Natural nuclear fission reactor
A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred.  The existence of this phenomenon was discovered in 1972 by French physicist Francis Perrin.  The conditions under which a natural nuclear reactor could exist were predicted in 1956 by P. Kuroda. The conditions found at Oklo were very similar to what were predicted.
At the only known location, three ore deposits at Oklo in Gabon, sixteen sites have been discovered so far at which self-sustaining nuclear fission reactions took place approximately 1.5 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time.

# History
In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the 235U isotope.  Normally the concentration is 0.7202%; these samples had only 0.7171%  a significant difference. This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat a l'energie atomique (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a 235U to 238U ratio as low as 0.440%.  Subsequent examination of other isotopes showed similar anomalies, such as Nd and Ru as described in more detail below.
This loss in 235U is exactly what happens in a nuclear reactor.  A possible explanation therefore was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on September 25 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.

# Fission product isotope signatures

## Nd
Neodymium and other elements were found with isotopic compositions different from what is customarily found on Earth. For example, natural neodymium contains 27% 142Nd; the Nd at Oklo contained less than 6% but contained more 143Nd. Subtracting the natural isotopic Nd abundance from the Oklo-Nd, the isotopic composition matched that produced by the fissioning of 235U.

## Ru
Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher 99Ru concentration than expected (27-30% vs. 12.7%). This anomaly could be explained by the decay of 99Tc to 99Ru. In the bar chart below the normal natural isotope signature of ruthenium is compared with that for fission product ruthenium which is the result of the fission of 235U with thermal neutrons. It is clear that the fission ruthenium has a different isotope signature. The level of 100Ru in the fission product mixture is low because of a long lived (half life = 1019 years) isotope of molybdenum. On the time scale of when the reactors were in operation very little decay to 100Ru will have occurred.

# Mechanism of the reactors
The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction.  After cooling of the mineral deposit, short-lived fission product poisons decayed, the water returned and the reaction started again.  These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported.
Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped within novel aluminium foams in the remnants of the natural reactor, in varying concentrations.  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 2 hours and 30 minutes.
A key factor that made the reaction possible was that, at the time the reactor went critical, the fissile isotope 235U made up about 3% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissile 238U.) Because 235U has a shorter half life than 238U, and thus decays more rapidly, the current abundance of 235U in natural uranium is about 0.7%. A natural nuclear reactor is therefore no longer possible on Earth.
The Oklo uranium ore deposits are the only known in which natural nuclear reactors existed.  Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water and physical conditions needed to support the chain reaction was unique to the Oklo ore bodies.
Another factor which probably contributed to the start of the Oklo natural nuclear reactor at 2 billion years, rather than earlier, was the increasing oxygen content in the earth's atmosphere.  Uranium is naturally present in the rocks of the earth, and the abundance of fissionable 235U was at least 3% or higher at all times prior to reactor startup.  However, uranium is soluble in water only in the presence of oxygen.  Therefore, the rising oxygen levels during the aging of earth may have allowed uranium to be dissolved and transported with groundwater to places where a high enough concentration could accumulate to form rich uranium ore bodies.  Without the new aerobic environment available on earth at the time, these concentrations probably couldn't have taken place.
It is estimated that secondary enrichment of the uranium in centimeter- to meter-sized veins consumed about six tons of 235U and elevated temperatures to a few hundred degrees Celsius. Remarkably, the non-volatile fission products have only moved a few centimeters in the veins during the last 1.5 billion years. This offers a case study of how radioactive isotopes migrate through the earth's crusta significant area of controversy as opponents of geologic nuclear waste disposal fear that releases from stored waste could end up in water supplies or be carried into the environment.

# Relation to the atomic fine-structure constant
The natural reactor of Oklo has been used to check if the atomic fine-structure constant \alpha might have changed over the past 2 billion years. That is because \alpha influences the rate of various nuclear reactions. For example, 149Sm captures a neutron to become 150Sm, and since the rate of neutron capture depends on the value of \alpha, the ratio of the two samarium isotopes in samples from Oklo can be used to calculate the value of \alpha from 2 billion years ago.
Several studies have analysed the relative concentrations of radioactive isotopes left behind at Oklo, and most (but not all) have concluded that nuclear reactions then were much the same as they are today, which implies alpha was the same too.

# In fiction
In Manifold: Space by Stephen Baxter, a far-future feudalist society exploits a naturally occurring fission reactor for power.