Gabon reactor

Retention of fission products and actinides in the Gabon reactors... 

As mentioned earlier, the natural abundance of U-235 is 0.7202 percent, but it has not always been that low. The half-lives of U-235 and U-238 are 700 million and 4.51 billion years, respectively. This means that U-235 must have been more abundant in the past, because it has a shorter half-life. In fact, at the time Earth was formed, the natural abundance of U-235 was as high as 17 percent Because the lowest concentration of U-235 required for the operation of a fission reactor is 1 percent, a nuclear chain reaction could have taken place as recently as 400 million years ago. By analyzing the amounts of radioactive fission products left in the ore, scientists concluded that the Gabon reactor operated about 2 billion years ago. 

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). 

It may seem unlikely that all these conditions could have been met, but at least one deposit of uranium ore has characteristics indicating that, long ago, it operated as a natural nuclear reactor. At Oklo in the Gabon Republic near the western coast of equatorial Africa (see photo), there are uranium deposits of high purity... 

Radioactivity, radioactive elements and nuclear reactors are found in nature. There are at least 14 natural fission reactors in the Oklo-Okelobon-do natural uranium formation in Gabon on the west coast of Africa. These fossil reactors had sufficient amounts of U-235 to allow chain reactions to... 

Gauthier-Lafaye, F., 2002, 2 billion year old natural analogs for nuclear waste disposal the natural nuclear fission reactors in Gabon (Africa). C. R. Physiquee 3 839-849. 

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... 

Hidaka, H. Holliger, P. 1998. Geochemical and neutronic characteristics of the natural fossil fission reactors at Oklo and Bamgombe, Gabon. Geochimica et Cosmochimica Acta, 62, 89-108. 

Savary, V. Pagel, M. 1997. The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16. Geochimica et Cosmochimica Acta, 61, 4479-4494. 

Stille, P., Gauthier-Lafaye, F. et al. 2003. REE mobility in groundwater proximate to the natural fission reactor at Bangombe (Gabon). Chemical Geology, 198, 289-304. 

Bros, R., Carpena, J., Sere, V. Beltritti, A. 1996. Occurence of Pu and fissiogenic rare earth elements in hydrothermal apatites from the fossil natural nuclear reactor 16 of Oklo (Gabon). Radio-chimica Acta, 74, 277-282. 

Bros, R., Hidaka, H., Kamei, G. Ohnuki, T. 2001. Retardation of fissiogenic REE in clays surrounding the Oklo reactor 2 (Gabon). In Proceedings of Atomic Energy Research Society of Japan Meeting, Sapporo, September 2001, 932. 

Eberly, P., Janeczek, J. Ewing, R. C. 1995. Precipitation of uraninite in chlorite-bearing veins of the hydrothermal alteration zone (argiles de pile) of the natural nuclear reactor at Bangombe, Republic of Gabon. Proceedings of Material Research Society Symposium, 353, 1195-1202. 

Natural fission reactors in the Franceville basin, Gabon A review of the conditions and results of critical event in a geological system. Geochimica Cosmochimica Acta, 60, 4831-4852. 

Hidaka, H., Holliger, P. Gauthier-Lafaye, F. 1999. Tc/Ru fractionation in the Oklo and Bangombe natural fission reactors (Gabon). Chemical Geology, 155, 323-333. 

Janeczek, J. Ewing, R. 1996. Florencite-(La) with fissiogenic REE from a natural fission reactor at Bagombe, Gabon. American Mineralogist, 81, 1263-1269. 

Uraninite a 2 Ga spent nuclear fuel from the natural fission reactor at Bangombe in Gabon, West Aftrica. Proceedings of Materia Research Society Symposium, 465, 1209-1218. 

Jensen, K. A. Ewing, R. C. 2001. The Oklelobondo natural fission reactor, southeast Gabon Geology, mineralogy and retardation of nuclear reaction... 

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

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. 

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