What is Nuclear Criticality?

The state in which a nuclear fission chain reaction is self-sustaining, with each generation of fission events producing exactly enough neutrons to maintain the reaction at a steady rate.

What category does Nuclear Criticality belong to?

Nuclear Criticality is a nuclear-physics concept in nuclear energy and SMR technology.

Nuclear Criticality — Nuclear & SMR Glossary

Nuclear criticality is the fundamental operating condition of every nuclear reactor, defined as the state in which the neutron population from fission reactions is self-sustaining. In a critical reactor, each fission event produces, on average, exactly enough neutrons to cause one subsequent fission, maintaining a constant power level. This balance is quantified by the effective neutron multiplication factor (k-effective or k_eff): when k_eff equals exactly 1.0, the reactor is critical; below 1.0 is subcritical (chain reaction dying out); above 1.0 is supercritical (power increasing). Reactor operators continuously adjust k_eff through control rod positioning, soluble boron concentration, or other reactivity control mechanisms to manage power output.

Achieving first criticality is the defining milestone in any new reactor's commissioning, marking the moment when the nuclear fuel assembly first sustains a chain reaction under controlled conditions. For the advanced reactor industry, upcoming criticality milestones are closely watched by investors and regulators alike. Radiant Industries is targeting first criticality of its Kaleidos microreactor at Idaho National Laboratory's DOME test facility on July 4, 2026. Kairos Power's Hermes demonstration reactor in Oak Ridge, Tennessee is progressing toward criticality in 2027. China's Linglong One (ACP100) in Hainan Province completed cold functional testing and non-nuclear commissioning in 2025, with first criticality expected ahead of commercial operation in H1 2026. China's HTR-PM at Shidaowan achieved first criticality in 2021 and has been in commercial operation since 2023.

The physics of criticality vary significantly between reactor types. Thermal-spectrum reactors (PWRs, BWRs, HTGRs) use a moderator to slow neutrons to thermal energies where fission cross-sections are highest, requiring lower fuel enrichment but larger core volumes. Fast-spectrum reactors (SFRs like TerraPower's Natrium and Oklo's Aurora, LFRs like Newcleo's LFR-AS-200) operate without a moderator, using high-energy neutrons that enable fuel breeding and actinide transmutation but require higher enrichment levels. Understanding and precisely controlling criticality through all operating conditions, including transients, loss of coolant, and seismic events, is the foundation of nuclear safety analysis and a central focus of every licensing submission to the NRC and international regulatory bodies.