The Oklo Reactor is a natural nuclear reactor discovered in Gabon in 1972. It was created by a rare combination of circumstances that caused a nuclear chain reaction to occur. The reactor stopped working a billion and a half years ago and is not dangerous. It is impossible for a natural reaction to occur on Earth today.
The Oklo Reactor is the only known natural nuclear reactor on Earth. If that sentence leaves you room for a pause, it should; normally, generating nuclear reactions requires a great deal of energy and effort, which makes it all the more remarkable that Earth has essentially succeeded in building its own reactor. Unlike man-made reactors, of course, the Oklo Reactor wasn’t used to power anything, but it’s certainly a point of scientific interest and attracted a lot of attention when it was discovered.
The discovery of the Oklo reactor occurred in 1972, when scientists were analyzing material samples from the Oklo uranium mine located in Gabon, Africa. The scientists noted that the isotope profile of the uranium from the site closely resembled that of spent nuclear fuel, suggesting that a nuclear chain reaction had occurred. However, they thought this wasn’t possible, so they were rather puzzled, until they investigated the site and discovered that a rare combination of circumstances actually “built” a nuclear reactor underground.
What happened at the Oklo reactor site is that the uranium deposits rested on a bed of sandstone and groundwater seeped through the uranium, picking up minerals along the way. As the earth’s crust shifted, the sandstone tilted, causing water to pool, pool, and create a thick deposit of uranium ore. A nuclear chain reaction began, evaporating groundwater, which allowed the reaction to finish. However, the groundwater built up again, starting the cycle over and over again.
Three sites in Oklo show signs that nuclear chain reactions have occurred, although not in recent memory; they stopped about a billion and a half years ago. Curiously, the normally very dangerous byproducts of nuclear reactions remained sequestered in the Oklo reactor, rather than spreading and contaminating the surrounding soil and groundwater.
Before you go looking for a natural nuclear reactor in your backyard, you might want to know that conditions on Earth today make a natural reaction impossible. Levels of the enriched isotopes needed to start chain reactions have dropped below the level where a chain reaction could start spontaneously, which is why uranium enrichment is such a big industry in energy-using nations nuclear.
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