subcriticality

Subcriticality refers to a state in a nuclear system where the neutron population cannot sustain a self-perpetuating chain reaction. In such systems, the effective multiplication factor, k_eff, is less than 1. Subcritical configurations require an external source of neutrons to maintain any level of activity or to achieve a steady state.

The concept of k_eff describes the average number of neutrons in one generation relative to the previous generation. If k_eff < 1, each generation produces fewer neutrons than the last, and the neutron population decays over time due to absorption and leakage. A subcritical system is contrasted with a critical system (k_eff = 1), where the neutron population remains constant, and a supercritical system (k_eff > 1), where it grows.

Subcritical systems are used in applications that emphasize inherent safety through external control of reactivity. External neutron sources, such as particle accelerators that drive spallation reactions, can sustain activity in subcritical assemblies. Accelerator-driven systems (ADS) and some research or waste-transmuting concepts rely on subcritical configurations to mitigate the risk of runaway reactions.

Measurement and monitoring of subcriticality employ methods such as Rossi-alpha, Feynman-alpha, and pulsed neutron source experiments to estimate k_eff and reactivity. Dynamic behavior is often described with point kinetics, highlighting how neutron population decays or responds to changes in the external source.

Overall, subcriticality defines a regime where ongoing neutron production depends on an external driver, offering potential safety advantages while enabling certain research and waste-management technologies.