Oklo Mines

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

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.

Main uranium mines of France

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

Some years later Gabon became an independent nation but mining operations in Oklo developed and the obtained material was sold to Europe. Over the next 40 years here was mined uranium which powered many nuclear power plants in the Western Europe. Now the mines have been exhausted and former mines are landscaped.

Where is the missing isotope of uranium?

Uranium has several isotopes and 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: it is the only naturally occuring fissile isotope – e.g. the only substance which can maintain nuclear fission chain reaction.

Thus – when uranium is mined, the only really important part of it 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%.

Was the valuable uranium-235 somehow stolen?

Natural nuclear reactor?

Further research by French physicists Henri Bouzigues, Francis Perrin and other scientists testified: something is not normal with the uranium ores from Oklo Mines. All samples had abnormal composition of uranium isotopes and also some other chemical elements had unusual isotopic compositions: increased percentage of 143Nd (neodimium) and 99Ru (ruthenium) isotopes. This could be explained only by the loss of the uranium isotope through chain reaction which led to increase of specific neodimium and ruthenium isotopes.

Existence of such 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 – before some 1.7 billion years, in Precambrian times. Nuclear reaction was caused by 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 atmosphere of Earth did not contain enough oxygen. Then the level of oxygen gradually increased: and only in the presence of oxygen 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 chain reaction started. Reaction warmed up the water and it boiled away, stopping the reaction for some hours. Then water returned and chain reaction started again. This continued for hundreds of thousands of years until the isotope composition changed and reaction could not be sustained anymore.

It is assumed that over a year these nuclear reactors produced some 15,000 MW of energy, temperatures reached some 200 – 300 °C.

Unique landmark

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

All the chemical byproducts of this natural nuclear reaction (including such exotic elements as lanthanum, neodymium, yttrium and others) have been preserved in intact form and provide much information about this unique process. This also shows 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.

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Oklo Mines -1.400021, 13.160274   Oklo Mines

Coordinates: 1.4000 S 13.1603 E
Categories: Rare natural materials
Values: Geology
Rank: 4
Address: Africa, Gabon, Haut-Ogooué, around Mounana town

Wonders of Gabon

Rainforest in Gabon
Rainforest in Gabon / Axel Rouvin, CC BY 2.0

Gabon is sparsely populated country which per 85% is covered with equatorial rainforest. Highlights of this country are impressive ecosystems including unusual relict rainforest which persisted during the ice age thanks to the fog from cold ocean streams nearby. In the country are located also impressive waterfalls and rapids.

Rare natural materials

Lava of Ol Doinyo Lengai volcano, Tanzania
Lava of Ol Doinyo Lengai volcano, Tanzania / Thomas Kraft, / CC BY-SA 3.0

There are places in the world, where one can find such materials which are not present anywhere else on Earth – or can be found just in a few places. Such landmarks are included in this category – category of rare natural materials.

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