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Roger Weller, geology instructor

by David Monagham
Physical Geology
Fall 2012

Oklo Reactors

How Nature Beat Humans to the Punch.


            The process of using radioactive uranium for energy has been a staple of the green and alternative energy movement since the early 20th century. This process has been as widely criticized as it has been lauded for its advancement in clean energy. Many would cite that nuclear fusion or fission for energy produces a dangerous by product and is inherently non-natural. What these people fail to realize is that nature has already learned the trick for creating energy from radioactive materials long before man ever appeared on this planet. In the area now known as Gabon Africa almost 2 billion years ago, natural forces made the only known naturally occurring nuclear reactor referred to as the Oklo Reactors.

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Location of Oklo reactors Today


            During the Proterozoic era, approximately 1.8 billion years ago, a concentration of Uranium-235 began to spontaneously react with itself producing the first and only naturally occurring nuclear reactor. This reactor was made possible for a number of reasons, first and foremost being the new abundance of oxygen in Earth’s atmosphere.  As life began to process the raw materials on Earth in bacterial format beginning around 3.6 billion years ago it produced oxygen as a byproduct. This oxygen was important in the formation of the Oklo reactors.  Oxygen is required in the fission process of uranium without the contributions of the organisms producing oxygen the reactors never would have formed.

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Stromatolites, early bacteria that produced oxygen required for Oklo Reactors.


     Scientists also believe that early bacteria had a part in concentrating the uranium in the area of the reactors so it could reach a critical mass, meaning that enough of the raw U-235 material existed in one location that it began to spontaneously react with itself.  Besides the appropriate amount of U-235 to reach critical mass and the presence of oxygen the other critical ingredient in the formation of the Oklo reactors was water, water allowed the reactor to maintain a semi stable cycle of heating and cooling that lasted for hundreds of thousands of years at a time in at least 16 known locations in the same area.

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Cross-section showing locations of the 16 known reactors at Oklo

            The reactors are believed to have operated on 3 hour cycles fueled by the constant influx of the water in the area. This cycle was determined through measurement of xenon gas isotopes that are found when a nuclear reaction takes place. By measuring these isotopes scientists estimate that the reactors operated in a 30 minute heating phase before the temperature of the reactor became too hot and evaporated all water in the area and entering into a cooling cycle lasting 2 hours and 30 minutes. This went on for hundreds of thousands of years at each site producing an average of 100kW of estimated energy, or energy comparable to that at modern man made research reactors.


            Francis Perrin, the French physicist, is credited with discovering the historical evidence of the reactors in 1972. He noticed a seemingly minute but significant discrepancy with the amount of U-235 remaining in the uranium ore that his company was mining. The difference in the amount is stated to be .003% different than what was to be expected from the site. While this amount is very minute it could have meant very grave consequences, it could mean that some other entity was mining the material for energy production or worse, nuclear weapons. By studying the surrounding material of the Oklo mine Perrin noticed a distinct increase in neyodymium and ruthenium isotopes commonly associated with nuclear reactors as well as traces xenon gas isotopes that occur during nuclear reactions. These findings led Perrin to conclude that there had been a naturally occurring nuclear reactor at the site of the mine many years ago.

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Francis Perrin

            Scientists are now looking to the Oklo reactors as an example of storage of the volatile byproducts produced during nuclear fission or fusion.  By studying how the products have reacted within the earth at the Oklo site for billions of years scientists are gaining increasing insight into the long term effects of how material produced at man-made reactors will react with their environments in the future. One such scientist, John Bradenburg, has used research and evidence gathered from the Oklo site to find what he believes to be an even larger naturally occurring Nuclear reactor -- on Mars. By studying what led Perrin to discover the Oklo reactors and recognizing patterns in gamma radiation Bradenburg believes that there may have been a similar natural nuclear reaction which used radioactive thorium as a source in the Mare Acidalium region on our neighboring planet. The implications could mean that there once was an abundance of oxygen on the planet and the organisms necessary to produce it as well as a readily available source of water to fuel the reaction process.  However this is still only a theory and it has yet to be proven. One thing is certain though, nature is full of surprises and new things are being learned everyday through the careful observation of the world with which we react.

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A proposed nuclear waste storage diagram based off of Oklo reactors.

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Image showing the former site of the Oklo mine

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Pictures showing exposed cores of Oklo reactors.