|Fig. 1: Map of Gabon. (Source: Wikimedia Commons)|
What comes to mind when one imagines a nuclear reactor? Images of large concrete cooling towers, steel containment vessels, and precisely controlled mechanisms likely spring to mind. However, nuclear reactors have been present since long before humans harnessed nuclear energy in the 20th century. Naturally occurring geological conditions have led to nuclear reactions on earth, which were discovered in 1972 at the adjacent Oklo and Okelobondo mines in the West-African country Gabon, shown in Fig. 1. . These reactors formed naturally, and self moderated for very long periods of time - in the case of Oklo, it is estimated that the reactor lasted up to 150,000 years. 
Andy Zhao writes that scientists discovered the Oklo and Okelobondo reactors when testing the percentage of U-235 found in ore extracted from the mines.  Certain regions had a lower percentage of U-235, which indicated that the isotope had reacted and decayed into other particles. It is estimated that these natural reactors produced about 100KW in 30 minute pulses, occurring every 3 hours.  There were up to 17 sites in the region where these nuclear reactions occurred, and they became critical around two billion years ago.  These reactors were moderated by groundwater that seeped into the ore.  Meshik expands on this moderating effect, which explains the cyclical on and off effect the reactors demonstrated. In the reactors, water allowed the nuclear reaction to be sustained. As the reaction proceeded, it generated heat, which would boiled the water. This cause the reactors to dry up and shut down, allowing them to cool. "As more water returned to the reaction zone, neutrons became properly moderated and fission once again resumed." 
It is no secret that man-made nuclear power has had a spotty track record since its invention. Regardless of its safety compared to other power generation methods, incidents do occur, often with very large scale consequences. However, the Oklo reactors functioned for hundreds of thousands of years without incident, before exhausting their supply of U-235 and going dormant. This is a great example of the effectiveness of passive safety systems, which act to keep a reactor safe without any human intervention. The moderating abilities of the water fed by the natural aquifers in the region not only enabled the reactor to function, but kept it from going into runaway.
A very particular set of conditions led to the creation of these reactors, conditions that have, as far as we know, been replicated in only one region of earth. The Oklo reactor deposits no longer exist, having been mined away.  However, these special accidents serve to highlight simultaneously how rare nuclear fission is, and somewhat counter intuitively how easily it can be accomplished under the right set of conditions, without any human intervention. This duality of the natural reactors makes them an incredibly interesting subject of study.
© Reed Kraus. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. P. Meshik, C. M. Hohenberg, and O. V. Pravdivtseva "Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon," Phys. Rev. Lett. 93, 182302 (2004).
 A.P. Meshik, "The Workings of an Ancient Nuclear Reactor," Scientific American, 26 Jan 09.
 A. Zhao, "Oklo: Nature's Nuclear Reactor," Physics 241, Stanford University, Winter 2016.