Criticality (status)

In the operation of a nuclear reactor, criticality or critical state is the state in which a nuclear chain reaction is self-sustaining but not growing. Subcriticality or subcritical state is the state in which a nuclear chain reaction is not self-sustaining. Supercriticality or supercritical state is the state in which a nuclear chain reaction is self-sustaining and growing. Sometimes, less preferably, criticality takes a wider definition, and refers to the any state in which a nuclear chain reaction is self-sustaining, no matter growing or not (encompassing criticality in strict definition and supercriticality).^[1]

In terms of reactivity, reactivity is 0 in criticality, less than 0 in subcriticality, greater than 0 in supercriticality.^[1] In terms of effective neutron multiplication factor (K_eff), K_eff is 1 in criticality, less than 1 in subcriticality, greater than 1 in supercriticality.