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

a) Chain reaction: In nuclear fission nearly three neutrons are produced when one uranium atom is destroyed. If they again participate in fusion reaction nine neutrons are produced. In next generation reaction, the neutron number becomes 27. In this process, the number of neutrons increases in geometric progression (GP) and the whole uranium is destroyed in few seconds. This type of self sustained fission
reaction is called chain reaction.
Neutron multiplication factor (K) :
It is defined as the ratio of number of neutrons produced in one generation to the number of neutrons in previous generation.
$\mathrm{K}=\frac{\text{ Number of neutrons in present generation }}{\text{ Number of neutrons in previous generation }}$
It is useful to understand the nature of nuclear reactions in a nuclear reactor.
For sustained chain reaction :
Neutral multiplication fact K $\ge$ 1