criticality

Criticality in nuclear engineering refers to the state where a nuclear chain reaction can be sustained. This occurs when the number of neutrons produced by fission events equals the number of neutrons lost through escape or absorption.12

States of a Nuclear Reactor

• Subcritical: The neutron population decreases, and the fission rate drops to zero. This means a chain reaction cannot be sustained.13
• Critical: The neutron population remains constant, leading to a steady fission rate. This is the desired state for normal operation in a nuclear reactor.14
• Supercritical: The neutron population increases over time, causing the fission rate and power output to rise.1

Achieving criticality requires a sufficient concentration and mass of fissile material, along with a suitable geometric configuration. If a chain reaction is sustained, a large amount of energy is released, forming the basis of nuclear power systems. Nuclear reactors are designed to maintain a controlled, critical state, unlike atomic bombs, which are designed for a rapid, supercritical chain reaction.521