Burnup

In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured both as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom) and as the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/tHM), or similar units.

Measures of burnup

Expressed as a percentage, burnup is simple: if 5% of the initial heavy metal atoms have undergone fission, the burnup is 5%. In reactor operations, this percentage is difficult to measure, so the alternative definition is preferred. This can be computed by multiplying the thermal power of the plant by the time of operation and dividing by the mass of the initial fuel loading. For example, if a 3000 MW thermal (equivalent to 1000 MW electric) plant uses 24 tonnes of enriched uranium (tU) and operates at full power for 1 year, the average burnup of the fuel is (3000 MW·365 d)/24 metric tonnes = 45.63 GWd/t, or 45,625 MWd/tHM (where HM stands for heavy metal, meaning actinides like thorium, uranium, plutonium, etc.).