Nuclear Reactions

Two consequences of nuclear reactions are the phenomena of nuclear fission and nuclear fusion. Both are important sources of nuclear energy, which may be used for peaceful or destructive purposes.

When a nucleus is bombarded with some sub-atomic particles such as a - particles, neutrons, protons etc, these particles are captured by the target nucleus, which then disintegrates. The new element formed has mass either slightly greater or slightly smaller than the parent element.

The process of splitting of a heavier nucleus (like that of U²³⁵) into a number of fragments of much smaller mass, by suitable bombardment with sub-atomic particles is called nuclear fission.

Of the three natural isotopes of uranium

nucleus undergoes nuclear fission when bombarded with slow neutrons. U²³⁶ is formed which being unstable, further breaks up in several different ways.

The tremendous amount of energy released during nuclear fission is because of the loss in mass. The sum of the masses of the fragments produced and neutrons released as a result of fission is less than the sum of the masses of target ²³⁵U and bombarding neutron. The loss in mass gets converted into energy according to Einstein equation

E = mc².

For e.g., we can calculus the loss of mass when - nuclide splits up into ¹⁴⁴Ba and ⁹⁰Kr along with the release of two neutrons.

Dm the mass defect or the mass converted into energy is given by

Dm = 236.127 - 235.846 = 0.281 amu

1 amu = 931.48 MeV

\ Energy released = 0.281 amu = 931.48 x 0.281

= 261.75 MeV

The neutrons emitted from the fission of first uranium atom hit other uranium nuclei and cause their fission resulting in the release of more neutrons, which further continue the fission process. In this way, a nuclear chain reaction sets up releasing tremendous amount of energy.

If the chain reaction takes place as mentioned, the energy released would be very high. This is the principle underlying the nuclear or atom bomb. However some of the released neutrons escape from the surface unused and do not involve in the chain process. It is found that the net amount of energy released from 1 kg of uranium 235 is equivalent to that available from 2 x 10⁴ kg of coal.

It has been found that a branching chain reaction is possible only with a quantity of larger than a certain critical amount (so that only a few neutrons escape).

The minimum amount of the fissionable material required so as to continue the chain reaction process under a given set of conditions is called the critical mass. If the mass of the material is more than the critical mass, it is referred to as super-critical mass whereas if the mass of the fissionable material is smaller than the critical mass, it is called sub-critical mass. The critical mass of U-235 is between 1 kg to 100 kg.

It must be noted that the naturally occurring uranium contains most U-238 isotope (about 99.3%) which is not fissionable with slow neutrons.