Burnup

In nuclear power technology, burnup is a measure of how much energy is extracted from a given amount of nuclear fuel. It may be measured in several ways (non-exhaustive):

• %FIMA (fissions per initial heavy metal atom): the fraction of fuel atoms that underwent fission, as a proportion of all heavy metal atoms;
• %FIFA (fissions per initial fissile atom): the fraction of fuel atoms that underwent fission, as a proportion of all fissile atoms;
• GWd/tHM: the energy released in gigawatt-days, per mass of initial fuel in metric ton of heavy metal.
• GWd/tU: the energy released in gigawatt-days, per mass of initial fuel in metric ton of uranium.

Here, "heavy metal" specifically means actinides like thorium, uranium, plutonium, etc.

The amount of initial fuel in the denominator is defined as all uranium, plutonium, and thorium isotopes, not including alloying or other chemical compounds or mixtures in the fuel charge.