Enrico Fermi switched on the first human-made nuclear reactor in 1942, but the first natural nuclear reactor on Earth occurred 1.7 billion years earlier.
A nuclear chain reaction occurs when a random neutron blunders into one of certain heavy isotopes. That neutron reacts with the isotope, causing it to split apart, and the split releases lots of energy and also some more neutrons. If those newly released neutrons hit other isotopes, more splits occur, more neutrons are released, and those neutrons trigger still more reactions… and that’s a classic chain reaction.
Such chain reactions only occur when there is enough of the right kind of material around to keep it going. Uranium can sustain a nuclear reaction, but only a specific isotope known as uranium 235. Only 0.72% of all the naturally occurring uranium on Earth is uranium 235, and this is why raw uranium must be processed to isolate the good stuff (“fissile material”) when setting up a nuclear reactor or making a nuclear bomb.
Another factor that affects the success of a nuclear chain reaction is the speed of those neutrons. When they are first freed from their uranium 235, neutrons are travelling at about 14,000 km/s. That’s too fast; a neutron travelling at this speed is much less likely to set off further reactions. So another necessary step is slowing down the neutrons so that they stick around long enough to react. Several materials – called neutron moderators – can slow them down: light water, heavy water, graphite, and more are used in nuclear reactors and nuclear bombs to promote the chain reaction.
To summarise, for a sustained nuclear reaction to take place you need enough a decent quantity of the right material plus something else to slow down the neutrons enough for that material to react. It’s almost impossible for such conditions to occur naturally on Earth. But, as it turns out, it’s not entirely impossible.
Cut to Gabon, 1972. The French were mining uranium near a town named Oklo, and noticed something extremely odd. The levels of uranium 235 were much lower than they should be: 0.60% rather than the expected 0.72%. Was someone stealing fissile material? (I’m looking at you, Christopher Lloyd…) Nope, something far stranger was going on.
At some point, perhaps 1.7 billion years ago, the Oklo uranium deposits had a higher concentration of uranium 235 than normal. Then they were flooded with water. This water acted as a natural neutron moderator, slowing down neutrons enough to create a low-grade chain reaction among the small amounts of uranium 235 in the ground. Natural nuclear power!
Further investigations found many tell-tale signs of an ancient nuclear reaction. It is thought that the heat given off by the reaction was enough to evaporate the surrounding water. Once that neutron moderator was gone the reaction stopped… only to kick off again once the water flowed back in. In Oklo this probably lasted for a few hundred thousand years.
We’ve never found any evidence of natural nuclear chain reactions on Earth outside of Oklo, and the conditions were so specific to that place and time that we probably never will. Fortunately.