Natural Nuclear Reaction Powered Ancient Geyser

Via LiveScience (Nov 2004) -

With all the complicated engineering and physics needed to build a nuclear reactor, it is rather remarkable that one turned on spontaneously two billion years ago.

Evidence for this natural reactor was found in 1972 at the Oklo mine in the West African country of Gabon. New research confirms that water regulated the nuclear reactions in a cyclic pattern similar to that in a geyser.

Alex Meshik and his colleagues at Washington University of St. Louis have determined that the Oklo reactor, which comprises several separate sites, ran for 30 minutes and then shut off for 2.5 hours, before starting over.

"The time is characteristic of water infiltrating rocks and then being boiled off once reactions started," Meshik told LiveScience.

When the water all boiled away, the reactions stopped until new water percolated back down. This geyser-like activity also prevented a runaway reaction.

"It's amazing it didn't explode," Meshik said. "Instead it released energy in short pulses."

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Water is very good at slowing down neutrons. Although scientists had long suspected that water was important for the Oklo reactor, the idea was not confirmed until Meshik's team looked at levels of xenon gas in the uranium deposits.

They realized that this xenon could only be trapped in the deposits if the reactor shut off on a regular basis - hence the geyser analogy. These results were published in a recent issue of Physical Review Letters.

Although water and uranium are not unique to Oklo, no other natural reactor has ever been found.

"It's very strange that something happened only once in nature," Meshik said. "But Oklo is very unique."

He explained that, after the fission process had finished, a geological shift caused the Oklo reactor to sink a few miles below the surface - where it was preserved from erosion. A few million years ago, another shift brought the uranium deposits back to the surface.

Other reactors could have turned on two billion years ago, only to have their evidence washed away in the intervening eons.