Definition : The splitting of a heavy nucleus into two or more nuclei of smaller masses is called nuclear fission.

This reaction was first carried out by two German scientists, Otto Hahn and Strassman in 1939. They observed that the nuclei of Uranium 235 disintegrated easily when they were bombarded with slow moving neutrons. One Uranium nucleus splits into two nuclei of smaller masses such as Barium and Krypton or Cesium and Rubidium. The energy released by the fission of one nucleus of Uranium 235 releases energy of 200 MeV (Million ElectronVolts). This amount of energy is equivalent to that obtained by burning 100 tonnes of pure Carbon in air.

This reaction can be represented as :

²³⁵₉₂U + ¹₀n ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + Energy

The products (the nuclei) formed are known as fission products. The mass of the products is slightly less than that of the original nucleus. This mass difference is converted into energy according to Albert Einstein's equation :

E = mc²

(where E stands for energy, m = loss of mass, c = velocity of light in vacuum.)

Chain Reaction :

In nuclear fission, neutrons are ejected. They strike the neighbouring nuclei and in turn, these nuclei undergo fission. This leads to a multiplying process called Chain reaction.

Uncontrolled Chain Reaction :

The bombardment of nuclei by neutrons in a chain reaction is self-accelerating. This results in a nuclear explosion liberating huge amounts of energy & radiations in a millionth of a second. Uncontrolled chain reaction is the underlying principle of an atom bomb.

Controlled Chain Reaction :

A Chain Reaction can be controlled by using Boron steel or Cadmium rods or Graphite blocks which absorb most of the neutrons. As a result, only a few neutrons are allowed to carry out fission.Controlled chain reaction forms the basis for nuclear reactors used for generating electricity and producing radioisotopes. Controlled chain reaction is also used in fuelling submarines.

See Also :

Reactors
Fusion

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