What is criticality of a nuclear reaction? How is it achieved?

Explanation

Criticality of a nuclear reaction means that the fissilon process is so controlled that a manageable constant power output is obtained. It can be done In two ways.
(i) Use of moderators: The commonly used moderator In the nuclear reactor is graphite. When the high-speed neutrons collide with the moderator, they lose some of thelr kinetic energy and slow down. lt is compulsory to slow down the speed ot neutrons produced because fast-moving neutrons cannot cause fission of U-235 effectively.
(ii) Useof neutron absorber: For energy fission, three neutrons are emitted. So, it Is necessary that extra neutrons are removed. For this, cadmium rods play an important role. These are inserted In the nuclear reactor in such a way that only a requisite number of neutrons are available for fission process to proceed in a controlled manner.