Natural fission reactors in the Franceville basin, Gabon: A review of the conditions and results of a {open_quotes}critical event{close_quotes} in a geologic system

Journal Article · Sun Dec 01 04:00:00 EST 1996 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/S0016-7037(96)00245-1· OSTI ID:543107

Gauthier-Lafaye, F ^[1]; Holliger, P ^[2]; Blanc, P L ^[3]

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1. CNRS, Strasbourg (France)
2. CEA-Cadarache, Saint-Paul-les-Durance (France)
3. Institut de Protection et de Surete Nucleaire, Fontenay-aux-Roses (France)

Natural nuclear fission reactors are only known in two uranium deposits in the world, the Oklo and Bangombe deposits of the Franceville basin: Gabon. Since 1982, five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative European program. New geological, mineralogical, and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2.0 Ga years. The Franceville basin and the uranium deposits remained geologically stable over a long period of time. Therefore, the sites of Oklo and Bangombe are well preserved. For the reactors, two main periods of actinide and radionuclides migration have been observed: during the criticality, under P-T conditions of 300 bars and 400-500{degrees}C, respectively, and during a distention event which affected the Franceville basin 800 to 900 Ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors. New isotopic analyses on uranium dioxides, clays, and phosphates allow us to determine their respective importance for the retention of fission products. The UO{sub 2} matrix appears to be efficient at retaining most actinides and fission products such as REEs, Y, and Zr but not the volatile fission products (Cd, Cs, Xe, and Kr) nor Rb, Sr, and Ba. Some fissiogenic elements such as Mo, Tc, Ru, Rh, Pd, and Te could have formed metallic and oxide inclusion in the UO{sub 2} matrix which are similar to those observed in artificial spent fuel. Clays and phosphate minerals also appear to have played a role in the retention of fissiogenic REEs and also of Pu. 82 refs., 21 figs., 12 tabs.

• OSTI ID: 543107
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 23 Vol. 60; ISSN 0016-7037; ISSN GCACAK
• Country of Publication: United States
• Language: English

Related Subjects

05 NUCLEAR FUELS
54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
CRITICALITY
GABON
NATURAL ANALOGUE
NATURAL URANIUM REACTORS
RADIOECOLOGICAL CONCENTRATION
RADIONUCLIDE MIGRATION
SEDIMENTARY ROCKS
STRATIGRAPHY
URANIUM
URANIUM DEPOSITS