Natural nuclear fission reactor

A fossil natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios - both of uranium and of (the stable daughter nuclides of) fission products. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda.[1] The phenomenon was discovered in 1972 in Oklo, Gabon by French physicist Francis Perrin under conditions very similar to what was predicted.

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

Oklo is the only known location for this in the world and consists of 16 sites with patches of centimeter-sized ore layers. Here self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging probably less than 100 kW of thermal power during that time.[2][3][4]

History

In May 1972 at the Tricastin uranium enrichment site at Pierrelatte in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, showed a discrepancy in the amount of the 235
U
 isotope. Normally the concentration is 0.72% while these samples had only 0.60%, a significant difference.[5] This discrepancy required explanation, as all civilian uranium handling facilities must meticulously account for all fissionable isotopes to ensure that none are diverted to the construction of nuclear weapons. 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 235
U
 concentration as low as 0.44%. 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 234
U did not deviate significantly in its concentration from other natural samples. Both depleted uranium and reprocessed uranium will usually have 234
U concentrations significantly different from the secular equilibrium of 55 ppm 234
U relative to 238
U. This is due to 234
U being enriched together with 235
U and due to it being both consumed by neutron capture and produced from 235
U by fast neutron induced (n,2n) reactions in nuclear reactors. In Oklo any possible deviation of 234
U concentration present at the time the reactor was active would have long since decayed away.

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

Neodymium

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

Ruthenium
Isotope signatures of natural ruthenium and fission product ruthenium from 235
U
 which had been subjected to thermal neutrons. The 100
Mo
 (an extremely long-lived double beta emitter) has not had time to decay to 100
Ru
 over the time since the reactors stopped working.

Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher 99
Ru
 concentration than otherwise naturally occurring (27–30% vs. 12.7%). This anomaly could be explained by the decay of 99
Tc
 to 99
Ru
. 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 235
U
 with thermal neutrons. It is clear that the fission ruthenium has a different isotope signature. The level of 100
Ru
 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 100
Ru
 will have occurred.

Mechanism

The natural nuclear reactor 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 humanmade 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 to complete a 3-hour cycle.[6] 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 only 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 235
U
 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 non-fissile 238
U
.) Because 235
U
 has a shorter half-life than 238
U
, and thus decays more rapidly, the current abundance of 235
U
 in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite.[7]

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.

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.[4] Uranium is naturally present in the rocks of the earth, and the abundance of fissile 235
U
 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 235
U
 and elevated temperatures to a few hundred degrees Celsius.[4][8] Most of the non-volatile fission products and actinides have only moved centimeters in the veins during the last 2 billion years.[4] Studies have suggested this as a useful natural analogue for nuclear waste disposal.[9]

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, 149
Sm
 captures a neutron to become 150
Sm
, 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 α was the same too.[10][11][12]

See also

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. 293 (5): 82–6, 88, 90–1. Bibcode:2005SciAm.293e..82M. doi:10.1038/scientificamerican1105-82. PMID 16318030.
  3. Mervine, Evelyn (July 13, 2011). "Nature's Nuclear Reactors: The 2-Billion-Year-Old Natural Fission Reactors in Gabon, Western Africa". blogs.scientificamerican.com. Retrieved July 7, 2017.
  4. 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.
  5. Davis, E. D.; Gould, C. R.; Sharapov, E. I. (2014). "Oklo reactors and implications for nuclear science". International Journal of Modern Physics E. 23 (4): 1430007–236. arXiv:1404.4948. Bibcode:2014IJMPE..2330007D. doi:10.1142/S0218301314300070. ISSN 0218-3013. S2CID 118394767.
  6. 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.
  7. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1257. ISBN 978-0-08-037941-8.
  8. 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 (1): 238–246. Bibcode:1980E&PSL..50..238D. doi:10.1016/0012-821X(80)90135-1.
  9. 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.
  10. New Scientist: Oklo Reactor and fine-structure value. June 30, 2004.
  11. 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. S2CID 118272311.
  12. Davis, Edward D.; Hamdan, Leila (2015). "Reappraisal of the limit on the variation in α implied by the Oklo natural fission reactors". Physical Review C. 92 (1): 014319. arXiv:1503.06011. Bibcode:2015PhRvC..92a4319D. doi:10.1103/physrevc.92.014319. S2CID 119227720.
  • Bentridi, S.E.; Gall, B.; Gauthier-Lafaye, F.; Seghour, A.; Medjadi, D. (2011). "Génèse et évolution des réacteurs naturels d'Oklo" [Inception and evolution of Oklo natural nuclear reactors]. Comptes Rendus Geoscience (in French). 343 (11–12): 738–748. Bibcode:2011CRGeo.343..738B. doi:10.1016/j.crte.2011.09.008.
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