The process of splitting of a nucleus of a heavy atom into a number of light nuclei with the liberation of large amount of energy and two or three neutrons is called nuclear fission.
Example: When U-235 is bombarded by a neutron it splits into barium, krypton and 2.5 neutrons per fission on an average.
Fission of U-235 Nucleus by a Thermal Neutron
Naturally occurring uranium exists in the form of two isotopes U-235 and U-238. The naturally occurring uranium contains 99.3% of U-238 and 0.7% of U-235. Out of these two isotopes only U-235 is readily fissionable i.e., it is very unstable. Whereas U-238 is not fissionable. It absorbs the neutrons and forms Pu-239 which is readily fissionable. All heavy atoms are highly unstable whereas all light nuclei are stable.
The stability of a nucleus is dependent on the type of force which is dominant. The two forces operating inside a nucleus are the electrostatic force of repulsion and the nuclear force of attraction (strong force).
It is the force of repulsion existing between the like charges (protons inside the nucleus).
It is the force of attraction existing between the nucleons i.e., between two protons, or two neutrons, or a proton and a neutron. Nuclear force is a short range force and it is effective only when the nucleons are closely packed inside the nucleus.In a heavy atom the size of the atom is big to accommodate more number of nucleons and hence interparticle distance increases and nuclear force decreases. In a heavy atom the electrostatic force of repulsion is dominant than the nuclear force of attraction and this explains why heavy atoms are unstable.
In a small atom the size of the atom is small and the nucleons are closely packed. Hence the nuclear force of attraction is dominant than the electrostatic force of repulsion. Thus small atoms are stable.
In 1936, Neils Bohr suggested that the nucleus might be like a droplet of dense liquid, composed of sub droplets of protons and neutrons.A uranium nucleus is spherical in shape. In this spherical nucleus, protons and neutrons are together because of the delicate balance existing between the nuclear force of attraction and electrostatic force of repulsion. When a slow moving neutron hits U-235, the nucleus elongates and a constriction appears. When the nucleus elongates the distance between nuclear particles increases and weakens both nuclear force and electrostatic force. But the electrostatic force of repulsion will be dominant and due to the repulsion between the protons the constriction deepens further. Due to the increased repulsion between the protons the nucleus splits into krypton, barium and three neutrons.
The fission reaction can take place in two ways:
In critical fission each fission of an unstable atom is allowed to retain just the right number of neutrons. This ensures that the number of atoms undergoing fission remains constant with time and does not go on multiplying endlessly.Enrico Fermi carried out the first critical fission reaction on 2nd December 1942.
One of the practical applications of nuclear energy or critical fission reaction is generation of electricity using a nuclear reactor.
An explosive fission reaction is that fission reaction which is deliberately allowed to go out of control by allowing all the neutrons produced during fission of an unstable atom to cause further fission, resulting in an explosion.Explosive fission reaction is used in an atom bomb.
A nuclear chain reaction occurs when on an average more than one neutron from a nuclear fission reaction causes another fission reaction. The reaction taking place in an atom bomb is an uncontrolled chain reaction. Whereas in a nuclear reactor it is a controlled chain reaction. In a controlled chain reaction of U-235 the two neutrons out of the three neutrons released are absorbed by control rods made either of cadmium or boron.
n - neutron
U - 235 Uranium
Ba - Barium
Kr - Krypton
E - Energy
Note :The number of neutrons released in a uncontrolled chain reaction of U-235 nuclei is given by 3n where n = 1,2,3,.....
A Schematic Diagram Showing Essential Steps in a Chain Reaction Based on Fission of U-235 by Thermal Neutrons. The Nuclear Reaction Produced by Fast Neutrons in U-238 Leading to Creation of Plutonium Pu-239 is also Shown Schematically
n - neutronU-235 Uranium
Ba - BariumKr - Krypton
E - EnergyControl rods are made of cadmium or boron which absorb neutrons.
The minimum amount of the fissionable substance required so as to continue the chain reaction under a given set of conditions is called critical mass.If the amount of fissionable substance is less than the critical mass then the chain reaction does not take place.
If the mass of the fissionable material is more than the critical mass, then the mass is called super critical mass.Explosive fission reaction takes place when the mass is super critical.
If the mass of the fissionable material is less than the critical mass then the mass is called sub-critical mass.
Note : Neutron is considered to be the ideal bombarding agent because being neutral in charge there is no force of attraction or repulsion between the nucleus and neutron.
These are slow moving neutrons. Slow moving neutrons are used to bring about the fission reaction. If we use fast moving neutrons, it will not be in contact with the fissionable material for sufficient time to bring about the fission reaction.