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Criticality (in the operation of a nuclear reactor)

Criticality (in the operation of a nuclear reactor) refers to the condition wherein a nuclear reactor is maintaining a self-sustaining nuclear chain reaction. This state is achieved when the reactor’s neutron population remains steady over time, meaning each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions. This condition is crucial for the stable and controlled release of energy necessary for power generation.

When a reactor reaches criticality, the effective multiplication factor (k-eff) equals one. This balance ensures that the rate of neutron production is equal to the rate of neutron loss due to absorption and leakage. Operating in a critical state allows the reactor to produce a consistent output of thermal energy, which can then be converted into electricity through various mechanisms, such as steam turbines.

Achieving and maintaining criticality involves precise control and monitoring of several parameters, including fuel composition, control rod positioning, coolant flow, and reactor geometry. Safeguarding the stability of these conditions is essential to prevent scenarios of supercriticality, where the reaction rate would increase, or subcriticality, where the reaction rate would diminish, potentially affecting the reactor’s efficiency and safety.