Nuclear chain reaction: In fission reaction, on the average three neutrons are emitted. These three neutrons can further produce fission of three more uranium nuclei and so on. This can lead to a chain reaction and within no time a large number of uranium can undergo fission resulting in the enormous amount of energy.
It is also possible that one or more neutrons produced during fission may escape out of uranium piece or may be absorbed and get lost. The chain reaction can be sustained only if, on the average, more than one neutron per fission are available to cause further fission as shown in Fig. The escape of neutrons which do not cause further fission depends upon following factors.
1. Leakage of neutrons from the system. If the size of the fissionable material is small, then the incident neutrons escape and the chain reaction will not be sustained. So for a sustained nuclear chain reaction, the number of neutrons lost must be equal to the number of neutrons produced and the system is said to be critical. For a material to be of critical size, its linear dimension, should be of the order of mean free path of the neutrons.
2. Presence of non-fissionable material in the system. The non-fissionable material present in the sample of U235 can slow down the incident neutrons and they get absorbed without causing further fission. Thus to have sustained chain reaction, the fissionable material should be free from such impurities.
3. Absorption of neutrons by U238. In a sample of uranium, there are three isotopes namely U233, U235 and U238 having abundance of 0.006%, 0.714% and 99.28%. If neutrons are made to bombard on such a material, they are slowed down by collision with U238 and are thus absorbed. U238 is also fissionable but require the energy of incident neutrons more than 1 MeV, where as U235 is fissionable with neutrons having energy of the order of 0.025 eV. Thus for self-sustained chain reaction to take place either U238 is seprated from U235 (this is called enriched uranium) or moderators be used to slow down the neutrons to energy about 0.025 eV to cause fission in U235.
