Oklo Reactor

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Something is not right about the story of this deposit. IT is NOT possible for water to start a nuclear reaction unless it is heavy water (Tritium). Tritium in a nuclear reaction undergoes fusion unlike uranium which under goes fission losing protons to become a lead isotope.

More likely the water boiled off because of a sustained reaction, refilling after with groundwater and/or condensate.

However the first fluids to contact an igneous deposit would be hydrothermal. If this deposit is formed of sands then it would have been placed by fluvial processes where most of the lighter components of the original rock would have been carried further downstream thus concentrating the heaver or uranium fraction. The concentration leading to a reaction.

I must point out ALL uranium decays. A large enough deposit would generate heat as well leading to the vaporization of groundwater.

This decay is constant not episodic and water only affects the temperature not the reaction rate. Since the decay of uranium is a constant it allows us to date igneous rocks by the uranium to lead isotope ratios. This principle is well known and doesn’t change.

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ROCK you’re quite wrong about the nuclear reaction at Oklo. It’s a light-water moderated reaction which is possible with low-enriched uranium, much like most nuclear power reactors. Heavy water (deuterium, not tritium) moderated reactors can run on present-day enriched uranium, like the CANDU reactors. The heavy water does not cause fusion either - that requires very different conditions. Fission of U-235 does not result in lead - you’re thinking of alpha decay. Fission results in much lighter (roughly half the weight) products.
At Oklo the reaction moderated by water caused the water to boil off, stopping the reaction and cooling down. Once water refills its moderation role, the reaction starts again. Water definitely affects the reaction rate.