Editing Nuclear reactor (section)

==Natural nuclear reactors==
{{Main|Natural nuclear fission reactor}}
Almost two billion years ago a series of self-sustaining nuclear fission "reactors" self-assembled in the area now known as [[Oklo]] in [[Gabon]], West Africa. The conditions at that place and time allowed a [[natural nuclear fission reactor|natural nuclear fission]] to occur with circumstances that are similar to the conditions in a constructed nuclear reactor.<ref>[http://video.google.com/videoplay?docid=-2334857802602777622 Video of physics lecture] – at Google Video; a natural nuclear reactor is mentioned at 42:40 mins into the video {{webarchive |url=https://web.archive.org/web/20060804021811/http://video.google.com/videoplay?docid=-2334857802602777622 |date=4 August 2006 }}</ref> Fifteen fossil natural fission reactors have so far been found in three separate ore deposits at the Oklo uranium mine in Gabon. First discovered in 1972 by French physicist [[Francis Perrin (physicist)|Francis Perrin]], they are collectively known as the [[Natural nuclear fission reactor|Oklo Fossil Reactors]]. Self-sustaining [[nuclear fission]] reactions took place in these reactors approximately 1.5&nbsp;billion years ago, and ran for a few hundred thousand years, averaging 100&nbsp;kW of power output during that time.<ref>Meshik, Alex P. (November 2005) [http://www.scientificamerican.com/article/ancient-nuclear-reactor/ "The Workings of an Ancient Nuclear Reactor."] {{Webarchive|url=https://web.archive.org/web/20150315120003/http://www.scientificamerican.com/article/ancient-nuclear-reactor/ |date=15 March 2015 }} ''Scientific American.'' p. 82.</ref> The concept of a natural nuclear reactor was theorized as early as 1956 by [[Paul Kuroda]] at the [[University of Arkansas]].<ref name="OCRWM">{{cite web|title=Oklo: Natural Nuclear Reactors |work=Office of Civilian Radioactive Waste Management |url=http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml |access-date=28 June 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060316101947/http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml |archive-date=16 March 2006 }}</ref><ref name="ANS1">{{cite web |title=Oklo's Natural Fission Reactors |work=[[American Nuclear Society]] |url=http://www.ans.org/pi/np/oklo |access-date=28 June 2006 |archive-date=30 March 2021 |archive-url=https://web.archive.org/web/20210330044447/https://ans.org/pi/np/oklo/ |url-status=live }}</ref>

Such reactors can no longer form on Earth in its present geologic period. Radioactive decay of formerly more abundant uranium-235 over the time span of hundreds of millions of years has reduced the proportion of this naturally occurring fissile isotope to below the amount required to sustain a chain reaction with only plain water as a moderator.

The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years, cycling on the order of hours to a few days.

These natural reactors are extensively studied by scientists interested in geologic [[radioactive waste]] disposal. They offer a case study of how radioactive isotopes migrate through the Earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment.