Oklo Natural Nuclear Reactors

Executive Summary

The Oklo natural nuclear reactors in Gabon, discovered in 1972, sustained uranium fission chain reactions about 2 billion years ago and remain the only confirmed natural reactors on Earth. Sixteen reactor zones across the Oklo, Okélobondo, and Bangombé uranium deposits reached criticality because natural uranium then held about 3.5 percent uranium 235 and groundwater slowed neutrons inside ore that contained very little neutron absorbing material. Boiling water shut each zone down and returning water restarted it, a cycle that xenon isotopes trapped in the mineral place at roughly 30 minutes of fission followed by about 2.5 hours of rest. Many fission products and actinides stayed within or near the ore for the full 2 billion years, which gives repository designers a field record that no laboratory test can replicate. See Sources 1, 3, and 20.

The subject carries commercial weight in 2026. A Department of Energy authorized isotope reactor built by a company named for the Gabon site reached criticality in Lockhart, Texas, on August 5, 2026, and the Finnish radiation regulator found no obstacle to an operating license for the Onkalo spent fuel repository on August 10, 2026. Onkalo is described as the first commercial final disposal facility for spent fuel, so its retention evidence must come from analogues, and Oklo supplies a record measured in billions of years. See Sources 7, 16, and 18.

Two legal instruments set the stakes. Section 63.311 of Title 10 of the Code of Federal Regulations limits dose to the reasonably maximally exposed individual to 15 millirem per year for the first 10,000 years and 100 millirem per year through one million years, and 42 U.S.C. § 2210 extends Price Anderson Act liability protection under the 2024 amendments through December 31, 2065. See Sources 9 and 15.

Detailed Findings

The Oklo record rests on four lines of evidence: isotope depletion in the ore, fission product signatures, xenon trapped in minerals, and neutronics simulations of the reactor zones. Each line was developed by separate laboratories over five decades, and they converge on the same operating history. See Sources 1, 2, and 3.

What are the Oklo natural nuclear reactors and where are they located?

The Oklo natural nuclear reactors are sixteen zones of self sustaining uranium fission in the Franceville basin of Gabon, spread across the Oklo, Okélobondo, and Bangombé deposits. See Source 20. One 2011 neutronics study counts approximately fifteen zones at the Oklo and Okélobondo mines. See Source 3.

Each zone is a lens of uranium ore a few tens of centimeters thick, set in sandstone and clay. The best studied zone, labeled RZ9, measures about 12 meters long and 7 meters wide. See Source 3. That study places operation near 1.95 billion years ago, and other compilations give a window between 1.7 and 2 billion years ago. See Sources 3, 19, and 21.

How was the Oklo natural reactor discovered in 1972?

Routine mass spectrometry on uranium hexafluoride samples at the Tricastin enrichment site in Pierrelatte, France, revealed uranium 235 depletion in Oklo ore in May 1972, and the French Atomic Energy Commission announced the finding on September 25, 1972. See Source 21.

Natural uranium on Earth carries 0.72 percent uranium 235. One early sample read 0.717 percent against the 0.720 percent standard, a gap small enough that investigators first looked for an artificial cause until physicist Francis Perrin and colleagues established that the process was natural. See Source 19. Some ore samples later measured as low as 0.44 percent, about 40 percent below normal. See Source 21.

The missing uranium 235 appeared as fission products. Natural neodymium contains 27 percent neodymium 142, while the neodymium extracted from Oklo ore contains less than 6 percent, and ruthenium 99 reaches 27 to 30 percent against 12.7 percent in nature because technetium 99 decayed into it. See Source 21.

Why did natural fission occur at Oklo about 2 billion years ago?

Natural fission occurred at Oklo because five conditions coincided: enriched uranium, concentrated ore, water, adequate size, and low neutron absorption. A reactor needs all five at once, and no other known deposit has met them. See Sources 2 and 3.

Enrichment came from radioactive decay. Uranium 235 decays with a half life of 704 million years and uranium 238 with a half life of 4.47 billion years, so the uranium 235 share of natural uranium stood near 3.5 percent about 1.95 billion years ago, compared with 0.72 percent today. See Source 3. Light water reactors run on fuel enriched to a similar range, which is why ordinary groundwater could serve as moderator.

Concentration required ore with at least about 20 percent uranium by weight in dehydrated form. See Source 3. Atmospheric oxygen had risen enough by then to dissolve uranium as the uranyl ion, carry it in groundwater, and deposit it where reducing conditions trapped it. See Sources 2 and 21.

Water slowed fast neutrons to thermal energies, where uranium 235 fission is most probable. See Source 20. Simulated critical radii ranged from 50 to 160 centimeters depending on geometry, and the best studied lens is several times wider than that range. See Source 3. The ore held little of the boron and rare earth material that absorbs neutrons. See Source 2.

In 1956 the physicist Paul Kuroda calculated that these conditions could produce a chain reaction in a uranium deposit, sixteen years before the discovery in France. See Source 4. At 0.72 percent, natural uranium cannot sustain a chain reaction in ordinary water, so a natural reactor cannot start today without heavy water or graphite as moderator. See Source 21.

How did the Oklo reactors regulate themselves for 150,000 years?

Water governed each Oklo reactor zone as both moderator and thermostat. Fission heated the zone, the wate