If k is the neutron multiplication factor of a subcritical core and S 0 is the number of neutrons coming per generation in the reactor from an external source, then at the instant when the neutron source is switched on, number of neutrons in the core will be S 0 . After 1 generation, this neutrons will produce k × × S 0 neutrons in the reactor and reactor will have a totality of k × × S 0 + S 0 neutrons considering the newly entered neutrons in the reactor. Similarly after 2 generation, number of neutrons produced in the reactor will be k × × ( k × × S 0 + S 0 ) + S 0 and so on. This process will continue and after a long enough time, the number of neutrons in the reactor will be,