when is a nuclear reactor said to be critical?

Question

it is related to generation of nuclear fission energy

Answer 1

A nuclear reactor is critical when the number of neutrons generated is just sufficient for a self sustaining reaction. Three neutrons are produced when uranium fissions. If <1 neutron interacts with another uranium nucleus the reaction is sub critical. If 1 of the 3 neutrons interacts with another uranium nucleus, the reaction is critical and proceeds at a constant rate. If >1 neutron interacts with another uranium nucleus the reaction rate grows exponentially and, if unchecked, will result in a large explosion.

Answer 2

a nuclear reactor is critical when there is no other option or choice of alternative energy. Unless we all want to go back to candle power.. :)