Why must a critical mass of fissionable material be present in order for a chain reaction to be set in motion and sustained?

Answer 1

Critical mass is dependent on several variables, six, specifically. The bottom line is that the fission rate be self-sustaining, in the absence of an outside contribution of neutrons. This is what we call KEffective >= 1. In essence, each fission generates one or more neutrons, each of which is capable of producing another fission. KEffective is the number of seconds that the reaction power doubles. In a typical power plant, KEffective is trimmed to be exactly 1, representing a constant output of energy that is carried away, usually by water, and used to make steam and turn an electric turbine. In a typical weapon, KEffective is pushed as far as it can go, causing super prompt criticality, which is a state where most of the six variables become immaterial, and you have a near total conversion of the core in a length of time short enough for the focused explosion to hold the core together against the massive forces of prompt dispersal, i.e. long enough for a nuclear detonation. (We are talking milliseconds or less, here.)