Burnup

A measure of how much energy has been extracted from nuclear fuel before it is removed from the reactor. It is usually stated in gigawatt-days per tonne of heavy metal (GWd/t), where heavy metal means the uranium or other actinides in the fuel, or as the percentage of heavy metal atoms that have fissioned. Megawatt-days per tonne (MWd/t) is a unit a thousand times smaller, and mixing the two up is a common error. Light-water reactor fuel is typically removed at roughly 40 to 60 GWd/t, about 5% of its heavy metal atoms fissioned. Higher burnup means fewer refuellings and less spent fuel by mass for the same energy, although each tonne of it is more radioactive. Every fuel form is qualified only up to the burnup it has been tested to: the US Department of Energy reports that TRISO particles reached a peak of 19% of heavy metal atoms fissioned in a 2009 test, about three times what current light-water fuel achieves.

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