Criticality in a nuclear reactor refers to the state in which a nuclear chain reaction is self-sustaining, meaning that each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions). In other words, criticality is the normal operating condition of a nuclear reactor, in which nuclear fuel sustains a fission chain reaction). When a reactor achieves criticality, it is said to be critical. The International Atomic Energy Agency defines the first criticality date as the date when the reactor is made critical for the first time, which is an important milestone in the construction and commissioning of a nuclear power plant). Criticality is desired and is certainly nothing to fear, as it is the state in which the reactor can operate at a steady power level.