How does a critical mass affect a nuclear chain reaction?

Critical mass is the minimum amount of fissile material required to sustain a nuclear chain reaction under specific conditions. Its effect on the reaction is determined by the multiplication factor, k, which represents the ratio of fissions in one generation to the preceding one.123

• Subcritical (k < 1): The mass is insufficient to sustain a chain reaction because too many neutrons escape or are absorbed without causing further fission.23
• Critical (k = 1): The mass is exactly sufficient to maintain a steady-state chain reaction, where the rate of neutron generation balances the rate of loss.3
• Supercritical (k > 1): The mass exceeds the critical amount, causing the number of fissions to increase at each step. This divergent reaction is the fundamental principle behind the atomic bomb, where the goal is to achieve a supercritical state rapidly to release a massive amount of energy.3

The critical mass is influenced by the material's type, density, shape, and the presence of neutron reflectors, which bounce escaping neutrons back into the core to sustain the reaction.23