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Oklo Mines – the only natural nuclear reactor

Flooded mine in Mounana, Oklo in Gabon
Flooded mine in Mounana, Oklo /
nicktsurikov, screenshot from Youtube video

WorldBlue  In short

First time when humans launched a nuclear chain reaction was in 1942, in Chicago. But there is one place on Earth where nature itself launched a natural nuclear reactor some 1.8 – 1.7 billion years ago: Oklo Mines in Gabon, Africa.

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GPS coordinates
1.4000 S 13.1603 E
Location, address
Africa, Gabon, Haut-Ogooué, around Mounana town

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WorldYellow In detail

Main uranium mines of France

In 1956, when Gabon still was a part of French Equatorial Africa, geologists discovered here rich uranium ores. These mines were located in the tropical forest not too far away from the oldest French town in Gabon – Franceville.

Some years later Gabon became an independent nation. Nevertheless, mining operations in Oklo developed, and the obtained material was sold to Europe. Over the next 40 years, miners were digging for uranium which powered many nuclear power plants in Western Europe. Now the mines are exhausted and former mines – landscaped.

Where is the missing isotope of uranium?

Uranium has several isotopes. In nature the proportion between these isotopes is quite exact: 99.27% consists of uranium-238, 0.72% – uranium-235 and also a little bit – 0.005% – uranium-234.

For nuclear energy and also for the production of nuclear weapons extremely important is uranium-235. This is the only naturally occurring fissile isotope – e.g. the only substance which can maintain nuclear fission chain reaction.

Thus the only really important part of uranium mine is these 0.72% of uranium-235.

Analysis of the uranium from Oklo Mines in France, Pierrelatte uranium enrichment facility in May – September 1972 showed that these ores contain less uranium-235 than normally: only 0.6% and sometimes as low as 0.296%.

Did someone steal the valuable uranium-235?

Natural nuclear reactor?

French physicists Henri Bouzigues, Francis Perrin, and other scientists continued research and proved that something is definitely not normal with the uranium ores from Oklo Mines. All samples had an abnormal composition of uranium isotopes. Some other chemical elements had unusual isotopic compositions too, such as increased percentage of 143Nd (neodymium) and 99Ru (ruthenium) isotopes. Scientists could explain this only by the loss of the uranium isotope through chain reaction leading to the increase of specific neodymium and ruthenium isotopes.

Existence of such a natural nuclear reactor was predicted by other scientists including Paul Kazuo Kuroda (USA) in 1956. And now French scientists discovered it in real life!

Geological history

Nuclear reaction here started long, long ago: some 1.7 billion years ago, in Precambrian times. This natural nuclear reactor formed due to several coincidences:

  • First: this was unusual uranium ore with high concentration of the 235U isotope reaching up to 3.1%. Such higher concentrations were possible in those times.
  • Second: reaction at first did not start because the atmosphere of Earth did not contain enough oxygen. Then the level of oxygen gradually increased: and only in the presence of oxygen did uranium became soluble in water. Thus groundwater made it possible to accumulate rich uranium ore deposits where the chain reaction could start.
  • Third: the uranium ore was soaked with groundwater. Water started to act as a neutron moderator and a chain reaction started. Reaction warmed up the water and it boiled away, stopping the reaction for some hours. Then the water returned and the chain reaction started again. This continued for hundreds of thousands of years until the isotope composition changed and the reaction could not be sustained anymore.

Rough estimates show that during one year these nuclear reactors produced some 15,000 MW of energy, and temperatures reached some 200 – 300 °C.

Unique landmark

Specialists have found several more such natural nuclear reactors nearby – but nowhere else in the world. Oklo Mines up to now is the only known place on Earth where the natural nuclear reactor has been found.

Nature has preserved in intact form all the chemical byproducts of this natural nuclear reaction including such exotic elements as lanthanum, neodymium, yttrium, and others. This helps us to obtain much information about this unique process. Also – Oklo Mines serve to us as proof that in safe geological conditions, nuclear waste products can be safely stored for… 1.7 billion years!


  1. George A. Cowan, A Natural Fission Reactor. Scientific American, Vol. 235, No. 1 (July 1976), pp. 36-47.

Oklo Mines are included in the following article:

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