In nuclear physics and engineering, criticality refers to the condition of a place where there is fissile material, specifying whether a nuclear fission chain reaction there can be sustained. The place can be an atomic (nuclear) bomb, the core of a nuclear reactor, or some other place where fissile material is stored or processed.
For a nuclear chain reaction to be sustained, there must be a minimum critical mass of fissile material. Furthermore, the criticality depends on the geometry of the material. When a critical mass of fissile material is sufficiently compacted, it reaches a critical or supercritical condition and a chain reaction starts up. This causes a multitude of neutrons to be released and creates nuclear fission products, which emit a high level of radiation, which can be harmful or fatal to people nearby. Therefore, unintended criticality is to be avoided. Such a criticality accident can occur if too much uranium or plutonium is brought together in one place. Nuclear reactors have copious radiation shielding and are in a reactor containment to avoid exposing personnel to radiation.