Nuclear Fission : An Explanation

Nuclear Fission is the process in which a heavy nucleus is splitted in two light nuclei nearly of the same size by bombarding it with slow neutrons. In a fission reaction, a tremendous amount of energy (190 MeV) is released.

The reason of the release of energy is that the sum of masses of the product nuclei is less than the sum of mass of parent nucleus and neutron, i.e., there is a loss of mass in this reaction. This loss in mass is converted into energy by the einsten's mass-energy equivalent relation.

Controlled an uncontrolled chained nuclear fission reaction-
We know that when slow neutrons are bombarded on Uranium-235, each uranium nucleus splits into two nearly equally  fragments, with a release of three new neutrons and tremendous amount of energy (290 MeV). These new neutron can fission the other uranium nuclei under the suitable conditions. Thus, a chain of fission is formed which once started, continues till entire uranium is consumed. Hence, the energy obtained from the nuclear fission continuously increases. Due to such chain reaction, a strong explosion of entire uranium occurs in very short interval of time and tremendously high energy is released which can be very harmful. This is the uncontrolled chain reaction. A nuclear bomb is based on it.

If the chain reaction is controlled by absorbing some of the neutrons emitted in the fission process by means of moderators (such as graphite, heavy water, etc), the energy obtained in fission can be utilised for constructive purposes. This is the process of a nuclear reactor.

USES OF FISSION - 
The tremendous amount of energy released in the process of fission can be used in two ways :

1) For destructive use :
The fission process is used in a nuclear bomb (some time also called an atom bomb, but the word atom bomb is a misnomer) where the energy is released is fast and uncontrolled.

2)For constructive use:
The fission process is used in a nuclear reactor where the rate of release of energy is slow and controlled. This energy is used to generate the electric power.

Characteristics of Nuclear Fission-
1) It does not occur by itself. For it, neutrons are bombarded on a heavy nucleus.

2) On bombardment of a heavy nucleus with neutrons, a tremendous amount of energy is released and the nucleus splits in two nearly equal fragments.

3)The rate of nuclear fission can be controlled.

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