Explain Nuclear Fission.

ANS:


* During an attempt to produce transuranic elements, Prof. Fermi and his coworkers observed that when neutrons are bombarded on a nucleus, β-particles are emitted.

* Hahn and Strassman found that 92U235 on bombardment by a neutron produces 56Ba139 which emits β-radiation and gets converted into 57La140.

* The splitting of a heavy nucleus into daughter elements spontaneously or as a result of bombardment by a neutron is called NUCLEAR FISSION.

* There are two categories of nuclear fission: (1) Prompt fission (2) Delayed fission.

* If an atom (like uranium) is bombarded by a high-energy neutron, its nucleus splits immediately. This is prompt (rapid) fission.

* In delayed (slow) fission, the bombarded neutron is absorbed by the target nucleus (uranium) and excites the nucleus which splits later on.

* During nuclear fission, the heavy nucleus obtained after fission has atomic mass 130 - 149 u and the light nucleus has the atomic mass 85 - 104 u.

* If the nucleus of a heavy element (or its isotope) splits without being bombarded by any particle, the fission is called 'Spontaneous fission'.

* During nuclear fission, some neutrons are released. For example, in the fission of 92U235, on an average 2 - 3 neutrons are released.

* During fission, a large amount of energy is released. Let us take the following example:

92U235 + 0n1 → 35Mo95 + 57La139 + 20n1

Here the sum of the atomic masses on the left hand side is

235.124 u + 1.009 u = 236.133 u

and the sum of the atomic masses on the right hand side is

94.946 u + 138.955 u + 2.018 u = 235.919 u

the difference in the total mass of reactants and products is called 'MASS DEFECT'. The mass defect in the above example is

236.133 u - 235.919 u = 0.214 u

This lost mass (mass defect) is converted into energy. According to the Einstein's mass-energy equation when 1 u mass is converted, 931.48 MeV energy is produced.

Thus, in the above example, 0.214 u mass is converted to produce 200 MeV energy which is equivalent to 3.2 x 10-11 Ws (joule) of energy.


* The fission of 1 kg of 92U235 produces 109 W-day or 1000 MW-day energy. If only 30% of this energy is converted into electrical energy, we get 300 MW-day energy for which 2500 tons of coal are needed.

* Electrical energy is produced by using chain reaction in which the neutrons produced in previous reaction are used to split uranium nuclei in subsequent steps. This type of fission is carried out in special type of vessels called Nuclear Reactor.