What is the definition of critical mass?

A nuclear chain reaction is a self-sustaining process where one nuclear reaction generates a minimum of one other nuclear reaction, which in turn produces more nuclear reactions. The most common example of this process is nuclear fission, where a nucleus splits into two smaller nuclei, which are accompanied by the release of energy and other particles, such as neutrons. If these released neutrons cause the fission of other nearby nuclei, the process continues and leads to a chain reaction.

Critical mass is the minimum amount of fissile material (such as uranium-235 or plutonium-239) required to sustain a nuclear chain reaction. The purpose of critical mass is to ensure that there are enough fissile materials present for the neutrons released by one fission event to, on average, cause at least one additional fission event. When the amount of fissile material is below the critical mass, there are not enough potential fission events to sustain a chain reaction, and the process will eventually halt. When there is more fissile material than the critical mass, the chain reaction becomes self-sustaining and, if uncontrolled, can lead to a rapid release of energy, such as in a nuclear explosion. In a controlled environment, such as a nuclear reactor, the critical mass is maintained to produce a steady energy output.