What does it mean for a nuclear reactor to be critical?

In the context of nuclear physics, a nuclear reactor is in a critical state when the neutron population within the core is sustained at a constant rate. This condition ensures that the fission rate remains steady, allowing the reactor to operate at a consistent power level.1

A nuclear chain reaction is self-sustaining when the ratio of neutrons causing new fissions to the number of parent neutrons is exactly 1. If this ratio falls below 1, the neutron population decreases and the reactor becomes subcritical; if it exceeds 1, the population increases, leading to a supercritical state.21

To maintain criticality, a reactor must meet specific requirements regarding the concentration of fissile material and the physical geometry of the core to prevent excessive neutron leakage. During operation, reactor control rods are adjusted to balance the neutron population, effectively maintaining this steady-state critical state.31