A useful measure of the performance in the nuclear fuel cycle is the energy obtained per unit mass of fuel, known as the fuel's burnup. The burnup is commonly specified in megawatt-days or gigawatt-days of thermal output per metric tonne of heavy metal (MWDT/MTHM or GWDT/MTHM). This is a cumbersome notation for repeated use, and we represent GWDT/MTHM in a more compact form as GWd/t (gigawatt-days per tonne). In standard energy units, 1 GWd = 8.64 x 1013 joules (J).
For U.S. reactors, as well as most reactors elsewhere, the "heavy metal" in the original fuel is uranium.20 The fuel is in the form of uranium oxide (UO2 ). About 12% of the mass of the fuel is oxygen and, therefore, there is a distinction between the mass of heavy metal and the mass of the fuel. The heavy metal in the spent fuel removed from the reactor is still primarily uranium, but it also includes isotopes of plutonium and—to a small extent— other transuranic elements. Typically, the mass of heavy metal is about 3% or 4% less in the spent fuel than in the initial fuel due to fission (including fission of plutonium isotopes).21
A 1000-MWe reactor, operating at a typical thermal efficiency of 32%, produces energy at the rate of 3125 MWt (where it is explicitly indicated that this is the thermal output). One gigawatt-year of electric power therefore represents a thermal output of 1141 GWd(t). If, for example, the average burnup in a reactor is 40 GWd/t, the fuel consumption is 28.5 tonnes of enriched uranium per gigawatt-year.
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