Criticality is the fundamental operating principle of every nuclear reactor. When a uranium or plutonium atom undergoes fission, it releases neutrons that can trigger fission in neighboring atoms. If, on average, exactly one neutron from each fission goes on to cause another fission, the chain reaction is self-sustaining — the reactor is "critical." If fewer than one neutron per fission causes another fission, the reaction dies out (subcritical). If more than one does, the reaction accelerates (supercritical), which is how reactors increase power during startup.
Reactor operators and safety systems carefully control criticality using control rods (which absorb neutrons), coolant properties, and fuel configuration. In normal operation, a reactor is maintained at exactly critical, with tiny adjustments made to match power output to demand. The concept of "delayed neutrons" — a small fraction of neutrons released slightly after fission rather than instantaneously — is what makes reactor control practical. Without delayed neutrons, the chain reaction would respond too quickly for mechanical control systems.
For SMR and advanced reactor designers, achieving and controlling criticality in novel geometries and fuel types is a fundamental design challenge. New reactor concepts must demonstrate predictable critical behavior through extensive computational modeling and, eventually, physical testing. The journey from design to first criticality is a major milestone for any reactor project — it proves the physics works as intended. TerraPower, Kairos Power, and other SMR developers are progressing toward first criticality of their demonstration reactors, representing pivotal moments for the advanced nuclear industry. For deeper coverage, see DeepTechIntel's nuclear section.