Geochemical Characterization of U Tailings (Bois Noirs Limouzat, France)

The uranium (U) mining site of Bois Noirs Limouzat located in the central France, produced 7,000 tons of U from 1959 to 1980. Tailings containing non extracted U and its radioactive progenies are currently stored under water in a basin extending over 18ha. To predict U (100-400 ppm) and ²²⁶Ra (10-70Bq.g^-1) mobility, a detailed chemical, mineralogical and radiological characterization of the phases is required. For this, tailings sampling was performed to collect undisturbed samples close of in situ conditions. Preliminary characterizations evidenced good correlation between facies (to link with the extraction mode), granulometry, U concentration and ²²⁶Ra activity. U and ²²⁶Ra are concentrated in the silty-clay facies mainly as inherited monazites and neoformation of uranyl phosphates known for their low solubility. Clay minerals and Fe-oxi-hydroxides are also encountered and may play an important role in radionuclide trapping by sorption processes. These preliminary results confirm the immobilization of U and ²²⁶Ra within the tailings, even though, additional experiments are needed to determine the extent of U and ²²⁶Ra sorbed on Fe-oxi-hydroxides and on clay minerals.