You know how a nuclear reactor works, right? You’re familiar with isotopic decay and nuclear chain reactions, aren’t you? No? Well, don’t fret; you don’t actually need to for this discussion.
Nuclear physics has a well-earned reputation for being impenetrable to the layman. It’s one of the most complicated and technically demanding areas of human expertise. And even the most knowledgeable among us don’t have all the answers. So how do we know that Mother Nature created a naturally occurring nuclear reactor some two billion years ago?
Perhaps we’re getting ahead of ourselves. There’s some fun stuff to explain first.
There’s a place in Africa called Oklo; it’s near Franceville in the Haut-Ogooué Province of Gabon. Oklo is known for its particularly rich deposits of nuclear fission material, namely uranium ore. And this is where our story begins. The Oklo uranium mines are the location of the first known natural nuclear reactor on the planet.
Warning: Science Ahead!
It is, however, important to know a few basic things about nuclear fission before we dive right in; first:
When a uranium-235 atom absorbs a neutron, its nucleus can split, releasing energy and several more neutrons. Those neutrons can be absorbed by other uranium-235 atoms, causing them to split and release still more neutrons. Under the right conditions, the process sustains itself, and then you’ve got a nuclear chain reaction.
Of course, a nuclear reaction requires more than just a few atoms bumping into one another. In the case of Oklo, we needed a rich source of uranium, enough U-235 in those deposits to sustain the reaction, and groundwater to slow the neutrons so that they can more readily be captured by the uranium-235 atoms.
All those many years ago, Oklo provided all of those conditions: the right geology, atomic population, water, and time. Evidence suggests that at least one of the reactor zones operated in a remarkable cycle: roughly 30 minutes of activity followed by about two and a half hours of shutdown, over a period of hundreds of thousands of years.
That pattern was supported by a cycle of evaporation: water slows the neutrons so they can be captured by the uranium-235 atoms; the reaction produces heat, which drives out the water. Without water, the reaction stops, and the heat dissipates, which allows water to return, starting the whole process over again.
So, how do we know all of this?
You could be forgiven for thinking that this all seems like speculation. How could we possibly know that this process happened when we’re talking about timescales in the billions of years? And this is where this story becomes really fascinating.
In 1972, in a laboratory in the Pierrelatte enrichment plant in France, technicians were routinely measuring the U-235 content of uranium originating from Gabon. They expected to find that the ore contained 0.7202% U-235, but the sample contained only 0.7171%. A difference of only 0.0031 percentage points.
You might be thinking: So what? That’s a minuscule difference. But in modern nuclear physics, uranium is necessarily measured with extreme precision, and this discrepancy was well outside the expected measurement error. So the technicians looked a little deeper, measuring other samples from the same region. And they eventually found uranium containing as little as about 0.3% U-235. That’s a much more significant difference, which prompted a deeper investigation.
Where had the missing U-235 gone?
The obvious conclusion was contamination somewhere in the processing chain. Perhaps someone had mistakenly mixed depleted uranium—uranium from which some U-235 had already been removed—into the samples. But further investigation determined that the Oklo samples also contained fission products, including neodymium and samarium. These fission products quite strongly suggested that these samples had already undergone fission. Someone, or something, had already used the fuel.
And since this was an ancient geological deposit rather than the remains of some human activity, the implication was extraordinary: the uranium deposit itself had once sustained nuclear fission.
Interestingly, in 1956, sixteen years before the discovery, Japanese-American chemist Paul Kuroda had worked out theoretically that a natural uranium deposit could, under the right circumstances, sustain a nuclear chain reaction. He identified virtually the same requirements as we discussed above.
So we know that a natural nuclear reactor is possible, but how do we know about the remarkably precise operating pattern of the Oklo reactor—something that happened billions of years ago?
In 2004, Alexander Meshik and colleagues examined xenon trapped in aluminum-phosphate minerals from one of the Oklo reactor zones. Xenon is particularly useful because several of its isotopes are produced through radioactive decay chains following uranium fission. Crucially, they don’t all appear at the same rate.
Think of it like this: while the reactor was active and hot, much of the xenon produced by fission escaped. But other radioactive products remained behind and continued to decay into xenon after the reactor shut down and cooled. Different isotopes appeared at different rates, leaving behind a kind of nuclear timestamp in the surrounding minerals.
By measuring those isotopes and calculating how long they would have taken to accumulate in those proportions, Meshik and his colleagues were able to reconstruct the reactor’s operating cycle. Thus, evidence suggests that one of the reactor zones operated roughly in a cycle of 30 minutes active, with 2-½ hours of downtime.
Don’t let the scientific language of this revelation overwhelm your sense of awe at the idea that nature actually made a working nuclear reactor billions of years before mankind even knew what such a thing was. What is perhaps even more amazing is the fact that humans are able to discern what happened so long ago. Noticing a subtle difference in the isotopic compositions of various elements was enough to make someone say, “Huh, that’s weird.” Moments like that have sparked some of science’s greatest discoveries. They inspire further investigation and a need to ask questions, interrogating what we already know in comparison to what we only think we know.
In the case of Oklo’s natural nuclear reactors, scientists have been able to confidently determine what conditions existed on Earth two billion years ago. Conditions that contributed to a process that, until then, seemed like the exclusive domain of human engineering.
The past may be inaccessible, but it isn’t necessarily unknowable.
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