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.
> I once read a horrifying fiction story about a pre-industrial culture that used nuclear reactions in open piles of uranium ore
The real world was worse than this fiction. It's easy to find pictures of old advertisements for radioactive products, in the years following the discovery of radioactivity. Radioactive pills "to boost your energy", radioactive cosmetics, radioactive false teeth... Now imagine what happened to the people that used these.
It seems less odd when one realizes how common uranium actually is. Today, it is about as common as tin. Knowing that, there were probably lots of undiscovered places where the density got high enough to fission. Much of the earth's internal heat comes from fission. We only get interested when it happens near the surface.
Uranium is one of the elements that are greatly enriched in the continental crust of the Earth (like also thorium and the rare-earth metals).
So in the continental crust the abundance of uranium is more than one hundred times greater than the average abundance of uranium in the entire Earth or in the Solar System.
Otherwise uranium would have been almost the least abundant chemical element, as it is on average in the entire Solar System.
Tin is among the metals that are depleted in the crust of the Earth. Elsewhere tin is a few hundred times more abundant than uranium.
A continental crust like on Earth can appear only on planets that have been melted, so that the light and heavy minerals have combined and separated by density, with the continental crust floating over the molten magma and then solidifying. Most of the Earth has been melted by the collision that has formed the Moon, but possibly also by other smaller collisions.
In most other places in the Solar System it will be unlikely to find so easily accessible and abundant uranium as here on Earth.
> Knowing that, there were probably lots of undiscovered places where the density got high enough to fission.
Density of U235, specifically… you could crystallize pure uranium in the Earth’s crust today, and it would not be enriched enough for a nuclear reactor.
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.
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
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.
The real world was worse than this fiction. It's easy to find pictures of old advertisements for radioactive products, in the years following the discovery of radioactivity. Radioactive pills "to boost your energy", radioactive cosmetics, radioactive false teeth... Now imagine what happened to the people that used these.
So in the continental crust the abundance of uranium is more than one hundred times greater than the average abundance of uranium in the entire Earth or in the Solar System.
Otherwise uranium would have been almost the least abundant chemical element, as it is on average in the entire Solar System.
Tin is among the metals that are depleted in the crust of the Earth. Elsewhere tin is a few hundred times more abundant than uranium.
A continental crust like on Earth can appear only on planets that have been melted, so that the light and heavy minerals have combined and separated by density, with the continental crust floating over the molten magma and then solidifying. Most of the Earth has been melted by the collision that has formed the Moon, but possibly also by other smaller collisions.
In most other places in the Solar System it will be unlikely to find so easily accessible and abundant uranium as here on Earth.
Density of U235, specifically… you could crystallize pure uranium in the Earth’s crust today, and it would not be enriched enough for a nuclear reactor.