Natural nuclear fission reactor

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Geological situation in Gabon leading to natural nuclear fission reactors
1. Nuclear reactor zones
2. Sandstone
3. Uranium ore layer
4. Granite

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.[1] The conditions found at Oklo were very similar to what was predicted.

At the only known location, three ore deposits at Oklo in Gabon, sixteen sites have been discovered at which self-sustaining nuclear fission reactions took place approximately 2 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time.[2][3]


[edit] 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 à l'énergie 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.

[edit] Fission product isotope signatures

[edit] Nd

A diagram showing the isotope signatures of natural neodymium and fission product neodymium from U-235 which had been subjected to thermal neutrons. Note that the Ce-142 (a long lived beta emitter) has not had time to decay to Nd-142 over the time since the reactors stopped working.

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.

[edit] Ru

A diagram showing the isotope signatures of natural ruthenium and fission product ruthenium from U-235 which had been subjected to thermal neutrons. Note that the Mo-100 (a long lived double beta emitter) has not had time to decay to Ru-100 over the time since the reactors stopped working.

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.

[edit] 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.[4]

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.[3] 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 nuclear reactions in the uranium in centimeter- to meter-sized veins consumed about five tons of 235U and elevated temperatures to a few hundred degrees Celsius.[3][5] Remarkably, most of the non-volatile fission products and actinides have only moved centimeters in the veins during the last 2 billion years.[3] This offers a case study of how radioactive isotopes migrate through the Earth's crust[6]—a 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.

[edit] 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, 149Sm captures a neutron to become 150Sm, 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 (but not all) have concluded that nuclear reactions then were much the same as they are today, which implies α was the same too.[7][8]

The 149Sm resonance is sensitive to changes in the proton-to-electron mass ratio, μ, as well as changes in α. It may be that some cancellation occurs between these two, and so a null result does not necessarily mean that both α and μ have been constant over this time period.

[edit] See also

[edit] In fiction

In the novel Camelot 30K by Robert L. Forward, natural nuclear fission is revealed as the power source (as opposed to solar power) of the biosphere of a Trans-Neptunian dwarf planet.

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

[edit] References

  1. ^ Kuroda, P. K. (1956). "On the Nuclear Physical Stability of the Uranium Minerals". Journal of Chemical Physics 25: 781–782; 1295–1296. doi:10.1063/1.1743058. 
  2. ^ Meshik, A. P. (November 2005). "The Workings of an Ancient Nuclear Reactor". Scientific American. 
  3. ^ a b c d Gauthier-Lafaye, F.; Holliger, P.; Blanc, P.-L. (1996). "Natural fission reactors in the Franceville Basin, Gabon: a review of the conditions and results of a "critical event" in a geologic system". Geochimica et Cosmochimica Acta 60 (25): 4831–4852. doi:10.1016/S0016-7037(96)00245-1. 
  4. ^ Meshik, A. P.; et al. (2004). "Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon". Physical Review Letters 93 (18): 182302. doi:10.1103/PhysRevLett.93.182302. 
  5. ^ De Laeter, J. R.; Rosman, K. J. R.; Smith, C. L. (1980). "The Oklo Natural Reactor: Cumulative Fission Yields and Retentivity of the Symmetric Mass Region Fission Products". Earth and Planetary Science Letters 50: 238–246. doi:10.1016/0012-821X(80)90135-1. 
  6. ^ Gauthier-Lafaye, F. (2002). "2 billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon (Africa)". Comptes Rendus Physique 3 (7–8): 839–849. doi:10.1016/S1631-0705(02)01351-8. 
  7. ^ New Scientist: Oklo Reactor and fine-structure value. June 30, 2004.
  8. ^ Petrov, Yu. V.; Nazarov, A. I., Onegin, M. S., Sakhnovsky, E. G. (2006). "Natural nuclear reactor at Oklo and variation of fundamental constants: Computation of neutronics of a fresh core". Physical Review C 74 (6): 064610. doi:10.1103/PHYSREVC.74.064610. 

[edit] External links

Coordinates: 1°23′40″S 13°09′39″E / 1.39444°S 13.16083°E / -1.39444; 13.16083

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