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In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured both as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom) and as the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/tHM), or similar units.
Measures of Burnup
Expressed as a percentage, burnup is simple: if 5% of the initial heavy metal atoms have undergone fission, the burnup is 5%. In reactor operations, this percentage is difficult to measure, so the alternative definition is preferred. This can be computed by multiplying the thermal power of the plant by the time of operation and dividing by the mass of the initial fuel loading. For example, if a 3000 MW thermal (equivalent to 1000 MW electric) plant uses 24 tonnes of enriched uranium (tU) and operates at full power for 1 year, the average burnup of the fuel is (3000 MW·365)/24 metric tonnes = 45.63 GWd/t, or 45,625 MWd/tHM (where HM stands for heavy metal, meaning actinides like uranium, plutonium, etc.).
Converting between percent and energy/mass requires knowledge of κ, the thermal energy released per fission event. A typical value is 193.7 MeV (3.1E-11 J) of thermal energy per fission (see Nuclear fission). With this value, the maximum burnup of 100%, which includes fissioning not just fissile content but also the other fissionable nuclides, is equivalent to about 909 GWd/t. Nuclear engineers often use this to roughly approximate 10% burnup as just less than 100 GWd/t.
The actual fuel may be any actinide that can support a chain reaction, including uranium, plutonium, and more exotic transuranic fuels. 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/tU. With newer fuel technology, and particularly the use of nuclear poisons, these same reactors are now capable of achieving up to 60 GWd/tU. After so many fissions have occurred, the build-up of fission products poisons the chain reaction and the reactor must be shut down and refueled.
Some more-advanced light-water reactor designs are expected to achieve over 90 GWd/t of higher-enriched fuel.
Fast reactors are more immune to fission-product poisoning and can inherently reach higher burnups in one cycle. In 1985, the EBR-II reactor at Argonne National Laboratory took metallic fuel up to 19.9% burnup, or just under 200 GWd/t.
In a power station, high fuel burnup is desirable for:
- Reducing downtime for refueling
- Reducing the number of fresh nuclear fuel elements required and spent nuclear fuel elements generated while producing a given amount of energy
- Reducing the potential for diversion of plutonium from spent fuel for use in nuclear weapons
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 disposed of 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.
One 2003 MIT graduate student thesis concludes that "the fuel cycle cost associated with a burnup level of 100 GWd/tHM is higher than for a burnup of 50 GWd/tHM. In addition, expenses will be required for the development of fuels capable of sustaining such high levels of irradiation. Under current conditions, the benefits of high burnup (lower spent fuel and plutonium discharge rates, degraded plutonium isotopics) are not rewarded. Hence there is no incentive for nuclear power plant operators to invest in high burnup fuels."
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- L. C. Walters (September 18, 1998). "Thirty years of fuels and materials information from EBR-II". Journal of Nuclear Materials (Elsevier) 270: 39–48. Bibcode:1999JNuM..270...39W. doi:10.1016/S0022-3115(98)00760-0.
- "Small Nuclear Power Reactors". Information Papers. World Nuclear Association. July 2008. Retrieved 2008-08-02.
- Etienne Parent (2003). "Nuclear Fuel Cycles for Mid-Century Deployment". MIT. p. 81.