Burn up meaning

In nuclear regulatory and contractual documents, burn up (also written burnup or burn-up) describes how much energy has been extracted from a reactor’s nuclear fuel for each unit of its mass, a metric used to set operating limits, support safety cases and classify spent fuel. It measures the thermal energy released relative to the mass of fuel, typically expressed as gigawatt days per tonne of heavy metal (GWd/tHM) or per tonne of fuel (GWd/t). The term is not generally defined in UK or Irish primary legislation or case law. It is a technical term of art used across nuclear site licences, environmental permits, transport approvals and fuel supply or reprocessing agreements, with precise definitions and methodologies set in the relevant instrument, standard or guidance (for example, IAEA documents). Higher burn up signifies greater energy extraction and affects decay heat, radiological inventory and criticality margins, which in turn drive licensing limits, package design, storage, reprocessing and disposal strategies, and reporting obligations. Usage is broadly consistent across England & Wales, Scotland, Northern Ireland and Ireland; while Ireland has no nuclear power stations, the metric arises in transport, cross‑border regulation and environmental assessment.