Transcript Reactor Outage Heat Sinks - Accidents and Heat Removal

Introduction to
Reactor Outage Heat Sinks
Dr. David Novog
Associate Professor, Dpt. Eng-Phys
DNGS Nuclear Site
Summary of Reactor Systems
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Main Heat Transport System
HTS Support Systems
– HTS Pressure and Inventory Control (Feed and Bleed)
– HTS Purification System
– Shutdown Cooling System (Maintenance Cooling System)
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Moderator System
Control Systems
– Boiler Pressure Control (BPC)
– Reactor Regulating System (RRS)
– Unit Power Control (UPC) ...
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Secondary Side System
– Feedwater and Main Steam Supply
– Turbines and control valves
– Condensor
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Annulus Gas System
Fuel Handling System
Containment
…
Importance of Heat Sinks
• The ultimate goal of the reactor is to produce heat in the fuel,
remove the energy to the heat transport system fluid, transport that
hot fluid to the boiler, generate steam and convert that steam to
electricity in the turbine.
• Once the energy is deposited in the boiler, the cooler fluid is then
recirculated back to the reactor by the main heat transport pumps.
• The term HEAT SINK is applied to the component in the heat
transport system which is used to deposit the energy produced in
the fuel.
• Even when not producing electrical power, a heat sink must always
be in place.
• What happens if the heat sink performance is degraded?
Reactor Internals
CANDU Schematic
Heat Sinks
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High Power:
– Above ~5%FP, the main boilers (steam generators) are the heat sink.
– NO back-up heat sink exists at these powers which is capable of receiving the
heat produced in the core.
– Therefore the main heat transport system pumps MUST be operating AND the
boilers MUST be fully functioning.
– Therefore loss of secondary side feedwater flow or Loss of BPC is considered in
accident analyses and reactor trip effectiveness demonstrated.
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Low Power:
– Low power corresponds to below ~5%FP down to cases at 10-7%FP and the
reactor is critical.
– In general boilers are still used as a heat sink, although SHUTDOWN COOLING
is capable of removing these power levels.
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Shutdown:
– Even if the reactor is completely shutdown (i.e., subcritical), effective heat sinks
MUST be in place to remove decay heat levels from the fuel.
– Fuel may still melt in several minutes if cooling is stopped, even at decay
heat levels.
Low Power / Shutdown Heat Sinks
• Under low power or shutdown conditions there are several options
to ensure a heat sink is in place:
– Main HTS pumps running and boilers in service.
– Shutdown cooling system is “valved in” and operational.
• Shutdown Cooling System:
– Since the SDC system can remove up to ~5%FP of heat, it allows for
maintenance of the heat transport system components as well as
secondary side systems.
– The HTS and Secondary Side systems can be taken out of service (i.e.
turned off, drained to lower elevations, …) since SDC is a separate
system which will ensure a heat sink is in place.
– Consists of pumps and heat exchangers which by-pass the usual route
and connect the inlet and outlet headers on each loop.
CANDU Main Heat Transport System
CANDU Shutdown Cooling System
Shutdown Cooling Schematic
Safety in the Low Power State
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CANDU reactors are designed to operate safely in either high or low power
states.
However, at low power or shutdown conditions, the safety case is made
more complicated due to the following:
– Instrumentation may be off-scale. i.e., some power measurements and flow
indications become irrational (as they are outside of their design requirements.
– Equipment may be taken out of service for maintenance.
– Sensing equipment may have higher uncertainty due to low signal strength and
noise.
– Some auto-shutdown systems are “conditioned out”, i.e., disabled, to allow for
maintenance.
• e.g., main HT pump maintenance requires the HTS system be drained to a certain level,
opening the system means the “auto-trip functions” may incorrectly think there has been
a Loss of Coolant Accident.
– Some auto-shutdown setpoints were designed for high power operation, may not
be effective at low power operation.
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Increased operator diligence is mandatory (e.g., GSS), and emergency heat
sinks are planned.
Emergency Heat Sinks
• There are several accident scenarios (electrical failures, loss of
shutdown cooling ,…) where forced circulation may be lost. This will
put the heat sink in jeopardy.
• For these events the fuel may remain cool due to some natural
circulation phenomena.
• A large amount of planning is done to ensure that at least Heat Sink
will be effective in keeping the fuel cool in the event of an
emergency (such as a loss of the SDC system).
• Often these employ Core Cooling in the Absence of Forced Flow
(CCAFF).
Core Cooling in the Absence of Forced Flow
• CCAFF Methodology:
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Single-phase thermosyphoning
Two-phase thermosyphoning
Intermittent Buoyancy Induced Flow (IBIF)
Continuous steam venting
• Note there are other emergency heat sinks such as the emergency
Water Supply, the ‘Outage Heat Sink Plan” will specify all the heat
sinks (normal and emergency) hich are applicable to a given outage.
Emergency Heat Sinks
• Single Phase Thermosyphoning
– Continuous heat input to the coolant raises the temperature in the fuel
channel.
– This induces a buoyancy force whereby the hot liquid rises to the steam
generator, deposits its heat. With a lower temperature this liquid then
continues back to the core.
– Here the buoyancy forces are sufficient to overcome the static pressure
caused be the elevation of the boilers with respect to the fuel channels.
– This is a continuous process and is capable of removing a substantial
amount of decay power.
– Does this work if the heat transport system is in a drained state? (i.e.,
for an outage where we need to do maintenance on the HTS pumps)
– If the boiler heats up (due to say a lack of cold feedwater input), will this
continue indefinitely?
Two-Phase Continuous Thermosyphoning
• Two-Phase thermosyphoning
– Similar to liquid thermosyphoning but where continuous boiling of the
liquid provides an additional buoyancy force for heat transfer to the
steam generators.
– Flow is still continuous in a given direction, but there may be some
vapour at the outlet of the channels.
Intermittent Buoyancy Induced Flow
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What happens if thermosyphoning breaks down?
Initially:
– What happens to the flow of coolant in the channel?
– What happens to the coolant temperature?
– Is the temperature change uniform throughout the channel?
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Boiling:
– Which channels would we expect to have boiling occur first?
• What dictates the amount of decay heat?
• What role does static pressure play (i.e., channel elevation)?
– Where along the channel would we expect vapour to initially form and why?
– Where would this vapour tend to accumulate?
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Fuel temperature:
– As elements become uncovered, what happens to the available heat transfer?
– What role does this play on the RATE of further void generation?
– What happens to an element temperature once its uncovered?
Phases of an IBIF Cycle I
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Loss of Forced Circulation
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Flow stagnation in the channel
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Channel reaches saturation
– Stratified condition.
– Steam is generated only in the liquid portion of the channel
– Uncovered elements begin to heat-up (also local heating of the pressure tube).
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Large vapour bubble grows outward from the center of the channel towards
the end-fittings.
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Bubble grows by void generation in the liquid portion of the channel.
Uncovered fuel begins to heat up.
Uncovered sections stop producing vapour.
Bubble gets condensed at the edges
IBIF Phases II
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Steam heats up the end fitting.
– End fitting is a very large mass of relatively cold material (during an outage
everything gets near 60C).
– It takes a while to heat up in order for the bubble to “push” itself by.
– This delays the venting process, as the longer the fuel is uncovered the hotter it
gets.
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The process continues until the steam creates a flow path to the feeders.
– Happens on one side or the other first. It does not necessarily follow the usual
flow direction.
– Once it starts to “vent” (vapour rushes out of that side of the channel) this
creates a pressure force which sucks in cold liquid from the opposite header.
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Venting!
Fuel Channel
IBIF Phases (Stagnation)
Power profile
IBIF Phases (Onset of boiling)
Power profile
Initial void
IBIF Phases
(Steam Propagation Towards the End Fittings)
Bubble Growth
IBIF Phases
(Steam Reaches End Fittings)
heat transfer from the steam to the end fitting
IBIF Phases
(Steam Venting and Refill)
IBIF Phases
(new cycle)
Typical IBIF Cycle
Heatup Period
450
Fuel is uncovered
400
Temperature [C]
350
Venting and
Quenching
300
250
Critical Heat Flux is reached.
200
150
Saturation Temperature
Nucleate Boiling
100
Natural Convection
50
0
0
200
400
600
Time [seconds]
800
1000
1200
IBIF Duration of the Cycle
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In general the longer the duration to venting - the higher the fuel and
pressure tube temperature
– Heat Capacity of the end-fittings (direct proportion).
– Channel Power (inverse proportion to venting time). However, the higher the
power the higher the fuel and pressure tube temperature.
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Each subsequent venting cycle is shorter, and in general has lower
temperatures
– Channel power is dropping (due to decay heat) each cycle.
– Venting time due to end fitting temperature.
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Note: the number of elements uncovered at the time of venting relies
on empirical data from full scale experiments.
Outage Heat Sink Planning
• Plan must consider:
– The core decay power and the main and back-up heat sinks.
• High power heat sink is the main HTS pumps and boilers.
• Low Power/Shutdown heat sink is some combination of main HTS
pumps/boilers, shutdown cooling pumps and H/X, EWS to the boilers.
– Under normal conditions and operating variations the above are more
than acceptable.
– Analysis is performed for emergency heat sinks to demonstrate that:
• The fuel does not get damaged due to overheating.
• The pressure tube temperature (absolute as well as gradients) remain such
that thermal stresses remain acceptable.
– Every outage must go through ALL reactor core configurations and list
the main, back-up and emergency heat sinks at ALL times.
High Power Example
• Under a Loss of Forced Cooling scenario:
– Following shutdown from full power, when the heat transport system
(HTS) is maintained full, single-phase thermosyphoning, with the
steam generators as the heat sink, is effective for decay heat removal in
the short term.
– Heat is removed through the steam generators to the secondary side by
the buoyancy induced flow, or through reflux condensation
– In the longer term, and during outages, the requirements for singlephase thermosyphoning will not be met and thermosyphoning will
break down.
– In the Absence of Forced Flow, we CANNOT rule out the possibility that
SOME Channels might be subjected to small net driving forces (i.e. very
low flow).
• Loss of Heat Sink
• Consequently, small flow through these channels
• Consequently, fuel and pressure tube heatup can occur.