alternate method of quickly shutting downthe reactor is the rapid injection of poison (con-centrated gadolinium nitrate solution) into themoderator through horizontal tubes that enterone side of the calandria and terminate as nozzlesthat span the calandria, between rows of fuelchannels. There are six shutdown system num-ber 2 poison injection nozzles in a CANDU 6reactor. This shutdown system employs an inde-pendent logic system that senses the requirementfor a reactor shutdown and opens fast-actingvalves located in the line between a high pres-sure helium tank and the poison tanks. Thereleased helium expels the poison from thetanks, through the injection nozzles into themoderator.

Similar trip parameters used to activate shut-down system number 1 also initiate a trip condi-tion on shutdown system number 2. The instru-mentation for these trips is however physicallyand electrically separate.

Valve Normally Open

Shutdown System Number 2Shutdown System Number 1

Helium Tank

Poison Tanks

Absorber Elements

Absorber Elements

Reactor

Moderator

Shutoff Units

Valve Normally Closed

48

Emergency Core Cooling (ECCS)

System Operation

The emergency core cooling system providesordinary water to the heat transport system tocompensate for the heavy water coolant lost in a postulated Loss of Coolant Accident (LOCA),and recirculates and cools the heavy water/light water mixture that collects in the reactorbuilding floor to the reactor headers to maintainfuel cooling in the long term.

The CANDU 6 ECCS has three stages of opera-tion: high, medium and low pressure. Systemoperation is triggered, on a loss-of-coolant acci-dent (LOCA), when the heat transport systempressure drops to 5.5 MPa (800 psia) and a loopisolation system (independent of ECCS logic)closes the applicable valves to isolate the twoHTS loops.

High Pressure (HP) OperationThe initial LOCA signal isolates the two HTSloops, opens the gas inlet, HP injection and theapplicable heat transport system H2O/D2O isola-tion valves simultaneously and also initiates therapid cooling of the steam generators: the latteris accomplished by opening the main steamsafety valves on the steam generator secondaryside, and discharging steam. Emergency coolant(ordinary water) is forced from the ECCS watertanks into the ruptured HTS loop

when pressure in that loop falls below the injec-tion pressure – 4.14 MPa (600 psia). This periodto ECC injection can take about 10 seconds for a maximum pipe-size break. Coolant escapingfrom the ruptured circuit collects in the reactorbuilding sump. Minimum time to empty thewater (maximum break) is 2.5 minutes. Theentire HP phase is initiated automatically.

When the ECCS water tanks reach a predeter-mined low level, the HP injection valves closeautomatically.

Medium Pressure (MP) Operation

The medium pressure stage consists of watersupplied from the dousing tank, and delivered tothe HTS headers via the ECC pumps. The valvesconnecting the dousing tank to the ECC pumpsare opened on the LOCA signal, while the MPinjection valves open on a delayed signal. Thewater in the dousing tank provided for MP ECCis sufficient for a minimum of 13 minutes opera-tion with the maximum design basis HTS break.

There are two ECC pumps each capable of pro-viding 100 per cent of the water needs at a pres-sure of 150 psia. Class IV electrical supply to theECCS pumps is backed up by Class III powerand the emergency power supply system.

Low Pressure Operation

As the dousing tank nears depletion, the valvesbetween the reactor building floor and the ECCpumps open. Water collected in the reactor base-ment is returned to the heat transport system via heat exchangers, to provide long term fuel cooling.

The heat exchanger maintains the temperature of the coolant flow at about 49°C. Temperatureof the water (D2O and H2O) from the sumpwould be about 66°C at the ECC pumps.

For small breaks decay heat is transferred to the steam generators and rejected via the mainsteam safety valves, which have a total steamflow capacity greater than that of the steamgenerators. For large breaks, the break itself acts as the heat sink in combination with ECC injection.

Safety Systems

ModeratorFuel

Pressure tube

Gas annulus

Calandria tube

49

Undamaged HTS Loop

During the ECCS operating sequence decay heatin the undamaged HTS loop is transferred to thesteam generators by natural circulation. Coolantlosses from this circuit prior to its isolation willbe less than 20 per cent. This inventory loss canbe made up by opening the isolating valves tothe feed circuit with the override control in thecontrol room or, if the circuit pressure fallsbelow that of the injection tanks, by the ECCS.

Backup Decay Heat Removal

In the very unlikely event that the emergencycore cooling system fails during or following aLOCA, decay heat is transferred from the fuel to the moderator by radiation and conduction.The centre element of the CANDU 6 fuel bundleis only 50 mm from the cool heavy water moderator (see fuel channel section shown on previous page): hence decay heat removal fromthe fuel following shutdown is assured withoutmelting the uranium dioxide, even if no coolantis present in the fuel channel.

Gas Tank

FromDousing

Tank

Outlet Header

InputValves

HighPressure

Valves

Medium Pressure Valves

Break

Outlet Header

HeatExchanger

ECCPumps

Reactor Building Floor

Valve Normally Open

ECC High Pressure Operation

Valve Normally Closed

Pump

Check Valve

Low Pressure

Medium Pressure

High Pressure

Pressurizer

ECCWaterTanks

IsolationValve

IsolationValve

ContainmentContainment comprises a number of systemsthat operate to provide a sealed envelope aroundthe reactor systems if an accidental radioactivityrelease occurs from these systems. The struc-tures and systems that form containment are:

• a lined, post-tensioned concrete containment structure

• an automatic dousing system

• air coolers

• a filtered air discharge system

• access airlocks

• an automatically initiated containment isolation system.

Systems Operation

If a large break in the heat transport systemoccurred, the building pressure would rise and,at an overpressure of 3.5 kPa (0.5 psig), wouldinitiate containment closure (if closure had notalready been initiated by an activity release sig-nal). Other sensors associated with the reactorwould have caused a reactor trip and ECCSoperation. The dousing system will start to oper-ate automatically at an overpressure of 14 kPa (2 psig) and stops when the pressure drops to 7 kPa (1 psig). The operation can be continuousor cyclic, dependent on the size of the break.

Condensation, on the building walls, and opera-tion of the building air coolers subsequentlyreduce the pressure from 7 kPa (1 psig) to aboutatmospheric. Vapour recovery dryers initiallyclean up the containment atmosphere when thedewpoint has reached about 16°C. This is fol-lowed by a fresh air purge that is dischargedthrough the dryers and reactor building ven-tilation system filter train, removing the par-ticulate and radioiodine activity prior to atmospheric release.

For a small break in the heat transport systemthe building coolers would condense dischargingheat transport system coolant and maintain thebuilding pressure at atmospheric level.

Gamma activity, if sensed in the ventilation discharge ducts and/or vapour recovery systemwill initiate signals that close the containmentdampers and valves to prevent activity releases.

A fission product release in a fuelling machineroom, caused by damaging one or more fuel elements, would be sensed in the ventilation discharge ducts and would initiate containment isolation.

The fuelling machine room, vaults and boilerroom can be purged through the reactor buildingventilation system filter train to remove the particulate and radioiodine activity prior toatmospheric venting.

50

Safety Systems

51

Dousing SprayVentilation Discharge

Isolation Valves

Reactor

Base Slab

Building Access Airlock

Ventilation Air

Dousing Tank

To Air Filter System

Irradiated Fuel Storage

Fuel Transfer Channel

52

The station is designed to prevent the loss ofD2O from the reactor systems. Special measuresare taken to recover and upgrade D2O whichdoes escape.

Provisions ensuring optimum D2O manage-ments are:

• use of welded joints, with the number ofmechanical joints in heavy water systems is kept to a minimum

• heavy water and light water systems are segregated as much as possible

• a D2O liquid recovery system is provided

• the building containing most heavy water systems is sealed and has a minimum throughventilation flow

• air entering and leaving the reactor building is dried to minimize D2O downgrading andloss respectively

• air within the building is maintained dry byclosed circulation and drying systems so thatany increase in humidity can be readily detected.Heavy water vapour removed by the dryers isrecovered and upgraded.

Deuteration and De-deuteration System

The spent resins from the ion exchange columnsof the heat transport system and the moderatorsystem contain D2O. To recover the D2O theresins are processed (de-deuteration – a down-ward flow of H2O through the resin beds) in the deuteration and de-deuteration system.

Similarly when ion exchange resins are receivedfrom the suppliers they are also processed(deuteration – an upward flow of D2O throughthe resin bed) to remove H2O.

The ion exchange resins from the heat transportand moderator systems are processed separatelyand in both of the processes some D2O is down-graded, collected and transferred to the D2Ocleanup system.

Cleanup System

The D2O cleanup system purifies the down-graded D2O collected in the station by removingmost of the impurities, with the exception of H2O.

The D2O cleanup system contains three ionexchange columns, one charcoal filter and two feed pumps.

The ion exchange columns remove lithium, ironand boron ions and other corrosion products.The columns also remove fission products thatmay be present.

The charcoal filter removes any oil present in theheavy water, as well as impurities such asamines and carbonates. The clean downgradedD2O is transferred to the D2O upgrading system.

Vapour Recovery System

A D2O vapour recovery system is provided inthe reactor building to maintain a dry atmos-phere in areas that may be subject to leakage.The areas are segregated into three groups, eachgroup serviced by one portion of the system.

AArreeaass aacccceessssiibbllee oonnllyy dduurriinngg rreeaaccttoorr sshhuuttddoowwnn• fuelling machine operating areas

• boiler room, including shutdown cooler areas

• moderator room (exclusive of enclosurearound equipment).

MMooddeerraattoorr aarreeaass• enclosure space around moderator equipment

AArreeaass aacccceessssiibbllee dduurriinngg rreeaaccttoorr ooppeerraattiioonn• fuelling machine maintenance locks

• fuelling machine auxiliary equipment room

• monitoring rooms.

Heavy Water (D2O) Management

D2O Collection System

This system is designed to collect D2O leakagefrom mechanical components that may occur in any area of the reactor building and to receive D2O drained from equipment prior to maintenance.

The D2O in the holding tank is transferred, bytwo pumps, to either the pressure and inventorycontrol system or, if downgraded, to the D2Ocleanup system.

D2O Upgrading System

The D2O upgrading system separates a mixtureof H2O and D2O into:

• an overhead distillate, richer in light waterthan the feed

• a bottom product, richer in heavy water thanthe feed.

The upgrading system accepts mixtures varyingfrom 2 per cent to 99 per cent D2O and upgradesthem to reactor grade 99.8 per cent D2O. Theoverhead distillate has a concentration of less than 2 per cent D2O.

D2O Supply System

The D2O supply system receives D2O from two sources:

• fresh D2O is received from tank trucks or drums

• upgraded D2O is received from the upgradingsystem.

The four storage tanks of the D2O supply systemcontain inventory for one moderator or one heattransport system.

The four tanks also act as a high isotopic D2Ostorage facility during normal reactor operation.

Valve Normally Open

Valve Normally Closed

Pump

Deuteration/ Dedeuteration

Low Tritium Storage

Heat Transport System

Vapour Recovery

D2O Collection

Pump

Filter

Ion Exchange Columns

D2O Cleanup

Deuteration/ Dedeuteration

Low Tritium Storage

Moderator System

D2O Upgrading

H2O

Air Pressure

Fresh D2O

D2O Supply

Moderator D2O

HTS D2O

To Heat Transport SystemTo Moderator

System

Vapour Recovery

D2O Collection

Pump

Filter

Ion Exchange Columns

D2O Cleanup

53

54

Overview

High neutron economy is the feature of theCANDU reactor that allows it to operate with a variety of low fissile content fuels. Severalpossible fuel cycles are illustrated in the follow-ing figure. These include natural uranium (NUcycle) and slightly enriched uranium (SEUcycle). Also, this feature of CANDU provides a unique synergy between CANDU and LightWater Reactors (LWRs) as there is sufficient fissile content in spent LWR fuel to provide new fuel in CANDU (Tandem cycle) as mixeduranium and plutonium oxide (MOX) fuel.Alternately, the recovered uranium from theLWR spent fuel can be used in CANDU withoutthe plutonium (RU cycle) to operate in synergywith LWRs that recycle the plutonium.

In addition to burning the products of conven-tional LWR fuel reprocessing, the CANDU reac-tor can operate on LWR spent fuel, refabricatedwithout chemical reprocessing (DUPIC Cycle), a process that is easier to safeguard againstdiversion of fissile material. This fuel cycle is particularly suitable for LWR owners andoperators that do not have access to indigenousresources of uranium and conventional spentfuel reprocessing technology.

The option of recycling reprocessed LWR fuelsin CANDU leads to higher energy output by 30 to 40 per cent compared with recycling usingLWRs alone. Furthermore, the recycling of plu-tonium in the LWR is limited in capacity by otherwise unacceptable changes in the dynamicbehaviour of the reactor. No such limitationexists with CANDU.

The high neutron economy of CANDU also hasimplications in the area of waste disposal, inparticular, in the reduction of the radiotoxicity of spent fuel. There is sufficient fissile content in the mixture of plutonium and the higheractinides (a by-product of LWR spent fuel

reprocessing) to be used as fuel in CANDU; noaddition of uranium is required. The absence ofuranium prevents the formation of plutonium.The fissile content of the transuranic mixdepletes rapidly due to the lack of plutoniumformation. As a result, the level of neutron fluxincreases rapidly in order to maintain reactorpower at the rated level. The high level of neu-tron flux is instrumental in transmuting andannihilating the toxic material. The CANDUreactor can therefore produce energy through thedestruction of toxic waste and without producingany such waste in the process, and falls into thecategory of “green” technology.

CANDU fuel cycle options of current interestinclude: natural uranium (NU), slightly enricheduranium (SEU), recovered uranium (0.9 per centU235) (RU), direct use of spent LWR fuel inCANDU (DUPIC), the thorium/U233 cycles andthe transuranic mix. The fuel cycle options areillustrated in the following figure, and are dis-cussed further in the following sub-sections.

An important feature of CANDU's versatility is that alternate fuel cycles can be implemented in operating CANDU reactors, with little or no equipment change. As a result, fuel cycle benefits do not require a big investment in newdesigns. This versatility also allows “reversibili-ty” of fuel cycle options. Even if a CANDU hasbeen optimized for a particular fuel cycle, theoperator can convert to alternate fuel cycles, if required. This is a major attraction of theadvanced fuel cycles, since it preserves theoption of national independence of enrichmentsupply even while using the advanced fuel.

The ability to switch to alternative nuclear fuelsthat become available in the global marketallows CANDU to take advantage of global fuelavailability, thereby reducing cost and providingincreased security of fuel supply.

Fuel Cycle Flexibility

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LWR

Enrichment

Dry Processing

Reprocessing

Uranium Mine

Thorium Cycle

Plutonium Cycle

Plutonium Cycle

Actinide Burning

Natural Fuel

Slightly Enriched Uranium (0.8 to 1.2%) Fuel

Enriched Uranium

Fuel

Direct Use 0.9%U 0.6%Pu

Recovered Uranium 0.9%

Natural Uranium

0.7%

CANDU fuel cycles (current and future)

Natural Uranium Fuel Cycle

CANDU Nuclear Power Plants now in operationor under construction utilize natural uranium(NU) fuel. The ability to burn natural uranium is a direct consequence of the excellent neutroneconomy provided by CANDU.

Historically, the ability to use NU fuel has hadmany benefits to CANDU owners, including lowfuelling costs compared to other nuclear powerplants, no reliance on supply of uranium enrich-ment or fuel reprocessing, and low uraniumresource consumption.

These benefits, coupled with competitive capital cost, have made natural uranium fuelledCANDU reactors a competitive electricity supplyoption. Although CANDU spent fuel volumesare greater than for LWRs, the economic impactis offset by the lower specific activity and theabsence of criticality concerns in handling andstorage. Thus, on-site dry spent-fuel storage hasbeen implemented with relative ease at operatingCANDU plants. The dry storage technology isalso applicable to the alternative fuel cyclesmentioned above.

Short Term Advanced Fuel Cycles – RU & SEU

The short-term SEU and RU cycles involve verylittle development and can be implemented intoexisting CANDU plants as no significant hard-ware changes are required. The choice of theenrichment level for the SEU fuel is dictatedprimarily by the limit placed on fuel dischargeburnup.

There is a specific enrichment level (and burnup)that maximizes the utilization of the naturaluranium which is the starting material for theSEU fuel. This enrichment level is approximately 1.2 wt per cent U235. This is due to the opposingeffects of enrichment and the production rate of plutonium (which is the source of more than50 per cent of the energy generated by naturaluranium CANDU fuel). The enrichment reducesthe production rate of plutonium but it alsoextends the fuel life in the core, which tends to increase the total energy contribution of theplutonium. A burnup of 22 MWd/kgU, which

requires 1.2 wt per cent enrichment to achieve,minimizes uranium resource requirement. Thespent fuel volume is reduced to 33 per cent ofthat of the natural uranium cycle as the burnup is increased by three times.

The RU cycle essentially uses a waste product of LWR spent fuel reprocessing since only theplutonium from the spent LWR fuel is currentlyrecycled into the LWR; Therefore the cost of the RU fuel material is potentially low. The exit burnup of this fuel in CANDU is between14 and 18 MWd/kgU (depending on the U235content of the RU which varies between 0.8 to1.0 wt per cent). The additional energy obtainedin CANDU is approximately 40 per cent ofinitial LWR fuel burnup. The reduction inCANDU spent fuel volume with the RU cycle isapproximately 50 per cent that of the natural uranium cycle.

DUPIC Cycle

The DUPIC cycle facilitates the use of spentLWR fuel in CANDU without the need forchemical reprocessing, via relatively simple fuel refabrication technologies.

The DUPIC cycle is particularly suitable forcountries that have spent LWR fuel and whichdo not have reprocessing technology, due to thesimplified refabrication technology utilized bythe DUPIC process.

The DUPIC cycle offers strategic benefits, suchas security of supply and protection from uranium and enrichment price changes. Thedevelopment of LWR-CANDU fuel cycles,such as DUPIC, transforms spent LWR fuelinventory from a “waste-disposal” cost into avaluable energy resource. A mix of LWRs andCANDUs incorporating the DUPIC fuel cyclewould generate electricity utilizing 40 per centless uranium resources than LWRs alone.

The DUPIC cycle has important non-proliferationadvantages; fissile plutonium is never separatedfrom the remaining heavy metals, so that nostream of material exists which could be divertedto non-peaceful use.

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Fuel Cycle Flexibility

Thorium Cycle

Thorium, which is abundant in many areas ofthe world, is a fertile material and produces a fissile material, U233, by neutron capture.Hence, thorium can largely displace uranium in CANDU operation.

Some fissile material is required to initially(fresh core) make the reactor critical. This fissilematerial can be NU fuel, SEU fuel, RU fuel orDUPIC fuel. Once the process of neutron cap-ture in thorium has started, significant quantitiesof U233 are produced and fissioned during thelife of the thorium fuel in CANDU. The produc-tion of U233 is highly efficient (more so than theproduction of plutonium in the natural uraniumcycle). There is, under optimum circumstances,almost one atom of U233 produced per atom ofU233 destroyed. This makes the CANDU reactora near-breeder as it breeds most of its own fuel.

There are essentially two types of thorium/U233cycles that can be used in CANDU, one withoutreprocessing spent fuel and one with reprocess-ing and recycling of the U233 that is extracted.The reprocessing option results in more efficientfuel utilization but fuel cost depends on repro-cessing cost. The fissile fuel production efficien-cy of the CANDU thorium/U233 cycle ensuressecurity of resources almost indefinitely.

Another advantage of the thorium/U233 cycle isthe lower radiotoxicity (by a factor of 10 to 100,compared with the uranium based cycles) of thespent fuel. This simplifies spent fuel disposalsignificantly. The lower radiotoxicity of thespent fuel is a result of the absence of U238which is the starting material for the productionof the transuranic actinides, the main contribu-tors to long-lived radiotoxicity of the spent uranium fuel.

The thorium fuel cycles are of immediate interestto countries that have thorium reserves whilelacking significant uranium reserves and are ofgeneral interest to all countries as the price ofuranium increases.

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Printed in Canada. May 2005. PP&I 1086