Natural nuclear fission reactor

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 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda.^[1] The conditions found were very similar to what was predicted.

Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time.^[2][3]

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 0.720%; these samples had only 0.717% – 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.

Fission product isotope signatures

Nd

Isotope signatures of natural neodymium and fission product neodymium from U-235 which had been subjected to thermal neutrons. 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.

Ru

Isotope signatures of natural ruthenium and fission product ruthenium from U-235 which had been subjected to thermal neutrons. 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.

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, 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 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 to complete a 3-hour cycle.^[4]

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 ²³⁵U made up about 3.1% 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 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 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. 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 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 ²³⁵U and elevated temperatures to a few hundred degrees Celsius.^[3][5] 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]

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]

See also

• Deep geological repository

References

1. ^ Kuroda, P. K. (1956). "On the Nuclear Physical Stability of the Uranium Minerals". Journal of Chemical Physics 25 (4): 781–782; 1295–1296. Bibcode 1956JChPh..25..781K. doi:10.1063/1.1743058. 
2. ^ Meshik, A. P. (November 2005). "The Workings of an Ancient Nuclear Reactor". Scientific American. http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor. 
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. Bibcode 1996GeCoA..60.4831G. 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. Bibcode 2004PhRvL..93r2302M. doi:10.1103/PhysRevLett.93.182302. PMID 15525157. 
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. Bibcode 1980E&PSL..50..238D. 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. Bibcode 2002CRPhy...3..839G. 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. arXiv:hep-ph/0506186. Bibcode 2006PhRvC..74f4610P. doi:10.1103/PHYSREVC.74.064610. 

External links

• The natural nuclear reactor at Oklo: A comparison with modern nuclear reactors, Radiation Information Network, April 2005
• Oklo Fossil Reactors
• NASA, Astronomy Picture of the day: Oklo, Fossile Reactor, Zone 15

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