For a chain reaction to occur each fission must produce at least more fission reaction(s)

For a chain reaction to occur each fission must produce at least one more fission reactions.
 Nuclear chain reactions are series of nuclear fission, which involves splitting of atomic nuclei, each initiated by a neutron produced in a preceding fission. Each fission produces 2 to 3 neutrons. At least one of these neutrons on average has to hit another nucleus and initiate another fission, and thus the number of fission increase with time.

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