# Physics:Criticality (status)

Short description: State of a nuclear reactor where the nuclear chain reaction is precisely self-sustaining

In the operation of a nuclear reactor, criticality is the state in which a nuclear chain reaction is self-sustaining—that is, when reactivity is zero. In supercritical states, reactivity is greater than zero.[1]

## Applications

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

The International Atomic Energy Agency 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.