Nuclear fission is the phenomenon of splitting of a heavy nucleus into two or more lighter nuclei. The heavier nucleus generally has a mass number (A) greater than 230 i.e. A > 230.In the process of the nuclear fission a certain mass disappears i.e. the sum of the masses of final products is found to be slightly less than the sum of the masses of the reactant components.
This difference in the masses is called the mass defect and it is represented by Δm. Therefore, as per mass energy relation given by Einstein the energy released in the nuclear fission is given byE = (Δm) c2
Nuclear Fission ExampleIn 1938, Hahn and Strassman discovered that when 92U235 is bombarded with the thermal neutrons then it splits up into 56Ba141 and 36Kr92 with the emission of 3 neutrons along with the 200 MeV of energy per fission. The neutrons produced after fission is called secondary neutrons.
The reaction is represented as92U235 + 0n1 `->` 56Ba141 + 36Kr92 + 3 0n1 + QThe mass defect in the above reaction is comes out to be 0.2513 amu.Thus the energy released per fission of 92U235= 0.2153 x 931 MeV = 200.4 MeVThus about 200 MeV of energy is released in the fission of a single 92U235 nucleus. This energy appears in the form of the γ-rays, kinetic energy of the fission fragments and the released neutrons. Further, such a huge amount of energy is released in about 10-9 second. In the case of the 92U235 fission the fission fragments are not always Ba and Kr but the other nuclides have also been obtained but the energy evolved is almost the same in all the cases.
Nuclear Chain Reaction formed due to Nuclear FissionOne of the important aspects of the nuclear fission is the nuclear chain reaction shown in fig.1 in which the 3 neutrons produced in the fission of a 92U235 are further used for the fission of three other 92U235 nuclei and so on.