What is a nuclear fission reaction?

Explanation

Nuclear fission: The process, in which a heavy nuclei splits into two or more! nuclei and tremendous amount of energy is released, is known as a nuclear fission.
For example : When a slow neutron strikes a uranium `(""_(92)U^(235))`  nucleus then it splits up into two light nuclei Barium `(""_(56)Ba^(141))`  and Krypton `(""_(36)Kr^(92))` and three neutrons are produced along with them.
`""_(92)U^(235) + ""_(0)n^(1) rarr ""_(56)Ba^(11) + ""_(36)Kr^(92) + 3""_(0)n^(1) +`  energy
  Chain reaction : A reaction, in which the projectile used to carry out the nuclear fission reaction gets multiplied and they further enhance the nuclear reaction vigourously, is called a nuclear chain reaction.

When a neutron strikes a nucleus of  U-235  heavy nucleus breaks into small fragments which fly apart at a very high speed. In addition two or three neutrons are also released. These neutrons react with other U-235 nuclei and process is repeated. Fission takes place in a very short time resulting in explosion.
Nuclear chain reaction is shown in Fig. A slow moving neutron hits a nucleus of U-235 atom. The atom breaks down to produce barium, krypton and three neutrons with release of about 200 MeV of energy. These three neutrons cause further fission of three more U-235 atoms producing nine neutrons. The process multiplies resulting in fission of almost all the atoms with release of lot of energy. If large number of U-235 atoms are packed together in a sample, the process of fission can multiply at very fast rate producing more and more energy. This type of reaction is called chain reaction.