NUCLEAR REACTOR

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                           Conventional thermal power stations use oil or coal as the source as the source of energy. The reserves of these fuels are becoming depleted in many countries and thus there is a tendency to seek alternative sources of energy. In a nuclear power station instead of a furnace there is a nuclear reactor, in which heat is generated by splitting atoms of radioactive material under suitable conditions. For economical use in a power system a nuclear power station generally has to be large and where large units are justifiable.

Nuclear power plants provide about 17 percent of the world's electricity. Some countries depend more on nuclear power for electricity than others. In France, for instance, about 75 percent of the electricity is generated from nuclear power, according to the International Atomic Energy Agency
so that . In the United States, nuclear power supplies about 15 percent of the electricity overall, but some states get more power from nuclear plants than others. There are more than 400 nuclear power plants around the world, with more than 100 in the United States.

Nuclear power station in India:
In India, it was Dr. H. J. Bhabha who put India on the road to nuclear research, more than two decades ago. At present India have   four nuclear power plants.
·        Tarapur
·        Rana Pratap Sagar
·        Kalpakkam
·        Narora


Tarapur: This is the first power plant of India. It has two boiling water reactors each of 200 Me W capacity and each uses enriched U as fuel.

Rana Pratap Sagar: It is situated at Rajasthan.

Kalpakkam: It is situated at Tamil Nadu.

Narora: It is at U. P.

Main parts of a nuclear power station:

The main parts of a nuclear power station are
·        Nuclear reactor
·        Heat exchanger
·        Steam turbine
·        Condenser
·        Generator

Working:
In a reactor heat is produced by the fissioning or splitting of uranium atoms. A cooling medium takes up this heat and delivers it to the heat exchanger, where steam for the turbine is raised. When the uranium atoms split, there is radiation as well, the reactor and its cooling circuit must be heavily shielded against radiation hazards.

Large electrical generating plants which provide most of our electricity all work on the same principle - they are giant steam engines. Power plants use heat supplied by a fuel to boil water and make steam, which drives a generator to make electricity. A generating plant's fuel, whether it is coal, gas, oil or uranium, heats water and turns it into steam. The pressure of the steam spins the blades of a giant rotating metal fan called a turbine. That turbine turns the shaft of a huge generator. Inside the generator, coils of wire and magnetic fields interact - and electricity is produced.

Parts of Nuclear Reactor:
1.     nuclear fuel
2.     reactor core
3.     moderator
4.     control rods
5.     reflector
6.     reactor vessel
7.     biological shielding
8.     coolant

Nuclear fuel:
Fuel of a reactor should be fissionable material which can be defined as a fissionable material which can be defined as an element or isotope whose nuclei can be caused to undergo nuclear fission nuclear bombardment and to produce a fission chain reaction.
The fuels used are: U238, U235, U 234, UO2
Fertile materials, those which can be transformed into fissile materials, cannot sustain chain reactions. When  a fertile material is hit by neutrons and absorbs some of them, it is converted to fissile material.U238 and Th 232 are examples of fertile materials used for reactor purposes.
Reactor core:
This contains a number of fuel rods made of fissile material.

Moderator:
This material in the reactor core is used to moderate or to reduce the neutron speeds to a value that increases the probability of fission occurring.

Control rods:
The energy inside the reactor is controlled by the control rod. These are in cylindrical or sheet form made of boron or cadmium.
These rods can be moved in and out of the holes in the reactor core assembly.

Reflector:
This completely surrounds the reactor core within the thermal shielding arrangement and helps to bounce escaping neutrons back into the core. This conserves the nuclear fuel.

Reactor vessel:
It is a strong walled container housing the core of the power reactor. It contains moderate, reflector, thermal shielding and control rods.

Biological shielding:
Shielding helps in giving protection from the deadly α- and β-particle radiations and γ-rays as well as neutrons given off by the process of fission within the reactor.

Coolant:
This removes heat from the core produced by nuclear reaction. The types of coolants used are carbon dioxide, air, hydrogen, helium, sodium or sodium potassium.

Principle of reactor control:
When a nucleus captures a neutron the resulting compound nucleus is unstable. It splits into two fragments, releases energy and ejects some neutrons. If conditions are favorable, neutrons ejected by the first fission may be captured by other nuclei and the chain reaction begins. If the energy output from a reactor is to be maintained constant, one neutron and not more than one from each fission must split another nucleus(multiplication factor, k=1)
Otherwise control of chain reaction will not be possible.
 The principal law of nuclear energy is   E = mc2
                             Where   W-Energy (joules)                      
                                m- Mass (kilograms)
                                c- Speed of light (3*108m/sec)

The main reactions inside a reactor are
238U92    +   1n0     à   239U92   +  γ
239U92   has a half life period of 23.5 min only and hence it is    unstable.
239U92    +   0e-1       à   239Np93
239Np93 again has a short half life and emits β-particles.
239Np93   +   0e-1     à  239Pu94

Types of reactors:
1.     boiling water reactor
2.     pressurized water reactor
3.     pressurize heavy water reactor
4.     gas cooled reactor
5.     advanced gas cooled reactor
6.     light water graphite reactor
7.     fast breeder reactor
8.     high temperature gas cooled reactor
9.     CANDU type reactor

What types of reactors are there?
All nuclear reactors operate on the same basic principle, but various designs are in use throughout the world.

Choice of cycle conversion:
1.     A well established method of conversion of heat due to nuclear reaction to electric power by the direct use of the coolant. The reactor heat is transferred to the coolant, which heats water to produce steam for driving the turbine or other heat engine.
2.     Another method for conversion of heat produced in the reactor to electric power is the direct use of liquid or as that cools the reactor to drive the turbine or other heat engine, which in turn drives the electric generator.
3.     Direct generation of electric current from the heat produced during the nuclear reaction. An example of this type of conversion is the production of electric current by means of thermocouples.
4.     Direct generation of electric current from electrons produced during a nuclear reaction.

Advantages of Nuclear Power Plant:
1.     Space requirement of a nuclear power plant is less as compared to other conventional power plants of equal size.
2.     A nuclear power plant consumes very small quantity of fuel. Thus fuel transportation cost is less and large fuel storage facility is not needed.
3.     There is increased reliability of operation.
4.     Nuclear power plants are not affected by adverse weather   conditions.
5.     Nuclear power plants are well suited to meet large power demands. They give better performance at higher load factors (80-90%).
6.     Materials expenditure on metal structures, piping, storage mechanisms are much lower for a nuclear power plant than a coal burning power plant.
7.     It does not require large quantity of water.

Disadvantages:
1.     Initial cost of nuclear power plant is higher as compared to hydro or steam power plant.
2.     Nuclear power plants are not well suited for varying load conditions.
3.     Radioactive wastes if not disposed carefully may have bad effect on the health of workers and other population.
4.     Maintenance cost of the plant is high.
5.     It requires trained personnel to handle nuclear power plants.

Nuclear and Chemical Accidents
1952
Dec. 12, Chalk River, nr. Ottawa, Canada: a partial meltdown of the reactor's uranium fuel core resulted after the accidental removal of four control rods. Although millions of gallons of radioactive water accumulated inside the reactor, there were no injuries.

1953
Love Canal, nr. Niagara Falls, N.Y.: was destroyed by waste from chemical plants. By the 1990s, the town had been cleaned up enough for families to begin moving back to the area.


1957
Oct. 7, Windscale Pile No. 1, north of Liverpool, England: fire in a graphite-cooled reactor spewed radiation over the countryside, contaminating a 200-square-mile area.
South Ural Mountains: explosion of radioactive wastes at Soviet nuclear weapons factory 12 mi from city of Kyshtym forced the evacuation of over 10,000 people from a contaminated area. No casualties were reported by Soviet officials.

1976
nr. Greifswald, East Germany: radioactive core of reactor in the Lubmin nuclear power plant nearly melted down due to the failure of safety systems during a fire.

1979
March 28, Three Mile Island, nr. Harrisburg, Pa.: one of two reactors lost its coolant, which caused overheating and partial meltdown of its uranium core. Some radioactive
later and gases were released. This was the worst accident in U.S. nuclear-reactor history

1984
Dec. 3, Bhopal, India: toxic gas, methyl isocyanate, seeped from Union Carbide insecticide plant, killed more than 2,000, injured about 150,000.

1986
April 26, Chernobyl, nr. Kiev, Ukraine: explosion and fire in the graphite core of one of four reactors released radioactive material that spread over part of the Soviet Union, eastern Europe, Scandinavia, and later western Europe. 31 claimed dead. Total casualties are unknown. Worst such accident to date.

1987
Sept. 18, Goiânia, Brazil: 244 people contaminated with cesium-137 from a cancer-therapy machine that had been sold as scrap. Four people died in worst radiation disaster in Western Hemisphere.

1999
Sept. 30, Tokaimura, Japan: uncontrolled chain reaction in a uranium-processing nuclear fuel plant spewed high levels of radioactive gas into the air, killing two workers and seriously injuring one other.

2004
Aug. 9, Mihama, Japan: non-radioactive steam leaked from a nuclear power plant, killing four workers and severely burning seven others.

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 ”.