subkriittinen

Subkriittinen (subcritical) describes a fissile system in which the effective neutron multiplication factor, keff, is less than 1. In such a state, a chain reaction cannot be sustained without an external source of neutrons; the neutron population decays from generation to generation.

Subcriticality can be engineered by increasing neutron absorption or loss relative to production, through materials and geometry: absorbing materials (boron, cadmium), moderators that slow neutrons, limits on fissile content, and arrangements that reduce neutron reflection and leakage. Subcritical configurations are used in safety analyses, experimental research, and accelerator-driven systems (ADS). In ADS, an external accelerator produces neutrons that drive fission in a subcritical core, providing a pathway to energy production without relying on a self-sustaining chain reaction.

Subcritical states are central to criticality safety in fuel handling and storage, ensuring that arrangements remain well below critical conditions under all credible scenarios. Researchers use subcritical assemblies and pulsed neutron experiments to study reactor physics, nuclear data, and material behavior while avoiding sustained criticality.

Key terminology includes keff, the effective multiplication factor; subcriticality means keff < 1. The extent of subcriticality is described by how far keff is below 1, with values typically well below 1 indicating greater stability and easier control. The concept is widely used in nuclear engineering, safety analysis, and reactor design.