Fun fact: any chunk of uranium you stumble across in our corner of existence has the exact same isotope ratio.
Except for that time when Mother Earth shat out her very own nuclear reactor.
Today’s Moment of Science… the all natural Oklo reactor.
In 1972, scientists at the French Alternative Energies and Atomic Energy Commission got a curious chunk of uranium from a mine in Gabon. Instead of 0.720%, their uranium-235 (U-235) was at a concentration of 0.717%. It may not sound like much, but it’s not like there are bits of uranium aging a smidge faster than others.
They checked their math, their equipment, their analysis; it was all fine. Then they checked other samples from Gabon. Many specimens had even lower concentrations of U-235. Given the size of the mine, they calculated that about 200kg of the blow-shit-up type uranium was possibly missing. Researchers went to the mine to get a clearer understanding of what happened because “unaccounted for uranium” is a bit of a sphincter clencher.
The uranium wasn’t there to be found. It was gone, as in it didn’t exist anymore.
Uranium-238 (U-238) and U-235 are the two main isotopes of uranium. At least in the known universe, uranium is currently composed of 99.27% U-238, with only 0.72% coming from U-235. I say currently because that ratio has changed over time. Both isotopes are undergoing radioactive decay, decomposing into other elements. But with a shorter half-life, U-235 is going away much faster. It’s theorized they were present in equal quantities when the element was blasted into existence.
In 1956, Japanese-American chemist Dr. Paul Kuroda proposed that, given the right conditions, a fission reaction could become self-sustaining in nature. The uranium deposit needed to be at least long enough for a fission-inducing neutron to travel, which is about two feet. There couldn’t be large quantities of elements like silver, boron, or lithium, which would absorb neutrons rather than letting them play with the more temperamental elements. There also had to be a neutron moderator- like water- to slow the neutrons. This made them more likely to smash into your radioactive atoms.
Lastly, you needed a metric fucktonne of U-235.
About two billion years ago, uranium was about 3.5% U-235, which is on par with the concentration used in some nuclear plants. Given all the deposits of uranium in the world, it would almost be strange if the conditions for a nature-made reactor never occurred.
The curious chunk of uranium with 0.717% U-235 was the result of a natural, self-sustaining nuclear reaction in what’s now West Africa.
(I say ‘now West Africa’ because this all happened like five supercontinents ago).
In total, sixteen defunct nuclear reactor sites in that region of Gabon have been identified. They burned for hundreds of thousands of years, possibly over a million, eventually coming to the end of their fuel’s fissibility and calling it a day.
Other than being, technical term, holy shartballs cool, what can we do with this? Well, somehow a nuclear reactor did its thing, stored its waste naturally for a few billion years, and didn’t Chernobyl the place. The area is safe and not a hotbed of radioactivity. With further research, scientists are looking into ways to mimic the Earth’s natural process of managing nuclear waste.
Could something like this happen today? The chances are somewhere between no and fuck no. This could only have happened in the planet’s youth when we still had higher natural concentration of U-235 kicking about.
If you’re worried about an unexpected nuclear reactor showing up, there was that one guy who built a nuclear reactor in his backyard. Mother Nature might be out of the ‘DIY nuclear reactor’ business, but watch out for Fellow Humans.
This has been your daily Moment of Science, reminding Gwyneth Paltrow that nuclear reactors are all natural.
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Smashing post. Thanks.
So Victor Appleton’s 1956 pseudo-scientific potboiler, Tom Swift in the Caves of Nuclear Fire, was based on Kuroda’s paper and made the lucky guess that they would occur in Africa.