Natural fission reactors in the Franceville basin, Gabon

Fascinating that there existed just the perfect geology for this to occur, and that there was also a uranium source.

Here is how the paper explains how Uranium concentrations of 15% occurred:

> In areas affected by intense hydraufracturing resulting in brecciation of the rock, uranium content may reach values as high as 15%. In such cases, mineralization is closely related to oxidation-reduction fronts with the development of Fe-oxides. In the brecciated, oxidized ores, uranium forms 1 to 10 cm size irregular patches in the matrix and fills the microfracture networks. It is thought that fission reactions started in this type of ore when the uranium content reached the critical mass and the other conditions for chain fission were met. To model the criticality of the reactors, various parameters have to be taken into account such as the contents of B and REEs which are poison for neutrons and the content of U which allows it to reach the critical mass, the porosity of the sandstones which controls the amount of water (which acts as regulator of the fission reaction), the mineralogical composition of the ore (which controls the amount of ele- ments having different cross section values) and the temper- ature which acts on the density of the water which is assumed to be the moderator for neutrons. Naudet (1991) has com- puted that at Oklo, the criticality could happen under two main conditions: (1) the mineralized sandstones must have been fractured in order to have an open porosity ranging between 10 and 15% and (2) fission reactions could start only in area having the highest uranium content ranging between 10 and 20%. Criticality was easily achieved in ore where sandstones had already lost some silica which at Oklo could have been in volume of around cubic meter of sand- stone with a 10% uranium content

This is such an interesting and weird phenomenon, in the context of the complexity of human made nuclear reactors.

I once read a horrifying fiction story about a pre-industrial culture that used nuclear reactions in open piles of uranium ore tended by slaves as a heat source, but cannot remember the name or author.

Paper, https://comptes-rendus.academie-sciences.fr/physique/item/10...

This 1976 SciAm article has a lot of info including photos of the mine: https://www.jstor.org/stable/24950391?seq=1

The use of plural really confused me. What little I knew of this previously, always seemed to talk in the singular. One place (Gabon) this happened to our knowledge. But, it happened more than once I find. I suppose for geological effects, time causes change, and sufficient change both physically (location) and pressure/temperature, and radiology, means the thing can happen again and again. Or, dispersed foci can react in shorter time, close to each other, until entropy sets in.

It is interesting to contrast this paper to what would happen if people wanted to put an artificial nuclear waste dump in Gabon. The response would be furious.