Chain Reactions
In order to see what we need, there is the criticality constant, k, defined as:
k = Pf * avg number of neutrons/fission
When kə, any reaction will die out, since we have less neutrons coming out than going in
When k=1, we have steady state, the operating point for a reactor (CRITICAL)
When kɭ, we have the start-up for a reactor, or if continued, the conditions for a bomb.