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 occurred in the past, was established by analysis of isotope ratios of uranium and of the fission products (and the stable daughter nuclides of those fission products). The first discovery of such a reactor happened in 1972 in Oklo, Gabon, by researchers from the French Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissile in gaseous uranium hexafluoride made from Gabonese ore.

Oklo is the only location where this phenomenon is known to have occurred, consisting of sixteen 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. 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. right|thumb|Geological situation in Gabon leading to natural nuclear fission reactors

Thus, the French 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 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. 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.

alt=A graph showing the exponential decay of Uranium-235 over time.|thumb|upright=1.2|Change of content of Uranium-235 in natural uranium; the content was 3.65% 2 billion years ago.

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. Most of the non-volatile fission products and actinides have only moved centimeters in the veins during the last 2 billion years. 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
  • Oklo Inc

References

Sources

<!-- this source is plainly important and should be cited in multiple places in the article -->

  • 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)