Nuclear and Hydel Power Plants

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    Introduction

    Life without electricity is unthinkable. This electricity is generated by power

    plants using coal, water etc., Use of nuclear energy as a power source is becoming a

    common trend in the world. This is due to the rapid depletion of conventional energy

    source. Fuel transportation network and large storage facility in case of thermal

    power plants and dependence on whether conditions in case of hydel power plants are

    the major hurdles faced by today's power sectors. Utilization of nuclear power helps

    to save a considerable amount of fossil fuels which can be used in other areas.

    Nuclear power engineering is basically concerned with the phenomenon taking place

    within the nucleus of the atoms.

    One of the outstanding attractions is the large amount of energy that can be

    released from a small mass of active material. Complete fission of one kg of uranium

    92U235 results in energy equivalent to 4500 tones of high grade coal or 1900 tones of

    oil. The economic advantage of nuclear power can be realized only at a load of

    about 75%.

    Nuclear energyThe energy released due to the split up of nucleus into two or more smaller

    nuclei is known as atomic or nuclear energy. An atom consists of a relatively heavy,

    positively charged nucleus and a number of much lighter negatively charged electrons

    that revolves in different orbits around the nucleus. These electrons are held in their

    orbits by electro static forces. The nucleus consist of sub particles nucleons which, in

    turn, has electrically neutral charge neutrons and positively charged protons. It is

    difficult to bring these protons together in a nucleus of an atom. There, it requires

    same to bring and keep the protons together in the nucleus of an atom. This energy is

    known as binding energy.

    The forces that hold the protons and neutrons inside the nucleus are million

    times as strong as the electrostatic forces holding the electrons to the nucleus. Thus,

    the binding energy is very large compared with chemical bond energy. Therefore, if a

    nucleus disintegrates then a very large amount of energy is released. This energy is

    due to the fission of neutrons and is used for power production in nuclear plants.

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    This energy is known as atomic energy or prompt energy or nuclear energy. In

    other words, the amount of energy what released now is only at the time of fission. At

    the time of fission, the neutrons are under fast moving conditions. If the reaction

    proceeds at this rapid movement, then maximum utilization of energy is not possible.

    So, fast moving neutrons are decelerated with the help of moderator to get more

    amount of energy. Thus, more energy is produced due to slow decay of the fission

    fragments into fission products and the non fission capture of excess neutrons in

    reactions (by means of reflectors). This produces energy much lesser than that of

    energy obtained by fission. So, the total energy produced per fission reaction is

    greater than the prompt energy and is about 200 MeV.

    Nuclear fission and fusion reactions

    Nuclear reaction plays a vital role in power production. So, it is necessary to

    study the basic concepts behind the operation of nuclear reactors. These reactions are

    three types namely fission, fusion and radioactivity. The fission and fusion concepts

    are well applicable to power production systems as these reactions will release

    enormous amount of heat energy than radioactivity.

    Nuclear fusion

    In nuclear fusion, two or more lighter nuclei (of similar electric charge) are

    made to collide to fuse them together in a plasma at high temperatures (above 108oC

    i.e., high particle velocities). As a result of this, a heavier nucleus is formed and the

    mass difference is transformed into enormous amount of heat which is much greater

    than that was produced by fission reaction. To cause fusion, the nuclei are necessary

    to accelerate to high kinetic energies by raising their temperature to hundreds of

    millions of degrees in order to overcome electrical repulsive forces.

    The tremendous energy produced in the sun and stars are by continuous fusion

    reactions. In the sun, quintillions of hydrogen nuclei collide with each another and

    fuse to become helium nuclei. During this, they lose a small amount of their original

    mass which is converted into extremely energetic particles, radiation and heat. This

    process is also known as thermo-nuclear fusion. For generating power by this process,

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    a controlled, relatively slow and sustained fusion is needed. The nuclear fusion is

    presently under experimentation and the results are expected by 2015.

    Nuclear fission

    Figure 1 : Nuclear fission reaction

    In nuclear fission, a high energy neutron is made to struck with a heavy nuclei.

    Due to this effect the heavy nucleus will split into two almost equal fragments of

    lighter atoms accompanied by release of heat plus two to three free neutrons. Fission

    can be caused by bombarding with high energy a particles, protons, deutrons, X -rays

    as well as neutrons. But, neutrons are most suitable for fission reaction because they

    are electrically neutral and thus do not require high kinetic energy to overcome the

    electrical repulsion from positively charged nuclei.

    One of the elements whose nucleus easily fissions is 92U235 . All the other are

    naturally available stable elements and hence splitting the nucleus of these atoms isnot as easy thing. Thus in a fission process, a high energy neutron is made to struck

    with heavy unstable uranium ( 92U235) nuclei. During fission reaction, the following

    results.

    1. The immediate (prompt) products of a fission reaction, such as ( 54Xe 140) xenon

    and strontium ( 38Sr94) are called fission fragments.

    2. Fission fragments are released along with other decay products ( , , etc.)arecalled fission products.

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    3. Two to three fast free neutrons accompanied by the release of heat for every

    bombardment.

    Controlled and uncontrolled chain reaction

    Figure 2 : Chain Reaction

    As discussed in nuclear fission reaction, when an unstable neutron enters the

    uranium 92U235 nucleus, the nucleus splits into two fragments and releases 2 to 3

    neutrons per fission accompanied with heat. The neutrons released during the fission

    can strike other nuclei of 92U235 and causes further fission reactions under favorable

    conditions. Thus the reaction becomes self-sustaining and results in continuous chain

    reaction. Thus, a chain reaction is a self-sustained, continuing sequence of fission

    reactions which is maintained under favorable conditions. This chain reaction may

    either occur in controlled manner or uncontrolled manner shown in figure 2.

    As discussed above, 2 to 3 neutron will be releasing as a result of fission

    reaction. As an average, 2.5 neutrons are ejected per neutron absorbed. Out of 2.5neutrons, nearly 0.2 to 0.3 neutrons is lost due to escape at the surface and the

    remaining is 2.2 neutrons. If these 2.2 neutrons are allowed to continue the chain

    reaction, an exponential increase in reaction rate can be noticed. As a result of this,

    enormous amount of heat energy will be evolved and such a type of chain reaction is

    known as uncontrolled chain reaction. This extremely large amount of energy is

    applied in atomic explosion.

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