The Oklo Natural Reactor
Later explainedSummary
In 1972 a French technician found that a batch of uranium ore was missing some of its uranium-235. The shortfall was tiny, three parts in a thousand, but in nuclear materials accounting a missing gram of fissile material is a serious matter, and it triggered an investigation.
The uranium had not been stolen. It had been used. Roughly 1.7 billion years ago, in a seam of rock in what is now Gabon, uranium ore went critical and ran as a nuclear reactor, on and off, for a few hundred thousand years, with nobody present to build it, fuel it, or moderate it.
What is documented
The discrepancy. In June 1972, routine mass spectrometry at the Pierrelatte enrichment plant in France showed uranium hexafluoride with a uranium-235 concentration of about 0.717 per cent. The figure for natural uranium, anywhere on Earth, in any ore, from any planet in the solar system, is about 0.720 per cent. The difference is around three thousandths of a per cent.
It should not have been possible. Uranium isotope ratios are fixed by the age of the solar system and are the same everywhere. A shortfall meant either an instrument fault or a diversion of fissile material.
The trace-back. The ore was traced to the Oklo mine in Gabon. Further sampling there found ore far more depleted than the original batch, in places down to around 0.44 per cent uranium-235. Something had consumed a substantial fraction of it.
The fission products. The decisive evidence was not the missing uranium but what had replaced it. The rock contained the characteristic isotopic signature of nuclear fission: neodymium, ruthenium and other elements present in the exact proportions that fission produces and in proportions found nowhere in ordinary rock. Xenon isotopes trapped in the mineral grains matched fission yields.
The ore had not merely lost uranium. It contained the ash.
The reactor zones. Sixteen or more distinct natural reactor zones have since been identified at Oklo and the nearby sites of Okelobondo and Bangombé.
How it ran. Reconstruction from the isotopic record indicates that the zones operated intermittently, in cycles, over a period of the order of hundreds of thousands of years, at an average thermal power of roughly a hundred kilowatts. This is a very small reactor. It is about the output of a domestic heating system, sustained for longer than modern humans have existed.
Leading explanations
The mechanism is understood, and it turns on two facts that were true then and are not true now.
The fuel was enriched, because the universe was younger. Uranium-235 decays faster than uranium-238. Its half-life is about 704 million years; uranium-238's is about 4.5 billion. Run the clock back 1.7 billion years and the ratio was very different: natural uranium at Oklo would have been around three per cent uranium-235.
Three per cent is not a coincidence. It is approximately the enrichment level of the fuel in a modern commercial power reactor. The Earth was, for a time, made of reactor fuel, and it has been decaying out of that condition ever since. Oklo could happen then. It cannot happen now, anywhere, because the fuel no longer exists.
Water was the moderator. A chain reaction needs the neutrons slowed down, and water does this. Groundwater percolating through the porous, uranium-rich sandstone at Oklo did the job that a modern reactor does with a deliberately engineered moderator loop.
It regulated itself, which is the best part. As the reaction ran, it heated the water. The water boiled away. With the moderator gone, the neutrons were no longer slowed, the chain reaction stopped, and the rock cooled. Groundwater seeped back in, and the reaction restarted.
The isotopic evidence is consistent with a cycle of roughly half an hour of criticality followed by a couple of hours of dormancy while the water returned, repeating for geological ages. Nobody built the control system. The physics supplied one.
Why nobody had predicted it. Somebody had. Paul Kuroda published the conditions required for a natural fission reactor in 1956, sixteen years before one was found, and the conditions he specified are essentially the ones that obtained at Oklo. His paper was a curiosity until it stopped being one.
What the popular version gets wrong
"A two-billion-year-old nuclear reactor: proof of ancient technology." It is the opposite of proof of technology. Its entire significance is that no technology was involved. Oklo is remarkable precisely because uranium, water and time did unassisted what it took the Manhattan Project to arrange deliberately.
"It exploded." It did not, and it could not. A reactor is not a bomb; sustaining a slow chain reaction and producing a nuclear detonation are different physical problems, and the second one is very hard. Oklo simmered at about a hundred kilowatts. It was, by any standard, an extremely boring reactor.
"Scientists cannot explain how it started." They can. The isotope ratios of the era, the porosity of the sandstone, the presence of groundwater, and Kuroda's 1956 conditions account for it completely.
"It could happen again." It cannot. Natural uranium is now about 0.72 per cent uranium-235 and will never again be three per cent. The window closed permanently, roughly a billion years ago, and the last natural reactor on Earth has already run.
Current status
Later explained. The registry keeps this record as its cleanest case of an explanation that is more astonishing than the anomaly it replaced.
And it turned out to be useful. Because the reactors at Oklo produced highly radioactive fission products, and because a great many of those products stayed more or less where they were formed for 1.7 billion years, Oklo is now studied as a natural experiment in exactly the problem that no one has solved: whether nuclear waste can be safely left in rock for geological time. The site has been running that experiment, unattended, since before multicellular life.
Sources
- Perrin, F., and Commissariat à l'énergie atomique (1972). Initial identification of the uranium-235 depletion at Pierrelatte and trace-back to Oklo.
- Kuroda, P. K. (1956). Theoretical conditions for the occurrence of a natural fission reactor, published sixteen years before one was found.
- International Atomic Energy Agency, proceedings on the Oklo phenomenon (Libreville, 1975 and later).
- Isotopic analyses of fission-product neodymium, ruthenium and xenon in the Oklo reactor zones.
- Subsequent literature on Oklo as a natural analogue for deep geological disposal of nuclear waste, and on constraints it places on variation in the fine-structure constant.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.