Criticality (status)

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Criticality is the state of a system where the nuclear chain reaction is precisely self-sustaining, that is, when the reactivity is zero. The term is also used for supercritical states, those in which the reactivity is greater than zero.[1]


In the context of a nuclear reactor, particularly in a nuclear power plant, criticality refers to the normal operating condition of a reactor, in which nuclear fuel sustains a fission chain reaction. A reactor achieves criticality, (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.[2]

The International Atomic Energy Agency also defines the first criticality date as the date when the reactor is made critical for the first time.[3] This is an important milestone in the construction and commissioning of a nuclear power plant.

See also[edit]


  1. ^ "Criticality" (PDF). IAEA Safety Glossary. International Atomic Energy Agency. 2007. p. 46. Retrieved 17 February 2014.
  2. ^ "Criticality". Glossary. US Nuclear Regulatory Commission. 11 December 2013. Retrieved 17 February 2014.
  3. ^ "First Criticality Date". Glossary. International Atomic Energy Agency. Retrieved 17 February 2014.