An ancient natural nuclear reactor that was churning away nearly 2 billion years ago sounds like a made-up myth. Perhaps it's because the word reactor suggests a manmade structure. Instead, the reactor is an area of natural uranium inside the Earth's crust, detected in Okla, Gabon.
Uranium is naturally radioactive, and the settings in this rocky area happened to be just accurate to do some nuclear reactions. The natural reactor must have annoyed nuclear scientists: The first nuclear reactor to provide electricity was build up in 1951, and this only made a small amount of energy. The pile of rock in the ground in Okla, instead, had generated nuclear power around 2 billion years ago!
It was discovered in 1972, when French researchers grabbed uranium ore from the mine in Gabon to examine its uranium content. Now, naturally, uranium rock consists of three types (isotopes) of uranium, each one with a different number of neutrons: for example Uranium 238, which is present in a huge amount, uranium 234, which is very rare, and uranium 235, which nuclear researchers are most concerned about because it can bear nuclear chain reactions.
One would imagine to find that the uranium ore is composed of 0.720% uranium 235 since that is the percentage found in several other rock samples from the Earth's crust, the Moon and even in several meteorites. So the French researchers found something suspicious: The uranium sample only enclosed 0.717% of uranium 235. What might seem to be a slight discrepancy of 0.003% is very important with regards to uranium.
It means that, back at the mine, nearly 200 kilograms of uranium 235 seemed to be missing. It hadn't been vanished or stolen. On the other hand, this missing 0.003% had gone through nuclear fission and divided into other atoms. This assumption may very well have furrowed some peaks as there are three very explicit conditions that a reactor requires to churn out energy uninterruptedly. And as even researchers had struggled to construct a nuclear reactor, it seemed doubtful that nature just happened to generate one completely by accident. Doubtful, but not impossible, because that is precisely what happened. The circumstances that the natural reactor happened to justify are as follows.
The first was possessing a good fraction of Uranium 235 to fuel the reaction. Though 0.720% may seem small, it's best for nuclear fission, and when the ore trials from Okla are linked to other samples from all over the world, it is possible that this was the percentage over 2 billion years ago when the reactor started.
The second was the source of neutrons. Uranium 235 decays naturally into thorium and discharges a neutron in the procedure. This neutron can then rustle towards other Uranium 235 atom and start the chain reaction.
Sadly, all good days are numbered, even for a naturally occurring reactor: The amount of uranium 235 got used up and the level was too low to withstand any more important reactions. The reactor ultimately slowed to a stop, leaving only a little amount of hints behind that it ever occurred – counting the mystery of the "missing uranium."