Fuel burnup

Burnup is the thermal energy extracted from a quantity of fuel before it is discharged, expressed in gigawatt-days per tonne of uranium, GWd/tU. A fuel assembly that spent three or four cycles in a light-water reactor core now leaves at 50 to 60 GWd/tU: each tonne of uranium produced the equivalent of 60 days of a 1,000 MW thermal reactor. Generation II reactors were designed for about 40 GWd/tU and now go beyond 60 with the same, improved, kind of fuel; the industry is aiming at 70.

Burnup is tied to enrichment. For the fuel to stay reactive after more time in the core it has to go in with more uranium-235: the move from 40 to over 60 GWd/tU came with enrichments rising from about 3.25% to 5%, the maximum that conventional fuel fabrication plants handle today. That is why designs aiming at higher burnup also need enrichments above 5% to be licensed.

Not every reactor plays the same game. The CANDU and the other heavy-water reactors burn natural uranium: their fuel is around 7.5 GWd/t, does not stay long in the core, and is replaced on power, bundle by bundle. The EPR, at the other end, was designed for 65 GWd/t. Higher burnup means fewer assemblies to buy, transport and store per kilowatt-hour, but each discharged assembly is hotter and more radioactive: the NRC calls fuel above about 45 GWd/tU high burnup fuel and evaluated its behaviour in dry storage separately.