Were they built by aliens? The answer is a bit less fantastical, but equally extraordinary.
Nuclear fission is an extraordinary feat of human ingenuity, but did you know it can also occur spontaneously? There are three different forms or “isotopes” of uranium that exist in perfect balance in natural occurring ores. Increase the abundance of one by as little as 3.5 percent, and a cascade of nuclear fissions will ignite below our feet.
Two billion years ago, in a region of Gabon called Oklo, that’s precisely what happened.
It all started in 1972, when scientists testing routine samples of uranium from a mine in Oklo noticed something a bit off — the concentration of uranium-235 was only 0.600 percent. This lower amount was characteristic of uranium that had already fissioned. So how did it wind up in a mine yet seemed untouched by human hands?
Further study of these ores revealed their startling life stories. A fortuitous coalescence of circumstances over the past two billion years created natural nuclear reactors, where nuclear fission had spontaneously occurred.
In order for these natural reactors to form, uranium deposits had be concentrated enough. This requirement was satisfied after the Great Oxidation Event, about 2.4 billion years ago, when cyanobacteria — sometimes called blue-green algae — produced enough oxygen to bring atmospheric levels from less than 1 percent to above 15 percent. All the extra atmospheric oxygen caused uranium to dissolve in water and eventually settle in highly concentrated deposits.
The second requirement for a spontaneous nuclear reaction is a higher ratio of one of uranium’s isotopes: uranium-235. While uranium-235’s present-day abundance is low, its relatively short half-life compared to the other isotopes means that it used to be more abundant earlier in Earth’s history. In certain zones of the Oklo deposits, the abundance reached over 3.5 percent—the concentration at which nuclear physicists enrich the isotope to achieve fission.
When uranium undergoes fission, a rogue neutron crashes into the uranium nucleus, causing it to split apart and release a massive amount of energy. The fission invariably knocks loose a few more neutrons, which creates a chain reaction that continues until the uranium-235 is depleted.
The percolating water in the deposits also acted as a neutron moderator, slowing them down enough so that they would actually collide with the uranium nuclei. As heat from the reaction boiled away the water, the reactors would turn off until enough had trickled back again. This built-in control mechanism kept them incredibly stable, so that they turned on and off every few hours without ever melting down or exploding.
So these natural nuclear reactors quietly ran for up to 1 million years, generating an average output of about 100 kilowatts—enough to power about a thousand lightbulbs. Eventually, the uranium-235 depleted to a low enough amount that it could no longer sustain fission, and the reactors lay dormant until they were discovered by the miners.
But the story doesn’t end there. We make a huge fuss about containing nuclear reactions, for good reason: the waste products are hugely radioactive and dangerous. Having weathered through 2 billion years of geologic temper tantrums, the Oklo reactors can offer some insight into managing nuclear contamination. Unfortunately, researchers will have to make do with samples, because the original uranium deposits containing the reactors have been completely mined since their discovery.
Along with observing how radioactive waste has behaved over long time periods, scientists studying these samples have had some other epiphanies. A 2014 paper claims that the Oklo data generates energy models that may call into question constants that were previously considered fundamental and, well, constant. The Oklo reactors not only serve as marvels of geology and examples of natural nuclear waste containment, but they are also challenging, and thus improving, our understanding of the forces of physics in a changing universe.