INTRODUCTION TO NUCLEAR POWER
STUDENT HANDOUT
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INTRODUCTION
The development of the Nuclear Power industry occurred within the last 60 years
of the 20th century. The major types of commercial Nuclear Reactors in use in the
United States are the Pressurized Water Reactor (PWR) and the Boiling Water
Reactor (BWR). These terms describe the basic differences in the method of heat
removal from the nuclear fuel, and will be covered in more detail later in this
lesson.
Today's large modern Nuclear Power Plants typically have the capacity of
generating more than 1,000 Megawatts of electrical energy. While these plants are
very complicated and sophisticated, there are many aspects that can be discussed
sufficiently in general terms so that we will be able to have an overall
understanding of how they work.
OBJECTIVES
•
LIST the four basic requirements for a Nuclear Reactor
•
DEFINE the terms PWR and BWR.
•
LIST two differences between a BWR and a PWR.
•
.
NAME three major buildings at a nuclear power site.
THE REACTOR AND ITS COMPONENTS
The real heart of the Nuclear Power Plant is the nuclear reactor, which we will
refer to as the reactor. The four basic requirements for any reactor are its fuel,
coolant, moderator, and control material.
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The discovery that the nucleus of a large atom can be fissioned or split into two or
more fragments by a neutron was made in the years preceding World War II. The
discovery offered no immediate possibility for a useful application until it was
further discovered that each fission of Uranium-235 also released approximately
2.4 neutrons. This surplus of neutrons made a self-sustaining or so-called chain
nuclear reaction theoretically possible. Once the principle of the chain reaction
was established, the possibility of constructing a nuclear reactor became
promising. During the afternoon of December 2, in 1942 the first reactor went into
operation in the football stadium under the west stands of Stagg Field at the
University of Chicago.
A pound of Uranium-235, when completely fissioned, will produce something on
the order of 3 x 1010 BTUs of energy. This is equivalent to the energy contained
in 3 billion pounds of coal. However this Uranium fuel cannot be used in a normal
power plant. It takes a specialized Nuclear Power Plant with many important
design features, and specialized support systems to utilize this special fuel.
Certain atoms can fission when struck by a neutron. This fission usually results in
the splitting of the nucleus into two parts of unequal mass. These parts are known
as fission fragments. Along with the fission fragments, some free neutrons are
also produced (the average number of these free neutrons being produced is
approximately 2.4 neutrons for each Uranium-235 fission event). They are called
free neutrons because they are loose and are not attached to any atoms or
nucleases.
In order for any reaction, chemical or nuclear to be practical, it must be selfsustaining. Most power today is produced from the combustion of natural gas, oil
or coal. In order to start combustion it is necessary to raise the fuel above its
ignition temperature. Once the combustion process has started the temperature of
the furnace in a Boiler becomes high enough so that the reaction remains selfsustaining as long as sufficient fuel and oxygen are continually added.
For each fission that occurs in a nuclear plant, there must be one net neutron
available from the previous fission event to cause the next fission. There are
several processes within the reactor that compete for these vital neutrons.
Materials within the core will absorb some of the neutrons, and some will leak out
of the sides. If either the absorption within the core or the leakage from the core is
excessive, there will not be an average of one neutron left available from each
fission to sustaining the nuclear reaction. If this happens the reactor will start
shutting down.
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The energy released by the fission process appears mainly in the form of the
kinetic energy of the fission fragments. As these fission fragments are still
contained within the fuel rods, the transfer of energy from the fission fragments
appears to us as an increase in the temperature of the fuel material. Heat is
transferred from the fuel elements to the reactor coolant by virtue of the
temperature difference between them, and thus the heat produced in the reactor
fuel is removed while the temperature of the coolant flowing around the fuel
increases.
Most reactors use Uranium Dioxide for fuel. The Uranium Dioxide molecule
contains one Uranium atom and two Oxygen atoms. The Uranium that is used is
generally 2-8% enriched in Uranium-235. Some reactors, however, use natural
Uranium fuel, and some use Uranium Carbide fuel.
TP 10 shows a typical Fuel Assembly. Reactor fuel is generally compacted into
pea sized pellets, which are inserted into hollow tubes or rods made of Zirconium.
The Zirconium rods contain and protect the fuel, and when the fuel pellets within
heat them, they transfer this heat to the whatever touches them. The fuel rods are
bundled together in fuel assemblies and inserted into the reactor as the nuclear
fuel.
In most reactors, ordinary purified water acting as coolant flows past the fuel rods
to remove the heat from the fuel, however some reactors in other countries use gas
as their coolant.
In most reactors, water serves as both the coolant and the moderator. The purpose
of the moderator is to slow down the fast neutrons produced in the fission process
to slow neutrons (thermal neutrons) so that they can cause additional fissions.
Thermal neutrons are more readily absorbed to produce fissions than fast neutrons
are. Some reactors use Graphite as the moderator and some use heavy water
(water with a different type [isotope] of Hydrogen in the molecule).
Control material, in the form of rods, are also inserted into the reactor to absorb
excess neutrons and, therefore, regulate the number of fission events. Control rods
come in different shapes and sizes, depending on the reactor type. Liquid control
material can also be added to the coolant to absorb neutrons.
NEUTRON LEAKAGE
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Neutron leakage from the core is a core surface effect; the production of neutrons
within the core is a volume effect. In order to have good fuel economy we must
minimize the neutron leakage.
In order to minimize leakage it is desirable to have as small a surface-to-volume
ratio as possible. Obviously, the best shape would be a sphere or ball. It can be
demonstrated that as a body becomes larger in volume the ratio of the surface area
to the volume area becomes smaller. Once the materials for use in a given reactor
have been chosen, the neutron economy of the reactor may be improved and the
leakage cut down merely by increasing its size.
If the reactor is too small the leakage will be excessive, and a chain reaction
cannot be sustained. The minimum size that will be able to achieve a selfsustaining reaction within it, is called the critical size for that design. Reactors are
built larger than critical size, so that they can run for long periods, and the excess
neutrons available due to the resultant lower leakage are usually controlled by
means of neutron absorbers in the core. These absorbers, are typically in the form
of control rods or liquid chemical solutions. A reactor operating in the United
States will typically contain anywhere from two to eight kilograms of highly
enriched Uranium fuel and ten or more tons of natural Uranium.
As a reactor is operated, the fission fragments build up within the fuel assemblies.
Many of the fission fragments act as absorbers of neutrons and are harmful to the
neutron economy. After a period of time the fission fragments may have built up
to the point where a self-sustaining nuclear reaction is no longer possible. At this
time some of the fuel assemblies in the reactor must be removed and replaced if
the reactor is going to continue operating
TYPES OF REACTORS
Reactors are classified on the basis of the coolant and the type of neutron that
causes most of the fission events. In terms of coolant, there are four reactor types:
(1) The Light Water Reactor, which uses ordinary purified water;
(2) The Heavy Water Reactor, which uses heavy water, often along with regular
water;
(3) The Gas-Cooled Reactor, which uses gas, usually Helium, and
(4) The Liquid Metal Reactor, which uses a liquid metal, usually Sodium
Potassium (NaK).
In terms of the neutron uses to cause most of the fission events, there are two
Reactor types:
(1) the Fast Reactor, which uses fast neutrons, and
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(2) the Thermal Reactor, which uses thermal neutrons.
The major Reactor of the Light Water Reactor type in the world today is the
Thermal Light Water Reactor. As stated previously, the coolant is ordinary
purified water and this reactor uses thermal neutrons to cause most of the fission
events. Two common types of the Thermal Light Water Reactor are the Boiling
Water Reactor and the Pressurized Water Reactor.
Pressurized Water Reactor
The most common Nuclear Power Plant is the Pressurized Water Reactor, or PWR
as it is commonly called. The PWR has both a primary and a secondary loop. (ref.
TP 23) The fluid in the primary loop is maintained in a liquid by intense pressure.
It remains in liquid form as it continues its repeating cycle throughout the loop. A
self-sustaining nuclear reaction occurs inside the core. This reaction heats the fuel
elements in the core, which in turn heats the fluid as it passes through the core.
After the fluid has left the Reactor core it proceeds to a heat exchanger that we call
a “Steam Generator”. In the steam generator the pressure in the open area is lower
than it is inside the primary loop. The primary coolant inside the tubes, heats the
lower pressure water outside the tubes to the boiling point. When this water boils
it forms steam. This steam then flows to a turbine that is used to drive an
electrical generator, which of course produces electrical power. After leaving the
turbine the steam goes to a “condenser” where it is condensed and returned by the
“feed pumps” to the steam generators, where the cycle starts all over again .
A PWR plant may have as many as two, three or four primary loops. The STP
Units each have four primary loops, with each primary loop having its own steam
generator. Steam leaving the steam generators, is equalized in pressure by a
common header that pipes the steam from all four steam generators to the turbine.
Water from the primary loop enters the reactor vessel, flows down the sides and
then up through the core, picking up the heat from the fuel as it flows over it. The
heated water leaves through the outlet nozzle and flows to the steam generator,
where it gives up its heat to the cooler water of the secondary systems.
In the PWR, there are essentially two basic ways to control the number of fission
events during normal operations: control rods and a liquid control material
(Boron) in the coolant.
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The control rods in a PWR are attached together in cluster assemblies and are
inserted into the Reactor from the top, while the rods in a BWR are inserted from
the bottom. The liquid control is a chemical (Boron) that is added to the water in
the Primary System to absorb neutrons and thus provide additional control.
Additional non-variable neutron controls can also be provides through the use of
neutron poisons installed with the fuel when it is loaded into the reactor vessel.
If any abnormal conditions occur, a reactor “trip” will cause the control rods to be
automatically released so that the force of gravity will drive them down into the
reactor. In addition, a backup system would inject increased amounts of the liquid
control material (liquid neutron absorbers) into the reactor if the trip system failed.
There are many other safety systems in a PWR, each with at least one redundant
backup system.
The entire primary loop and the steam generators are enclosed within the Reactor
Containment Building to protect personnel from nuclear radiation coming from
within the reactor core during normal operations. This containment also protects
the general public and plant personnel from the effects of a major system failure
during the unlikely event of a primary system failure.
A PWR plant includes the Containment Building, the Turbine Building , the
Auxiliary Building, and the Fuel Handling Building. The Reactor Containment
Building is a large, dome-shaped structure.
The Boiling Water Reactor
The Boiling Water Reactor (BWR) is the other of the two major types of Thermal
Light Water Reactors.
A BWR plant includes the Containment, the Turbine, the Auxiliary Building, and
the Fuel Handling Building. The Containment Building is sometimes a large,
square structure.
The BWR also contains the four major components necessary for a reactor - fuel,
coolant, moderator, and control material. The BWR also includes three other
components that are not found in the other major reactor types. These are the:
(1) Moisture Separators and
(2) Steam Dryers, which together separate steam from water, and
(3) Jet Pumps, which increase flow through the fuel.
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TP 24 shows a typical BWR flowpath inside the reactor. Feedwater enters the
reactor vessel and travels through the jet pumps, where its velocity is increased. It
then flows up through the reactor core, picking up heat from the fuel as it goes. In
the upper regions of the core, the water boils, producing steam. The steam is
separated from the moisture within it by the “moisture separators” and “steam
dryers”. The steam then leaves the reactor vessel, and the moisture (water) falls
back down to mix with the feedwater entering the reactor vessel.
The steam leaving the reactor vessel flows to the “turbine”, where it causes the
turbine and the attached electrical generator to turn, thus producing electricity.
The steam leaving the turbine is condensed back to water in the “condenser”, and
the water is then pumped back to the reactor. The major difference between the
PWR and the BWR is that the coolant (water) flowing through the BWR reactor is
allowed to boil in the reactor, directly producing steam, and therefore does not
need or use a "steam generator" as a PWR does.
In the BWR, there are two basic ways to control the number of fission events
during normal operations - control rods and recirculation flow. The control rods
in a BWR are inserted from the bottom of the reactor (remember, this is also
considered a major difference, as in the PWR the control rods are inserted from the
top).
The recirculation flow system is also shown in TP 24. This system controls the
rate that the water flows through the core. As the flow in this system is increased,
water flows faster through the core, and fewer steam bubbles are allowed to form.
Since water is a better moderator than steam, with this fast flow, more neutrons are
slowed down to thermal energies, and there will be more fissions. If recirculation
flow is decreased, more steam bubbles are formed, so in this condition there would
be less moderation and fewer fissions would occur.
If any abnormal conditions occur in the BWR, a reactor trip or scram will cause
the control rods to be inserted automatically up into the reactor. In addition, a
backup system would inject a liquid control substance into the reactor if the trip
system failed. There are many other safety systems in the BWR, each with at least
one redundant backup system.
OTHER REACTOR DESIGNS
Another type of commercial reactor that uses light water as the coolant, but
Graphite as the moderator is a design from the former Soviet Union, the RBMK.
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This variant of the Boiling Water reactor design uses Steam Generators (called
Steam Drums in the RBMK), the design is shown in TP 28) . Originally intended
as a small military plutonium production design, the RBMK was brought into
commercial use because its modular design was found to be easy to scale up,
manufacture and install. It is a boiling water reactor that uses numerous but
identical, power producing chambers or modules, all installed parallel to each
other. These individual modules are used together, instead of a large common
reactor vessel. This reactor type is not licensable in the United States as a
commercial reactor, and it is not found in commercial applications outside the
Commonwealth of Independent States (CIS), [the former Soviet Union].
The Heavy-Water Reactor, a PWR variant is shown in TP 29. The hydrogen
molecules in “heavy water” water have an added neutron in their nucleus.
Because the Hydrogen atoms in heavy water already have a neutron, they do not
normally take any of the free neutrons that are available from the fission process.
This increases the number of available neutrons in the core to a sufficient quantity,
such that natural (NON-enriched) " Uranium may be used as the fuel. A further
advantage of the Heavy Water Reactor is that they are designed to allow refueling
while still operating at full power.
The High-Temperature Gas-Cooled Reactor shown in TP’s 32 & 33 uses Helium
gas as the coolant. Helium is blown through the reactor core, where it picks up
heat from the fuel. The high-temperature Helium then circulates through a heat
exchanger, where it gives up its heat to water. The primary reason for using
Helium as the coolant is that Helium is an inert gas; it will not react with other
materials. The rest of the cycle is the same basic steam/water cycle described for
the Light Water Reactors. The water turns to steam and the steam flows to the
turbine. An evolution of this reactor type is the Advanced Gas Cooled Reactor.
This reactor uses high temperature Carbon Dioxide as the coolant instead of
Helium. Both of these types were developed extensively in Great Briton, with the
earlier type going all the way back to the 1950s (it is the oldest commercial
nuclear design in operation).
For fuel, the Gas-Cooled Reactors uses enriched Uranium fuel. Graphite is used
as the moderator and control rods containing neutron absorbers are used to control
the fission process.
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The next two Reactor types are not in commercial or even common use at present,
but technology may develop to the point where their use is more widespread.
These two Reactor types are (1) the Breeder Reactor and (2) the Fusion Reactor.
The Breeder Reactor (TP’s 37 through 39) works on the principle that it will
produce, or breed, more fuel than it started with*. When certain types of materials
absorb neutrons, the eventual results are atoms that will readily fission. For
example, Uranium-238 will absorb neutrons and eventually produce Plutonium239, which will readily fission. These reactors use fast neutrons for fission and
use either high temperature liquid metal or liquid salt as their coolants.
All Reactors (breeders and non-breeders) that have Uranium-238 or Thorium-232
in them will produce fissionable fuel. However, in the Light Water Reactors, more
fuel is used up than is produced.
The final type of reactor is the Fusion Reactor. (TP’s 41 through48) Some
scientists call this reactor the energy of the future. In the fusion process, two light
Hydrogen Isotopes are fused together to form helium and a free neutron. As this
occurs, energy is emitted. One problem with the fusion reactor is that the
temperature required to fuse two atoms together is around 100,000,000°F. The fuel
supply for this type of reactor, however is unlimited; it is a type of Hydrogen that
is found in the ocean. Experimental facilities have shown the feasibility of fusion
power, and it is expected that technology will develop to the point where fusion
may be a prime source of power sometime during this century.
Major Buildings
Modern power plants generally contain several main buildings or structures. The
five major ones are:
1. The Containment Building, a massive structure that houses the Reactor and
associated equipment, and is capable of withstanding hurricanes, earthquakes,
and other adverse events of nature;
*Note: A breeder reactor is not a "perpetual motion machine". It can only make
fissionable fuel from the non-readily fissionable Uranium-238 and Thorium-232
fuel that is already in the core. Thus the fuel supply will run out just as in a
normal reactor, when there is an insufficient amount of material left to fission and
maintain the necessary steady state power level.
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2. The Turbine Building, which houses the Turbine, the Generator, the
Condenser, pumps, and the other equipment that supports these systems;
3. The Auxiliary Building, which houses support systems such as the Ventilation
Systems, the Water Purifying Systems, and the Water Disposal Systems,
4. The Fuel Handling Building, which houses the fuel transfer equipment and fuel
storage pits, as well as pit cooling and purifying equipment, and
5. The Emergency Diesel Generator Buildings that contain the emergency power
sources that might be needed to help shut down the reactor if normal power
sources were all unavailable.
The plant site also contains a large Switchyard, where the generated electrical
power is fed into circuit breakers that route the electricity to areas where it is used.
The plant site also includes administrative buildings and security buildings.
All plants have some means of cooling the water from the condenser. Some plants
use cooling towers, while others such as the South Texas Project use a reservoir or
other large body of water such as a lakes, or a river.
A Nuclear Power Plant requires a central location to provide control for the
complicated operations involved in producing safe, reliable, and economical
electrical power. The Control Room, which is manned 24 hours a day by
Federally Licensed Operators, contains the controls and monitoring devices
associated with every major system in the plant: switches, indicating lights,
gauges, and other devices that tell the status of the plant at a glance. There are
also Annunciator Panels that alert the operator to the existence of abnormal
conditions. A computer provides additional specific information such as the
temperature and pressure of the systems, and aids the interaction between the
operator and the plant.
SUMMARY:
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The major commercial Nuclear Power Plants in use in the world are the Thermal
Light Water Reactors. This type includes the BWR and PWR or Boiling Water
Reactor and Pressurized Water Reactor. In both versions, the major buildings are
the (1) Containment Building which encloses the reactor and supplies steam to (2)
the Turbine Building to drive the Turbine Generator and other steam loads, (3) the
Auxiliary Building, which encloses equipment and systems necessary for plant
operation, (4) the Fuel Handling Building, which is used to handle, store, and cool
fuel assemblies, and (5) the Emergency Diesel Generator Building, which is the
structure that protects the emergency electrical generators. The Reactor and
support systems are operated from a centrally located Control Room.
Common to all reactors is the need for fuel, coolant, moderator, and control
material. The BWR boils the coolant in the reactor into steam, which then turns
the turbine generator. The PWR uses a secondary loop as the steam cycle, with the
reactor coolant pressurized in the primary loop as the heat source. Both the BWR
and PWR insert a control material to regulate the amount of neutrons allowed to
cause fission in the reactor. A BWR plant in the United States inserts control rods
up into the reactor while a PWR inserts its control rods down into the reactor.
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