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Nuclear Fission

The process of splitting of a heavy nucleus into two lighter nuclei of comparable masses (after bombardment with a energetic particle) with liberation of energy is called nuclear fission.
  • The phenomenon of nuclear fission was discovered by scientist Ottohann and F. Strassman and was explained by N. Bohr and J.A. Wheeler on the basis of liquid drop model of nucleus.
Fig. 5
Fission reaction of U235
  • The energy released in U235 fission is about 200 MeV or 0.8 MeV per nucleon.
  • By fission of 92U235, on an average 2.5 neutrons are liberated. These neutrons are called fast neutrons and their energy is about 2 MeV (for each). These fast neutrons can escape from the reaction so as to proceed the chain reaction they are need to slow down.
  • Fission of U235 occurs by slow neutrons only (of energy about 1 eV) or even by thermal neutrons (of energy about 0.025 eV).
  • 50 kg of U235 on fission will release ≈ 4 × 1015 J of energy. This is equivalence to 20,000 tons of TNT explosion. The nuclear bomb dropped at Hiroshima had this much explosion power.
  • The mass of the compound nucleus must be greater than the sum of masses of fission products.
  • The (binding energy/A) of a compound nucleus must be less than that of the fission products.
  • It may be pointed out that it is not necessary that in each fission of uranium, the two fragments 56Ba and 36Kr are formed but they may be any stable isotopes of middle weight atoms.
  • Some other U235 fission reactions are:
    Many more
  • The neutrons released during the fission process are called prompt neutrons.
  • Most of the energy released appears in the form of kinetic energy of fission fragments.
Fig. 6

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