A SEMINOR REPORT ON NUCLEAR REACTOR.pdf

7/27/2019 A SEMINOR REPORT ON NUCLEAR REACTOR.pdf

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CHAPTER 01

INTRODUCTION

1.1 WHAT IS A NUCLEAR REACTOR?

A nuclear reactor is a system that contains and controls sustained nuclear

chain reactions. Reactors are used for generating electricity, moving aircraft carriers

and submarines, producing medical isotopes for imaging and cancer treatment, and

for conducting research.

Fuel, made up of heavy atoms that split when they absorb neutrons, is placed

into the reactor vessel (basically a large tank) along with a small neutron source. The

neutrons start a chain reaction where each atom that splits releases more neutrons that

cause other atoms to split. Each time an atom splits, it releases large amounts of

energy in the form of heat. The heat is carried out of the reactor by coolant, which is

most commonly just plain water. The coolant heats up and goes off to a turbine to

spin a generator or drive shaft. So basically, nuclear reactors are exotic heat sources.

Fig. 1.1 Nuclear Reactor


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CHAPTER 02

MECHANISM

Fig.2.1 Fission Reaction

A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn

splits into fast-moving lighter elements (fission products) and free neutrons. Though

both reactors and nuclear weapons rely on nuclear chain-reactions, the rate of

reactions in a reactor occurs much more slowly than in a bomb.

Just as conventional power-stations generate electricity by harnessing

the thermal energy released from burning fossil fuels, nuclear reactors convert the

thermal energy released from nuclear fission.

2.1 FISSION

When a large fissile atomic nucleus such as uranium-235 orplutonium-

239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into

two or more lighter nuclei, (the fission products), releasing kinetic energy, gamma

radiation, and free neutrons. A portion of these neutrons may later be absorbed by

other fissile atoms and trigger further fission events, which release more neutrons, and

so on. This is known as a nuclear chain reaction.
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To control such a nuclear chain reaction, neutron poisons and neutron

moderators can change the portion of neutrons that will go on to cause more

fission. Nuclear reactors generally have automatic and manual systems to shut the

fission reaction down if monitoring detects unsafe conditions.

Commonly-used moderators include regular (light) water (in 74.8% of the

world's reactors), solid graphite (20% of reactors) and heavy water(5% of reactors).

Some experimental types of reactor have used beryllium, and hydrocarbons have been

suggested as anotherpossibility.

2.2 HEAT GENERATION

The reactor core generates heat in a number of ways:

1. The kinetic energy of fission products is converted to thermal energy when

these nuclei collide with nearby atoms.

2. The reactor absorbs some of the gamma raysproduced during fission and

converts their energy into heat.

3. Heat is produced by the radioactive decay of fission products and materials

that have been activated by neutron absorption. This decay heat-source will

remain for some time even after the reactor is shut down.

A kilogram ofuranium-235 (U-235) converted via nuclear processes releases

approximately three million times more energy than a kilogram of coal burned

conventionally (7.2 1013 joulesper kilogram of uranium-235 versus 2.4 107 joules

per kilogram of coal).

2.3 COOLING

A nuclear reactor coolant usually water but sometimes a gas or a liquid

metal (like liquid sodium) ormolten salt is circulated past the reactor core to

absorb the heat that it generates. The heat is carried away from the reactor and is then

used to generate steam. Most reactor systems employ a cooling system that is

physically separated from the water that will be boiled to produce pressurized steam

for the turbines, like the pressurized water reactor. But in some reactors the water for
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the steam turbines is boiled directly by the reactor core, for example the boiling water

reactor.

2.4 REACTIVITY CONTROL

The power output of the reactor is adjusted by controlling how many neutrons

are able to create more fission.

Control rods that are made of a neutron poison are used to absorb neutrons.

Absorbing more neutrons in a control rod means that there are fewer neutrons

available to cause fission, so pushing the control rod deeper into the reactor will

reduce its power output, and extracting the control rod will increase it.

In some reactors, the coolant also acts as a neutron moderator. A moderator

increases the power of the reactor by causing the fast neutrons that are released from

fission to lose energy and become thermal neutrons. Thermal neutrons are more likely

than fast neutrons to cause fission, so more neutron moderation means more power

output from the reactors. If the coolant is a moderator, then temperature changes can

affect the density of the coolant/moderator and therefore change power output. A

higher temperature coolant would be less dense, and therefore a less effective

moderator.

In other reactors the coolant acts as a poison by absorbing neutrons in the

same way that the control rods do. In these reactors power output can be increased by

heating the coolant, which makes it a less dense poison. Nuclear reactors generally

have automatic and manual systems to scram the reactor in an emergency shutdown.

These systems insert large amounts of poison (often boron in the form ofboric acid)

into the reactor to shut the fission reaction down if unsafe conditions are detected or

anticipated.

2.5 ELECTRICAL POWER GENERATION

The energy released in the fission process generates heat, some of which can

be converted into usable energy. A common method of harnessing this thermal

energy is to use it to boil water to produce pressurized steam which will then drivea steam turbine that turns an alternatorand generates electricity.
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CHAPTER 03

COMPONENTS OF NUCLEAR REACTOR

3.1 COMPONENTS

The essential parts of a nuclear reactor are:

1. NUCLEAR FUEL :The nuclear fuel used in a nuclear reactor is the

enriched 92U235.The nuclear fuel is sealed in a long ,narrow metal tubes called fuel

rods . The enriched 92U235ensures that at least one of the neutrons produced by a

fission reaction has a good chance of causing fission in another92U235 nucleus.

2.MODERATOR: The neutron released by fission normally move very fast .At this

high speed , the chance of a neutron being captured by another92U235 nucleus is very

small , If the neutron is slowed , its chance of capture is much better . In order to slow

down the fast fission neutrons, a moderator is used.

3. CONTROL RODS :In order to control the rate at which fission reaction occurs ,

control rods of neutron - absorbing material (eg. cadmium) are used .The control rodskeep the net rate of production of neutrons to the required level by capturing the

necessary proportion of neutrons before they initiate fission. When the control are

moved upward out of the reactor , the number of neutrons left to produce fission is

increased .On the other hand , when the control rods are lowered , the number of

neutrons producing fission is decreased .

4. COOLANT: The propose of the coolant is to removed heat from the reactor core

and take it to the place of its utilization eg. Steam turbine.

5. PROTECTIVE SHIELD: In a nuclear reactor, many types of harmful radiations

are emitted .In order to prevent these radiations from reaching the persons working

near the reactor; the reactor is enclosed in thick concrete walls.


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CHAPTER 04

CLASSIFICATION OF NUCLEAR REACTORS

4.1 CLASSIFICATION BY COOLENT

4.1.1 PRESSURIZED WATER REACTOR

The radioactive water in the reactor core is kept under extreme pressure so that

it does not boil when it hits 100 degrees Celsius but continues to absorb heat. This

super-hot water goes through a heat exchanger which transfers the heat to non-

radioactive water. This water forms super-heated steam which is used to power the

turbines of the power station.

Fig.4.1 Pressurized Water Reactor

4.1.2 BOILING WATER REACTOR

In this reactor the water in the core is not pressurized and so it boils into steam

in the core. This water is then piped out to the turbines where it is used to generate

electricity. Upon cooling, the water is returned to the core. While this reactor type

saves somewhat on the cost of pressurizing the core, it does mean that the radioactive

water from the core is passed through the turbines which then also become


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contaminated with radiation. This reactor model carries a far greater cleanup cost

when it is dismantled as there are far more heavily radiated components.

4.2 Boiling Water Reactor

4.1.3 SODIUM COOLED FAST REACTOR

The first electricity-producing nuclear reactor in the world was SFR .As the

name implies, these reactors are cooled by liquid sodium metal. Sodium is heavier

than hydrogen, a fact that leads to the neutrons moving around at higher speeds

(hence fast). These can use metal or oxide fuel, and burn anything you throw at them

(thorium, uranium, plutonium, higher actinides).

1. Can breed its own fuel, effectively eliminating any concerns about uranium

shortages

2.

Can burn its own waste

3. Metallic fuel and excellent thermal properties of sodium allow for passively

safe operation -- the reactor will shut itself down and cool decay heat without

any backup-systems working (or people around), only relying on physics

(gravity, natural circulation, etc.).
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4.

Fig. 4.3 Sodium Cooled Fast Reactor

4.1.4 CANADA DEUTERIUM-URANIUM REACTORS (CANDU)

CANDUs are a Canadian design found in Canada and around the world. They

contain heavy water, where the Hydrogen in H2O has an extra neutron (making it

Deuterium instead of Hydrogen). Deuterium absorbs many fewer neutrons than

Hydrogen, and CANDUs can operate using only natural uranium instead of enriched.

Require very little uranium enrichment.

Can be refueled while operating, keeping capacity factors high (as long as the

fuel handling machines dont break).

Are very flexible, and can use any type of fuel.

Some variants have positive coolant temperature coefficients, leading to safety

concerns.

Neutron absorption in deuterium leads to tritium production, which is

radioactive and often leaks in small quantities.

Can theoretically be modified to produce weapons-grade plutonium slightly

faster than conventional reactors could be.
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.

Fig. 4.4 Canada Deuterium-Uranium Reactors (CANDU)

4.1.5 HIGH TEMPERATURE GAS COOLED REACTOR

HTGRs use little pellets of fuel backed into either hexagonal compacts

or into larger pebbles (in the prismatic and pebble-bed designs). Gas such as helium

or carbon dioxide is passed through the reactor rapidly to cool it. Due to their low

power density, these reactors are seen as promising for using nuclear energy outside

of electricity: in transportation, in industry, and in residential regimes. They are not

particularly good at just producing electricity.

1. Can operate at very high temperatures, leading to great thermal efficiency

(near 50%!) and the ability to create process heat for things like oil refineries,

water desalination plants, hydrogen fuel cell production, and much more.

2. Each little pebble of fuel has its own containment structure, adding yet another

barrier between radioactive material and the environment.

3. High temperature has a bad side too. Materials that can stay structurally sound

in high temperatures and with many neutrons flying through them are hard to

come by.


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4. If the gas stops flowing, the reactor heats up very quickly. Backup cooling

systems are necessary.

5. Gas is a poor coolant, necessitating large amounts of coolant for relatively

small amounts of power. Therefore, these reactors must be very large toproduce power at the rate of other reactors.

6. Not as much operating experience.

4.2 CLASSIFICATION BY PHASE OF FUEL

1. Solid fueled

2. Fluid fueled

3. Aqueous homogeneous reactor

4. Molten salt reactor

5. Gas fueled

4.3 CLASSIFICATION BY USE

1. Electricity

o

Nuclear power plants including small modular reactors

2. Propulsion,

o Nuclear marine propulsion

o Various proposed forms ofrocket propulsion

3. Other uses of heat

o Desalination

o Heat for domestic and industrial heating

o Hydrogen production for use in a hydrogen economy

4. Production reactors fortransmutation of elements
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CHAPTER 05

NUCLEAR FUEL CYCLE

5.1 NUCLEAR FUEL CYCLE

Thermal reactors generally depend on refined and enriched uranium. Some

nuclear reactors can operate with a mixture of plutonium and uranium .The process by

which uranium ore is mined, processed, enriched, used, possibly reprocessed and

disposed of is known as the nuclear fuel cycle.

Less than 1% of the uranium found in nature is the easily fissionable U-

235 isotope and as a result most reactor designs require enriched fuel. Enrichmentinvolves increasing the percentage of U-235 and is usually done by means ofgaseous

diffusion orgas centrifuge. The enriched result is then converted into uranium

dioxidepowder, which is pressed and fired into pellet form. These pellets are stacked

into tubes which are then sealed and called fuel rods. Many of these fuel rods are used

in each nuclear reactor.

Most BWR and PWR commercial reactors use uranium enriched to about 4%

U-235, and some commercial reactors with a high neutron economy do not require thefuel to be enriched at all (that is, they can use natural uranium). According to

the International Atomic Energy Agency there are at least 100 research reactors in the

world fueled by highly enriched (weapons-grade/90% enrichment uranium). Theft

risk of this fuel (potentially used in the production of a nuclear weapon) has led to

campaigns advocating conversion of this type of reactor to low-enrichment uranium

(which poses less threat of proliferation).

Fissile U-235 and non-fissile but fissionable and fertile U-238 are both used in

the fission process. U-235 is fissionable by thermal (i.e. slow-moving) neutrons. A

thermal neutron is one which is moving about the same speed as the atoms around it.

Since all atoms vibrate proportionally to their absolute temperature, a thermal neutron

has the best opportunity to fission U-235 when it is moving at this same vibrational

speed. On the other hand, U-238 is more likely to capture a neutron when the neutron

is moving very fast. This U-239 atom will soon decay into plutonium-239, which is

another fuel. Pu-239 is a viable fuel and must be accounted for even when a highlyenriched uranium fuel is used. Plutonium fissions will dominate the U-235 fissions in
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some reactors, especially after the initial loading of U-235 is spent. Plutonium is

fissionable with both fast and thermal neutrons, which make it ideal for either nuclear

reactors or nuclear bombs.

Most reactor designs in existence are thermal reactors and typically use water

as a neutron moderator (moderator means that it slows down the neutron to a thermal

speed) and as a coolant. But in a fast breeder reactor, some other kind of coolant is

used which will not moderate or slow the neutrons down much. This enables fast

neutrons to dominate, which can effectively be used to constantly replenish the fuel

supply. By merely placing cheap enriched uranium into such a core, the non-

fissionable U-238 will be turned into Pu-239, "breeding" fuel.

In thorium fuel cycle thorium-232 absorbs a neutron in either a fast or thermal

reactor. The thorium-233 beta decays to protactinium-233 and then to uranium-233,

which in turn is used as fuel. Hence, like uranium-238, thorium-232 is a fertile

material.

5.2 FUELING OF NUCLEAR REACTORS

The amount of energy in the reservoir ofnuclear fuel is frequently expressed

in terms of "full-power days," which is the number of 24-hour periods (days) a reactor

is scheduled for operation at full power output for the generation of heat energy. The

number of full-power days in a reactor's operating cycle (between refueling outage

times) is related to the amount offissile uranium-235 (U-235) contained in the fuel

assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at

the beginning of a cycle will permit the reactor to be run for a greater number of full-

power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent"

and is discharged and replaced with new (fresh) fuel assemblies, although in practice

it is the buildup ofreaction poisons in nuclear fuel that determines the lifetime of

nuclear fuel in a reactor. Long before all possible fission has taken place, the buildup

of long-lived neutron absorbing fission byproducts impedes the chain reaction. The

fraction of the reactor's fuel core replaced during refueling is typically one-fourth for

a boiling-water reactor and one-third for a pressurized-water reactor. The disposition

and storage of this spent fuel is one of the most challenging aspects of the operation of
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a commercial nuclear power plant. This nuclear waste is highly radioactive and its

toxicity presents a danger for thousands of years.

Not all reactors need to be shut down for refueling; for example, pebble bed

reactors, RBMK reactors, molten salt reactors, Magnox, AGRand CANDU reactors

allow fuel to be shifted through the reactor while it is running. In a CANDU reactor,

this also allows individual fuel elements to be situated within the reactor core that are

best suited to the amount of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its burn up, which

is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn

up is commonly expressed as megawatt days thermal per metric ton of initial heavy

metal.
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CHAPTER 06

SAFTY

6.1 SAFETY

To achieve optimum safety, nuclear plants in the western world operate using

a 'defense-in-depth' approach, with multiple safety systems supplementing the natural

features of the reactor core. Key aspects of the approach are:

1. High-quality design & construction,

2. Equipment which prevents operational disturbances or human failures and

errors developing into problems,

3. Comprehensive monitoring and regular testing to detect equipment or operator

failures,

4. Redundant and diverse systems to control damage to the fuel and prevent

significant radioactive releases,

5. Provision to confine the effects of severe fuel damage (or any other problem)

to the plant itself.

6. These can be summed up as: prevention, monitoring, and action (to mitigate

consequences of failures).

The safety provisions include a series of physical barriers between the

radioactive reactor core and the environment, the provision of multiple safety

systems, each with backup and designed to accommodate human error. Safety

systems account for about one quarter of the capital cost of such reactors. As well as

the physical aspects of safety, there are institutional aspects which are no less

important - see following section on international collaboration.


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CHAPTER 07

CONCLUSION

Widely used nuclear energy can be of great benefit for mankind. It can

bridge the gap caused by inadequate coal and oil supply. It should be used to as much

extent as possible to solve power problem. With further developments, it is likely that

the cost of nuclear power stations will be lowered and that they will soon be

competitive. With the depletion of fuel reserves and the question of transporting fuel

over long distances, nuclear power stations are taking an important place in the

development of the power potentials of the nations of the world today in the context

of the changing pattern of power .


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CHAPTER 08

BIBLIOGRAPHY

1. www.google.com

2. www.wikipedia.com

3. www.seminarpapers.com

4. NUCLEAR POWER PLANT BY A. K. RAJA
http://www.google.com/http://www.google.com/http://www.wikipedia.com/http://www.wikipedia.com/http://www.seminarpapers.com/http://www.seminarpapers.com/http://www.seminarpapers.com/http://www.wikipedia.com/http://www.google.com/

Documents

A SEMINOR REPORT ON NUCLEAR REACTOR.pdf