Decay Heat meaning

In nuclear regulatory and environmental practice, decay heat describes the residual heat generated by radioactive decay in a shut-down reactor and in spent fuel or other radioactive materials. It persists after fission stops and must be removed to prevent overheating and potential fuel damage. This is a technical term used across multiple legal contexts rather than a definition fixed by statute or case law. It commonly appears in safety cases, nuclear site licence documentation and regulator guidance (including the ONR Safety Assessment Principles). Legally, it underpins dutyholder obligations to provide and maintain effective residual heat removal, monitoring and containment during shutdown, defuelling, storage, transport and decommissioning. It informs design-basis and severe-accident analyses, emergency planning (for example under REPPIR/NI equivalents), and environmental permitting and BAT demonstrations for radioactive substances activities. Usage and meaning are broadly consistent across England & Wales, Scotland and Northern Ireland within the GB nuclear licensing regime. In Ireland, although there are no nuclear power reactors, the concept is used in radiological protection, radioactive waste management and emergency preparedness under national legislation implementing the EU Basic Safety Standards. Practitioners should address decay heat when preparing operating rules, decommissioning plans and conditions of authorisation for cooling systems and storage facilities.