Editing Nuclear fuel cycle (section)

==Front end==
<gallery>
Image:Uranium ore square.jpg|'''1 [[Uranium ore]]''' – the principal raw material of nuclear fuel
Image:Yellowcake.jpg|'''2 [[Yellowcake]]''' – the form in which uranium is transported to a conversion plant
Image:UF6 square.jpg|'''3 [[Uranium hexafluoride|UF<sub>6</sub>]]''' – used in enrichment
Image:Nuclear fuel pellets.jpeg|'''4 [[Nuclear fuel]]''' – a compact, inert, insoluble solid
</gallery>

===Exploration===
A deposit of uranium, such as [[uraninite]], discovered by geophysical techniques, is evaluated and sampled to determine the amounts of uranium materials that are extractable at specified costs from the deposit. Uranium reserves are the amounts of ore that are estimated to be recoverable at stated costs.

Naturally occurring uranium consists primarily of two isotopes U-238 and U-235, with 99.28% of the metal being U-238 while 0.71% is U-235, and the remaining 0.01% is mostly U-234. The number in such names refers to the [[isotope]]'s atomic [[mass number]], which is the number of [[proton]]s plus the number of [[neutron]]s in the [[atomic nucleus]].

The atomic nucleus of U-235 will nearly always fission when struck by a [[free neutron]], and the isotope is therefore said to be a "[[fissile]]" isotope. The nucleus of a U-238 atom on the other hand, rather than undergoing fission when struck by a free neutron, will nearly always absorb the neutron and yield an atom of the isotope U-239. This isotope then undergoes natural radioactive decay to yield Pu-239, which, like U-235, is a fissile isotope. The atoms of U-238 are said to be fertile, because, through neutron irradiation in the core, some eventually yield atoms of fissile Pu-239.

===Mining===
{{Main|Uranium mining}}
Uranium ore can be extracted through conventional mining in open pit and underground methods similar to those used for mining other metals. [[In-situ leach]] [[mining]] methods also are used to mine uranium in the [[United States]]. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. Uranium ores in the United States typically range from about 0.05 to 0.3% uranium oxide (U<sub>3</sub>O<sub>8</sub>). Some uranium deposits developed in other countries are of higher grade and are also larger than deposits mined in the United States. Uranium is also present in very low-grade amounts (50 to 200 parts per million) in some domestic [[phosphate]]-bearing deposits of marine origin. Because very large quantities of phosphate-bearing rock are mined for the production of wet-process [[phosphoric acid]] used in high analysis [[fertilizer]]s and other phosphate chemicals, at some phosphate processing plants the uranium, although present in very low concentrations, can be economically recovered from the process stream.

===Milling===
When Uranium is mined out of the ground it does not contain enough pure uranium per pound to be used. The process of milling is how the cycle extracts the usable uranium from the rest of the materials, also known as tailings. To begin the milling process the ore is either ground into fine dust with water or crushed into dust without water.<ref name=":0">{{Cite book |last=Hore-Lacy |first=Ian |title=Uranium for nuclear power: resources, mining and transformation to fuel |date=2016 |publisher=Woodhead Publishing is an imprint of Elsevier |isbn=978-0-08-100307-7 |series=Woodhead Publishing series in energy |location=Duxford, UK}}</ref> Once the Materials have been physically treated, they then begin the process of being chemically treated by being doused in acids. Acids used include hydrochloric and nitrous acids but the most common acids are sulfuric acids. Alternatively if the material that the ore is made of is particularly resistant to acids then an alkali is used instead.<ref>{{Cite journal |last1=Edwards |first1=C. R. |last2=Oliver |first2=A. J. |date=September 2000 |title=Uranium processing: A review of current methods and technology |url=https://link.springer.com/10.1007/s11837-000-0181-2 |journal=JOM |language=en |volume=52 |issue=9 |pages=12–20 |doi=10.1007/s11837-000-0181-2 |bibcode=2000JOM....52i..12E |issn=1047-4838|url-access=subscription }}</ref> After being treated chemically the uranium particles are dissolved into the solution used to treat them. This solution is then filtered until what solids remain are separated from the liquids that contain the uranium. The undesirable solids are disposed of as [[tailings]].<ref>{{Cite report |url=http://dx.doi.org/10.2172/1342847 |title=Uranium Mining and Milling |last=Karpius |first=Peter |date=2017-02-02 |publisher=Office of Scientific and Technical Information (OSTI)|doi=10.2172/1342847 }}</ref> Once the solution has had the tailings removed the uranium is extracted from the rest of the liquid solution,  in one of two ways, solvent exchange or [[ion exchange]]. In the first of these a solvent is mixed into the solution. The dissolved uranium binds to the solvent and floats to the top while the other dissolved materials remain in the mixture. During ion exchange a different material is mixed into the solution and the uranium binds to it. Once filtered the material is panned out and washed off.<ref name=":0" /> The solution will repeat this process of filtration to pull as much usable uranium out as possible. The filtered uranium is then dried out into U<sub>3</sub>O<sub>8</sub> uranium. The milling process commonly yields dry powder-form material consisting of natural uranium, "[[yellowcake]]", which is sold on the uranium market as U<sub>3</sub>O<sub>8</sub>. Note that the material is not always yellow.

===Uranium conversion===
Usually milled uranium oxide, U<sub>3</sub>O<sub>8</sub> ([[triuranium octoxide]]) is then processed into either of two substances depending on the intended use.

For use in most reactors, U<sub>3</sub>O<sub>8</sub> is usually converted to [[uranium hexafluoride]] (UF<sub>6</sub>), the input stock for most commercial uranium enrichment facilities. A solid at room temperature, uranium hexafluoride becomes gaseous at 57&nbsp;°C (134&nbsp;°F). At this stage of the cycle, the uranium hexafluoride conversion product still has the natural isotopic mix (99.28% of U-238 plus 0.71% of U-235).

There are two ways to convert uranium oxide into its usable forms uranium dioxide and uranium hexafluoride; the wet option and the dry option. In the wet option the yellowcake is dissolved in nitric acid then extracted using tributyl phosphate. The resulting mixture is then dried and washed resulting in uranium trioxide.<ref>{{Cite journal |date=October 2019 |journal=World Journal of Nuclear Medicine |volume=18 |issue=4 |doi=10.1055/s-012-53210 |issn=1450-1147|doi-access=free }}</ref> The uranium trioxide is then mixed with pure hydrogen resulting in [[uranium dioxide]] and dihydrogen monoxide or water. After that the uranium dioxide is mixed with four parts hydrogen fluoride resulting in more water and uranium tetrafluoride. Finally the end product of uranium hexafluoride is created by simply adding more fluoride to the mixture.<ref>{{Cite book |title=Uranium for nuclear power: resources, mining and transformation to fuel |date=2016 |publisher=Elsevier |isbn=978-0-08-100307-7 |editor-last=Hore-Lacy |editor-first=Ian |series=Woodhead publishing series in energy |location=Waltham, MA}}</ref>

For use in reactors such as [[CANDU]] which do not require enriched fuel, the U<sub>3</sub>O<sub>8</sub> may instead be converted to [[uranium dioxide]] (UO<sub>2</sub>) which can be included in [[ceramic]] fuel elements.

In the current nuclear industry, the volume of material converted directly to UO<sub>2</sub> is typically quite small compared to that converted to UF<sub>6</sub>.

===Enrichment===<!-- This section is linked from [[Nuclear proliferation]] -->
{{Main|Enriched uranium}}
[[Image:Nuclear Fuel Cycle.png|330px|thumb|'''Nuclear fuel cycle''' begins when uranium is mined, enriched and manufactured to nuclear fuel (1) which is delivered to a nuclear power plant. After usage in the power plant the spent fuel is delivered to a reprocessing plant (if fuel is recycled) (2) or to a final repository (if no recycling is done) (3) for geological disposition. In [[nuclear reprocessing|reprocessing]] 95% of spent fuel can be recycled to be returned to usage in a nuclear power plant (4).]]

The natural concentration (0.71%) of the fissile isotope U-235 is less than that required to sustain a nuclear chain reaction in [[light water reactor]] cores.  Accordingly, UF<sub>6</sub> produced from natural uranium sources must be enriched to a higher concentration of the fissionable isotope before being used as nuclear fuel in such reactors. The level of enrichment for a particular nuclear fuel order is specified by the customer according to the application they will use it for: light-water reactor fuel normally is enriched to 3.5% U-235, but uranium enriched to lower concentrations is also required. Enrichment is accomplished using any of several methods of [[isotope separation]]. [[Gaseous diffusion]] and [[gas centrifuge]] are the commonly used uranium enrichment methods, but new enrichment technologies are currently being developed.

The bulk (96%) of the byproduct from enrichment is [[depleted uranium]] (DU), which can be used for [[armor]], [[kinetic energy penetrator]]s, [[radiation shielding]] and [[Sailing ballast|ballast]]. As of 2008 there are vast quantities of depleted uranium in storage. The [[United States Department of Energy]] alone has 470,000 [[tonne]]s.<ref>{{cite web |url= http://web.ead.anl.gov/uranium/faq/storage/faq16.cfm |title= How much depleted uranium hexafluoride is stored in the United States? |work= Depleted UF6 Management Information Network |access-date= 2008-01-15 |archive-url= https://web.archive.org/web/20071223063911/http://web.ead.anl.gov/uranium/faq/storage/faq16.cfm |archive-date= 2007-12-23 |url-status= dead }}</ref> About 95% of depleted uranium is stored as [[uranium hexafluoride]] (UF<sub>6</sub>).

===Fabrication===
{{Main|Nuclear fuel}}
For use as nuclear fuel, enriched uranium hexafluoride is converted into [[uranium dioxide]] (UO<sub>2</sub>) powder that is then processed into pellet form. The pellets are then fired in a high temperature [[sintering]] [[Industrial furnace|furnace]] to create hard, [[ceramic]] pellets of [[enriched uranium]]. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The pellets are stacked, according to each [[nuclear reactor core]]'s design specifications, into tubes of corrosion-resistant metal [[alloy]]. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.

The alloy used for the tubes depends on the design of the reactor. [[Stainless steel]] was used in the past, but most reactors now use a [[zirconium alloy]]. For the most common types of reactors, [[boiling water reactor]]s (BWR) and [[pressurized water reactor]]s (PWR), the tubes are assembled into bundles<ref>{{cite web |url= http://www.pplweb.com/NR/rdonlyres/F63D7386-A57E-46C6-90A5-857D513B0254/0/seic_plantguide.pdf |title= Susquehanna Nuclear Energy Guide |publisher= PPL Corporation |access-date= 2008-01-15 |url-status= dead |archive-url= https://web.archive.org/web/20071129121008/http://www.pplweb.com/NR/rdonlyres/F63D7386-A57E-46C6-90A5-857D513B0254/0/seic_plantguide.pdf |archive-date= 2007-11-29 }}</ref> with the tubes spaced precise distances apart. These bundles are then given a unique identification number, which enables them to be tracked from manufacture through use and into disposal.