Criticality safety

Criticality safety relates to accident prevention and protection from an uncontrolled nuclear fission chain reaction, or a criticality accident.

The reactivity of nuclear system expresses the departure of that system from criticality, i.e. the state in which each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions. A positive reactivity addition indicates a move towards supercriticality (power increase). A negative reactivity addition indicates a move towards subcriticality (power decrease).

Criticality is the state of a nuclear reactor when enough neutrons are created by fission to make up for those lost by leakage or absorption such that the number of neutrons produced in fission remains constant.

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NEA work on this topic

Exchanging information on national programmes, co-ordinating priorities of common interest, developing resources and facilitating information are essential to advancing criticality safety in nuclear science.

The Working Party on Nuclear Criticality Safety (WPNCS) deals with technical and scientific issues relevant to criticality safety. Specific areas of interest include investigations concerning static and transient configurations encountered in the nuclear fuel cycle, such as fuel fabrication, transport, separation processing and storage.