Mobility of Uranium, Thorium and Lanthanides Around the Bangombe Natural Reactor (Gabon)
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MOBILITY OF URANIUM, THORIUM AND LANTHANIDES AROUND THE BANGOMBE NATURAL NUCLEAR REACTOR (GABON) R. BROS*, F. GAUTHIER-LAFAYE**, P. LARQUE**, J. SAMUEL**, P. STILLE** * CEA, CEN Cadarache, DCC/DSD/SCS, Bat. 307, 13108 Saint Paul-lez-Durance, France
** CNRS, CGS, 1, rue Blessig, 67000 Strasbourg, France.
ABSTRACT New mineralogical and isotopic studies were carried out on samples from the Bangombe natural nuclear reactor. This reactor is located at shallow depth in the weathering profile and has been subjected to severe supergene alteration. Textural evidence indicates partial dissolution of uraninite in the Bangomb6 ore related to precipitation of Fe-Ti oxi-hydroxides and clay minerals (kaolinite and metahalloysite). As a consequence of the alteration of the uraninite, uranium and fissiogenic rare earth elements were released in the clayey border of the reactor, whereas radiogenic 232 Th remained confined in the close vicinity of the core. A retention effect is also evidenced, under reducing conditions, in the black shales located above the reactor. INTRODUCTION The behaviour of radioactive nuclides in geological environments is of great interest because of its relevance to the potential return of radioactive wastes to the biosphere. The long-
term behaviour of radionuclides which cannot be examined by laboratory experiments can be studied in existing natural analogues. The Olo uranium ore deposit in Gabon is a unique natural analogue because fission reactions spontaneously occurred 2 billion years ago and criticality was maintained over a period of 105-106 years [1,2]. This led to the production of high amounts of fission and neutron capture products which show isotopic anomalies (e.g. Zr, Mo, Ru, Ag, Sn, Te and lanthanides). Isotopic measurements allow characterization of the neutronic parameters of the reactors and define the geochemical behaviour of radioactive nuclides in the deposit [3,4,5,6,7]. The confinement of radionuclides in the ore depends mainly on several parameters such as the stability of the U02 matrix, the diffusion constants, the complexation in aqueous solutions, the pH and the redox conditions of the fluids, the adsorption by organic molecules (bitumen) and minerals (clays, oxydes, phosphates). In this paper, we present new results on the mineralogy and the isotope geochemistry of the Bangomb6 reactor. The Bangombe deposit is located about 20 km from the Oklo ore. Most importantly, it is located at shallow depth (10-12 m) in the weathering profile. This allows us to study the stability of a reactor that has been subjected to severe supergene alteration in a wet tropical climate. We stress three aspects, namely the alteration of the U02 matrix and that of the enclosing shales and sandstones, the migrations of U, Th and REE induced by this alteration, and the influence of redox conditions on the mobility of these nuclides.
Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
1188
SETTING AND MATERIAL At BangombC, the uranium occurs at the top of the FA Formation, which cons
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