A process in which bombardment of a nucleus by fast moving neutrons results in its break up in two smallest nuclei of almost equal mass with the liberation of large amount of energy is called Nuclear Fission reaction.
Many nuclear reactions lakes place when the nuclear of different atoms are bombarded by a-particles, protons, rays etc.
The product atom in these reactions has approximately similar mass and charge as that of its parent nucleus. It was also found that the neutron bombardment on U235 nucleus resulted into two nuclei of almost equal mass and a large amount of energy was liberated. A chain reaction is also possible only when the volume of fissionable material exceeds its critical size.
This fission reaction may be of two types
(i) Controlled fission reaction and
(ii) Uncontrolled fission reaction
In controlled fission reaction, the fissionable material is carried out for continuous production of energy. The fissionable material is mixed with a moderator such as graphite or heavy water which slows down the neutrons. The rate of fission chain reaction is controlled by using rods of cadmium which absorbs the neutrons.
Where as in uncontrolled fission reaction, unlimited chain fission reaction takes place after initiation with neutrons. This fission reaction produces enormous amounts of energy until the whole fissionable material is finished. Presently, fission reaction is being used on a large scale production of nuclear energy in nuclear power plant.