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 some French scientists took uranium ore from the mine in Gabon to test its
uranium content. 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.
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 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."
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