Editing Radioactive waste (section)

=== Nuclear fuel cycle ===
{{Main|Nuclear fuel cycle|Spent nuclear fuel}}
{{see also|Nuclear power}}

==== Front end ====
Waste from the front end of the [[nuclear fuel cycle]] is usually alpha-emitting waste from the extraction of uranium. It often contains radium and its decay products.

[[Uranium dioxide]] (UO<sub>2</sub>) concentrate from mining is a thousand or so times as radioactive as the [[granite]] used in buildings. It is refined from [[yellowcake]] (U<sub>3</sub>O<sub>8</sub>), then converted to [[uranium hexafluoride]] gas (UF<sub>6</sub>). As a gas, it undergoes [[enriched uranium|enrichment]] to increase the [[U-235]] content from 0.7% to about 4.4% (LEU). It is then turned into a hard [[ceramic]] oxide (UO<sub>2</sub>) for assembly as reactor fuel elements.<ref>{{cite book |last=Cochran |first=Robert |url=http://www.new.ans.org/store/i_350015 |title=The Nuclear Fuel Cycle: Analysis and Management |publisher=American Nuclear Society |year=1999 |isbn=0-89448-451-6 |location=La Grange Park, Illinois |pages=52–57 |language=en-us |access-date=2011-09-04 |archive-url=https://web.archive.org/web/20111016144333/http://www.new.ans.org/store/i_350015 |archive-date=2011-10-16 |url-status=dead}}</ref>

The main by-product of enrichment is [[depleted uranium]] (DU), principally the [[Uranium-238|U-238]] isotope, with a U-235 content of ~0.3%. It is stored, either as UF<sub>6</sub> or as U<sub>3</sub>O<sub>8</sub>. Some is used in applications where its extremely high density makes it valuable such as [[anti-tank]] [[KE-penetrator|shells]], and on at least [[Pen Duick|one occasion]] even a sailboat [[keel]].<ref>{{cite web |url=http://www.janes.com/defence/news/jdw/jdw010108_1_n.shtml |title=Global Defence News and Defence Headlines – IHS Jane's 360 |url-status=live |archive-url=https://web.archive.org/web/20080725032900/http://www.janes.com/defence/news/jdw/jdw010108_1_n.shtml |archive-date=2008-07-25}}</ref> It is also used with plutonium for making [[mixed oxide fuel]] (MOX) and to dilute, or [[enriched uranium#Downblending|downblend]], highly enriched uranium from weapons stockpiles which is now being redirected to become reactor fuel.

==== Back end ====
{{See also|Nuclear reprocessing}}
The back-end of the nuclear fuel cycle, mostly spent [[fuel rod]]s, contains [[fission product]]s that emit beta and gamma radiation, and [[actinide]]s that emit [[alpha particle]]s, such as [[uranium-234]] (half-life 245&nbsp;thousand years), [[neptunium-237]] (2.144&nbsp;million years), [[plutonium-238]] (87.7&nbsp;years) and [[americium-241]] (432 years), and even sometimes some neutron emitters such as [[californium]] (half-life of 898 years for californium-251). These isotopes are formed in [[nuclear reactor]]s.

It is important to distinguish the processing of uranium to make fuel from the [[nuclear reprocessing|reprocessing]] of used fuel. Used fuel contains the highly radioactive products of fission (see high-level waste below). Many of these are neutron absorbers, called [[neutron poison]]s in this context. These eventually build up to a level where they absorb so many neutrons that the chain reaction stops, even with the control rods completely removed from a reactor. At that point, the fuel has to be replaced in the reactor with fresh fuel, even though there is still a substantial quantity of [[uranium-235]] and [[plutonium]] present. In the United States, this used fuel is usually "stored", while in other countries such as Russia, the United Kingdom, France, Japan, and India, the fuel is reprocessed to remove the fission products, and the fuel can then be re-used.<ref>{{cite web |title=Recycling spent nuclear fuel: the ultimate solution for the US? |url=http://analysis.nuclearenergyinsider.com/operations-maintenance/recycling-spent-nuclear-fuel-ultimate-solution-us |url-status=bot: unknown |archive-url=https://web.archive.org/web/20121128101318/http://analysis.nuclearenergyinsider.com/operations-maintenance/recycling-spent-nuclear-fuel-ultimate-solution-us |archive-date=28 November 2012 |access-date=2015-07-29 |website=Nuclear Energy Insider}}</ref> The fission products removed from the fuel are a concentrated form of high-level waste as are the chemicals used in the process. While most countries reprocess the fuel carrying out single plutonium cycles, India is planning multiple plutonium recycling schemes <ref name="reprocess">{{cite web |title=Continuous Plutonium Recycling In India: Improvements in Reprocessing Technology |url=http://www.dailykos.com/story/2009/6/13/742039/-Continuous-Plutonium-Recycling-In-India:Improvements-in-Reprocessing-Technology. |url-status=dead |archive-url=https://web.archive.org/web/20110606130212/http://www.dailykos.com/story/2009/6/13/742039/-Continuous-Plutonium-Recycling-In-India:Improvements-in-Reprocessing-Technology. |archive-date=2011-06-06 |website=dailykos.com}}</ref> and Russia pursues closed cycle.<ref>{{Cite web |url=https://world-nuclear.org/information-library/country-profiles/countries-o-s/russia-nuclear-fuel-cycle.aspx |title=Russia's Nuclear Fuel Cycle &#124; Russian Nuclear Fuel Cycle - World Nuclear Association}}</ref>

==== Fuel composition and long term radioactivity ====
[[File:Activityofuranium233.jpg|thumb|upright=1.4|Activity of [[Uranium-233|U-233]] for three fuel types. In the case of MOX, the U-233 increases for the first 650 thousand years as it is produced by the decay of [[Np-237]] which was created in the reactor by absorption of neutrons by U-235.]]
{{See also|Spent nuclear fuel|High-level waste}}
{{Main|Long-lived fission product}}
[[File:Activitytotal1.svg|thumb|upright=1.4|Total activity for three fuel types. In region 1, there is radiation from short-lived nuclides, in region 2, from [[Sr-90]] and [[Cs-137]], and on the far right, the decay of Np-237 and U-233.]]
The use of different fuels in nuclear reactors results in different [[spent nuclear fuel]] (SNF) composition, with varying activity curves. The most abundant material being U-238 with other uranium isotopes, other actinides, fission products and activation products.<ref name=":1">{{Cite web |title=Radioactivity : Spent fuel composition |url=https://www.radioactivity.eu.com/site/pages/Spent_Fuel_Composition.htm |url-status=dead |archive-url=https://web.archive.org/web/20200923005202/https://www.radioactivity.eu.com/site/pages/Spent_Fuel_Composition.htm |archive-date=2020-09-23 |access-date=2021-08-10 |website=www.radioactivity.eu.com}}</ref>

Long-lived radioactive waste from the back end of the fuel cycle is especially relevant when designing a complete waste management plan for SNF. When looking at long-term radioactive decay, the actinides in the SNF have a significant influence due to their characteristically long half-lives. Depending on what a [[nuclear reactor]] is fueled with, the actinide composition in the SNF will be different.

An example of this effect is the use of [[nuclear fuel]]s with [[thorium]]. Th-232 is a fertile material that can undergo a neutron capture reaction and two beta minus decays, resulting in the production of fissile [[uranium-233|U-233]]. The SNF of a cycle with thorium will contain U-233. Its radioactive decay will strongly influence the long-term [[radioactive decay|activity]] curve of the SNF for around a million years. A comparison of the activity associated to U-233 for three different SNF types can be seen in the figure on the top right. The burnt fuels are thorium with reactor-grade plutonium (RGPu), thorium with weapons-grade plutonium (WGPu), and [[Mixed oxide fuel]] (MOX, no thorium). For RGPu and WGPu, the initial amount of U-233 and its decay for around a million years can be seen. This has an effect on the total activity curve of the three fuel types. The initial absence of U-233 and its daughter products in the MOX fuel results in a lower activity in region 3 of the figure at the bottom right, whereas for RGPu and WGPu the curve is maintained higher due to the presence of U-233 that has not fully decayed. Nuclear reprocessing can remove the actinides from the spent fuel so they can be used or destroyed (see {{section link|Long-lived fission product|Actinides}}).

==== Proliferation concerns ====
{{See also|Nuclear proliferation|Reactor-grade plutonium}}

Since uranium and plutonium are [[nuclear weapons]] materials, there are proliferation concerns. Ordinarily (in spent nuclear fuel), plutonium is [[reactor-grade plutonium]]. In addition to [[plutonium-239]], which is highly suitable for building nuclear weapons, it contains large amounts of undesirable contaminants: [[plutonium-240]], [[plutonium-241]], and [[plutonium-238]]. These isotopes are extremely difficult to separate, and more cost-effective ways of obtaining fissile material exist (e.g., uranium enrichment or dedicated plutonium production reactors).<ref>{{cite web |url=http://www.world-nuclear.org/info/inf15.html |title=Plutonium |author=World Nuclear Association |date=March 2009 |access-date=2010-03-18 |url-status=dead |archive-url=https://web.archive.org/web/20100330221426/http://www.world-nuclear.org/info/inf15.html |archive-date=2010-03-30}}</ref>

High-level waste is full of highly radioactive [[fission products]], most of which are relatively short-lived. This is a concern since if the waste is stored, perhaps in deep geological storage, over many years the fission products decay, decreasing the radioactivity of the waste and making the plutonium easier to access. The undesirable contaminant Pu-240 decays faster than the Pu-239, and thus the quality of the bomb material increases with time (although its quantity decreases during that time as well). Thus, some have argued, as time passes, these deep storage areas have the potential to become "plutonium mines", from which material for nuclear weapons can be acquired with relatively little difficulty. Critics of the latter idea have pointed out the difficulty of recovering useful material from sealed deep storage areas makes other methods preferable. Specifically, high radioactivity and heat (80&nbsp;°C in surrounding rock) greatly increase the difficulty of mining a storage area, and the enrichment methods required have high capital costs.<ref>{{cite web |url=http://nci.org/s/sp121495.htm |title=A Perspective on the Proliferation Risks of Plutonium Mines |author=Lyman, Edwin S. |publisher=[[Nuclear Control Institute]] |date=December 1994 |access-date=2015-11-25 |url-status=dead |archive-url=https://web.archive.org/web/20151125225922/http://nci.org/s/sp121495.htm |archive-date=2015-11-25}}</ref>

Pu-239 decays to U-235 which is suitable for weapons and which has a very long half-life (roughly 10<sup>9</sup> years). Thus plutonium may decay and leave uranium-235. However, modern reactors are only moderately enriched with U-235 relative to U-238, so the U-238 continues to serve as a [[Denaturation (fissile materials)|denaturation]] agent for any U-235 produced by plutonium decay.

One solution to this problem is to recycle the plutonium and use it as a fuel e.g. in [[fast reactor]]s. In [[integral fast reactor|pyrometallurgical fast reactors]], the separated plutonium and uranium are contaminated by actinides and cannot be used for nuclear weapons.