Two billion years ago parts of an African uranium deposit spontaneously underwent nuclear fission.
Sometime a bit less than 2 billion years ago, and lasting for about 300,000 years, the Oklo reactors held a series of stable nuclear fission reactions.
Upon their discovery, the central question about these reactors was simply: How could they possibly work? One early hypothesis was that the fundamental physical constants that restrict nuclear reactions today may have been different 2 billion years ago. Analysis of the wastes at Oklo, running all the way up to this very month, suggest that the physical constants have indeed been constant all along. That means the reactor would have needed a fissionable isotope in the same concentrations we require today.
If we have such trouble making raw uranium usable in stable fission reactions, how could an inanimate planet possibly do it?
Well, it didn’t. Back when these Oklo reactors first fired themselves up, Earth was barely half as old as it is today. That means that less time had passed since its initial formation from bits of galactic dust and rock. As a result, fewer radioactive half-lives had played out, and unstable isotopes were found in much higher concentrations.
Scientists estimate the Oklo reactors would have had samples with roughly 3.6% uranium-235.
What was found in Oklo surprised everyone gathered there, the site where the uranium originated from is actually an advanced subterranean nuclear reactor that goes well beyond the capabilities of our present scientific knowledge. Researchers believe that this ancient nuclear reactor is around 1.8 billion years old and operated for at least 500,000 years in the distant past. Scientists performed several other investigation at the uranium mine and the results were made public at a conference of the International Atomic Energy Agency. According to News agencies from Africa, researchers had found traces of fission products and fuel wastes at various locations within the mine area.
Meanwhile, French nuclear scientists carried out a detailed survey of the Oklo site, discovering not just one reactor zone but up to 17 of them over an area of several tens of square kilometres. Some of these were close to the surface and so had been influenced by weathering processes, while others were at depths of up to 400 metres and were more or less pristine.