How Earth Built Its Own Power Plant 2 Billion Years Ago
New research illuminates the remarkable Oklo phenomenon, where geological processes created the world's only known natural nuclear reactor—offering insights for modern nuclear waste storage and reactor design
By Claude Anthropic and Stephen Pendergast | Scientific American
Deep in the rainforests of Gabon, West Africa, lies one of the most extraordinary geological phenomena on Earth: the remnants of a nuclear reactor that operated for hundreds of thousands of years—without any human engineering. The Oklo natural nuclear reactor, discovered in 1972, represents the only known instance of sustained nuclear fission occurring spontaneously in nature, fundamentally challenging our understanding of what's possible in geological systems.
A Puzzling Discovery
The story began with an anomaly that nearly caused an international nuclear security incident. In May 1972, at the Tricastin uranium enrichment site at Pierrelatte, France, routine mass spectrometry comparing UF6 samples from the Oklo mine showed a discrepancy in the amount of the 235U isotope. Where natural uranium typically contains exactly 0.720% uranium-235, the Oklo samples showed only 0.60%. This was a significant difference—the samples bore 17% less 235U than expected.
The shortage was alarming for multiple reasons. All civilian uranium handling facilities must meticulously account for all fissionable isotopes to ensure that none are diverted into the construction of unsanctioned nuclear weapons. Further, as fissile material is the reason for mining uranium in the first place, the missing 17% was also of direct economic concern.
French physicist Francis Perrin, working at the nuclear fuel processing facility, led the investigation that would ultimately revolutionize our understanding of natural nuclear processes. A series of measurements of the relative abundances of the two most significant isotopes of uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Some samples showed uranium-235 concentrations as low as 0.44%—nearly 40% below normal levels.
The Reactor Revealed
On 25 September 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. The evidence was overwhelming: isotopic signatures of fission products including neodymium, ruthenium, and xenon gas trapped in mineral formations perfectly matched those produced in artificial nuclear reactors.
The sixteen individual Oklo reactor zones are currently the only known examples of natural nuclear reactors. There, self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, during the Statherian period of the Paleoproterozoic. Scientists have identified 17 separate reactor zones in the main Oklo site, with an additional 20 zones discovered in the nearby Bangombe area.
Perfect Timing and Conditions
The Oklo reactor could only have existed during a narrow window in Earth's history. Natural nuclear reactors are not possible today because the natural uranium enrichment of 0.72% is too low to allow criticality with light-water moderation. However, uranium-235 decays faster than uranium-238, meaning concentrations were higher in the past.
Approximately 2 billion years ago, the percentage of U-235 in natural uranium was around 3%, significantly higher than today's 0.720%. This elevated concentration was crucial as it allowed for a sustained nuclear chain reaction, something that is meticulously replicated in the fuel enrichment process of contemporary nuclear reactors.
The Oklo reactors are thought to have formed during the Great Oxidation Event (GOE), which occurred between 2.14 and 1.95 billion years ago, when rising oxygen levels allowed uranium to dissolve and concentrate into rich ore deposits. Without this planetary oxygenation event, such reactors would not have been possible.
Self-Regulating Nuclear Power
What makes Oklo truly remarkable is its self-regulating mechanism. The reactor operated in predictable cycles governed by groundwater flow and thermal dynamics. The water acted in Oklo as a moderator, absorbing the neutrons, controlling the chain reaction. As fission reactions heated the uranium deposits, groundwater would boil away, removing the neutron moderator and shutting down the reaction. When temperatures cooled, water would return and restart the process.
The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down (exponentially decreasing residual decay heat) to complete a 3-hour cycle.
Recent modeling studies published in Progress in Nuclear Energy in 2025 have provided new insights into these thermal cycles. This simulation reproduces key features of the Oklo reactors found in the literature (the cyclic boiling and flow of water in and out of the reactor; the characteristic three-hour cycle time; the total energy released by the reaction), gives greater insight into their development and evolution, and demonstrates a non-cyclic, non-boiling regime of behaviour in the later stages of reactor operation that has not previously been described.
Natural Nuclear Waste Storage
Perhaps most importantly for modern nuclear science, Oklo demonstrates the feasibility of long-term geological nuclear waste storage. Radioactive fission products remained essentially immobile for 2 billion years, migrating only millimeters from their points of origin. This natural containment has provided crucial data for designing modern nuclear waste repositories.
The reactor zones produced an estimated 15,000 megawatt-days of thermal energy over their operational lifetime—equivalent to powering a small city for a year. Fission in the ore at Oklo continued off and on for a few hundred thousand years and probably never exceeded 100 kW of thermal power.
Modern Applications and Research
The Oklo discovery has direct applications for contemporary nuclear technology. Samples of Oklo donated to Vienna's Natural History Museum continue to provide research opportunities. Rock samples from Oklo, some of them recovered during drilling campaigns, are stored in the headquarters of France's nuclear power and renewable energy company Orano.
Interestingly, the Oklo name has found new life in modern nuclear technology. California-based Oklo Inc., named after the natural reactor, is developing small modular reactors that can use recycled nuclear fuel. Oklo Inc. announced that it has completed the first end-to-end demonstration of its advanced fuel recycling process as part of an ongoing $5 million project in collaboration with Argonne and Idaho National Laboratories. The company recently signed major agreements, including a non-binding Master Power Agreement with Switch, a data center provider, to deploy 12 GW of Oklo projects through 2044. The agreement marks one of the largest corporate power agreements in history.
Scientific Legacy
The Oklo natural reactor fundamentally changed how scientists think about nuclear processes and geological systems. It demonstrated that complex, self-regulating nuclear reactions can arise through purely natural processes—no intelligence or engineering required.
Experts suspect there may have been other such natural reactors in the world, but these must have been destroyed by geological processes, eroded away or subducted — or simply not yet found. The unique geological stability of the Franceville Basin in Gabon preserved this remarkable natural experiment for billions of years.
Today, as the world grapples with nuclear waste storage and reactor safety, the lessons from Oklo remain more relevant than ever. This ancient reactor proves that under the right conditions, nuclear materials can be safely contained and managed over geological timescales—offering hope for sustainable nuclear energy solutions.
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