ACADs (08-006) Covered
1.1.4.6
1.2.1
1.2.2.3
1.1.1.1
Keywords
Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), primary loop,
reactivity, reactivity control, reactivity accidents, control rod drop, fuel failure,
Description
This PowerPoint presentation is an overview of Basic Reactor Operations including
reactor plant parameters, types of reactors, reactivity control, reactor start up and
shutdown, reactivity accidents, and fuel failures.
Supporting Material
OBJECTIVES
• Describe the basic operation of a Pressurized
Water Reactor (PWR).
• List the advantages and disadvantages of a PWR.
• Describe the basic operation of a Boiling Water
Reactor (BWR).
• List the advantages and disadvantages of a BWR.
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OBJECTIVES
• Discuss reactor parameters that are
monitored in a PWR and their important in
the safe operation of the plant.
• Discuss reactor parameters that are
monitored in a BWR and their important in
the safe operation of the plant.
• Discuss reactivity control and reactor response
to control rods, boron, and fission product
poisons.
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OBJECTIVES
• Describe a basic reactor startup and
shutdown.
• Describe various types of reactivity accidents.
• Describe fuel failures, including its causes and
consequences.
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REACTOR OPERATIONS
TYPES OF REACTORS
TYPES OF REACTORS (PWR)
• A Pressurized Water Reactor (PWR) has two
separate loops.
• Primary Loop
– Water is heated in the reactor core and pumped
through steam generator tubes, where it gives
up heat to the secondary side water, causing it
to flash to steam.
– Water in the primary loop is maintained at a high
temperature and pressure to prevent unwanted
boiling in the core.
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TYPES OF REACTORS (PWR)
A pressurizer is used to maintain the pressure in
the primary loop.
– Pressurizer heaters are used to raise pressure, and
a spray nozzle is used to lower pressure.
– Pressurizer pressure is the same as in the primary
loop, but water and steam temperature is
maintained about 100°F higher.
– It is maintained at saturation temperature for the
normal reactor pressure.
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TYPES OF REACTORS (PWR)
• Secondary Loop
– The secondary loop in a PWR takes the water that
flashes to steam around the outside of the tubes
in the steam generator and pipes it to a
turbine/generator, where it is converted to
electricity for use by the grid.
– The unused steam that exits the
turbine/generator is changed back into water in a
condenser and pumped back to the steam
generator to complete the cycle.
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ADVANTAGES OF A PWR
• In a PWR, the primary and secondary water
never come in direct contact with each other.
• As a result of this, the secondary side steam
and water are not radioactive as they are in a
Boiling Water Reactor.
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ADVANTAGES OF A PWR
• The advantages of the secondary side
components not being contaminated are
that:
– no shielding is required around secondary side
components.
– maintenance of secondary side components is
easier because they do not have to be
decontaminated prior to work.
– The overall Man-Rem dose received during an
outage is lower.
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DISADVANTAGES OF A PWR
• More expensive to built initially because there are
many more components.
• Higher pressures and temperatures in the primary
system.
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PRESSURIZED WATER REACTOR
Let’s look at a graphic example.
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TYPES OF REACTORS (BWR)
• In a Boiling Water Reactor (BWR), water in the
core is flashed directly to steam.
• The steam is piped to a turbine/generator,
where it is converted to electricity for use by
the grid.
• The unused steam that exits the
turbine/generator is changed back into water
in a condenser and pumped back to the
reactor vessel to complete the cycle.
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ADVANTAGES OF A BWR
• Cheaper to build initially - not as many
components.
• Lower temperature and pressure in the
reactor system.
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DISADVANTAGES OF A BWR
• Additional shielding required around a
secondary side components because of N-16
gammas.
• All steam and water leaks are radioactive and
cause contamination.
• Maintenance is more difficult because of
internal contamination.
• Man-Rem doses during outages are higher.
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BOILING WATER REACTOR
Let’s look at a graphic example.
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BOILING WATER REACTOR
Reactor
Building
Drywell
(Primary
Containment)
Reactor
Vessel
Torus or
Suppression
Pool
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REACTOR OPERATIONS
REACTOR PLANT PARAMETERS
REACTOR PARAMETERS (PWR)
• There are several very important parameters
that Operators must monitor continuously to
ensure safe operation of a Pressurized Water
Reactor (PWR) plant.
• For a PWR:
– TAVERAGE or TAVE
– THOT or Th
– TCOLD or TC
– Reactor Pressure
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TEMPERATURE (PWR ONLY)
THOT OR TH IS
MEASURED AT
THE OUTLET OF
THE REACTOR.
TCOLD OR TC IS
MEASURED AT
THE INLET OF
THE REACTOR
TAVERAGE IS:
TAVE
TH  TC

2
Pressurizer temperature is normally 100
degrees higher than hot leg temperature.
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PRESSURE (PWR ONLY)
It is important to
monitor reactor
coolant and
pressurizer
pressure.
PT
PT
Too high can cause
leaks and ruptures.
Too low can cause
boiling in the core
and cavitation of the
reactor coolant
pumps.
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FLOW (PWR ONLY)
Forced flow, using
reactor coolant
pumps, is necessary
for removing heat
from the reactor
core.
Flow measurement
helps determine
when flow is
inadequate.
PWR’s are designed for natural circulation flow at greatly
reduced power levels. This is an abnormal situation.
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REACTOR PARAMETERS (BWR)
• There are several very important
parameters that Operators must monitor
continuously to ensure safe operation of a
Boiling Water Reactor (BWR) plant.
• For a BWR:
– Reactor Pressure Vessel Level
– Reactor Pressure
– Reactor Flow
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REACTOR VESSEL LEVEL (BWR)
In a BWR, steam is produced
directly in the reactor vessel as
feedwater comes in contact with
the fuel rods.
It is very important to precisely
maintain reactor vessel water level
for two reasons:
First, to prevent uncovering the core
which would reduce the removal of
heat from the fuel.
Second, to prevent water from
covering the moisture separators and
steam dryers.
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REACTOR VESSEL LEVEL (BWR)
STEAM
SEPARATORS
AND
DRYERS
Uncovering the core can cause
core meltdown.
Covering the steam separators and
dryers at the top of the vessel can
cause moisture carryover in the
steam supply of the main turbine.
REACTOR
CORE
This can cause extensive damage
to the turbine.
Level is maintained in the reactor
pressure vessel at approximately
this point.
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REACTOR PRESSURE (BWR)
PT
Because the steam and water in a BWR
reactor pressure vessel are maintained at
saturation, control of pressure is critical.
Pressure must be maintained high
enough in the reactor vessel to prevent
excessive boiling in the core.
RPV pressure is controlled at approximately
1000 psig by adjusting steam inlet pressure
to the main turbine.
In a saturated system, 1000 psig
equates to approximately 545ºF.
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REACTOR FLOW (BWR)
In a BWR, recirc
pumps are used to
increase flow
through the core.
STEAM OUT
FEEDWATER IN
RECIRC LOOP
Increasing flow
prevents steam
RECIRC
bubbles from
PUMP
forming around the
fuel rods at high
power levels.
Because water is a better heat transfer medium than steam, this
allows a higher power level to be achieved over natural circulation
flow.
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REACTOR OPERATIONS
REACTIVITY CONTROL
REACTIVITY
• Reactivity is defined as “the fractional
change in neutron population in each
generation”. ()
• Remember from our previous lessons that
in a self-sustaining, stable reactor, the
number of neutrons in one generation will
equal the number of neutrons in the next
generation.
• We said that a ratio of 1 : 1 would
represent this.
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REACTIVITY
• If we want to increase reactor power, we must
make the ratio greater than 1 : 1, or
something like 1 : 1.003.
• The .003 in the above ratio represents a
positive rate of change of neutron population
in each generation, which means reactor
power is increasing.
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REACTIVITY
• In this example, generation #1 has 1000
neutrons, then generation #2 would have 1003
neutrons, generation #3 would have 1007
neutrons, and so on.
• We have added +.003 reactivity to the original
stable reactor.
• This is known as “positive reactivity addition”.
• “Negative reactivity addition” is just the
opposite.
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REACTIVITY
• Adding positive reactivity to the reactor does
not mean that reactor power will
automatically start increasing.
• For example, a reactor has a ratio of 1 :
.997 neutron population from one generation
to the next.
• If we add .003 positive reactivity, the ratio will
change to 1 : 1 (the ratio of a stable reactor).
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REACTIVITY MANIPULATIONS
Note that power
level continues to
increase or
decrease until an
equal amount of
opposite
reactivity is
added to
stabilize the
power level.
REACTIVITY ADDITIONS
+.003
-.003
-.005
+.005
P1
P0
Also note that we have added both positive and negative
reactivity to the reactor in this example.
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REACTIVITY CONTROL
• In a PWR, operators control reactivity in two
ways:
– Using the control rods. Pulling the control rods
out of the core adds positive reactivity and driving
them into the core adds negative reactivity.
– Changing the boric acid concentration in the
reactor coolant system. Increasing the
concentration adds negative reactivity and
decreasing the concentration adds positive
reactivity.
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REACTIVITY CONTROL
• In a BWR, operators control reactivity in two
ways:
– Using the control rods. Pulling the control rods
out of the core adds positive reactivity and driving
them into the core adds negative reactivity.
– Changing the flow of water through the core.
Increasing flow strips steam bubbles from the
core. This causes less neutron leakage which
equates to adding positive reactivity.
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REACTOR OPERATIONS
REACTOR STARTUP AND SHUTDOWN
REACTOR STARTUP
• Ensure all supporting systems are running.
– Reactor Coolant Pumps (Recirc pumps for a BWR)
– Secondary Systems
• Condensate
• Circulating Water
• Etc.
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REACTOR STARTUP
• Withdraw control rods in the prescribed
sequence until the reactor is critical.
• Using control rods, raise the temperature of
the reactor system slowly (typically <100°F/hr
to prevent thermal stress on the reactor
vessel) until at normal operating temperature.
• Start up necessary secondary system steamdriven components. (Feed pumps, SJAE’s, etc.)
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REACTOR STARTUP
• Bring the turbine-generator up to speed.
• Synchronize the generator with the grid.
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REACTOR SHUTDOWN
• Essentially the reverse of startup.
• Of concern is the removal of residual heat and
decay heat.
• If the reactor is to be cooled to ambient, a
preset cooldown rate must be followed to
prevent thermal stresses to the reactor vessel.
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REACTOR OPERATIONS
REACTIVITY ACCIDENTS
REACTIVITY ACCIDENTS
•
•
•
•
Dropped control rod (BWR)
Uncontrolled rod withdrawal (PWR)
Increase in Reactor Pressure
Cold Water Accident
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DROPPED CONTROL ROD (BWR)
• Rod becomes stuck at top of core and CRD
uncouples.
• CRD withdrawn fully.
• Rod becomes unstuck and falls freely.
• Results:
– Large, positive reactivity addition to core
– Possible localized fuel damage
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UNCONTROLLED ROD
WITHDRAWAL/DROPPED ROD (PWR)
• Uncontrolled addition or removal of
reactivity from the core caused by either a
dropped rod or a continuously withdrawn
rod.
• Dropped rod(s) or misaligned rods do not
cause core damage.
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UNCONTROLLED ROD
WITHDRAWAL/DROPPED ROD (PWR)
• Continuously withdrawn rod(s) can cause:
– Large, positive reactivity additions to core
– Possible localized fuel damage
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REACTOR PRESSURE INCREASE
• Consequences of Rx. Pressure increase transient
includes:
 Catastrophic failure of vessel or piping
 Lifting of PORV’s/SRV’s
 Turbine Trip
 Main Steam Line Isolation Valve Closure
 Loss of Feedwater with a minimization of energy
removal capability.
 Failure of Residual Heat Removal shutdown cooling
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COLD WATER ACCIDENT
• A cold slug of water rapidly injected into the
reactor vessel causes large positive reactivity
addition. Causes include:
– Loss of Feedwater heating
– Feedwater controller failure - max demand
– Inadvertent Emergency Core Cooling System
actuations
– Inadvertent Safety/Relief Valve operation
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REACTOR OPERATIONS
FUEL FAILURES
FUEL FAILURES
• Causes of fuel failures:
– reactivity accidents (including operator error)
– Manufacturing defects
– Corrosion
• Indication of fuel failures:
– Increased activity in coolant
– Increased off-gas activity
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FUEL FAILURES
• Consequences of fuel failures
 Operation at reduced power levels to meet Tech
Spec off gas release limits.
 Forced or prolonged outages to replace damaged
fuel.
 Increased cost of handling and/or repairing damage
fuel.
 Increased cost of replacement fuel.
 Increased exposure and contamination.
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