Criticality (nuclear)

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. In a simple use of the adjective, critical describes a place where a nuclear fission chain reaction is sustained, whether intended or not. In a technical sense, criticality is a number, a quantity symbolized by δ commonly describing the condition of a nuclear reactor at a given time. If the criticality of a place = 0, then the condition is critical and a nuclear chain reaction is sustained at a constant level at that place. If the criticality is positive, then the condition is supercritical and the chain reaction is increasing at that place. If the criticality is negative, then the condition is subcritical and a chain reaction is decreasing or not occurring at all at that place. Before an atomic bomb blows up, it is subcritical with no chain reaction sustained. When the bomb is triggered, it becomes strongly supercritical, the chain reaction rate skyrockets emitting tremendous energy, and the bomb blows up.

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.