2 candu design - · pdf file2: candu design b. rouben ... differences are primarily in the...
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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