What does the term "criticality" refer to in nuclear reactors?

The term "criticality" specifically refers to the condition in a nuclear reactor when a fission reaction becomes self-sustaining. This state is crucial because it indicates that the number of fission reactions occurring is sufficient to maintain a constant population of neutrons without the need for additional external neutron sources. When a reactor is at criticality, it achieves a balance between the rate of neutron production through fission and the rate of neutron loss due to absorption and leakage.

Understanding this concept is essential for reactor operations and safety, as ensuring the reactor operates at or near the critical level is vital for controlled power generation. Operating below this level is termed "subcritical," while operating above it is referred to as "supercritical," which can lead to increasing reactivity and potential safety hazards if not controlled.