Discover How Nature Designed a 2 Billion-Year-Old Nuclear Reactor: The Astonishing Science Behind It

“This can’t be possible,” physicist Francis Perrin thought in May 1972. He was at a nuclear fuel processing plant in southern France, looking at a dark piece of uranium ore from Gabon, Africa. This sample held a surprising secret that challenged everything known about natural uranium.

Typically, uranium has a consistent mix of isotopes: uranium-238, uranium-234, and uranium-235. In nature, uranium-235 makes up 0.720% of all uranium. But Perrin’s Gabon sample had only 0.717%. Although this seems like a tiny difference, it raised big questions. Had someone tampered with it? Was it linked to an ancient civilization? Or was it something more mysterious?

The mystery deepened during further investigation. Some uranium samples from the Oklo region showed even less uranium-235—down to 0.4%. This significant drop indicated that something extraordinary had happened to the ore. Eventually, scientists concluded that this uranium had undergone fission, the same process used in nuclear reactors. But this wasn’t due to human activity or aliens; it was a natural occurrence from two billion years ago. Nature had created its nuclear reactor.

For about 40 years, France mined uranium in Gabon, a former colony. It was an exciting discovery, but no one understood its true nature at first. When Perrin studied the sample, he confirmed it underwent fusion back when Earth was still young.

Man-made reactors rely on a controlled reaction, where uranium-235 atoms split when struck by neutrons. This releases energy and additional neutrons that can split more atoms. To sustain this reaction, enriched uranium is used, requiring precise engineering. It’s hard to believe such a process could occur naturally.

Yet two billion years ago in Oklo, the right conditions existed: the right amount of uranium, sufficient water, and stable geology allowed for a controlled fission reaction.

In 1956, chemist Paul K. Kuroda suggested that natural fission reactors could form under specific conditions. Though his ideas received some attention, many believed it was improbable. Kuroda estimated a uranium deposit needs to be about 0.66 meters thick to sustain a natural fission reaction. Back then, uranium-235 was about 3% of natural uranium, similar to the enriched uranium used today. Groundwater played a crucial role as a moderator, slowing down neutrons and facilitating ongoing reactions.

Peter Woods from the International Atomic Energy Agency explained, “The water acted as a moderator, absorbing neutrons and controlling the chain reaction.” Without contaminants like boron, which can halt fission, the Oklo site was perfectly suited for this natural event.

The ancient reactor didn’t operate continuously. Researchers found that it worked in cycles. Groundwater infiltrated the uranium, moderated the neutrons, and allowed fission to occur. This reaction heated the water until it boiled away as steam. Once the water was gone, fission would stop, but once more groundwater seeped in, the cycle would restart. This went on for hundreds of thousands of years.

“The circumstances of time, geology, and water came together for this to happen,” Woods remarked. “The detective story has been successfully solved.”

Research showed that over several hundred thousand years, the Oklo reactor produced about 15,000 megawatt-years of energy, comparable to running a large reactor for ten years.

In 1975, scientists worldwide gathered in Libreville, Gabon, to discuss the “Oklo Phenomenon.” Nature had harnessed nuclear power long before humans imagined it. Although initial theories aligned with observations, proving what transpired was complex. There were four natural reactor sites all linked to the same geological structure.

The breakthrough came with the study of xenon gas, trapped in the minerals at Oklo. Different isotopes of xenon form during fission, providing clues about the reactor’s stability. Research showed that the fission reactions at Oklo were remarkably stable, based on the ratios of trapped xenon. Eventually, as uranium-235 was consumed, the fuel supply dropped below what was needed for continuous fission.

Today, the mines in Oklo are depleted, but the legacy of the world’s only known natural nuclear reactor endures. Samples from Oklo are displayed in museums, including the Natural History Museum in Vienna. There may be other natural reactors awaiting discovery, while humanity focuses on its own fission reactors.

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