Basic Nuclear Physics- Reactor Operations

ACADs (08-006) Covered

KeywordsPressurized Water Reactor (PWR), Boiling Water Reactor (BWR), primary loop, reactivity, reactivity control, reactivity accidents, control rod drop, fuel failure,

DescriptionThis 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

1.1.4.6 1.2.1 1.2.2.3 1.1.1.1

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.

FEN-BET-I766 Rev. 0 2

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.


OBJECTIVES

• Describe a basic reactor startup and shutdown.

• Describe various types of reactivity accidents.

• Describe fuel failures, including its causes and consequences.


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.



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.



• 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.


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.


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.FEN-BET-I766 Rev. 0 10

DISADVANTAGES OF A PWR

• More expensive to built initially because there are many more components.

• Higher pressures and temperatures in the primary system.


PRESSURIZED WATER REACTOR


Let’s look at a graphic example.

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.


ADVANTAGES OF A BWR

• Cheaper to build initially - not as many components.

• Lower temperature and pressure in the reactor system.


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.


BOILING WATER REACTOR


Let’s look at a graphic example.

BOILING WATER REACTOR


Drywell (Primary

Containment)

Torus or Suppression

Pool

Reactor Vessel

Reactor Building

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 - Reactor Flow– THOT or Th

– TCOLD or TC

– Reactor Pressure


TEMPERATURE (PWR ONLY)


2

TTT CH

AVE

TCOLD OR TC IS MEASURED AT THE INLET OF THE REACTOR

TAVERAGE IS:

Pressurizer temperature is normally 100 degrees higher than hot leg temperature.

THOT OR TH IS MEASURED AT THE OUTLET OF THE REACTOR.

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.

FLOW (PWR ONLY)


Forced flow, using reactor coolant pumps, is necessary for removing heat from the reactor core.

PWR’s are designed for natural circulation flow at greatly reduced power levels. This is an abnormal situation.

Flow measurement helps determine when flow is inadequate.

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


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.

REACTOR VESSEL LEVEL (BWR)


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.

This can cause extensive damage to the turbine.

REACTOR CORE

STEAM SEPAR-ATORS AND DRYERS

Level is maintained in the reactor pressure vessel at approximately this point.

REACTOR PRESSURE (BWR)


Because the steam and water in a BWR reactor pressure vessel are maintained at saturation, control of pressure is critical.

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.

PT

Pressure must be maintained high enough in the reactor vessel to prevent excessive boiling in the core.

REACTOR FLOW (BWR)


FEEDWATER IN

STEAM OUT

RECIRC LOOP

RECIRC PUMP

In a BWR, recirc pumps are used to increase flow through the core.

Increasing flow prevents steam bubbles from 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.

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.


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.


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. FEN-BET-I766 Rev. 0 31

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).


REACTIVITY MANIPULATIONS


P0

P1

+.003 -.003 -.005 +.005

REACTIVITY ADDITIONSNote that power level continues to increase or decrease until an equal amount of opposite reactivity is added to stabilize the power level.

Also note that we have added both positive and negative reactivity to the reactor in this example.

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.


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.


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.


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 steam-driven components. (Feed pumps, SJAE’s, etc.)


REACTOR STARTUP

• Bring the turbine-generator up to speed.• Synchronize the generator with the grid.


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.


REACTOR OPERATIONS

REACTIVITY ACCIDENTS

REACTIVITY ACCIDENTS

• Dropped control rod (BWR)• Uncontrolled rod withdrawal (PWR)• Increase in Reactor Pressure• Cold Water Accident


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


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.


UNCONTROLLED ROD WITHDRAWAL/DROPPED ROD (PWR)

• Continuously withdrawn rod(s) can cause:– Large, positive reactivity additions to core– Possible localized fuel damage


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


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


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


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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Basic Nuclear Physics- Reactor Operations