Nuclear Power Plant: Menace or endowment

A Thesis Presented

by

ABIR CHOWDHURY

S.M. EFTEHAR UDDIN CHY

and

SHOEB MAHMOOD

to

The Department of Electrical and Electronic Engineering

of

International Islamic University Chittagong

In Partial Fulfillment of the Requirements

For the course EEE-3607 (Solid State Devices)

Specializing in Electrical and Electronic Engineering

May, 2011

ABSTRACT

The world's natural resources are being consumed at an alarming rate. As these resources

diminish, people will be seeking alternative sources by which to generate electricity for

heat and light. The only practical short term solution for the energy-crisis is nuclear

power. Energy sources currently being used are hydroelectricity, wind turbines, solar

power, fossil fuels and nuclear power, and now also hydrogen fuel cells. There is much

controversy over the health and safety issues of using nuclear power, especially after

Three Mile Island and the Chernobyl disasters. Nuclear power, however is not as safe as

burning coal, gas or oil in a factory it is in fact, much more dangerous, There are dangers

associated with a nuclear power plant which far out weigh the benefits to society as a

whole and in part to the community living and working around the power plant. Nuclear

energy is a comparatively new source of energy. The first nuclear power plant was

commissioned in June 1954 in Obninsk, Russia. Nuclear power plants generate only

about 11% of the world's electricity. There are around 316 nuclear power plants in the

world that create 213,000 megawatts of electricity.

PREFACE

This Thesis Paper intends to make a brief conception of Nuclear Energy and Nuclear

Power Plant. It is submitted as a partial fulfillment of the course „Solid State Devices‟ for

B.Sc. degrees in Electrical and Electronic Engineering from International Islamic

University Chittagong. In our thesis, we tried to describe the structure, operations and

impact of Nuclear Power Plant. This is a summarized collection of all necessary data to

know about Nuclear Power Plant. We also tried to signify the merits of Nuclear Energy

over its bad impact.

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ACKNOWLEDGEMENTS

We have many people to thank, including our parents, for their indirect help during the

writing of this thesis. Their confidence in our ability and willingness to let us learn in such

a prestigious department has been greatly appreciated.

Thanks also go to the course instructor who was willing to teach us different

important subjects through this thesis writing. Without him, this thesis would have

been diminished greatly. It was a pleasure both learning from him and getting to

know him as a helpful teacher. He always gives his full effort to teach us the course

materials and does not hesitate in front of us about anything. This thesis writing will

bear us fruit, when we face our Graduation Thesis.

At last we want to show our gratitude to The Almighty Allah for helping us regarding

this thesis writing.

ABIR CHOWDHURY, ET083031

S.M. EFTEHAR UDDIN CHY, ET083014

SHOEB MAHMOOD, ET083025

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TABLE OF CONTENTS

Topic Page

ACKNOWLEDGEMENTS……………………………………………………………. 1

CHAPTER 1: INTRODUCTION……………………………………………………… 4

1.1. Properties of Energy…………………………………………………………….. 4

1.2. History of Nuclear Energy………………………………………………………. 5

1.3. Power plant: A standard form of Energy Conversion…………………………….7

1.4. Nuclear fission……………………………………………………………………8

CHAPTER 2: NUCLEAR REACTOR…………………………………………………11

2.1. Nuclear Power Reactor…………………………………………………………...11

2.2. Components of a nuclear reactor…………………………………………………12

2.3. Fuelling a nuclear power reactor…………………………………………………15

2.4. The power rating of a nuclear power reactor……………………………………..17

2.5. Efficiency of Nuclear energy……………………………………………………..19

CHAPTER 3: CLASSIFICATION……………………………………………………...21

3.1. Basic classification of Nuclear Power Reactor……………………………………21

3.2. Commercial classification of Nuclear Power Reactor…………………………….23

CHAPTER 4: OPERATION…………………………………………………………….31

4.1. Lifetime of nuclear reactors……………………………………………………….31

4.2. Load-following capacity…………………………………………………………..32

4.3. Primary coolants…………………………………………………………………..32

CHAPTER 5: WASTE MANAGEMENT……………………………………………….36

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Topic Page

5.1. Safe place for High-level waste……………………………….............................36

5.2. Reprocessing……………………………………………………………………..36

5.3. Repository………………………………………………………………………...37

5.4. Dry cask………………………………………………………………………......38

5.5. The cooling pools…………………………………………………………………38

CHAPTER 6: SAFETY AND SECURITY……………………………………….…….39

6.1. Environmental Impact……………………………………………………………39

6.2. Safety of Nuclear Power Reactors………………………………………………..40

6.3. Nuclear plants and security issues: Nuclear Policies and Regulations…………...41

6.4. Advantages and disadvantages of Nuclear power……………………………….42

REFERENCES…………………………………………………………………………43

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CHAPTER 1: INTRODUCTION

1.1. Properties of Energy

The characteristics of energy can be categorized from different point of views:

1) As a commodity:

1. Supply

2. Demand

3. Price

2) As an ecological resource:

1. Delectability

2. Environmental impact

3. Future sustainability

4. Frugality

5. Market bystanders

6. Future generations

3) As a social necessity:

1. Availability

2. Distribution

3. Equity

4) As a strategic material:

1. Geopolitics

2. Foreign energy

3. Security

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1.2. History of Nuclear Energy

Ancient Greek philosophers first developed the idea that all matter is made of atoms.

During the 18th and 19th centuries, scientists conducted experiments to unlock the

secrets of the atom. In 1904, British physicist Ernest Rutherford wrote, “If it were ever

possible to control at will the rate of disintegration of the radio elements, an enormous

amount of energy could be obtained from a small amount of matter.”

One year later, Albert Einstein developed his theory of the relationship between mass and

energy. Einstein‟s mathematical representation of his theory, E=mc2, related the amount

of energy that could be derived from a mass if it were transformed to energy. In 1938,

Lise Meitner and Otto Hahn first provided the first experimental evidence of the release

of energy from fission.

The world‟s first self-sustained nuclear fission chain reaction occurred on December 2,

1942, in a squash court under the University of Chicago‟s Stagg Field. Enrico Fermi‟s

reactor, Chicago Pile 1, was built of six tons of uranium metal, 34 tons of uranium oxide,

nearly 400 tons of graphite bricks (to moderate the reaction) and cadmium rods to absorb

free neutrons. After World War II, following the success of the Manhattan Project that

developed the atomic bomb, the U.S. began to use nuclear energy for non-military

purposes.

The first reactor to generate electricity was an experimental breeder reactor run by the

U.S. government in Arco, Idaho, beginning on December 20, 1951. Breeder reactors

differ from commercial light-water reactors by using a fast neutron process that produces,

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or breeds, more fuel than it consumes. Civilian commercial nuclear reactors in the U.S.

are all light-water reactors, which use ordinary water to cool the reactor cores.

The first civilian nuclear power plant began generating electricity at Santa Susana,

California on July 12, 1957. The first large-scale commercial nuclear power plant in the

U.S. began operating on December 2, 1957, in Shippingport, Pennsylvania and continued

to operate until it was shut down in 1982.

1.3. Power plant: A standard form of Energy Conversion

There are many forms of energy conversions:

• Chemical to thermal: fire.

• Chemical to electromagnetic: a candle.

• Chemical to electrical: battery.

• Chemical to mechanical: rocket.

• Thermal to mechanical: heat engine.

• Electromagnetic to chemical: photosynthesis.

• Electromagnetic to thermal: greenhouse effect.

• Chemical to thermal to mechanical to electrical: a power plant.

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1.3.1. Types of Power Plants

Most of the world's electricity is generated by either thermal or hydroelectric power

plants.

1) Thermal power plant: Thermal power plants use fuel to boil water which makes

steam. The steam turns turbines that generate electricity. The majority of thermal power

plants burn fossil fuels because thermal power plants are cheaper to maintain and have to

meet less of the governments requirements compared to nuclear power plants. Fossil

fuels are coal and oil. The downfall of using fossil fuels is that they are limited. Fossil

fuels are developed from the remains of plants and animals that died millions of years

ago. Burning fossil fuels has other downfalls, too. All the burning that is required to turn

the turbines releases much sulfur, nitrogen gases, and other pollutants into the

atmosphere.

2) Hydroelectric power plant: The cleanest, cheapest, and least polluting power plant is

the hydroelectric power plant. Hydroelectric power plants use the great force of rushing

water from a dam or a waterfall to turn the turbines. The main reason most countries use

thermal versus the hydroelectric is because their countries don't have enough

concentrated water to create enough energy to generate electricity.

3) Nuclear power plant: Nuclear energy is a form of energy that is created through the

reaction and exchange of electrons. It is stored in the nuclei of atoms and released either

through fission (splitting atoms) or fusion (joining atoms). Electron magnetic energy is

concerned with the wavelengths of waves such as converting gamma rays to radio waves.

When a free neutron splits a nucleus, energy is released along with free neutrons, fission

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fragments that give off beta rays, and gamma rays. A free neutron from the nucleus that

just split splits another nucleus. This process continues on and is called a chain reaction.

Under controlled conditions, the rate of this chain reaction can be kept at a constant rate.

This produces high temperatures but is not allowed to react out of control as in a nuclear

bomb. The heat produced is used to turn water into steam, the steam then turns a turbine

and generator, creating electricity.

1.4. Nuclear fission

The nucleus is the centre of the atom which is normally made up of the same number of

protons as it has neutrons. However, some very large nuclei in certain isotopes have an

imbalance. They can often be found with too many neutrons, and this imbalance will

result in the nucleus becoming unstable.

Uranium-235 is a radioactive substance which due to its large size and unstable state can

undergo induced fission. Its nucleus can be split into smaller atoms when induced by a

neutron. This process will release two or three neutrons, depending on how the atom

splits. These new neutrons can then initiate the decomposition of the nuclei of other

atoms of Uranium. Propagation by the chain reaction releases more neutrons and causes

further nuclear splits.

1.4.1. Nuclear chain reactions

A chain reaction refers to a process in which neutrons released in fission produce an

additional fission in at least one further nucleus. This nucleus in turn produces neutrons,

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and the process repeats. If the process is controlled it is used for nuclear power or if

uncontrolled it is used for nuclear weapons.

Figure: Nuclear chain reactions

U235 + n → fission + 2 or 3 n + 200 MeV

If each neutron releases two more neutrons, then the number of fissions doubles each

generation. In that case, in 10 generations there are 1,024 fissions and in 80 generations

about 6 x 1023

(a mole) fissions.

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1.4.2. The by-product of nuclear fission

Radioactive, or nuclear, waste is the by-product of nuclear fission.

Once every year, one third of the nuclear fuel in a reactor is replaced with fresh fuel. The

used-up fuel is called spent fuel. Spent fuel is highly radioactive and is the primary form

of high-level nuclear waste. High-level radioactive waste is the by-product of commercial

nuclear power plants generating electricity, and from nuclear materials production at

defense facilities. This high-level waste must be isolated in a safe place for thousands of

years so its radioactivity can die down and not be harmful to people and the environment.

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CHAPTER 2: NUCLEAR REACTOR

2.1. Nuclear Power Reactor

Most nuclear electricity is generated using just two kinds of reactors which were

developed in the 1950s and improved since. New designs are coming forward and some

are in operation as the first generation reactors come to the end of their operating lives.

Over 16% of the world's electricity is produced from nuclear energy, more than from all

sources worldwide in 1960.

A nuclear reactor produces and controls the release of energy from splitting the atoms of

certain elements. In a nuclear power reactor, the energy released is used as heat to make

steam to generate electricity. The principles for using nuclear power to produce

electricity are the same for most types of reactor. The energy released from continuous

fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to

produce steam. The steam is used to drive the turbines which produce electricity.

In the world's first nuclear reactors about two billion years ago, the energy was not

harnessed since these operated in rich uranium ore bodies for a couple of million of years,

moderated by percolating rainwater. Those at Oklo in West Africa, each less than 100

kilowatts, consumed about six tons of that uranium.

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2.2. Components of a nuclear reactor

There are several components common to most types of reactors:

1. Fuel: Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are arranged

in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.

The Fuel Sheath are designed to:

contain the uranium dioxide under normal operating conditions,

minimize neutron absorption,

minimize corrosion and hydrogen/deuterium pickup,

minimize strain effects,

minimize resistance to heat transfer,

minimize hydraulic head loss,

Withstand normal operating (including refueling) loads.

2. Moderator: This is material in the core which slows down the neutrons released from

fission so that they cause more fission. It is usually water, but may be heavy water or

graphite. To provide the necessary operating conditions the moderator system performs

the following functions:

(1) Removes the heat that is continuously generated in the moderator and maintains a

controlled bulk temperature in the calandria.

(2) Maintains the chemical purity within specified limits by providing a means for

diverting a stream through a purification loop.

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(3) Allows short term and long term reactivity control by providing a means for

injection and removal of neutron absorbing chemicals.

(4) The supply, drainage and sampling of the heavy water.

(5) Maintains a controlled bulk temperature in the calandria by providing sufficient

heavy water cooling flow through the heat exchangers and ensures adequate net pump

suction head for the pumps under all normal and upset reactor operating conditions.

(6) Maintains the moderator level in the calandria within the design operating level

during normal operation.

(7) Maintains moderator level within design limits to minimize cover gas compression

and hydrostatic pressures on the lower calandria tubes during upset conditions.

(8) Provides adequate circulation during maintenance and normal shutdown with one

moderator system circuit operating.

(9) Serves as a heat sink with adequate circulation for heat removal following a loss-of-

coolant accident coincident with loss of emergency core cooling, with or without Class

IV power.

3. Control rods: These are made with neutron-absorbing material such as cadmium,

hafnium or boron, and are inserted or withdrawn from the core to control the rate of

reaction, or to halt it. In some PWR reactors, special control rods are used to enable the

core to sustain a low level of power efficiently.

4. Coolant: A liquid or gas circulating through the core so as to transfer the heat from it.

. In light water reactors the water moderator functions also as primary coolant. Except in

BWRs, there is secondary coolant circuit where the steam is made.

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5. Pressure vessel or pressure tubes: Usually a robust steel vessel containing the reactor

core and moderator/coolant, but it may be a series of tubes holding the fuel and

conveying the coolant through the moderator.

6. Steam generator: Part of the cooling system where the primary coolant bringing heat

from the reactor is used to make steam for the turbine. Reactors may have up to four

"loops", each with a steam generator.

Figure: Simplified Arrangement of a Generator Coupled to a Turbine Drive

7. Containment: The structure around the reactor core which is designed to protect it

from outside intrusion and to protect those outside from the effects of radiation in case of

any malfunction inside. It is typically a meter-thick concrete and steel structure.

To achieve this overall function, the containment system includes the following related

safety functions:

i) Isolation: to ensure closure of all openings in the containment when an accident

occurs.

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ii) Pressure/activity reduction: to control and assist in reducing the internal pressure and

the inventory of free radioactive material released into containment by an accident.

iii) Hydrogen control: to limit concentrations of hydrogen/deuterium within containment

after an accident to prevent potential detonation.

iv) Monitoring: to monitor conditions within containment and the status of containment

equipment, before, during and after an accident.

In addition, the containment structure also serves the following functions:

i) limits the release of radioactive materials from the reactor to the environment during

normal operations,

ii) provides external shielding against radiation sources within containment during

normal operations and after an accident,

iii) Protects reactor systems against external events such as tornados, floods, etc.

2.3. Fuelling a nuclear power reactor

Most reactors need to be shut down for refueling, so that the pressure vessel can be

opened up. In this case refueling is at intervals of 1-2 years, when a quarter to a third of

the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have

pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be

refueled under load by disconnecting individual pressure tubes.

If graphite or heavy water is used as moderator, it is possible to run a power reactor on

natural instead of enriched uranium. Natural uranium has the same elemental composition

as when it was mined (0.7% U-235, over 99.2% U-238), enriched uranium has had the

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proportion of the fissile isotope (U-235) increased by a process called enrichment,

commonly to 3.5 - 5.0%. In this case the moderator can be ordinary water, and such

reactors are collectively called light water reactors. Because the light water absorbs

neutrons as well as slowing them, it is less efficient as a moderator than heavy water or

graphite.

During operation, some of the U-238 is changed to plutonium, and Pu-239 ends up

providing about one third of the energy from the fuel.

In most reactors the fuel is ceramic uranium oxide (UO2 with a melting point of 2800°C)

and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are

typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the

zirconium being hard, corrosion-resistant and permeable to neutrons. Numerous rods

form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor

core. In the most common reactors these are about 3.5 to 4 meters long.

Burnable poisons are often used (especially in BWR) in fuel or coolant to even out the

performance of the reactor over time from fresh fuel being loaded to refueling. These are

neutron absorbers which decay under neutron exposure, compensating for the progressive

build up of neutron absorbers in the fuel as it is burned. The best known is gadolinium,

which is a vital ingredient of fuel in naval reactors where installing fresh fuel is very

inconvenient, so reactors are designed to run more than a decade between refueling.

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2.4. The power rating of a nuclear power reactor

Nuclear power plant reactor power outputs are quoted in three ways:

Thermal MWt, which depends on the design of the actual nuclear reactor itself,

and relates to the quantity and quality of the steam it produces.

Gross electrical MWe, indicates the power produced by the attached steam turbine

and generator, and also takes into account the ambient temperature for the

condenser circuit (cooler means more electric power, warmer means less). Rated

gross power assumes certain conditions with both.

Net electrical MWe, which is the power available to be sent out from the plant to

the grid, after deducting the electrical power needed to run the reactor (cooling

and feed-water pumps, etc.) and the rest of the plant. This (as also actual gross

MWe) varies slightly from summer to winter, so normally the lower summer

figure, or an average figure, is used. If the summer figure is quoted plants may

show a capacity factor greater than 100% in cooler times. Some design options,

such as powering the main large feed-water pumps with electric motors (as in

EPR) rather than steam turbines (taking steam before it gets to the main turbine-

generator), explains some gross to net differences between different reactor types.

The EPR has a relatively large drop from gross to net MWe for this reason.

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Figure: Power flow of a nuclear reactor

The relationship between these is expressed in two ways:

Thermal efficiency %, the ratio of gross MWe to thermal MW. This relates to the

difference in temperature between the steam from the reactor and the cooling

water. It is often 33-37%.

Net efficiency %, the ratio of net MWe achieved to thermal MW. This is a little

lower, and allows for plant usage.

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2.5. Efficiency of Nuclear energy

The majority, around 85%, of the energy gained from nuclear fission is the kinetic energy

of the products. In solid fuel, particles can only move a very short distance. Therefore the

kinetic energy is converted into heat as the particles are hitting against each other. The

other 15% of the energy is gained from the Gamma rays emitted during the fission

process, and from the kinetic energy of the neutrons released.

The time taken to capture and split the neutron is minute, taking only 1×10-12

seconds.

The energy gained by splitting an atom comes from the fact that the products formed

from the fission, together with the neutrons weigh less than the original product. The

change in mass appears in the form of energy, and follows Einstein‟s equation E=mc2.

The decay of a single Uranium-235 atom releases on average 200 million electron volts,

the equivalent to 3.204×10-11

joules of energy. In contrast, 4 electron volts are released

per molecule of carbon dioxide in the combustion of fossil fuels. To compare obtainable

energy content between fossil fuels and nuclear fuel, „a pound of highly enriched uranium

is equal to something on the order of a million gallons of gasoline‟. So it can be seen that

this is a very compact source of energy.

The reason for the large amount of energy released is because the forces involved in

nuclear reactions are much greater than those involved in chemical reactions. Uranium is

a very dense metal at 18.95g/cm3 and the nucleus of a Uranium atom is very dense

compared to the whole atom. The protons and neutrons are held very tightly together and

20

the electrons orbiting the nucleus are comparatively far away, so this shows how the

bonds involved are so much stronger.

Nuclear fission is a very efficient source of energy because of the low amounts of waste

products. Combustion of fossil fuels produces waste products such as ash and toxic

fumes. This reduces the amount of usable energy produced by reaction, and therefore

lowers its efficiency.

Uranium is found in most rocks, at 0.000002% concentration. The Uranium found in the

earths crust contains 99.3% Uranium-238 and 0.7% Uranium-235. Another possible

source to extract Uranium from is seawater; the key is to find it in quantities that are

economical for extraction.

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CHAPTER 3: CLASSIFICATION

3.1. Basic classification of Nuclear Power Reactor

3.1.1. Primitive reactors

The world's oldest known nuclear reactors operated at what is now Oklo in Gabon, West

Africa. About 2 billion years ago, at least 17 natural nuclear reactors achieved criticality

in a rich deposit of uranium ore. Each operated at about 20 kW thermal. At that time the

concentration of U-235 in all natural uranium was 3.7 percent instead of 0.7 percent as at

present. (U-235 decays much faster than U-238, whose half-life is about the same as the

age of the Earth.) These natural chain reactions, started spontaneously by the presence of

water acting as a moderator, continued for about 2 million years before finally dying

away.

During this long reaction period about 5.4 tons of fission products as well as 1.5 tons of

plutonium together with other transuranic elements were generated in the ore body. The

initial radioactive products have long since decayed into stable elements but close study

of the amount and location of these has shown that there was little movement of

radioactive wastes during and after the nuclear reactions. Plutonium and the other

transuranics remained immobile.

3.1.2. Advanced reactors

Several generations of reactors are commonly distinguished. Generation I reactors were

developed in 1950-60s and very few are still running today. They mostly used natural

uranium fuel and used graphite as moderator. Generation II reactors are typified by the

present US fleet and most in operation elsewhere. They typically use enriched uranium

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fuel and are mostly cooled and moderated by water. Generation III are the Advanced

Reactors, the first few of which are in operation in Japan and others are under

construction and ready to be ordered. They are developments of the second generation

with enhanced safety.

Generation IV designs are still on the drawing board and will not be operational before

2020 at the earliest, probably later. They will tend to have closed fuel cycles and burn the

long-lived actinides now forming part of spent fuel, so that fission products are the only

high-level waste. Many will be fast neutron reactors.

More than a dozen (Generation III) advanced reactor designs are in various stages of

development. Some are evolutionary from the PWR, BWR and CANDU designs above,

some are more radical departures. The former include the Advanced Boiling Water

Reactor, a few of which are now operating with others under construction. The best-

known radical new design is the Pebble Bed Modular Reactor, using helium as coolant, at

very high temperature, to drive a turbine directly.

Considering the closed fuel cycle, Generation 1-3 reactors recycle plutonium (and

possibly uranium), while Generation IV are expected to have full actinide recycle.

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3.2. Commercial classification of Nuclear Power Reactor

3.2.1. Pressurized Water Reactor (PWR)

This is the most common type, with over 230 in use for power generation and several

hundred more employed for naval propulsion. The design of PWRs originated as a

submarine power plant. PWRs use ordinary water as both coolant and moderator. The

design is distinguished by having a primary cooling circuit which flows through the core

of the reactor under very high pressure, and a secondary circuit in which steam is

generated to drive the turbine.

Figure: inner view of Pressurized water reactor

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a

large reactor would have about 150-250 fuel assemblies with 80-100 tons of uranium.

Water in the reactor core reaches about 325°C; hence it must be kept under about 150

times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a

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pressuriser (see diagram). In the primary cooling circuit the water is also the moderator,

and if any of it turned to steam the fission reaction would slow down. This negative

feedback effect is one of the safety features of the type. The secondary shutdown system

involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water here boils in the heat

exchangers which are thus steam generators. The steam drives the turbine to produce

electricity, and is then condensed and returned to the heat exchangers in contact with the

primary circuit.

Figure: external view of Pressurized water reactor

3.2.2. Boiling Water Reactor (BWR)

This design has many similarities to the PWR, except that there is only a single circuit in

which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils

in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in

25

the top part of the core as steam, and hence with less moderating effect and thus

efficiency there. BWR units can operate in load-following mode more readily then

PWRs.

The steam passes through drier plates (steam separators) above the core and then directly

to the turbines, which are thus part of the reactor circuit. Since the water around the core

of a reactor is always contaminated with traces of radio nuclides, it means that the turbine

must be shielded and radiological protection provided during maintenance. The cost of

this tends to balance the savings due to the simpler design. Most of the radioactivity in

the water is very short-lived, so the turbine hall can be entered soon after the reactor is

shut down.

Figure: inner view of boiling water reactor

26

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in

a reactor core, holding up to 140 tons of uranium. The secondary control system involves

restricting water flow through the core so that more steam in the top part reduces

moderation.

Figure: external view of boiling water reactor

3.2.3. Pressurized Heavy Water Reactor (PHWR or CANDU)

The PHWR reactor design has been developed since the 1950s in Canada as the CANDU,

and more recently also in India. It uses natural uranium (0.7% U-235) oxide as fuel,

hence needs a more efficient moderator, in this case heavy water (D2O).

The moderator is in a large tank called a calandria, penetrated by several hundred

horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy

water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR,

the primary coolant generates steam in a secondary circuit to drive the turbines. The

27

pressure tube design means that the reactor can be refueled progressively without shutting

down, by isolating individual pressure tubes from the cooling circuit.

Figure: simplified diagram of Pressurized Heavy Water Reactor

A CANDU fuel assembly consists of a bundle of 37 half meter long fuel rods plus a

support structure, with 12 bundles lying end to end in a fuel channel. Control rods

penetrate the calandria vertically, and a secondary shutdown system involves adding

gadolinium to the moderator. The heavy water moderator circulating through the body of

the calandria vessel also yields some heat. Newer PHWR designs such as the Advanced

Candu Reactor (ACR) have light water cooling and slightly-enriched fuel. CANDU

reactors can readily be run on recycled uranium from reprocessing LWR used fuel, or a

blend of this and depleted uranium left over from enrichment plants. About 4000 MWe of

PWR can then fuel 1000 MWe of CANDU capacity, with addition of depleted uranium.

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3.2.4. Advanced Gas-cooled Reactor (AGR)

These are the second generation of British gas-cooled reactors, using graphite moderator

and carbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in

stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and

then past steam generator tubes outside it, but still inside the concrete and steel pressure

vessel. Control rods penetrate the moderator and a secondary shutdown system involves

injecting nitrogen to the coolant.

Figure: simplified diagram of Advanced Gas-cooled Reactor

The AGR was developed from the Magnox reactor, also graphite moderated and CO2

cooled, and two of these are still operating in UK. They use natural uranium fuel in metal

form. Secondary coolant is water.

3.2.5. Light water graphite-moderated reactor (RBMK)

This is a Soviet design, developed from plutonium production reactors. It employs long

(7 meter) vertical pressure tubes running through graphite moderator, and is cooled by

29

water, which is allowed to boil in the core at 290°C, much as in a BWR. Fuel is low-

enriched uranium oxide made up into fuel assemblies 3.5 meters long. With moderation

largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron

absorption without inhibiting the fission reaction and a positive feedback problem can

arise, which is why they have never been built outside the Soviet Union.

3.2.6. Fast neutron reactors (FNR)

Some reactors (only one in commercial service) do not have a moderator and utilize fast

neutrons, generating power from plutonium while making more of it from the U-238

isotope in or around the fuel. While they get more than 60 times as much energy from the

original uranium compared with the normal reactors, they are expensive to build. Further

development of them is likely in the next decade, and the main designs expected to be

built in two decades are FNRs. If they are configure to produce more fissile material

(plutonium) than they consume they are called Fast Breeder Reactors (FBR). See also

Fast Neutron Reactors and Small Reactors papers.

3.2.7. Floating nuclear power plants

Apart from over 200 nuclear reactors powering various kinds of ships, Rosatom in Russia

has set up a subsidiary to supply floating nuclear power plants ranging in size from 70 to

600 MWe. These will be mounted in pairs on a large barge, which will be permanently

moored where it is needed to supply power and possibly some desalination to a shore

settlement or industrial complex. The first has two 40 MWe reactors based on those in

icebreakers and will operate at Vilyuchinsk, Kamchatka peninsula, to ensure sustainable

electricity and heat supplies to the naval base there from 2013. The second plant of this

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size is planned for Pevek on the Chukotka peninsula in the Chaun district of the far

northeast, near Bilibino. Electricity cost is expected to be much lower than from present

alternatives.

Table: Nuclear power plants in commercial operation

Reactor type Main

Countries Number GWe Fuel Coolant Moderator

Pressurized Water

Reactor (PWR)

US, France,

Japan, Russia,

China

265 251.6 enriched

UO2 water water

Boiling Water

Reactor (BWR)

US, Japan,

Sweden 94 86.4

enriched

UO2 water water

Pressurized Heavy

Water Reactor

'CANDU' (PHWR)

Canada 44 24.3 natural

UO2

heavy

water

heavy

water

Gas-cooled Reactor

(AGR & Magnox) UK 18 10.8

natural U

(metal),

enriched

UO2

CO2 graphite

Light Water

Graphite Reactor

(RBMK)

Russia 12 12.3 enriched

UO2 water graphite

Fast Neutron

Reactor (FBR) Japan, Russia 2 1.0

PuO2 and

UO2

liquid

sodium none

Other Russia 4 0.05 enriched

UO2 water graphite

TOTAL 439 386.5

GWe = capacity in thousands of megawatts (gross)

Source: Nuclear Engineering International Handbook 2010

31

CHAPTER 4: OPERATION

4.1. Lifetime of nuclear reactors

Most of today's nuclear plants which were originally designed for 30 or 40-year operating

lives. However, with major investments in systems, structures and components lives can

be extended, and in several countries there are active programs to extend operating lives.

In the USA most of the more than one hundred reactors are expected to be granted license

extensions from 40 to 60 years. This justifies significant capital expenditure in upgrading

systems and components, including building in extra performance margins.

Some components simply wear out, corrode or degrade to a low level of efficiency.

These need to be replaced. Steam generators are the most prominent and expensive of

these, and many have been replaced after about 30 years where the reactor otherwise has

the prospect of running for 60 years. This is essentially an economic decision. Lesser

components are more straightforward to replace as they age. In CANDU reactors,

pressure tube replacement has been undertaken on some plants after about 30 year‟s

operation.

A second issue is that of obsolescence. For instance, older reactors have analogue

instrument and control systems. Thirdly, the properties of materials may degrade with

age, particularly with heat and neutron irradiation. In respect to all these aspects,

investment is needed to maintain reliability and safety. Also, periodic safety reviews are

undertaken on older plants in line with international safety conventions and principles to

ensure that safety margins are maintained.

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4.2. Load-following capacity

Nuclear power plants are essentially base-load generators, running continuously. This is

because their power output cannot readily be ramped up and down on a daily and weekly

basis, and in this respect they are similar to most coal-fired plants. However, in some

situations it is necessary to vary the output according to daily and weekly load cycles on a

regular basis, for instance in France, where there is a very high reliance on nuclear power.

While BWRs can be made to follow loads reasonably easily without burning the core

unevenly, this is not as readily achieved in a PWR. The ability of a PWR to run at less

than full power for much of the time depends on whether it is in the early part of its 18 to

24-month refueling cycle or late in it, and whether it is designed with special control rods

which diminish power levels throughout the core without shutting it down. Thus, though

the ability on any individual PWR reactor to run on a sustained basis at low power

decreases markedly as it progresses through the refueling cycle, there is considerable

scope for running a fleet of reactors in load-following mode.

4.3. Primary coolants

The advent of some of the designs mentioned above provides opportunity to review the

various primary coolants used in nuclear reactors. There is a wide variety - gas, water,

light metal, heavy metal and salt:

1. Water or heavy water must be maintained at very high pressure (1000-2200 psi, 7-

15 MPa) to enable it to function above 100°C, as in present reactors. This has a

major influence on reactor engineering. However, supercritical water around 25

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MPa can give 45% thermal efficiency - as at some fossil-fuel power plants today

with outlet temperatures of 600°C, and at ultra supercritical levels (30+ MPa)

50% may be attained.

2. Helium must be used at similar pressure (1000-2000 psi, 7-14 MPa) to maintain

sufficient density for efficient operation. Again, there are engineering

implications, but it can be used in the Brayton cycle to drive a turbine directly.

3. Carbon dioxide was used in early British reactors and their AGRs. It is denser

than helium and thus likely to give better thermal conversion efficiency. There is

now interest in supercritical CO2 for the Brayton cycle.

4. Sodium, as normally used in fast neutron reactors, melts at 98°C and boils at

883°C at atmospheric pressure, so despite the need to keep it dry the engineering

required to contain it is relatively modest. However, normally water/steam is used

in the secondary circuit to drive a turbine (Rankine cycle) at lower thermal

efficiency than the Brayton cycle.

5. Lead or lead-bismuth eutectic in fast neutron reactors are capable of higher

temperature operation. They are transparent to neutrons, aiding efficiency, and

since they do not react with water the heat exchanger interface is safer. They do

not burn when exposed to air. However, they are corrosive of fuel cladding and

steels, which originally limited temperatures to 550°C. With today's materials

650°C can be reached, and in future 700°C is in sight, using oxide dispersion-

strengthened steels. A problem is that Pb-Bi yields toxic polonium (Po-210)

activation products. Pb-Bi melts at a relatively low 125°C (hence eutectic) and

34

boils at 1670°C, Pb melts at 327°C and boils at 1737°C but is very much more

abundant and cheaper to produce than bismuth, hence is envisaged for large-scale

use in the future, though freezing must be prevented. The development of nuclear

power based on Pb-Bi cooled fast neutron reactors is likely to be limited to a total

of 50-100 GWe, basically for small reactors in remote places. In 1998 Russia

declassified a lot of research information derived from its experience with

submarine reactors, and US interest in using Pb or Pb-Bi for small reactors has

increased subsequently. The Hyperion reactor will use lead-bismuth eutectic

which is 45% Pb, 55% Bi.

6. Molten fluoride salt boils at 1400°C at atmospheric pressure, so allows several

options for use of the heat, including using helium in a secondary Brayton cycle

with thermal efficiencies of 48% at 750°C to 59% at 1000°C, or manufacture of

hydrogen.

Low-pressure liquid coolants allow all their heat to be delivered at high temperatures,

since the temperature drop in heat exchangers is less than with gas coolants. Also, with a

good margin between operating and boiling temperatures, passive cooling for decay heat

is readily achieved. The removal of passive decay heat is a vital feature of primary

cooling systems, beyond heat transfer to do work. When the fission process stops, fission

product decay continues and a substantial amount of heat is added to the core. At the

moment of shutdown, this is about 6% of the full power level, but it quickly drops to

about 1% as the short-lived fission products decay. This heat could melt the core of a

35

light water reactor unless it is reliably dissipated. Typically some kind of convection

flow is relied upon.

Figure: heat transfer for different primary coolants

Producing steam to drive a turbine and generator is relatively easy, and a light water

reactor running at 350°C does this readily. As the above section and Figure show, other

types of reactor are required for higher temperatures. A 2010 US Department of Energy

document quotes 500°C for a liquid metal cooled reactor (FNR), 860°C for a molten salt

reactor (MSR), and 950°C for a high temperature gas-cooled reactor (HTR). Lower-

temperature reactors can be used with supplemental gas heating to reach higher

temperatures, though employing an LWR would not be practical or economic. The DOE

said that high reactor outlet temperatures in the range 750 to 950°C were required to

satisfy all end user requirements evaluated to date for the Next Generation Nuclear Plant.

36

CHAPTER 5: WASTE MANAGEMENT

5.1. Safe place for High-level waste

The name of the safe place that the Department of Energy of US Government is trying to

make is called a repository. But until a repository is made, spent fuel and high-level

waste is being stored in temporary storage facilities called dry casks and cooling pools.

By the end of the year 2000, there will be more than 40,000 metric tons of high-level

waste in casks and storage pools. There will also be more than 8,000 metric tons of high-

level waste from defense programs. The high-level waste from defense programs is

currently being stored in Idaho, South Carolina, and Washington.

Dry casks and cooling pools are being used to store spent fuel in power plants

everywhere. Dry casks and cooling pools are only meant to be temporary storage

facilities until a permanent repository is made. The need for a permanent disposal for

high-level radioactive waste is becoming more urgent every year because the dry casks

and cooling pools at nuclear power plants are filling up.

5.2. Reprocessing

Reprocessing is the chemical process by which uranium and plutonium are recovered

from spent fuel. This means that it is the recycling process of high-level waste. The

reason private industries aren't reprocessing their high-level waste is because

reprocessing costs more than mining and making new fuel. Several countries that actually

care about their environment reprocess their high-level waste.

37

Figure: block diagram of uranium reprocessing

(source: US Department of Energy)

5.3. Repository

A repository is a storage facility that stores high-level nuclear waste deep underground so

the waste can not harm or effect people or the environment. The guidelines of a

repository are mainly if the geologic location will work out. To make sure that the

repository would be able to stay unscathed for thousands of years, scientists in all areas of

science are making predictions of what could happen over the time period.

A rem is a unit scientist use to measure radiation exposure. Over a person‟s lifetime, they

usually receive 7-14 rems of natural sources of radiation, such as cosmic rays and

ultraviolet rays from the sun. On a single exposure of 5-75 rems, there are few to no

38

noticeable symptoms. For someone to receive 75-200 rems of exposure, vomiting,

fatigue, and loss of appetite would occur. Recovery would take a few weeks. If someone

were to be exposed to more than 300 rems, severe changes in blood cells and hemorrhage

takes place. If someone were to receive more than 600 rems, symptoms would be hair

loss, loss in your body‟s ability to fight infection and usually results in death. The main

reason for building a repository is to keep people and the environment safe from deadly

radiation.

5.4. Dry cask

A dry cask is a concrete of steel container that protects the outside world from its

radioactive innards. A cooling pool is a pool of water that the spent fuel is put into. The

water is a radioactive shield and coolant.

5.5. The cooling pools

The cooling pools are a type of concrete warehouse. Inside the warehouses are steel

caskets containing the spent fuel rods and cooling pools. Scientists say that the cooling

pool prevents the spent fuel to explode, but the extreme weight of the fuel inside the

warehouses might cause the structures to rupture, especially in the case of an earthquake.

The cooling pools were supposed to contain no more than 400 fuel assemblies,

approximately 80,000 rods. The pools contain over four times as much of the spent fuel

that they're supposed to. Nearly all of the nation‟s older power plants are in this state of

overload.

39

CHAPTER 6: SAFETY AND SECURITY

6.1. Environmental Impact

The increased acceptance of nuclear power is not without criticism and challenges.

Critics of nuclear power cite the potential environmental impact of accidents at nuclear

reactors, ranging from a catastrophic meltdown of a reactor core to minor accidents that

release relatively small amounts of radioactivity into the environment.

On March 28, 1979, Pennsylvania‟s Three Mile Island‟s Unit 2 suffered a partial

meltdown of its reactor core. According to a report by the Nuclear Regulatory

Commission, equipment failures, design-related problems and human error led to this, the

nation‟s most serious commercial nuclear accident. No lives were lost as a result of the

accident. Following the accident, NRC improved the level of safety at reactor sites by

increased safety regulations inspection procedures.

On April 26, 1986, the world‟s most significant nuclear accident occurred in the Ukraine,

then part of the Soviet Union. A sudden surge of power in the Unit 4 reactor at the

Chernobyl nuclear power plant caused an explosion and fire that destroyed the reactor

and released massive amounts of radioactive material into the surrounding area. The

accident was caused by breaches of technical operating procedures as well as inadequate

safety systems. About 116,000 people were evacuated from the surrounding area. The

death toll from the explosion and immediate aftermath is officially 30, with 28 deaths due

to radiation exposure among power plant employees and firemen.

In addition, nuclear power plants use large quantities of water for cooling purposes.

Depending upon the plant type, electricity generation from nuclear power requires

40

withdrawals of between zero and 17,590 gallons per million Btu of heat produced. This is

the amount of water extracted from a water source; most of the water withdrawn is

returned to that source. Water consumption refers to the portion of those withdrawals that

is actually used and no longer available. Nuclear energy consumes between zero and 211

gallons of water for each million Btu of heat energy produced.

6.2. Safety of Nuclear Power Reactors

From the outset, there has been a strong awareness of the potential hazard of both nuclear

criticality and release of radioactive materials. There have been three major reactor

accidents in the history of civil nuclear power - Three Mile Island, Chernobyl and

Fukushima. One was contained without harm to anyone, the next involved an intense fire

without provision for containment, and the third severely tested the containment,

allowing minor release of radioactivity. These are the only major accidents to have

occurred in over 14,400 cumulative reactor-years of commercial operation in 32

countries. The risks from western nuclear power plants, in terms of the consequences of

an accident or terrorist attack, are minimal compared with other commonly accepted

risks. Nuclear power plants are very robust.

In relation to nuclear power, Safety is closely linked with Security and in the nuclear field

also with Safeguards.

Safety focuses on unintended conditions or events leading to radiological releases

from authorized activities. It relates mainly to intrinsic problems or hazards. Safety is

achieved through "defense in depth".

41

On the other hand, Security focuses on the intentional misuse of nuclear or other

radioactive materials by non-state elements to cause harm. It relates mainly to external

threats to materials or facilities.

Safeguards focus on restraining activities by states that could lead to acquisition

of nuclear weapons. It concerns mainly materials and equipment in relation to rogue

governments.

6.3. Nuclear plants and security issues: Nuclear Policies and Regulations

The philosophy of physical protection of a nuclear power plant is well described by the

Institute of Nuclear Materials Management. The following diagram describes the

essential components of physical protection of a plant.

1) Physical protection

i) Determination of protection objectives

ii) Facility characterization and assets identification

iii) Threat definition and characterization

iv) Target identification

v) Protection area design

(1) Exclusion zone

(2) Protected area

(3) Vital area

(4) Material access area

vi) Detection system (interior and exterior)

42

vii) Delay system design

viii) Response system design (force-on-force, etc)

2) Regulation guidance and communication

3) Licensing requirements by NRC

4) International treaties

6.4. Advantages and disadvantages of Nuclear power

6.4.1. Advantages

Nuclear power generation does emit relatively low amounts of carbon dioxide (CO2). The

emissions of green house gases and therefore the contribution of nuclear power plants to

global warming is therefore relatively little.

This technology is readily available, it does not have to be developed first.

It is possible to generate a high amount of electrical energy in one single plant.

6.4.2. Disadvantages

The problem of radioactive waste is still an unsolved one.

High risks: It is technically impossible to build a plant with 100% security.

The energy source for nuclear energy is Uranium. Uranium is a scarce resource, its

supply is estimated to last only for the next 30 to 60 years depending on the actual

demand.

43

REFERENCES

1. Power plant Engineering; New Age International Publishers.

2. The History of Nuclear Energy; US Department of Energy;

http://nuclear.gov/pdfFiles/History.pdf.

3. Power Reactors; U.S. Nuclear Regulatory Commission;

http://www.nrc.gov/reactors/power.html.

4. The Current Status, Safety, and Transportation of Spent Nuclear Fuel; National

Academy of Engineering.

5. Waste Management and Disposal; World Nuclear Association; http://www.world-

nuclear.org/how/wastemanag.html.

6. Nuclear Power Plant Systems and Operation by Dr. George Bereznai.

7. Nuclear Physics and Reactor Theory, Volume 1 & 2; DOE FUNDAMENTALS

HANDBOOK; U.S. Department of Energy.

8. The Fission Process and Heat Production; Nuclear Regulatory Commission;

http://www.nrc.gov/reading-rm/basic-ref/teachers/02.pdf.

9. The International Nuclear Event Scale; International Atomic Energy Agency;

http://www.iaea.org/Publications/Factsheets/English/ines.pdf.

10. Thermodynamics: An Engineering Approach; McGraw-Hill, New York.

11. Pressurized water reactor systems; Nuclear Regulatory Commission, USA.

12. THE ENERGY REPORT, MAY 2008; Texas Comptroller of Public Accounts.