Burnup

Burnup is a measure of how much energy has been extracted from nuclear fuel relative to its initial quantity of fissile heavy metal. It is commonly expressed as energy units per mass, such as megawatt-days per kilogram of uranium (MWd/kgU) or gigawatt-days per metric ton of uranium (GWd/MTU). The higher the burnup, the more energy has been produced from the same amount of fuel.

Calculation and meaning: Burnup is defined as the integral of reactor power over irradiation time divided by the initial mass of heavy metal in the fuel: Bu = ∫ P dt / m0. It reflects the reactor’s irradiation history and is influenced by power level, burnup shaping, and fuel design. Burnup is distinct from enrichment and is not a direct measure of age; it is a measure of energy extraction.

Operational use: Burnup informs fuel design, loading strategies, and fuel management, including when to reload or rearrange assemblies. It also affects spent fuel characteristics, such as decay heat, radiotoxicity, and the buildup of fission products that influence reactivity and cooling requirements. In shielding and safety analyses, burnup data support licensing and safety margins. Burnup credit is the practice of accounting for reduced reactivity of spent fuel in criticality analyses of transportation and storage systems.

Measurement and prediction: Predictive burnup values are obtained with depletion calculations using reactor physics codes and fuel depletion models. Actual burnup for a given fuel assembly can be inferred from measurements, such as spent-fuel gamma spectroscopy or destructive assays, though these methods vary in accuracy and practicality.

Typical values and limits: In commercial reactors, discharged fuel burnups commonly range from roughly 30 to 60 GWd/MTU, with advanced fuel designs reaching higher levels. Very high burnup can influence fuel performance, cladding integrity, and fission-gas behavior.