Editing Breeder reactor (section)

=== Nuclear waste ===
{{Actinides vs fission products}}
In broad terms, spent nuclear fuel has three main components. The first consists of [[nuclear fission product|fission products]], the leftover fragments of fuel atoms after they have been split to release energy. Fission products come in dozens of elements and hundreds of isotopes, all of them lighter than uranium. The second main component of spent fuel is transuranics (atoms heavier than uranium), which are generated from uranium or heavier atoms in the fuel when they absorb neutrons but do not undergo fission. All transuranic isotopes fall within the actinide series on the [[periodic table]], and so they are frequently referred to as the actinides. The largest component is the remaining uranium which is around 98.25% uranium-238, 1.1% uranium-235, and 0.65% uranium-236. The U-236 comes from the non-fission capture reaction where U-235 absorbs a neutron but releases only a high energy [[gamma ray]] instead of undergoing fission. 

The physical behavior of the fission products is markedly different from that of the actinides. In particular, fission products do not undergo fission and therefore cannot be used as nuclear fuel. Indeed, because fission products are often [[neutron poison]]s (absorbing neutrons that could be used to sustain a chain reaction), fission products are viewed as nuclear 'ashes' left over from consuming fissile materials. Furthermore, only seven [[long-lived fission product]] isotopes have [[Half-life|half-lives]] longer than a hundred years, which makes their geological storage or disposal less problematic than for transuranic materials.<ref>{{cite web |title=Radioactive Waste Management |publisher=World Nuclear Association |url=http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Nuclear-Wastes/Radioactive-Waste-Management |access-date=19 September 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130921055655/http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Nuclear-Wastes/Radioactive-Waste-Management/ |archive-date=21 September 2013}}</ref>

With increased concerns about nuclear waste, breeding fuel cycles came under renewed interest as they can reduce actinide wastes, particularly plutonium and minor actinides.<ref name="world-nuclear">{{cite web |title=Supply of Uranium |publisher=World Nuclear Association |url=http://world-nuclear.org/info/inf75.html |access-date=11 March 2012 |url-status=live |archive-url=https://web.archive.org/web/20130212223705/http://www.world-nuclear.org/info/inf75.html |archive-date=12 February 2013}}</ref> Breeder reactors are designed to fission the actinide wastes as fuel and thus convert them to more fission products. After [[spent nuclear fuel]] is removed from a light water reactor, it undergoes a complex decay profile as each nuclide decays at a different rate. There is a large gap in the decay half-lives of fission products compared to transuranic isotopes. If the transuranics are left in the spent fuel, after 1,000 to 100,000 years the slow decay of these transuranics would generate most of the radioactivity in that spent fuel. Thus, removing the transuranics from the waste eliminates much of the long-term radioactivity of spent nuclear fuel.<ref name="aps">{{cite journal |last=Bodansky |first=David |title=The Status of Nuclear Waste Disposal |journal=Physics and Society |volume=35 |issue=1 |publisher=American Physical Society |date=January 2006 |url=http://www.aps.org/units/fps/newsletters/2006/january/article1.html |access-date=30 July 2012 |url-status=live |archive-url=https://web.archive.org/web/20080516010935/http://www.aps.org/units/fps/newsletters/2006/january/article1.html |archive-date=16 May 2008}}</ref>

Today's commercial light-water reactors do breed some new fissile material, mostly in the form of plutonium. Because commercial reactors were never designed as breeders, they do not convert enough uranium-238 into plutonium to replace the uranium-235 consumed. Nonetheless, at least one-third of the power produced by commercial nuclear reactors comes from fission of plutonium generated within the fuel.<ref>{{cite web |title=Information Paper 15 |publisher=World Nuclear Association |url=http://www.world-nuclear.org/info/inf15.html |access-date=15 December 2012 |url-status=dead |archive-url=https://web.archive.org/web/20100330221426/http://www.world-nuclear.org/info/inf15.html |archive-date=30 March 2010}}</ref> Even with this level of plutonium consumption, light water reactors consume only part of the plutonium and minor actinides they produce, and nonfissile [[isotopes of plutonium]] build up, along with significant quantities of other minor actinides.<ref name="SCALE5">{{cite web |title=SCALE 5 Analysis of BWR Spent Nuclear Fuel Isotopic Compositions for Safety Studies |work=ORNL/TM-2010/286 |publisher=OAK RIDGE NATIONAL LABORATORY |author1=U. Mertyurek |author2=M. W. Francis |author3=I. C. Gauld |url=http://info.ornl.gov/sites/publications/Files/Pub27046.pdf |access-date=25 December 2012 |url-status=live |archive-url=https://web.archive.org/web/20130217043714/http://info.ornl.gov/sites/publications/Files/Pub27046.pdf |archive-date=17 February 2013}}</ref>

Breeding fuel cycles attracted renewed interest because of their potential to reduce actinide wastes, particularly various isotopes of plutonium and the minor actinides (neptunium, americium, curium, etc.).<ref name="world-nuclear" /> Since breeder reactors on a closed fuel cycle would use nearly all of the isotopes of these actinides fed into them as fuel, their fuel requirements would be reduced by a factor of about 100. The volume of waste they generate would be reduced by a factor of about 100 as well. While there is a huge reduction in the ''volume'' of waste from a breeder reactor, the [[specific activity|''activity'']] of the waste is about the same as that produced by a light-water reactor.<ref>{{cite web |title=Fast Breeder Reactors |url=https://fas.org/rlg/3_15_2010%20Fast%20Breeder%20Reactors%201.pdf |url-status=live |archive-url=https://web.archive.org/web/20160329001222/http://fas.org/rlg/3_15_2010%20Fast%20Breeder%20Reactors%201.pdf |archive-date=29 March 2016 |access-date=4 June 2016}}</ref>

Waste from a breeder reactor has a different decay behavior because it is made up of different materials. Breeder reactor waste is mostly fission products, while light-water reactor waste is mostly unused uranium isotopes and a large quantity of transuranics. After spent nuclear fuel has been removed from a light-water reactor for longer than 100,000 years, the transuranics would be the main source of radioactivity. Eliminating them would eliminate much of the long-term radioactivity from the spent fuel.<ref name="aps" />

In principle, breeder fuel cycles can recycle and consume all actinides,<ref name="sustainablenuclear" /> leaving only fission products. As the graphic in this section indicates, fission products have a peculiar "gap" in their aggregate half-lives, such that no fission products have a half-life between 91 and 200,000 years. As a result of this physical oddity, after several hundred years in storage, the activity of the [[radioactive waste]] from an FBR would quickly drop to the low level of the [[long-lived fission products]]. However, to obtain this benefit requires the highly efficient separation of transuranics from spent fuel. If the [[nuclear reprocessing|fuel reprocessing]] methods used leave a large fraction of the transuranics in the final waste stream, this advantage would be greatly reduced.<ref name="Argonne" />

The FBR's fast neutrons can fission actinide nuclei with even numbers of both protons and neutrons. Such nuclei usually lack the low-speed "thermal neutron" [[doppler broadening|resonances]] of fissile fuels used in LWRs.<ref>{{cite web |title=Neutron Cross Sections4.7.2 |url=http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_2.html |url-status=dead |archive-url=https://web.archive.org/web/20130101101959/http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_2.html |archive-date=1 January 2013 |access-date=17 December 2012 |publisher=National Physical Laboratory}}</ref> The thorium fuel cycle inherently produces lower levels of heavy actinides. The fertile material in the thorium fuel cycle has an atomic weight of 232, while the fertile material in the uranium fuel cycle has an atomic weight of 238. That mass difference means that thorium-232 requires six more neutron capture events per nucleus before the transuranic elements can be produced. In addition to this simple mass difference, the reactor gets two chances to fission the nuclei as the mass increases: First as the effective fuel nuclei U233, and as it absorbs two more neutrons, again as the fuel nuclei U235.<ref>{{cite web |last=David |first=Sylvain |author2=Elisabeth Huffer |author3=Hervé Nifenecker |title=Revisiting the thorium-uranium nuclear fuel cycle |url=http://www.europhysicsnews.org/index.php?option=article&access=standard&Itemid=129&url=/articles/epn/pdf/2007/02/epn07204.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070712172902/http://www.europhysicsnews.org/index.php?option=article&access=standard&Itemid=129&url=%2Farticles%2Fepn%2Fpdf%2F2007%2F02%2Fepn07204.pdf |archive-date=12 July 2007 |access-date=11 November 2018 |publisher=europhysicsnews}}</ref><ref>{{cite web |title=Fissionable Isotopes |url=http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/fission.html |url-status=live |archive-url=https://web.archive.org/web/20121108064526/http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html |archive-date=8 November 2012 |access-date=25 December 2012}}</ref>

A reactor whose main purpose is to destroy actinides rather than increasing fissile fuel-stocks is sometimes known as a '''burner reactor'''. Both breeding and burning depend on good neutron economy, and many designs can do either. Breeding designs surround the core by a [[breeding blanket]] of fertile material. Waste burners surround the core with non-fertile wastes to be destroyed. Some designs add neutron reflectors or absorbers.<ref name="Hoffman" />