URANIUM MIGRATION THROUGH INTACT SANDSTONE CORES

Uranium is often considered to be a mobile radioelement in the natural environment owing to its tendency to form stable complexes with a num ber of aqueous anions, particularly in oxidising milieu. A series of i nfiltration experiments were devised to investigate this migration beh aviour under rigidly controlled laboratory conditions. Intact cores of Permo-Triassic Clashach Sandstone were pre-equilibrated with syntheti c groundwater solutions and continuous flow-through of uranium monitor ed together with Ph and concentrations of other ions. Prior to perform ing each experiment a simulation was carried out using a one-dimension al coupled chemical transport code, encompassing a thermodynamic descr iption of the electrical double layer. These calculations together wit h electron microscopy indicated the potential role played by iron oxyh ydroxide grain coatings in retarding the uranium plume. Thus, a second series of experiments was initiated on pre-acidified cores from which all surface exposed iron had been removed, allowing an assessment of the retention capacity of non-ferric components. Taken together, the d ata clearly illustrate the strong affinity of aqueous uranium species for natural surfaces even under strongly oxidising conditions. The suc cess of the model in predicting a priori the dominant trends in uraniu m migration behaviour is encouraging and may aid in prioritising analy tical requirements for investigations in more complex geochemical situ ations than those studied here.