In supercritical states, reactivity is greater than zero.
[1] 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.