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In nuclear power technology, burnup is a measure of how much energy is extracted from a given amount of nuclear fuel. It may be measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial heavy metal atom) or %FIFA (fissions per initial fissile atom) as well as the actual energy released per mass of initial fuel in gigawatt-days/metric ton of Heavy metals (GWd/tHM), or similar units. The amount of initial fuel in the denominator is defined as all uranium, plutonium, and thorium isotopes, not including alloying or other chemical compounds or mixtures in the fuel charge.
Converting between percent and energy/mass requires knowledge of κ, the thermal energy released per fission event. A typical value is 193.7 Electronvolt () of thermal energy per fission (see Nuclear fission). With this value, the maximum burnup of 100%FIMA, 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 (meaning it is fissile), 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.
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.
The Deep Burn Modular Helium Reactor (DB-MHR) might reach 500 GWd/t of transuranic elements.
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.
On the other hand, there are signs that increasing burnup above 50 or 60 GWd/tU leads to significant engineering challengesEtienne Parent. Nuclear Fuel Cycles for Mid-Century Deployment, MIT, 2003. and that it does not necessarily lead to economic benefits. Higher-burnup fuels require higher initial enrichment to sustain reactivity. Since the amount of separative work units (SWUs) is not a linear function of enrichment, it is more expensive to achieve higher enrichments. There are also operational aspects of high burnup fuels that are associated especially with reliability of such fuel. The main concerns associated with high burnup fuels are:
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.
Unprocessed used fuel from current light-water reactors consists of 5% fission products and 95% actinides (most of it uranium), and is dangerously radiotoxic, requiring special custody, for 300,000 years. Most of the long-term radiotoxic elements are transuranic, and therefore could be recycled as fuel. 70% of fission products are either stable or have half-lives less than one year. Another six percent (129I and 99Tc) can be transmuted to elements with extremely short half-lives (130I: 12.36 hours; 100Tc: 15.46 seconds). 93Zr, having a very long half-life, constitutes 5% of fission products, but can be alloyed with uranium and transuranics during fuel recycling, or used in zircalloy cladding, where its radioactivity is irrelevant. The remaining 20% of fission products, or 1% of unprocessed fuel, for which the longest-lived isotopes are 137Cs and 90Sr, require special custody for only 300 years. Therefore, the mass of material needing special custody is 1% of the mass of unprocessed used fuel. In the case of or this "special custody" could also take the form of use for food irradiation or as fuel in a radioisotope thermoelectric generator. As both the native elements strontium and caesium and their oxides—chemical forms in which they can be found in oxide or metal fuel—form soluble hydroxides upon reaction with water, they can be extracted from spent fuel relatively easily and into a solid form for use or disposal in a further step if desired. If tritium has not been removed from the fuel in a step prior to this aqueous extraction, the water used in this process will be contaminated, requiring expensive isotope separation or allowing the tritium to decay to safe levels before the water can be released into the biosphere.
Plutonium and other transuranic isotopes are produced from uranium by neutron absorption during reactor operation. While it is possible in principle to remove plutonium from used fuel and divert it to weapons usage, in practice there are formidable obstacles to doing so. First, fission products must be removed. Second, plutonium must be separated from other actinides. Third, fissionable isotopes of plutonium must be separated from non-fissionable isotopes, which is more difficult than separating fissionable from non-fissionable isotopes of uranium, not least because the mass difference is one atomic unit instead of three. All processes require operation on strongly radioactive materials. Since there are many simpler ways to make nuclear weapons, nobody has constructed weapons from used civilian electric power reactor fuel, and it is likely that nobody ever will do so. Furthermore, most plutonium produced during operation is fissioned. To the extent that fuel is reprocessed on-site, as proposed for the Integral Fast Reactor, opportunities for diversion are further limited. Therefore, production of plutonium during civilian electric power reactor operation is not a significant problem.
A study sponsored by the Nuclear Energy University Programs investigated the economic and technical feasibility, in the longer term, of higher burnup.
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