Natural uranium as such cannot sustain a nuclear chain reaction. Therefore, naturally occurring uranium is first processed (by physical processes of separation which depend on relative mass,
e.g., ultra centrifugal techniques) to increase the percentage of fissionable uranium-235 in it a sample containing 2-5% U-235 or U-236 can be used.
(b) Fission of U-235 (or U-236) (Role of moderators):
Fission of U-235 takes place satisfactorily when the bombarding neutron is slow. Energy is released when slow moving neutron bombards a U-235 nucleus (fission). This gives energy and more fast moving neutrons. Now these fast moving neutrons do not cause further fission.
So that the fission process becomes continuous, speed of the ejected neutrons has to be decreased. A substance which slows down the speed of neutrons in the nuclear reactor to a level to cause effective fission of U-235 is called a moderator.
Commonly used moderators are graphite or heavy water. Graphite or heavy water is placed around the uranium rods.
Neutrons ejected first strike with the moderator, lose their kinetic energy and get slowed down. These slow neutrons bombard the uranium-235 nuclei and the process of liberation of energy and more of neutrons continue.
(c) Neutron absorbent or controlling rods (Attainment of criticality):
As seen above an ordinary fission reaction produces a large number of neutrons which promotes chain reaction thus releasing tremendous amount of energy which becomes uncontrollable and leads to explosion.
In order that the energy released may be tapped and utilized properly, fission of U-235 should take place in a controlled manner.
This requires removal of excess neutrons which are being produced. The excess neutrons are removed by using cadmium or boron rods because these elements have the ability to absorb neutrons and thus make them ineffective. Care should be taken that all the neutrons produced are not absorbed.
The length of the cadmium (or boron) rods is so adjusted in the reactor that the fission reaction is neither slow nor fast.
(If the cadmium rod is inserted deeply, all the neutrons produced are absorbed and if they are withdrawn from the fuel, then all the neutrons will be available for fission and cause explosion or chain reaction).
Some pipes containing coolant are embedded in the reactor to take out the heat energy produced in it. Generally liquid sodium metal is used as the coolant which removes heat energy in a very short time. In some reactors, carbon dioxide or water is also used.
(e) Production of electricity:
The coolant pipes are passed through a tank half filled with water. The heat from the coolant is transferred to water and the hot pressurized steam is passed through the turbines.
When the turbines rotate, the dynamo fixed on its shaft starts working and electricity is generated.