Natural nuclear fission reactor. A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The idea of a nuclear reactor existing in situ within an ore body moderated by groundwater was briefly explored by Paul Kuroda in 1956. The existence of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, are established by analysis of isotope ratios of uranium and of the fission products (and the stable daughter nuclides of those fission products). The first such fossil reactor was first discovered in 1972 in Oklo, Gabon, by researchers from the French Alternative Energies and Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissile  in gaseous uranium made from Gabonese ore.

Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 sites with patches of centimeter-sized ore layers. 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. 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. Life on Earth at this time consisted largely of sea-bound algae and the first eukaryotes, living under a 2% oxygen atmosphere. However even this meager oxygen was likely essential to the concentration of uranium into fissionable ore bodies, as uranium dissolves in water only in the presence of oxygen. Before the planetary-scale production of oxygen by the early photosynthesizers groundwater-moderated natural nuclear reactors are not thought to have been possible.

Discovery of the Oklo fossil reactors 
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  isotope. Where the usual concentrations of  were 0.72% the Oklo samples showed only 0.60%. This was a significant difference—the samples bore 17% less  than expected. This discrepancy required explanation, as 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. 

Thus the French Alternative Energies and Atomic Energy Commission (CEA) began an investigation. 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. Further investigations into this uranium deposit discovered uranium ore with a  concentration as low as 0.44% (almost 40% below the normal value). Subsequent examination of isotopes of fission products such as neodymium and ruthenium also showed anomalies, as described in more detail below. However, the trace radioisotope  did not deviate significantly in its concentration from other natural samples. Both depleted uranium and reprocessed uranium will usually have  concentrations significantly different from the secular equilibrium of 55 ppm  relative to . This is due to  being enriched together with  and due to it being both consumed by neutron capture and produced from  by fast neutron induced (n,2n) reactions in nuclear reactors. In Oklo any possible deviation of  concentration present at the time the reactor was active would have long since decayed away.  must have also been present in higher than usual ratios during the time the reactor was operating, but due to its half life of  years being almost two orders of magnitude shorter than the time elapsed since the reactor operated, it has decayed to roughly  its original value and thus basically nothing and below any abilities of current equipment to detect.

This loss in  is exactly what happens in a nuclear reactor. A possible explanation was that the uranium ore had operated as a natural fission reactor in the distant geological past. Other observations led to the same conclusion, and on 25 September 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

Neodymium 
The neodymium found at Oklo has a different isotopic composition to that of natural neodymium: the latter contains 27% , while that of Oklo contains less than 6%. The  is not produced by fission; the ore contains both fission-produced and natural neodymium. From this  content, we can subtract the natural neodymium and gain access to the isotopic composition of neodymium produced by the fission of . The two isotopes  and  lead to the formation of  and  by neutron capture. This excess must be corrected (see above) to obtain agreement between this corrected isotopic composition and that deduced from fission yields.

Ruthenium 

Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher  concentration than otherwise naturally occurring (27–30% vs. 12.7%). This anomaly could be explained by the decay of  to . In the bar chart, the normal natural isotope signature of ruthenium is compared with that for fission product ruthenium which is the result of the fission of  with thermal neutrons. The fission ruthenium has a different isotope signature. The level of  in the fission product mixture is low because fission produces neutron rich isotopes which subsequently beta decay and  would only be produced in appreciable quantities by double beta decay of the very long-lived (half life  years) molybdenum isotope . On the timescale of when the reactors were in operation, very little (about 0.17 ppb) decay to  will have occurred. Other pathways of  production like neutron capture in  or  (quickly followed by beta decay) can only have occurred during high neutron flux and thus ceased when the fission chain reaction stopped.

Mechanism 
The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with groundwater, which could act as a moderator for the neutrons produced by nuclear fission. A chain reaction took place, producing heat that caused the groundwater to boil away; without a moderator that could slow the neutrons, however, the reaction slowed or stopped. The reactor thus had a negative void coefficient of reactivity, something employed as a safety mechanism in human-made light water reactors. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials, coupled with the build-up of neutron poisons, no longer could sustain a chain reaction.

Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped 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 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. Xenon-135 is the strongest known neutron poison. However, it is not produced directly in appreciable amounts but rather as a decay product of iodine-135 (or one of its parent nuclides). Xenon-135 itself is unstable and decays to caesium-135 if not allowed to absorb neutrons. While caesium-135 is relatively long lived, all caesium-135 produced by the Oklo reactor has since decayed further to stable barium-135. Meanwhile, xenon-136, the product of neutron capture in xenon-135 decays extremely slowly via double beta decay and thus scientists were able to determine the neutronics of this reactor by calculations based on those isotope ratios almost two billion years after it stopped fissioning uranium.

A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope  made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was  and roughly 55 ppm , neither of which is fissile by slow or moderated neutrons.) Because  has a shorter half-life than , and thus decays more rapidly, the current abundance of  in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite.

The Oklo uranium ore deposits are the only known sites 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, as far as is currently known, to the Oklo ore bodies. It is also possible that other natural nuclear fission reactors were once operating but have since been geologically disturbed so much as to be unrecognizable, possibly even "diluting" the uranium so far that the isotope ratio would no longer serve as a "fingerprint". Only a small part of the continental crust and no part of the oceanic crust reaches the age of the deposits at Oklo or an age during which isotope ratios of natural uranium would have allowed a self sustaining chain reaction with water as a moderator.

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 fissile  was at least 3% or higher at all times prior to reactor startup. Uranium is soluble in water only in the presence of oxygen. Therefore, increasing oxygen levels during the aging of the 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 could not have taken place.

It is estimated that nuclear reactions in the uranium in centimeter- to meter-sized veins consumed about five tons of  and elevated temperatures to a few hundred degrees Celsius. Most of the non-volatile fission products and actinides have only moved centimeters in the veins during the last 2 billion years. Studies have suggested this as a useful natural analogue for nuclear waste disposal. The overall mass defect from the fission of five tons of  is about . Over its lifetime the reactor produced roughly  in thermal energy, including neutrinos. If one ignores fission of plutonium (which makes up roughly a third of fission events over the course of normal burnup in modern human-made light water reactors), then fission product yields amount to roughly  of technetium-99 (since decayed to ruthenium-99),  of zirconium-93 (since decayed to niobium-93),  of caesium-135 (since decayed to barium-135, but the real value is probably lower as its parent nuclide, xenon-135, is a strong neutron poison and will have absorbed neutrons before decaying to  in some cases),  of palladium-107 (since decayed to silver),  of strontium-90 (long since decayed to zirconium), and  of caesium-137 (long since decayed to barium).

Relation to the atomic fine-structure constant 
The natural reactor of Oklo has been used to check if the atomic fine-structure constant α might have changed over the past 2 billion years. That is because α influences the rate of various nuclear reactions. For example,  captures a neutron to become , and since the rate of neutron capture depends on the value of α, the ratio of the two samarium isotopes in samples from Oklo can be used to calculate the value of α from 2 billion years ago.

Several studies have analysed the relative concentrations of radioactive isotopes left behind at Oklo, and most have concluded that nuclear reactions then were much the same as they are today, which implies that α was the same too.

See also 
 Deep geological repository
 Geology of Gabon
 Mounana

References

Sources

External links 

 The natural nuclear reactor at Oklo: A comparison with modern nuclear reactors, Radiation Information Network, April 2005
 Oklo Fossil Reactors
 
 הכור הגרעיני של הטבע (in Hebrew language)

Nuclear reactors
Geography of Gabon
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Nuclear fission
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Nuclear chemistry