For the nuclear fission process how is critical mass important in a chain reaction

 Critical mass is the minimum amount of fissible material that must be available before nuclear chain reaction can be sustained. The mass of a given fissible material that has the ability to sustain the continuous reaction. When the mass is subcritical chain reaction cannot occur and when it is supercritical it will subsequently return to the critical state ( equilibrium). Therefore the available mass of fissible material determine if a nuclear chain fission reaction can be sustained or not.  nuclear chain reaction is sustained  only at critical mass.