Critical / Criticality meaning

In nuclear regulatory practice, critical or criticality describes the condition in which a body of fissile material sustains a nuclear chain reaction on its own. A system is critical when neutrons produced by fission are created at the same rate as they are lost through absorption or leakage; it is subcritical if the reaction dies away and supercritical if it grows. The term is technical rather than a defined statutory term, but it is used across legislation, licence conditions and regulator guidance. Criticality is central to nuclear safety, licensing and compliance. Dutyholders must show in safety cases and risk assessments that operations, storage, processing and transport of fissile material remain subcritical with adequate margins, and plan for the unlikely event of a criticality accident. It informs design controls (for example, limits on mass, geometry, moderation and spacing) and emergency arrangements. In the UK, this arises under the Nuclear Installations Act 1965 licensing regime, ONR licence conditions and guidance, the Ionising Radiations Regulations 2017, REPPIR 2019 and the Carriage of Dangerous Goods regulations. In Ireland, the EPA (Office of Radiological Protection) regulates under legislation transposing the Euratom Basic Safety Standards and dangerous goods transport rules. Usage is broadly consistent across England & Wales,...