NUCLEAR CHAIN REACTIONS

NUCLEAR CHAIN REACTIONS
Definition
A fission reaction, where the neutrons from the previous step continue to propagate and repeat the reaction is called nuclear chain reaction.
Suppose if we initiate fission of 92U235 nucleus by one neutron (obtained either by cosmic rays or by radioactive source) the reaction will liberate three neutrons. These three neutrons will in turn causes the fission of other three 92U235 nucleus and will liberate three more neutrons in the fission of each 92U235 i.e., nine neutrons will be obtained and further it give rise to 27 neutrons and so on.
A nuclear chain reaction continues till the whole of 92U235 nucleus are fissioned. It is also accompanied by the liberation of huge amount of energy called nuclear energy or atomic energy.
Some of the neutrons, released in the fission of U235, may escape from the surface to the surroundings or may be absorbed by U235 present as impurity. This will result in breaking of the chain and the amount of energy released will be less than expected.
For a nuclear chain reaction to continue, sufficient amount of U235 must be present to capture the neutrons, otherwise neutrons will escape from the surface.

Critical Mass
The minimum amount of fissionable material (U235) required to continue the nuclear chain reactor is called critical mass.
The critical mass of U235 lies between 1 kg to 100kg.

a. Super critical mass
If the mass of the fissionable material (U235) is more than the critical mass, it is called super critical mass.

b. Sub critical mass
If the mass of the fissionable material is smaller than the critical mass, it is called sub critical mass. Thus the mass greater or lesser than the critical mass will hinder the propagation of the chain reaction.

Nuclear energy
The enormous amount of energy released during the nuclear chain reaction of heavy isotope like U235 or Pu239 is called nuclear fission energy or nuclear energy.
The fission of U235 or Pu239 occurs instantaneously, producing enormous amount of energy in the form of heat and radiation.

Cause of the release of energy
The enormous amount of energy released during the nuclear fission is due to the loss in some mass, when the reaction takes place. It has been observed that during nuclear fission, the sum of the masses of the products formed is slightly less than the sum of masses of target species and bombarding neutron. The loss in mass gets converted into energy according to Einstein equation
E = mc2
where
C = velocity, m = loss in mass and E = energy.