Nature Built a Nuclear Reactor Two Billion Years Before We Did

Every so often I run into a fact that stops me dead, and I have to go and read the actual paper before I'll let myself believe it. This is one of those. About 1.7 billion years ago, in what is now Gabon, a lump of rock started splitting atoms on its own. No engineers. No control rods. No people, because there weren't any yet. Just uranium, water, and the right conditions all arriving at the same place at the same time. A nuclear reactor that switched itself on, ran in slow pulses for a few hundred thousand years, and then quietly switched itself off when the fuel ran low. We didn't build the first reactor. We were about two billion years late to that.

Here is how we found out, which I think is almost as good as the fact itself.

A number that was wrong by a rounding error

In 1972, at a French enrichment plant at Pierrelatte, an analyst was doing the kind of routine isotope check that nobody expects to turn up anything. Natural uranium has a very fixed recipe. Wherever you dig it up on Earth, whether it's from Canada or Australia or Namibia, 0.720% of it is the isotope uranium-235. That consistency is one of those quiet, dependable facts you can build an industry on. The sample from the Oklo mine came back at 0.717%.

That's a tiny difference. But it should have been impossible. The U-235 fraction in natural uranium does not vary, because the stuff has been mixing and settling in the Earth's crust for billions of years and there's no ordinary chemical or geological process that picks out one isotope of uranium over another. Something had removed a sliver of the U-235 from this particular ore. And once people started looking properly, they found a lot more missing than that first sample suggested. In some zones the ore had been stripped down well below half of what it should have held.

There is really only one thing that eats U-235 specifically, and we'd been doing it ourselves for less than thirty years at that point. The ore had been a reactor.

What "enrichment" actually means, from the ground up

Let me back up, because the whole thing hangs on understanding why that 0.720% number matters. If you've ever heard the phrase "enriched uranium" and nodded along without being totally sure what it meant, you are in good company and I'll try to do this without any jargon.

Uranium comes in two main flavours, or isotopes. U-238 is the common one, about 99.3% of the total. U-235 is the rare one, that 0.7%. They're chemically identical, but only U-235 is easy to split, and splitting is the whole game. When a U-235 atom gets hit by a wandering neutron it breaks apart, releases energy, and throws out a couple more neutrons. If those neutrons go on to split more U-235 atoms, you get a chain reaction. That's a reactor, and at the extreme it's a bomb.

The trouble is that natural uranium today, at 0.7% U-235, is too weak to sustain that chain on its own under most conditions. There just isn't enough of the splittable stuff packed in. This is why we "enrich" it: we go through an enormous industrial effort to nudge that fraction up to around 3 to 5% for a power reactor. The whole sprawling business of centrifuges and gas diffusion exists to shift one number from 0.7 to about 4. So the obvious objection to a natural reactor is the obvious one: where did the enriched fuel come from? Nature doesn't run centrifuges.

It doesn't have to. It just has to wait.

The twist: deep time did the enrichment

Both isotopes are radioactive, which means both are slowly decaying away. But they decay at different rates, and this is the crucial part. U-235 decays noticeably faster than U-238. So if you wind the clock back, the rare isotope was less rare. Run the numbers back about 1.7 billion years and natural uranium contained roughly 3% U-235.

Sit with that for a second. Three percent. That is, near enough, modern reactor-grade fuel. Back then, every uranium deposit on Earth was walking around enriched, by nothing more exotic than the passage of time. The fuel we sweat to manufacture today was just lying in the ground, ready to go, for anywhere it could find the other ingredients. Oklo is the one place we know of where it did.

Three ingredients in the same room

A rich seam of uranium on its own won't do it. You need a few more things to line up, and at Oklo they did.

First, the ore had to be concentrated enough. Oklo had unusually rich uranium deposits, thick veins of the stuff laid down by ancient rivers and groundwater rather than thinly scattered. Second, the surrounding rock had to be clean. Certain elements are what physicists call neutron poisons: they soak up the wandering neutrons before those neutrons can go on and split anything, and they kill a chain reaction stone dead. Oklo's ore happened to be low in those. And third, you need a moderator.

A moderator is one of those concepts that sounds backwards the first time you hear it. The neutrons flying out of a fission event are too fast and energetic to reliably split the next atom. To keep the chain going you actually need to slow them down, and the cheapest, most abundant thing in the universe that does this well is plain water. The hydrogen in water bounces the neutrons around until they're ambling along slowly enough to be captured. At Oklo, groundwater seeped down through the porous ore and did exactly that job. Water was the moderator. The same water you'd drink.

The natural on/off switch

And here's the part I find genuinely lovely, because it's so self-correcting it almost looks designed. The reaction needed the water to keep going. But the reaction also produced heat. Enough heat, and the water in the ore boils off into steam. Steam is far less dense than liquid water, so it's a much worse moderator, and with the moderator gone the chain reaction stalls and stops. No water, no reaction. The rock then cools, fresh groundwater seeps back in, the moderator returns, and the whole thing flickers back to life.

So it ran in pulses. The best estimates I've seen suggest something like thirty minutes of activity followed by a couple of hours dormant while the water came back, over and over. On, off, on, off, a geological pilot light cycling away in the dark for a few hundred thousand years. And it was modest. The average output was on the order of 100 kilowatts, which is to say a slow simmer, roughly enough to run a few dozen kettles. I want to stress that, because "natural nuclear reactor" conjures something violent and it really wasn't. There was no explosion, no crater, no glowing pit. Just warm, damp rock quietly fissioning whenever the conditions allowed.

The accidental gift to waste science

There's a payoff to all this that goes well beyond being a beautiful curiosity, and it lands right on the thing people worry about most with nuclear power: the waste.

A reactor doesn't just burn fuel, it makes new stuff. Fission products, and heavier elements like plutonium, get created right there in the ore. At Oklo, all of that was made roughly two billion years ago and then left completely alone, sitting in wet rock, with no engineered barrier of any kind. If you wanted to design an experiment to find out whether buried nuclear waste actually stays put over geological timescales, you could not build a better one, and nobody had to. It just happened, and we get to read the results.

And the results are, broadly, encouraging. A surprising amount of the fission products and even the plutonium barely moved. Over two billion years, much of it migrated only a few metres, held in place by the surrounding clays and the chemistry of the rock. Geologists genuinely cite Oklo as a natural analog when they argue that a deep geological repository can contain waste for the immense spans of time involved.

I want to be fair-minded here, though, because this is exactly the sort of fact that gets stretched too far. Oklo is not a blanket guarantee that any hole you dig will hold waste forever. Its specific clay-and-rock chemistry, its particular groundwater, the kinds of elements involved, none of that automatically transfers to a different site with different geology. It's a powerful real-world data point, not a universal proof. And I'll be honest about my own limits: I'm a hobbyist who reads the geochemistry, not a geochemist. Some of the finer details of how mobile certain elements were, and why, are still argued over by people who actually do this for a living. I'm reporting the headline, not refereeing the field.

We only saw it because we'd done it ourselves

The thing I keep coming back to is this. The Oklo deposit is now mostly mined out, dug up and run through the fuel cycle like any other ore. We found the fingerprint of a two-billion-year-old reactor in the one window of history where we'd both invented the instruments to measure isotopes that precisely and learned what a depleted U-235 reading actually means. A century earlier, that 0.717% would have meant nothing to anyone. We recognised the signature of a natural reactor only because we'd just finished learning to build the artificial kind. We knew what it was because we'd made one too.

Nature got there first by about two billion years, ran it cleaner and more patiently than we manage, and then tidied up after itself. I find that humbling in the best way.

If you want to go deeper, the IAEA has written about Oklo as a natural analog for waste disposal, and there's a good body of geochemistry literature on the Oklo and Bangombe deposits if you fancy a long evening. I read the reports so you don't have to, but honestly, this is one where you might want to.