Just read an interesting article about a natural, self-sustaining nuclear reaction that occured here on Earth...........

[NNadir]

4. Thank you. You have it right. I've written about Oklo many times, here and elsewhere.

I would link some of multiple writings on this topic, but it appears Journals are off line for maintenance.

Oklo has been studied for a long time as an indicator of the migration of fission products in uncontrolled circumstances, which involved water flows through the reactor cores in porous sandstone. It is known that most did not migrate very far, with some exceptions, notably technetium, which migrated as the soluble TcO₄^- ion, all of which has decayed to ruthenium. Even this did not migrate more than a few kilometers. (The Oklo formations show isotopic distributions for ruthenium reflecting this property, with the ⁹⁹Ru isotope being separated from the other fission product ruthenium isotopes.)

The cyclic behavior was tied to thermal feedback loops which is the primary control in human made thermal reactors, technically called "a negative void coefficient." Basically when water boils it can no longer moderate neutrons as well as when it is in the liquid state.

Ancient uranium was what we would now call "Low Enriched Uranium" because much of the shorter lived ²³⁵U (t_1/2 = 700 million years) had not decayed as much as it has now. Billions of years ago the fraction of ²³⁵U was on the order of between 3% and 4%. The migration of uranium became possible with the emergence of oxygen in the planet atmosphere.

A current case wherein we can follow the migration of fission products and actinides are the Hanford Reservation in Washington State, where the plutonium for American Nuclear Weapons was manufactured, as well as the Nevada National "Security" Reservation where nuclear weapons were tested underground.

I do have access to one post of mine on the migration of actinides at that site, as well as at Hanford, that I've written here, in which Oklo is mentioned with references: 828 Underground Nuclear Tests, Plutonium Migration in Nevada, Dunning, Kruger, Strawmen, and Tunnels

That (rather long and desultory) post contains reference to some scientific papers on Oklo.

Thanks again for your kind note.