Chain Reaction In Nuclear Fission_Part 01
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Chain Reaction in Nuclear Fission: When slow neutrons are bombarded on uranium, each uranium nucleus is broken in to two almost equal parts and along with it enormous energy and two or three trash neutrons are emitted. If the conditions are favorable, these neutrons take part in the fission.This leads to a chain of nuclear fissions which continues till the whole of the Uranium is consumed. Thus the energy produced keep on multiplying.We have seen that about 200 MeV MeV- mega electron volts of energy is released due to fission of 1 Uranium nucleus. In this chain reaction, the number of nuclei undergoing fission increases very rapidly, hence the energy produced acquires an enormous amount very soon."The chain reaction is a series of nuclear fissions in which the neutrons produced in each fission cause additional fissions."This chain reaction liberate tremendous amount of energy within short interval of time. But sustaining a self--propagating chain reaction is not easy in practice, because the progressive chain reaction is inhibited due to the following reasons.A Ordinary uranium consists of three isotopes 92U238 92 Uranium 238, 92U236 92 Uranium 236 and 92U235 92 Uranium 235. The major part of ordinary uranium is the isotope 92U238 92 Uranium 238 the remaining are rare. Now, 92U238 92 Uranium 238 can be fissioned only with fast neutrons 1 MeV while 92U235 can be fissioned with slow as well as slow and fast neutrons. Neutrons having energy less than 1 MeV are absorbed by 92U238 through these low energy neutrons can take part is the fission of 92U235 but the probability of their absorption by U238 is much more. Thus the fresh neutrons released in the fission of ordinary uranium being absorbed by U238 are not able continue the chain reaction. This difficulty can be removed in the following two ways: The lighter isotope U235 can be separated from the ordinary uranium by diffusion method. The fission of U235 is now possible by neutrons of any energy high or low. So chain reaction continuous as shows in above.The second way is to slow down the neutrons by any method. So that their energy remains 0.03 eV This can be achieved with help of moderators, which reduce the speed of neutrons very rapidly. Graphite and heavy water are good moderators.The second difficulty in maintaining the chain reaction is that the fast neutrons released by the fission of U235 travel a distance of about 10 centimeter in the substance before they are slow down and fission other nuclei.Therefore, to continue the chain--reaction, the size of the fissionable substance should be bigger than a certain critical size.
