The unit GWd/MTU is the (average) thermal output, multiplied by the time of operation, and divided by the mass of fuel involved. This gives a rough measure of the number of nuclear fission events that have taken place within the fuel.
The actual fuel may be uranium, plutonium, or a mixture of these or of either or both of these with thorium. This fuel content is often referred to as the heavy metal to distinguish it from other metals present in the fuel, such as those used for cladding. The heavy metal is typically present as either metal or oxide, but other compounds such as carbides or other salts are possible.
Generation II reactors were typically designed to achieve about 40 GWd/MTU. With newer fuel technology, and particularly the use of burnable poisons, these same reactors are now capable of achieving up to 60 GWd/MTU. Some more-advanced reactor designs are expected to achieve over 90 GWd/MT of higher-enriched fuel, and eventually over 200 GWd/MT. The Deep Burn Modular Helium Reactor (DB-MHR) might reach 500 GWd/MT of transuranic elements. Complete fission of all heavy metal in a breeder reactor, not just fissile content but also any fissionable or fertile material, would yield around 1,000 GWd/MT.
In a power station, high fuel burnup is desirable for:
It is also desirable that burnup should be as uniform as possible both within individual fuel elements and from one element to another within a fuel charge. In reactors with online refuelling, fuel elements can be repositioned during operation to help achieve this. In reactors without this facility, fine positioning of control rods to balance reactivity within the core, and repositioning of remaining fuel during shutdowns in which only part of the fuel charge is replaced may be used.
In once-through nuclear fuel cycles such as are currently in use in much of the world, used fuel elements are buried whole as high level nuclear waste, and the remaining uranium and plutonium content is lost. Higher burnup allows more of the fissile 235U and of the plutonium bred from the 238U to be utilised, reducing the uranium requirements of the fuel cycle.
In once-through nuclear fuel cycles, higher burnup reduces the number of elements that need to be buried. However short-term heat emission, one deep geological repository limiting factor, is predominantly from medium-lived fission products, particularly 137Cs and 90Sr. As there are proportionately more of these in high-burnup fuel, the heat generated by the spent fuel is roughly constant for a given amount of energy generated.
Similarly, in fuel cycles with nuclear reprocessing, the amount of high-level waste for a given amount of energy generated is not closely related to burnup. High-burnup fuel generates a smaller volume of fuel for reprocessing, but with a higher specific activity.
Burnup is one of the key factors determining the isotopic composition of spent nuclear fuel, the others being its initial composition and the neutron spectrum of the reactor. Very low fuel burnup is essential for the production of weapons-grade plutonium for nuclear weapons, in order to produce plutonium that is predominantly 239Pu with the smallest possible proportion of 240Pu and 242Pu.