Fascinating that there existed just the perfect geology for this to occur, and that there was also a uranium source.<p>Here is how the paper explains how Uranium concentrations of 15% occurred:<p>> 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