An ancient nuclear reactor that was churning away 2 billion years ago sounds like a fabricated myth. Maybe it's because the term reactor implies a manmade structure. Instead, the reactor is a region of natural uranium within the Earth's crust, found in Okla, Gabon. Uranium is naturally radioactive, and the conditions in this rocky area happened to be just right to cook up some nuclear reactions.
The natural reactor must have annoyed nuclear scientists: The first nuclear reactor to produce electricity started up in 1951, and this only produced a trivial amount of energy. The pile of rock in the ground in Okla, on the other hand, had created nuclear power around 2 billion years ago!
It was discovered in 1972, when some French scientists took uranium ore from the mine in Gabon to test its uranium content. Now, typically, uranium ore is made up of three types (isotopes) of uranium, each one with a different number of neutrons: Uranium 238, which is the most abundant, uranium 234, which is the rarest, and uranium 235, which nuclear scientists are most interested in because it can sustain nuclear chain reactions.
One would expect to find that the uranium ore is composed of 0.720% uranium 235 since that is the percentage found in other rock samples from the Earth's crust, the Moon and even meteorites. However, these French scientists found something fishy: The uranium sample only contained 0.717% of uranium 235. What might seem to be a minor discrepancy of 0.003% is very significant with regards to uranium.
That meant that, back at the mine, around 200 kilograms (around 440 pounds) of uranium 235 appeared to be missing. It hadn't been lost or stolen. Instead, this missing 0.003% had undergone nuclear fission and split into other atoms. This conclusion may very well have furrowed some brows since there are three very specific conditions that a reactor needs to churn out energy continuously. And as even scientists had struggled to create a nuclear reactor, it seemed unlikely that nature just happened to create one purely by accident.
Unlikely, but not impossible, because that is exactly what happened.
The conditions that the natural reactor happened to fulfill are as follows.
The first was owning a good percentage of Uranium 235 to fuel the reaction. While 0.720% might seem small, it's perfect for nuclear fission, and when the ore samples from Okla are compared to other samples from all over the world, it is likely that this was the percentage over 2 billion years ago when the reactor started.
The second condition is a source of neutrons. Uranium 235 decays naturally into thorium and releases a neutron in the process. This neutron can then whoosh towards another Uranium 235 atom and start the fission process. Nuclear fission is when an atom breaks apart into smaller atoms, usually releasing energy in the process. This neutron fuses with a Uranium 235, and together they combine to form Uranium 236, which is unstable. The Uranium 236 is then overwhelmed with instability and splits, creating a variety of smaller, stable atoms and some neutrons. These neutrons can then happily shoot towards other atoms and continue the chain reaction.
The nuclear reactor had a supply of a regulating substance as well: a flow of natural groundwater. As the atoms started to split, they released neutrons as well as energy. The water would slow down the neutrons, but the energy would heat up the water. After a time, the water would get so hot that it would start to boil off. Eventually, enough of the water would have boiled away until there wasn't enough left in the reactor to slow down the neutrons. The neutrons started shooting off into the ground without reacting with anything, and the reaction would stop. Then, the natural flow of groundwater would trickle in until there was enough water to start the whole process again. This watery cycle probably continued for hundreds of thousands of years.
Sadly, all good days are numbered, even for a happy natural reactor: The levels of uranium 235 got used up and the level was too low to sustain any more meaningful reactions. The reactor eventually slowed to a stop, leaving only a few traces behind that it ever existed – including the enigma of the "missing uranium."