Université de Strasbourg

Symposium: Oklo natural nuclear reactors - 50th anniversary

June 28, 2022
From 14:00 until 18:00
Salle de Conférence - ISIS, 8 allée Gaspard Monge, Strasbourg

View of the open pit of Oklo during exploration of the reactors (1973). Image: Benoît Gall.

“This deposit [at Oklo] is no less unique, and certainly more irreplaceable, than
the most valued specimens from the Moon and Mars.”

Gauthier-Lafaye et al., 1997. The last natural nuclear fission reactor. Nature, vol. 387: 337.

In June 1972, at the Tricastin uranium enrichment site at Pierrelatte in France, a routine mass spectrometry analysis that compared samples from the Oklo mine - located in Gabon on the west coast of Africa - detected a puzzling discrepancy. The concentration of uranium-235 was significantly lower than would be expected, suggesting a disappearance of U-235. Given that U-235 can be used to build nuclear bombs, an explanation was urgently needed. Further investigations resulted in surprising findings which suggested the presence of processes exactly like those in a nuclear reactor – but natural, not man-made. On 25 September 1972, the French Alternative Energies and Atomic Energy Commission (CEA) announced its finding that self-sustaining nuclear chain reactions had occurred on Earth about two billion years ago.  

Oklo is the only known location for this in the world. Despite their modest power output, the Gabon natural nuclear reactors are remarkable because they spontaneously began operating around two billion years ago, and continued to do so in a stable manner for several hundreds of thousands of years. In addition, at the Gabon reactors, many of the radioactive products of the nuclear fission have been contained for two billion years, providing a natural site of long-term geologic storage for nuclear waste. 


This year - 2022 - marks the 50th anniversary of the discovery of the Oklo natural nuclear reactor. In honour of this occasion, a special symposium will take place on the afternoon of 28 June 2022, at the University of Strasbourg. It will highlight the Oklo phenomenon with presentations from historical actors in this scientific adventure, and will cover this amazing topic from its discovery and very first questions up to the most modern points of view on these reactors.

It will also address the question of the influence of the Great Oxidation Event 2.1 billion years ago, as discovery of the Gabonionta fossils [1] was made only a few dozen kilometres from these reactors.

 Gabonionta from a mine near the OKLO reactors. Image: B. Gall 

[1] El Albani A, Bengtson S, Canfield DE, Riboulleau A, Rollion Bard C, et al. (2014) The 2.1 Ga Old Francevillian Biota- Biogenicity, Taphonomy and Biodiversity. PLOS ONE 9(6)- e99438.

This symposium is part of the 10th International Conference on High Level Environmental Radiation Areas (ICHLERA-2022), which takes place on 27-30 June 2022, also in Strasbourg. The event is organised by Benoît Gall and colleagues, and co-funded by USIAS. 

For more information, contact Benoît Gall (email)

Oklo mine site illustration

Localisation of the OKLO reactors. Geological map of the Oklo area of the Franceville basin (left). The drawing on the right locates reactors 1 to 9 on a 3D perspective of the Oklo deposit. Informations complémentaires A; B. Gall et al. à paraître dans PLOS One 

More information about the Oklo natural nuclear reactor: 



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