Illite in the Oklo Natural Fission Reactors in Gabon: Considerations for Cs Containment
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ILLITE IN THE OKLO NATURAL FISSION REACTORS IN GABON: CONSIDERATIONS FOR Cs CONTAINMENT
1 1 NAGY 2, GYULA SZAB6 , JUDIT GUCZI , BARTHOLOMEW 4 3 EWING C. RODNEY AND JANUSZ JANECZEK
1 Fredlric Joliot-Curie National Research Institute for Radiobiology and Radiohygiene, 1221 Budapest, Anna utca 5, H-1775 Budafok 1, P.O. Box 101, Hungary 2 Laboratory of Organic Geochemistry, Department of Geosciences, The University of Arizona, Tucson, Arizona 85721, USA 3 Uniwersytet Slaski, Wydzial Nauk o Ziemi, 41-200 Sosnowiec, UI. Bedzinska 60, Poland 4 Department of Earth and Planetary SciencesUniversity of New Mexico, Albuquerque, New Mexico 87131, USA
ABSTRACT
The -2 Ga old Oklo, Okelobondo and Bangomb6 natural reactors in the Republic of Gabon contain solid graphitic bitumens and clay minerals, both of which have effected the containment, or partial containment, of 235U and several fission products. In laboratory experiments, sorption of 134 Cs by illite, and illite coated with petroleum was measured in aqueous NaCI solutions to simulate subsurface (connate) waters in sedimentary rocks. Elevated temperatures and increasing salinity of the NaCI solutions facilitated the removal of sorbed cesium from illite. INTRODUCTION
Small uranium-rich pockets in the uranium ore deposits at Oklo, Okelobondo and Bangomb6 in Gabon became natural nuclear reactors 1968 ± 50 Ma ago [1]. They maintained criticality for 105-106 years, with water acting as a moderator. The seventeen known natural
fission reactors at Oklo, Okelobondo and Bangomb6 are found in small pockets of hydrotherrnally altered sandstones that contain abundant uraninite and authigenic clays, mainly illite and
Mg, Mg-Fe and Mg-Al chlorites. Eleven of the natural reactors also contain abundant solid, graphitic bitumen in heterogeneous textures. Uraninites enclosed in and protected by the hydrophobic bitumen contain several fission products which could not effectively migrate through the bitumen [2]. Solid, graphitic bitumen may act as a physical barrier to radionuclide loss. Inclusions of galena in clays at some distance from the reactor's core suggest that lead migrated further distances (by at least a meter) from uraninite embedded in clays than from uraninite enclosed in the solid bitumen. Still, clay minerals may act as chemical barriers to radionuclide migration. The sorption of cesium on minerals, soils and sedimentary rocks has been studied repeatedly, because 137Cs is a significant radioactive component of nuclear waste produced by the nuclear power industry. 137Cs is a hazardous radionuclide because it is highly soluble in water. Therefore it may enter the biosphere by transport in groundwater. The migration of cesium may be restricted at waste repositories by sorption on minerals, rocks and backfill materials. Studies of Cs sorption on minerals and rocks have been performed to predict Cs mobility in rocks surrounding radioactive waste storage sites [3,4,5,6,7]. However, few investigations have dealt with Cs sorption on individual minerals which are used for backfil
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