Explain the terms 'chain reaction' and 'multiplication factor'. How is a chain reaction sustained ?

Explanation

Chain reaction : In nuclear fission nearly three neutrons are produced when one uranium atom is destoryed. If they again participate in fission reaction nine neutrons are produced. In next generation the neutrons becomes 27. In this process the number of neutrons increases in geometric progression and the whole uranium is destroyed in few seconds. This type of self sustained fission reaction is called reaction.
Neutron multiplication fator (K) , Neutron multiplilcation factor is defined as the ratio of number of neutrons produced in one generation to the number of neutrons in previous generation.
`K= ("Number of neutrons in present generation")/("Number of neutrons in previous generation")`
Neutron multiplication factor is useful to understand the nuclear reactions in a nuclear reactor.
To sustained chain reaction :
1. Neutron multiplication factor `K ge 1`.