Natural nuclear fission reactor
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The Francevillian basin |
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A natural nuclear fission reactor is a
Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 sites with patches of centimeter-sized
Gabon was a French colony when the first analyses of the subsoil were carried out by the CEA from the MABA base in Franceville, more precisely by its industrial direction which later became
France almost immediately opened mines, managed by the “Compagnie des Mines d'Uranium de Franceville” (COMUF), to exploit the resources, near the village of Mounana. After independence in 1960, the state of Gabon received a small share of the company's profits.
The "Oklo phenomenon" was discovered in June 1972 by the laboratory at the uranium enrichment plant in Pierrelatte, France. Routine analysis of a sample of natural uranium revealed a slight but abnormal deficit of uranium 235 (235U)6. The normal proportion of 235U is 0.7202%, whereas this sample showed only 0.7171%. As the quantities of fissile isotopes are precisely catalogued, this discrepancy had to be explained, so an investigation was launched by CEA on samples from all the mines operated by CEA in France, Gabon and Niger, and at all stages of ore processing and uranium purification.
For uranium and 235U analyses, CEA's Production Division relies on the Analytical Laboratory at the Pierrelatte plant and on CEA's Central Analysis and Control Laboratory at Cadarache, headed by Michele Neuilly, where Jean François Dozol is in charge of mass spectrometry analyses.
Discovery of the Oklo fossil reactors
Analyses carried out at Pierrelatte and Cadarache showed that magnesium uranates (or yellow cakes) from Gabon had a variable but constant 235U depletion. On July 7, 1972, researchers at Cadarache discovered an anomaly in uranium ore from Oklo in Gabon. Its 235U content was much lower than usual.[6] Isotopic analyses revealed the origin of the 235U depletion: the depleted uranium came from Oklo ore in Gabon, mined by COMUF. A systematic analysis campaign was then carried out in the Cadarache and Pierrelatte laboratories (uranium content measurements, isotopic content measurements). On Oklo samples, Cadarache analysts noted a 235U depletion for magnesium uranate from the Mounana plant (235U = 0.625%) and an even greater depletion for a magnesium uranate (Oklo M) (235U = 0.440%): Oklo 310 and 311 ores have uranium contents of 12% and 46% respectively, and 235U contents of 0.592% and 0.625%.
In this context, J.F. Dozol took the initiative of analyzing magnesium uranate and ore samples from Oklo on the AEI MS 702 Spark Source Mass Spectrometer (SSMS).
The advantage of the SSMS is its ability to produce substantial quantities of ions from all the elements present in the electrodes. The electrodes, between which a spark is generated, have to be conductive (to achieve this, Oklo samples were mixed with high-purity silver). All the isotopes in the sample, from lithium to uranium, are plotted on a photo plate. On examining the plate (see below), J.F. Dozol noted in particular the very high uranium content of Oklo 311 ore:
- elements present in significant quantities around masses 85-105 and 130-150, corresponding to the two bumps of 235U fission yields. (The mass distribution of fission products follows a "camel's hump" curve, with two maxima),
- the last lanthanides (holmium to lutetium) are not detected (beyond mass166). In nature, all 14 lanthanides are found; in nuclear fuel, having undergone fission reactions, the isotopes of the last lanthanides are not detected.
The next step is isotopic analysis of certain elements on a thermal ionization mass spectrometer, after chemical separation of neodymium and samarium. From the first analyses of Oklo "M" uranate and "Oklo 311" ore, it is clear that neodymium and samarium have an isotopic composition much closer to that found in irradiated fuel than to that of the natural element. The detection of 142Nd and 144Sm isotopes not produced by fission indicates that these elements are also present in the natural state, from which their contribution can be subtracted.[7][8]
These results were passed on to neutron scientist Jean Claude Nimal (CEA Saclay), who estimated the neutron flux received by the analyzed sample on the basis of its 235U deficit. This made it possible to estimate the neutron capture by the isotopes 143Nd and 145Nd, leading to the additional formation of 144Nd and 146Nd respectively. This excess must be subtracted to obtain fission yields for uranium 235.[9] As can be seen from the table below, the fission yields (M) agree with the results corrected (C) for the presence of natural neodymium and neutron capture.[10][11]
Nd | 143 | 144 | 145 | 146 | 148 | 150 |
---|---|---|---|---|---|---|
C/M | 0.99 | 1.00 | 1.00 | 1.01 | 0.98 | 1.06 |
Fission product isotope signatures
Neodymium
The neodymium found at Oklo has a different isotopic composition to that of natural neodymium: the latter contains 27% 142Nd, while that of Oklo contains less than 6%. The 142Ndis not produced by fission; the ore contains both fission-produced and natural neodymium. From this 142Ndcontent, we can subtract the natural neodymium and gain access to the isotopic composition of neodymium produced by the fission of 235U. The two isotopes 143Ndand 145Ndlead to the formation of 144Ndand 146Ndby neutron capture, and this excess must be corrected (see above) to obtain perfect agreement between this corrected isotopic composition and that deduced from fission yields.
Ruthenium
Similar investigations into the isotopic ratios of
Ru
will have occurred. Other pathways of 100
Ru production like neutron capture in 99
Ru or 99
Tc (quickly followed by beta decay) can only have occurred during high neutron flux
Mechanism
The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with
Fission of uranium normally produces five known isotopes of the fission-product gas
A key factor that made the reaction possible was that, at the time the reactor went
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
and roughly 55 ppm 234
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.[13]
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
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.[citation needed
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.
Relation to the atomic fine-structure constant
The natural reactor of Oklo has been used to check if the atomic
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
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.[16][17][18]
See also
References
- .
- PMID 16318030.
- ^ Mervin, 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.
- ^ .
- ^ 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
- ^ L'aval du cycle nucléaire, senat.fr/rap/97-612/97-61252
- ^ Jean-Francois Dozol, Michele Neuilly « Isotopic analysis of the rare earths contained in the Oklo ores », IAEA; Vienna; Symposium on the Oklo phenomenon; Libreville, Gabon; 23 Jun 1975; IAEA-SM--204/29, vol. Proceedings series;, no IAEA-SM--204/29, 1975, p. 357-369
- PMID 37934987.
- ^ M. E. Meek and B. F. Rider,, « », Vallecitos Nuclear Center, Pleasanton, Calif., NEDO-12154-1. Nouvelle edition, 1974
- ^ J.C. Nimal, « Historical simulations of Oklo cores », Radiation Protection Dosimetry, vol. Volume 199, Issue18, novembre 2023, p. 2262-2268
- ^ P. Girard, Compte rendu SFEN, vol. CR Conf 17 octobre 2018-Marseille, 2018
- PMID 15525157.
- ISBN 978-0-08-037941-8.
- .
- .
- ^ New Scientist: Oklo Reactor and fine-structure value. June 30, 2004.
- S2CID 118272311.
- S2CID 119227720.
Sources
- 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. .