2015 January 1

2: CANDU Design

B. Rouben

McMaster University

Nuclear Power Plant Systems & Operation

EP 4P03/6P03

2015 Jan-Apr

Outline

The CANDU Evolution

CANDU vs. PWR

CANDU 6 Design

2015 January 2

Qinshan1,2

NPD DouglasPoint

Bruce A1,2,3,4 Darlington

1,2,3,4

Pickering A1,2,3,4

Cernavoda1

Wolsong2,3,4

1970-1990

Bruce B1,2,3,4

Pickering B1,2,3,4

G-2Pt Lepreau

EmbalseWolsong 1

1990-2000

CANDU9

2000-2010

1960-1970

CANDU 6

Proto-

types

1945

ZEEP

CANDU 9

CANDU Evolution AdvancedCANDUReactorandBeyond

CANDU Development Builds on a Strong History

2015 January 3

SCWR (CANDU X)

Spin out new technologyto improve existing plants

Evolution of CANDU Reactors

Current GenerationCANDU 6

Advanced CANDU ReactorACR-1000

2015 January 4

CANDU vs. PWR

Two different design philosophies

CANDU (CANada Deuterium Uranium)

Neutron economy

Natural uranium fuel

Separated heavy water (D2O) coolant and moderator

Distributed core (fuel channels)

Pressurized Water Reactor (PWR)

235U enriched fuel

Combined light water coolant and moderator

Integrated core (pressure vessel)

2015 January 5

CANDU and PWR

Reactor Coolant Systems Comparison

2015 January 6

CANDU-PWR Similarities/Differences

CANDU is a P(H)WR – Pressurized Heavy Water Reactor.

Key features of CANDU and PWR are the same or very similar:

UO2 fuel in long zirconium-clad elements

High-pressure reactor coolant

‘Light bulb’ steam generator

Large concrete reactor containment structure

Turbine generator steam plant

Auxiliary systems (condenser water supply, pump-house, etc.)

Differences are primarily in the reactor core design.

2015 January 7

Reactor-Core Design

CANDU• Natural uranium fuel

• Heavy water coolant

• Heavy water moderator

• Separate coolant

and moderator

• Pressure tubes

• Small, simple fuel bundle

• On-power fuelling

• Boron reactivity control in

moderator

PWR• Enriched 235U uranium fuel

• Light water coolant

• Light water moderator

• Coolant and moderator

are same medium

• Pressure vessel

• Large, complex fuel assembly

• Off-power fuelling

• Boron reactivity control in

coolant

2015 January 8

Different Enrichment Options

2015 January 9

CANDU-6 Plant

Turbine Building

Reactor Containment Building

2015 January 10

Main CANDU Reactor Systems

Reactor Assembly

Fuel and Fuel Channel

Heat Transport System

Shutdown Cooling System

Pressure and Inventory Control System

Moderator System

Special Safety Systems

2015 January 11

CANDU 6

1. Reactor face

2. Reactor coolant pump

3. Steam generator

4. Fuelling machine carriage

5. Moderator heat exchanger

6. Dousing water system

7. Dousing water tank 5

1

3

2

4

6

7

2015 January 12

Reactor Assembly

The reactor assembly contains the reactor core and

the reactivity control devices. Major components of

the reactor assembly are:

Calandria Vessel

End-Shields

Shield Tank

Fuel Channels

Reactivity Control Devices

2015 January 13

CANDU-6 Reactor Assembly

2015 January 14

CANDU 6 Calandria with Pressure Tubes Installed

2015 January 15

Calandria Vessel

Low-pressure tank

Includes calandria tube and supports pressure tubes

Contains heavy water moderator

Contains reactivity control devices and shutdown systems

Embedded in light-water reactor vault (which provides radiation shielding)

Provides passive emergency heat sink in the event of a loss-of-coolant accident + loss-of-emergency cooling

2015 January 16

CANDU-6 ReactorVault

2015 January 17

CANDU-6 Calandria for Qinshan

2015 January 18

Pressure-Tube Core Design

Sub-divided reactor coolant system, no large pressure vessel.

Cool moderator separated from hot coolant.

Zr-2.5%Nb pressure tubes constitute CANDU ‘pressure vessel’.

Individual pressure tubes are replaceable.

Modular component – allows scaling of reactor size

Zirconium alloy provides neutron economy.

Interstitial reactivity devices (between fuel channels).

Distributed core allows cooling to be maintained with failure of the small diameter reactor coolant system components (a pressure tube or feeder pipe).

2015 January 19

Fuel-Channel Arrangement

2015 January 20

Heavy

Heavy-Water Coolant

2015 January 21

CANDU Fuel

Natural uranium (~0.7% 235U).

High-density uranium oxide (UO2) fuel

pellets in Zircaloy-4 cladding.

‘Collapsible’ cladding under normal

operating conditions.

Short (0.5 m) fuel elements arranged in

cylindrical fuel bundles.

2015 January 22

CANDU 37–Element Fuel Bundle

2015 January 23

Heat-Transport System

Two independent circuits arranged in figure-of-8 configuration with pumps and steam generators.

System components sized to minimize D2O inventory.

All core-external circuit components located above core

Facilitates passive natural circulation thermosyphoning in the event of loss of pumped flow.

Prevents draining of the reactor by failure of piping.

Entire reactor coolant system pressure boundary inside containment.

2015 January 24

CANDU-6 Heat-Transport System

2015 January 25

CANDU 6 Heat Transport System

2015 January 26

Reactor Face End Fittings and Feeders

2015 January 27

Reactor Coolant Parameters

Outlet header pressure 10 MPa

Outlet header temperature 310ºC

Outlet header steam quality (max.) 4.0%

Inlet header temperature 266ºC

Secondary Side Conditions

Steam pressure 4.7 MPa

Steam quality <0.25% moisture

Feedwater temperature 187ºC

CANDU-6 Heat-Transport System Design

2015 January 28

Qinshan Steam Generator

2015 January 29

STEAM GENERATOR

Tube Bundles

2015 January 30

Shutdown Cooling System

Long-term heat removal system.

7% of full power heat removal capability at full

pressure.

1.2% of full power heat removal capability when

depressurized.

Can connect to reactor coolant system at full

pressure.

Heat sink when steam generators unavailable.

2015 January 31

Shutdown Cooling System

2015 January 32

Heat-Transport Pressure and Inventory

Control System

2015 January 33

Moderator System

Low temperature (< 80oC), low pressure system.

Independent of reactor coolant system.

Normal heat removal is ~4% of full power.

Potential heat sink if Emergency Core Cooling is unavailable during a Loss-of-Coolant Accident (LOCA).

Contains shutdown systems located outside of high-pressure heat transport system.

Low temperature moderator cannot add to pressure of containment (add energy) during a LOCA.

2015 January 34

Moderator System

2015 January 35

On-Power Refuelling

Regular, routine fuel bundle insertion (fresh fuel) and removal (spent fuel) on power.

Maintains a constant power shape in the core.

Maintains an equilibrium fuel burnup (steady state source term for potential releases).

Shutdown system effectiveness does not change during a fueling cycle.

Permits on-power removal of failed fuel.

Keeps radioactive contamination in coolant system low.

Minimizes dose to workers.

2015 January 36

Reactivity-Control Philosophies

2015 January 37

2015 January 38

CANDU

6

Reactor

(700-

MWe

Class)Ion Chambers

Guide Tubes for

Reactivity

Devices and In-

Core Flux

Detectors

2015 January 39

Calandria, Showing Fuel Channels

2015 January 40

Long-Term Reactivity Control

For long-term maintenance of reactivity: Refuelling is required because reactivity eventually

decreases as fuel is irradiated: fission products accumulate and total fissile content decreases.

In CANDU 6, average refuelling rate ~ 2 channels per Full-Power Day (FPD), using the 8-bundle-shiftrefuelling scheme (8 new bundles pushed in channel, 8 irradiated bundles pushed out).

4-bundle-shift and 10-bundle-shift refuelling schemes have also been used in other CANDUs.

Selection of channels is the job of the station physicist.

2015 January 41

Fuelling machines at both ends of the

reactor remove spent fuel, insert new fuel

2015 January 42

Reactor Regulating System

The reactivity devices used for control

purposes by the Reactor Regulating System

(RRS) in the standard CANDU-6 design

are the following:

14 liquid-zone-control compartments (H2O

filled)

21 adjuster rods

4 mechanical control absorbers

moderator poison.

2015 January 43

CANDU Reactivity Devices

All reactivity devices are located or introduced into guide

tubes permanently positioned in the low-pressure

moderator environment.

These guide tubes are located interstitially between rows

of calandria tubes (see next Figure).

Maximum positive reactivity insertion rate achievable by

driving all control devices together is about 0.35 mk/s,

well within the design capability of the shutdown

systems.

2015 January 44

Special Safety Systems

There are in addition two spatially, logically, and

functionally separate special shutdown systems (SDS):

SDS-1, consisting of 28 cadmium shutoff rods which

fall into the core from above

SDS-2, consisting of high-pressure poison injection into

the moderator through 6 horizontally oriented nozzles.

Each shutdown system can insert > 50 mk of negative

reactivity in approximately 1 s.

Next Figure summarizes the reactivity worths and

reactivity-insertion rates of the various CANDU-6

reactivity devices.

2015 January 45

REACTIVITY WORTHS

OF CANDU REACTIVITY DEVICES

Function Device Total Reactivity

Worth (mk)

Maximum

Reactivity

Rate (mk/s)

Control 14 Zone

Controllers

7 0.14

Control 21 Adjusters 15 0.10

Control 4 Mechanical

Control Absorbers10 0.075(driving)

- 3.5 (dropping)

Control Moderator Poison — -0.01

(extracting)

Safety 28 Shutoff Units -80 -50

Safety 6 Poison-

Injection Nozzles

>-300 -50

2015 January 46

Liquid Zone Controllers

For fine control of reactivity:

14 zone-control compartments, containing variable amounts of light water (H2O used as absorber!)

The water fills are manipulated:

all in same direction,

to keep reactor critical for steady operation, or

to provide small positive or negative reactivity to increase or decrease power in a controlled manner

differentially, to shape 3-d power distribution towards desired reference shape

2015 January 47

Liquid Zone-Control Units

2015 January 48

Liquid Zone-Control Compartments

2015 January 49

Mechanical Control Absorbers

For fast power reduction:

4 mechanical absorbers (MCA), tubes of cadmium

sandwiched in stainless steel – physically same as

shutoff rods.

The MCAs are normally parked fully outside the core

under steady-state reactor operation.

They are moved into the core only for rapid reduction of

reactor power, at a rate or over a range that cannot be

accomplished by filling the liquid zone-control system

at the maximum possible rate.

Can be driven in pairs, or all four dropped in by gravity

following release of an electromagnetic clutch.

2015 January 50

X = Mechanical Control Absorbers

2015 January 51

Adjuster Rods

When refuelling unavailable (fuelling machine “down”) for long period, or for xenon override:

21 adjuster rods, made of stainless steel or cobalt (to produce 60Co for medical applications).

Adjusters are normally in-core, and are driven out (vertically) when extra positive reactivity is required.

The reactivity worth of the complete system is about 15 mk.

Maximum rate of change of reactivity for 1 bank of adjusters is < 0.1 mk per second.

The adjusters also help to flatten the power distribution, so that more total power can be produced without exceeding channel and bundle power limits.

2015 January 52

Top View Showing Adjuster Positions

2015 January 53

Face View Showing Adjuster Positions

2015 January 54

Moderator Poison

Moderator poison is used to compensate for

excess reactivity:

in the initial core, when all fuel in the core is

fresh, and

during and following reactor shutdown, when

the 135Xe concentration has decayed below

normal levels.

Boron is used in the initial core, and gadolinium

is used following reactor shutdown. Advantage

of gadolinium is that burnout rate compensates

for xenon growth.

2015 January 55

CANDU Special Shutdown Systems

Two independent,

fully capable

shutdown systems:

SDS-1 (rods enter

core from top)

SDS-2 (injection of

neutron “poison”

from side.

2015 January 56

SDS-1

SDS-1: 28 shutoff rods, tubes consisting of cadmium

sheet sandwiched between two concentric steel

cylinders.

The SORs are inserted vertically into perforated

circular guide tubes which are permanently fixed in the

core.

See locations in next Figure.

The diameter of the SORs is about 113 mm.

The outermost four SORs are ~4.4 m long, the rest

~5.4 m long.

2015 January 57

Top View

Showing

Shutoff-Rod

Positions

(SA 1 – 28)

2015 January 58

SDS-2

SDS-2: high-pressure injection of solution of gadolinium into the moderator in the calandria.

Gadolinium solution normally held at high pressure in vessels outside of the calandria. Concentration is ~8000 g of gadolinium per Mg of heavy water.

Injection accomplished by opening high-speed valves which are normally closed.

When the valves open, the poison is injected into the moderator through 6 horizontally oriented nozzles that span the core (see next Figure).

Nozzles inject poison in four different directions in the form of a large number of individual jets.

Poison disperses rapidly throughout large fraction of core.

2015 January 59

Positions of Liquid-Poison-Injection Nozzles

CANDU Control Systems

Extensive computer controls used in operation

since Douglas Point.

Safety shutdown systems automated.

2015 January 60

Reactor Control

Digital computer control

Reactor Regulating System

HTS pressure & inventory

Steam generator pressure & inventory

Turbine run-up

Fuelling machines

Alarms, displays

Dual computer system

Failure to fail-safe configuration

Availability >99% (each computer)

2015 January 61

Station Instrumentation Control

2015 January 62

2015 January 63

END