PRELIMINARY STUDIES OF GROUNDWATER-FLOW AND MIGRATION OF URANIUM ISOTOPES AROUND THE OKLO NATURAL REACTORS (GABON)

In specific zones of the Oklo uranium deposit, criticality was natural ly reached 2 Ga ago. This site thus provides a unique opportunity to s how whether the stable nuclear reaction end-products have remained or not in the vicinity of the reactor zones after the termination of nucl ear reactions. In addition, the evaluation of the stability of the ura ninite matrix over very long periods of time provides information on t he possible long-term stability of waste forms such as spent fuel. The Commission of the European Communities initiated in 1991 the Oklo Nat ural Analogue Programme, a part of which is devoted to present-day mig ration studies. The Swedish Nuclear Fuel and Waste Management Company (SKB) supports this programme, with special interest in the Bangombe r eactor, a shallow reaction zone possibly affected by surficial alterat ion processes. The Oklo study comprises hydrogeology, groundwater chem istry, isotopic analyses (environmental isotopes, U series, U-235/U-23 8), and modelling. Two sites are being thoroughly investigated: the le ss perturbed OK84 reactor zone in Okelobondo (200 m south from Oklo) a nd the Bangombe reactor zone, 30 km south of Oklo. We focus our study on uranium migration from these reactor zones, using tracers such as t he U-235/U-238 isotope ratio. After preliminary field campaigns, a con ceptual model was constructed, both for Okelobondo and Bangombe. For t his purpose, groundwaters have been characterised for three years in d ifferent areas around Oklo: Okelobondo groundwaters in mines and boreh oles and surface waters, and Bangombe, both in boreholes and surface w aters. Detailed investigations were then conducted in order to validat e our conceptual models, and finally to enabling us to model U migrati on from the reaction zones, and to evaluate the performance assessment of deep geological disposal of radioactive wastes. After the presenta tion of regional and local geology and hydrogeology, we give a complet e description and interpretation of groundwater chemical evolution bot h in Okelobondo and Bangombe. It concerns major-element behaviour, red ox control and uranium behaviour. Preliminary conclusions for the conc eptual model drawn from hydrogeological studies and from groundwater c hemistry agree closely. In addition, we have successfully tested the u se of U-235/U-238 isotope ratios in groundwaters to evaluate the migra tion of uranium from reactor zones.