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 self-

sustaining. 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 self-

sustaining 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 self-

sustaining 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 Plutonium-

239, 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.