Natural reactors

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Introducing the values into the equation, using a minimum Kd-value of >300, gives a retention factor of >750. If this value is combined with a representative water transport time from repository to recipient (>1000 years for a distance >100 m), the transport equation indicates that it will take the plutonium >750,000 years to reach the recipient which is the water man may use. This estimate is supported by findings at the ancient natural reactor site at Oklo in Gabon (67). 

Toulhoat, P., Gallien, J. P. et al. 1996. Preliminary studies of groundwater flow and migration of uranium isotopes around the Oklo Natural Reactors (Gabon). Journal of Contaminant Hydrology, 21, 3-17. 

The Oklo natural reactors in Gabon are the best natural analogues for assessing the geological behaviour of fission products and actinides (see also Gauthier-Lafaye et al. 2004). Elements that were compatible with the U ore structure were retained, whereas elements that... 

Indications from both microscopic analyses of metallic particles from corrosion tests and evidence from the Oklo natural reactors indicate that performance assessment calculations should not assume 99Tc is easily mobilized. It is entirely inappropriate to use "Tc release as a marker for fuel corrosion because Tc is not located in the fuel matrix. The TEM examinations of corroded e-particles have shown that Mo is preferentially leached from these phases, a behaviour that is similar to the one observed at Oklo. It is interesting to note that laboratory dissolution of billion-year old 4d-metallic particles for a chemical analysis required a corrosive mix of peroxide and acid (Hidaka Holliger 1998) similar to the experience at SNF reprocessing plants. It is doubtful that the oxidation potential at the surface of an aged fuel will be sufficient to move Tc(0) from the e-metal particles. 

Pourcelot, L. Gauthier-Lafaye, F. 1999. Hydrothermal and supergen clays of the Oklo natural reactors conditions of radionuclide release, migration and retention. Chemical Geology, 157, 155-174. 

This paper is devoted to the sorption of uranyl, which exhibits a complex aqueous and surface chemistry. We review briefly the sorption behaviour of An in the environment, and illustrate the variety of environmental processes using published data of uranyl sorption in the Ban-gombe natural reactor zone. After summarizing the general findings of the mechanisms of An sorption, we then focus particularly on the current knowledge of the mechanisms of uranyl sorption. A major area of research is the influence of the aqueous uranyl speciation on the uranyl surface species. Spectroscopic data of U(VI) sorbed onto silica and alumina minerals are examined and used to discuss the role of aqueous uranyl polynuclear species, U02(0H)2 colloids and uranyl-carbonate complexes. The influence of the mineral surface properties on the mechanisms of sorption is also discussed. 

Uranyl sorption processes in the Bangombe natural reactor zone... 

Del Nero, M., Salah, S., Miura, T., Clement, A. Gauthier-Lafayf., F. 1999a. Sorption/deso-rption processes of uranium in clayey samples of the Bangombe natural reactor zone, Gabon. Radiochimica Acta, 87, 135-149. 

Salah, S. 2000. Weathering Processes at the Natural Reactor of Bangombe (Gabon). Identification and Geochemical Modeling of the Retention and Migration Mechanisms of Uranium and Rare Earth Elements. PhD thesis, Strasbourg, France. 

Information on the interaction of radionuclides with ground-water in deeply-buried, high-level, long-term "waste repositories" is available at only a few locations. One is the OKLO natural reactor in Gabon which has for over 1. 7 billion years retained some of the radionuclides also present in nuclear wastes (5). Another is the Nevada test Site, where radionuclides were first deposited underground on September 19, 1967 during the 1.7 kt... 

The natural reactors at Oklo have been in operation for about 10 y, probably with intermissions, depending on the presence of water. The neutron flux density, the power level and the temperature were relatively low (< 10 cm s < 10 kW and about 400-600 °C, respectively). About 6 tons of have been consumed and about 1 ton of Pu has been produced. Since then, the latter decayed into... 

Analysis of the natural reactors at Oklo gives valuable information about the migration behaviour of fission products and actinides in the geosphere. Uranium and the lanthanides have been redistributed locally. Plutonium produced in the Oklo reactors did not move during its lifetime from the site of its formation 85-100% of the lanthanides, 75-90% of the Ru and 60-85% of the Tc were retained within the reactor zones. Small amounts of U, lanthanides, Ru and Tc moved with the water over distances of up to 20-50 m. 

Unusually high fission track densities are found in the vicinity of nuclear explosions and at the natural reactors at Oklo. 

Cuney M. and Mathieu R. (2000) Extreme fight rare earth element mobilization by diagenetic fluids in the geological environment of the Oklo natural reactor zones, EranceviUe Basin, Gabon. Geology 28, 743—746. 

Natural Analogues for the Long-term Behavior of Radionuclides in the Environment—The Ohio Natural Reactor... 

The following section provides detailed information concerning the transport of radionuclides associated with two very different field analogues the Chernobyl reactor accident and the Oklo Natural Reactor. These examples span wide temporal and spatial scales and include the rapid geochemical and physical processes important to nuclear reactor accidents or industrial discharges as well as the slower processes important to the geologic disposal of nuclear waste. 

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