Editing Uranium (section)

===Enrichment===
{{Main|Enriched uranium}}
[[File:Gas centrifuge cascade.jpg|thumb|Cascades of [[gas centrifuge]]s are used to enrich uranium ore to concentrate its fissionable isotopes.|alt=A photo of a large hall filled with arrays of long white standing cylinders.]]
In nature, uranium is found as uranium-238 (99.2742%) and uranium-235 (0.7204%). [[Isotope separation]] concentrates (enriches) the fissile uranium-235 for nuclear weapons and most nuclear power plants, except for [[gas cooled reactor]]s and [[pressurized heavy water reactor]]s. Most neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the [[nuclear chain reaction]]. The concentration and amount of uranium-235 needed to achieve this is called a '[[critical mass]]'.

To be considered 'enriched', the uranium-235 fraction should be between 3% and 5%.<ref>{{cite web |title=Uranium Enrichment |url=http://web.ead.anl.gov/uranium/guide/depletedu/enrich/index.cfm |access-date=11 February 2007 |publisher=Argonne National Laboratory |archive-url=https://web.archive.org/web/20070124232415/http://web.ead.anl.gov/uranium/guide/depletedu/enrich/index.cfm |archive-date=24 January 2007 |url-status=dead}}</ref> This process produces huge quantities of uranium that is depleted of uranium-235 and with a correspondingly increased fraction of uranium-238, called depleted uranium or 'DU'. To be considered 'depleted', the {{sup|235}}U concentration should be no more than 0.3%.<ref name="paducah">{{cite news |url=http://www.wise-uranium.org/dhap991.html |title=Depleted Uranium: a by-product of the Nuclear Chain |access-date=31 July 2009 |publisher=Laka Foundation |author=Diehl, Peter |url-status=dead |archive-url=https://archive.today/20130113114319/http://www.wise-uranium.org/dhap991.html |archive-date=13 January 2013}}</ref> The price of uranium has risen since 2001, so enrichment tailings containing more than 0.35% uranium-235 are being considered for re-enrichment, driving the price of [[depleted uranium hexafluoride]] above $130 per kilogram in July 2007 from $5 in 2001.<ref name="paducah" />

The [[gas centrifuge]] process, where gaseous [[uranium hexafluoride]] ({{chem|UF|6}}) is separated by the difference in molecular weight between {{sup|235}}UF{{sub|6}} and {{sup|238}}UF{{sub|6}} using high-speed [[centrifuge]]s, is the cheapest and leading enrichment process.{{sfn|Emsley|2001|p=478}} The [[gaseous diffusion]] process had been the leading method for enrichment and was used in the [[Manhattan Project]]. In this process, uranium hexafluoride is repeatedly [[diffusion|diffused]] through a [[silver]]-[[zinc]] membrane, and the different isotopes of uranium are separated by diffusion rate (since uranium-238 is heavier it diffuses slightly slower than uranium-235).{{sfn|Emsley|2001|p=478}} The [[molecular laser isotope separation]] method employs a [[laser]] beam of precise energy to sever the bond between uranium-235 and fluorine. This leaves uranium-238 bonded to fluorine and allows uranium-235 metal to precipitate from the solution.{{sfn|Emsley|2001|p=479}} An alternative laser method of enrichment is known as [[atomic vapor laser isotope separation]] (AVLIS) and employs visible [[tunable laser]]s such as [[dye laser]]s.<ref>{{cite book |editor=[[F. J. Duarte|Duarte, F. J.]] |editor2=Hillman, L. W. |title=Dye Laser Principles |publisher=Academic |date=1990 |page=413 |isbn=978-0-12-222700-4 |url=http://www.opticsjournal.com/dlp.htm |url-status=dead |archive-url=https://web.archive.org/web/20100917020215/http://www.opticsjournal.com/dlp.htm |archive-date=17 September 2010}}</ref> Another method used is liquid thermal diffusion.<ref name="SciTechEncy" />

The only significant deviation from the {{sup|235}}U to {{sup|238}}U ratio in any known natural samples occurs in [[Oklo]], [[Gabon]], where [[natural nuclear fission reactor]]s consumed some of the {{sup|235}}U some two billion years ago when the ratio of {{sup|235}}U to {{sup|238}}U was more akin to that of [[low enriched uranium]] allowing regular ("light") water to act as a [[neutron moderator]] akin to the process in humanmade [[light water reactor]]s. The existence of such natural fission reactors which had been theoretically predicted beforehand was proven as the slight deviation of {{sup|235}}U concentration from the expected values were discovered during [[uranium enrichment]] in France. Subsequent investigations to rule out any nefarious human action (such as stealing of {{sup|235}}U) confirmed the theory by finding isotope ratios of common [[fission product]]s (or rather their stable daughter nuclides) in line with the values expected for fission but deviating from the values expected for non-fission derived samples of those elements.