Subkritisch

Subkritisch, or subcritical, is a cross-disciplinary descriptor for a state in which a process cannot sustain itself without external input. In physics and engineering, the term is used wherever a self-amplifying chain reaction or wave propagation must be driven by an external source to continue. The exact criterion depends on the discipline.

In nuclear reactor physics, subcritical describes a system with an effective neutron multiplication factor k_eff less than 1. Without an external neutron source, the neutron population—and the reaction—will decay. Subcritical systems may be used for research or waste transmutation in accelerator-driven systems (ADS), where a particle accelerator produces neutrons by spallation to sustain the reaction.

In materials science, subcritical crack growth refers to the slow propagation of a crack under loads below the material's critical resistance, K_IC. The rate is influenced by environment, loading, and temperature, and can lead to failure after long service times even though the applied stress is below the nominal critical level.

In fluid dynamics, subcritical flow occurs when a channel flow has a Froude number below 1, meaning waves can travel upstream and the flow is relatively tranquil. Subcritical conditions affect dam design, spillway operation, and river hydraulics.

Assessing subcriticality involves measuring how far a system is from criticality and ensuring safety margins. In nuclear contexts, this includes reactivity control and k_eff monitoring; in materials, monitoring crack growth; in hydraulics, managing flow regimes.