Nuclear Fission and Nuclear chain reaction:

Uranium exists in different isotopes of U²³⁸, U²³⁴ ,  and U²³⁵. Out of these, U²³⁵ is the most unstable. When unstable heavy nucleus is bombarded with high energy neutrons, it splits up roughly into two equal fragments and about 2.5 neutrons are released and a large amount of energy is produced (nearly 200 million electron volts). This process is called Nuclear fission.

Also, the nuclear fission is followed by the emission of several high energy neutrons per neutron bombarded. This is the essential condition for self-sustaining the chain reaction. Thus the neutrons produced by nuclear fission are very fast moving neutrons and can be made to bombard other nuclei of U²³⁵ to enable a chain reaction. When a large number of fission occurs, enormous amount of heat is produced. The chain reaction is shown in Fig. 3.3.

Out of 2.5 neutrons released is fission of each nuclei of U²³⁵, one neutron is used to sustain the chain reaction, about 0.9 neutron is captured by U²³⁸ which gets converted into fissionable material, Pu²³⁹. And about 0.6 neutron is partly absorbed by control rod material, coolant moderator, and partly escape from the reactor.

If U²³⁵ (or) U²³⁸ is used in the reactor core, it produces fissionable material Pu²³⁹

i.e. U²³⁵ + U²³⁸ + neutron = Pu²³⁹.

Similarly, if thorium is used in the reactor core, it produces fissionable material U²³³.

i.e. Th²³² + neutron = U²³³

Thus, Pu²³⁹ and U²³³ are fissionable materials and they can be used as nuclear fuel and are known as secondary fuel, and U²³⁵ and Th²³² are known as primary fuels.

The chain reaction produces constant rate of heat energy and it may continue only if the neutron liberated by fission, balance the disposal of neutron by different ways as given below.

1. Escape of neutrons from the fissionable material.
2. Fission captured by U²³⁵, U²³⁸ and Pu²³⁹ and U²³³.
3. Non-fission captured by moderator, control rods fission fragments and by impurities.

When the neutrons produced in the chain reaction are less than the neutron disposed off in different ways, then the chain reaction will stop.