Capability of KD model to predict radionuclides behaviour and transport in unsaturated column under steady flow conditions

In the frame of the IRSN Chernobyl Pilot Site Project, a waste trench containing hot particles was instrumented to study the fate of radioactive pollutants. Results have shown the formation of a 90Sr plume in the downstream aquifer, mainly present in the eolian layer after 15 years. Laboratories experiments were performed to analyze the transport of 90Sr and 137Cs in a eolian sand both in saturated and unsaturated conditions. Batch experiments were performed to study the chemical equilibrium state of the soil/solution system under static conditions. Stirred flow-through reactor experiments were performed to study the kinetics and reversibility of sorption reactions at the surface of solid particles under dynamic conditions. Column experiments in the structured porous media were also performed in saturated and unsaturated flow conditions. Experimental data pointed out a nonlinear, instantaneous and reversible sorption process of 90Sr. 137Cs exhibits a nonlinear behavior with a partial irreversible sorption process. A suitable cation-exchange model was used to describe the solute/soil reactions. The model described successfully the behavior of 90Sr and 137Cs in both saturated and unsaturated flow conditions in the range of water content studied. From these observations, propositions can be made to explain the discrepancies observed between the data obtained from static (batch) and dynamic (reactor and column) experiments. According to model calculation, effective Kd type approach to describe both radionuclides transport properties is discussed. (author)