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 water boiled away, the chain reaction stopped, and cooling let groundwater return to restart it. See Source 1. Xenon isotopes trapped in aluminum phosphate grains record the rhythm: roughly 30 minutes of criticality followed by about 2.5 hours of cooling, a three hour cycle. See Sources 1 and 21. Average thermal power stayed below 100 kilowatts. About five tons of uranium 235 fissioned over the life of the zones, releasing roughly 420 petajoules, an energy equivalent to about 100 megatons of TNT. See Source 21. Estimates of total duration range from nearly 100,000 years through about 150,000 years to a few hundred thousand years, depending on the zone and the method. See Sources 3, 20, and 21. On a nuclear submarine the reactor operator learns to respect xenon 135, a fission product that absorbs neutrons so strongly that a reactor shut down for several hours can refuse to restart until the xenon decays. The same element, preserved in mineral, recorded the operating history of a reactor that ran two billion years before anyone built a control panel. The loss of moderator through boiling that stopped each Oklo cycle is the negative void feedback that engineers design into light water reactors today. What do the Oklo reactors show about fission product and plutonium retention? Oklo shows that many fission products and actinides stayed within or near the reactor zones for about 2 billion years, held in uraninite and aluminum phosphate minerals with clay around the zones. Phosphate rich minerals such as apatite retained actinides and fission products effectively, and uranium showed little or no migration. See Sources 8 and 20. Retention varied by element. Rare earth fission products and uranium largely stayed, and some volatile and soluble elements dispersed, so analogue studies report retention element by element. See Sources 2 and 8. Plutonium 239, produced when uranium 238 captured neutrons, has a half life of 24,100 years, so none remains. Its decay product, uranium 235, stayed in the ore, and that excess marks where plutonium once sat. See Source 2. Neodymium belongs to the fission products, and the Oklo neodymium isotope mix carries the fission signature described above. See Source 21. The analogy has limits. Oklo was a natural ore body at modest temperatures. An engineered repository such as Onkalo adds iron copper canisters, a bentonite buffer, and clay backfill, and spent fuel gives off decay heat for centuries. See Source 18. The Oklo record validates chemical retention in a geological setting and does not test canister corrosion or bentonite performance. How did Oklo test whether physical constants change over time? Oklo isotope ratios let physicists bound any change in the fine structure constant over two billion years, and analyses published through 2014 conclude that nuclear reactions then were much the same as they are today. See Sources 5 and 21. That result applies a reactor built by geology to a question in fundamental physics, and it supports the use of present day nuclear data to model the zones. How does Oklo connect to current reactor developers and waste policy? Oklo Inc., founded in 2013, takes its name from the Gabon site. See Source 21. The Nuclear Regulatory Commission denied the Aurora combined license application of the company on January 6, 2022, without prejudice, under docket 05200049. See Source 6. The Department of Energy selected the company for its Reactor Pilot Program in May 2025, and the Groves Isotope Test Reactor reached criticality at 9:19 p.m. Eastern time on August 5, 2026. See Sources 7, 16, and 21. Groves is a pool type, water cooled, nonpressurized reactor of 15 megawatts thermal using metal fuel, the fifth Department of Energy authorized advanced reactor to reach criticality in summer 2026 and the first on private land. See Sources 7 and 16. The Aurora powerhouse broke ground at Idaho National Laboratory in September 2025, and the Idaho Radiochemistry Laboratory received an NRC materials license. See Source 17. Finland supplies the disposal counterpart. On August 10, 2026, the Finnish Radiation and N