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 equationE = 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 byDm = 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.
Nuclear energy is the energy released when certain changes take place in the nucleus of an atom. Nuclear energy is partly renewable and partly non-renewable source of energy.
Nuclear fission and nuclear fusion are the two types of reactions that release nuclear energy:a) Nuclear fusion - is renewable because hydrogen needed for this process is available in plenty in nature.
b) Nuclear fission - is non-renewable because uranium needed for this process is radioactive and has only limited existence.