Uranium retention in a Callovo-Oxfordian clay rock formation: From laboratory-based models to in natura conditions

For the performance assessment of radioactive waste disposal, it is critical to predict the mobility of radionuclides in the geological barrier that hosts it. A key challenge consists of assessing the transferability of current knowledge on the retention properties deduced from model systems to in natura situations. The case of the redoxsensitive element uranium in the Callovo-Oxfordian clay formation (COx) is presented herein. Extensive experimental work was carried out with respect to parameters affecting uranium speciation (pH, P-CO2, [Ca] and redox potential) with illite, COx clay fraction and raw COx claystone. The "bottom-up " approach implemented, with illite and montmorillonite as reactive phases, quantitatively explains the adsorption results of U(VI) and U (IV) on COx. While retention is high for U(IV) (Rd-10(4) L kg(-1)), it remains very low for U(VI) (Rd-4 L kg (-1)) due to the formation of soluble ternary Ca(Mg)-U(VI)-carbonate complexes. The applicability of the sorption model was then assessed by comparing predictive analyses with data characterizing the behavior of naturally-occurring U (< 3 mg kg(-1)). The COx clay phase is the largest reservoir of naturally-occurring U (-65%) but only a small fraction appears to be adsorbed (-1%). Under representative site conditions (especially with respect to reducing conditions), we have concluded that ternary U(VI) complexes control U speciation in solution while U(IV) surface species dominate U adsorption, with Rd values > 70 L kg(-1).