a seminor report on nuclear reactor.pdf
TRANSCRIPT
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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/