Characterization and modeling of groundwater flow and transport around the nuclear natural reactors at Oklo, Gabon

The purpose of the study of the Oklo Natural Analogue - Transport in the Reactor for field to identify and model current groundwater flow which may mobilize elements produced in the fossil fission reaction zones. The first important step is to understand the circulation patter, of the water involved in transporting the elements and then to describe the geochemical system by which it is governed. Hydrogeologic studies were carried out at two sites: a deep site at Okelobondo which offers an opportunity to study potential radionuclide migration from the reaction zone, over a distance of 400 m. Through very heterogeneous layers and a superficial site at Bangombe with a more homogeneous layering but situated in a area of meteoric alteration. The rationale and scope of performed field studies are presented together with results of hydrogeological characterization, conceptualizations (at different scales) and results of modelling of groundwater flow and transport. After an overview of the geology and hydrogeology which inspired our treatment of the flow in the near far fields of the reaction zones, the different mathematical models used are described. The Okelobondo site is discussed first. For this site we used a hydrodynamic model together with a model of the migration of natural environmental tracers. The results lead us to propose a flow pattern associated with transfer times between the reaction zone and the surface discharge zone. Then a similar approach is used for Bangombe. For this site, we added a geochemistry model describing the migration of uranium in the vicinity of the reaction zone using the METIS, CHIMERE and STELE software, developed in the MIRAGE programme. The isotopic ratio 235U/238U was used as a natural, original, tracer of the reactor zone. (author)