1keff

1keff is a term used in nuclear reactor physics to describe the critical state of a reactor when the effective multiplication factor, keff, equals one. The effective multiplication factor is the ratio of neutrons produced by fission in one generation to neutrons lost through absorption and leakage in the preceding generation. When keff is equal to one, the neutron population remains constant from generation to generation, and the reactor is considered critical. If keff is less than one, the reactor is subcritical and the chain reaction tends to die out; if keff is greater than one, the reactor is supercritical and the neutron population, and usually the power, increases over time.

In practice, keff is determined through a combination of reactor design calculations and experimental measurements. It is influenced by factors such as fuel composition, moderator and reflector properties, geometry, and temperature. keff can be estimated with neutron transport and diffusion calculations, validated by criticality experiments and neutron-physics measurements. Reactivity, a related concept, measures the departure of keff from unity and is used to describe how far a system is from 1keff.

Maintaining 1keff is essential for safe, steady-state operation of a nuclear reactor. Operators adjust reactivity through control devices, soluble poisons, and moderator conditions to keep keff at or near unity. The concept is foundational in reactor kinetics, safety analysis, and criticality control across research reactors and commercial power plants.