|Fig. 1: The U-235 isotope of uranium, found in unusually low amounts in the Oklo deposits. (Source: Wikimedia Commons)|
Man-made nuclear reactors function by executing a difficult nuclear reaction. In the power plants, scientists split uranium atoms releasing energy as heat and releasing neutrons to split other atoms. This process is formally known as nuclear fission. Although a high degree of engineering, physics, and acute, detailed attention went into building a nuclear reactor, a discovery in 1972 revealed nuclear fission has been around for a lot longer, 2 billion years to be exact.  Nature had already perfected the process of nuclear fusion in a small region of West Africa, Gabon.
In 1972, the Commissariat LEnergie Atomique laboratory made an incredible discovery during a routine isotopic analyses of uranium samples obtained from a conversion factory.  All atoms of a specific chemical element, in this case uranium, have the same chemical properties, but may differ in weight. The range of weights of an element are known as isotopes. One of the samples collected had an abnormal isotopic composition, the percentage of U-235 (see Fig. 1) in the samples of uranium from the Oslo deposit in Gabon was 0.717% slightly below the 0.720% found in most terrestrial uranium.  The abnormal uranium sample was traced to a mining site in the Franceville basin of Gabon, to the Oklo deposit.  The unexpectedly low concentration of U-235 suggested that at some point in history natural nuclear fission of U-235 occurred at the site which came to be known as the Oklo Fossil Reactors. 
The Oklo reactor began operating about 2 billion years ago and operated for a period as long as 0.6 to 1.5 million years, shutting itself down when water necessary for neutron moderation depleted.  Once the natural reactors burned out, the highly radioactive waste they generated was held in place deep under Oklo by the granite, sandstone, and clays surrounding the areas. Plutonium has moved less than ten feet from where it was formed almost two billion years ago contained in the sedimentary rocks that kept them from being dissolved or spread by groundwater.
The Oklo phenomenon gives scientists an opportunity to examine the results of a natural two billion year experiment, one that cannot be duplicated in the lab. By analyzing the remnants of these ancient nuclear reactors and understanding how underground rock formations contained the waste, scientists studying Oklo can apply their findings to containing nuclear waste today. The rock types and other aspects of the geology at Oklo differ from those at Yucca Mountain.  But this information could prove useful in the design of a repository at Yucca Mountain, should it be judged suitable for one.
© Paige Voigt. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
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 H. J. Marvin, "Nuclear Fission Reactors as Energy Sources for the Giant Outer Planets," Naturwiss. 79, 7 (1992).