Simulation of Transport of Uranium (VI) Species Through the Bed of Bentonite
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K.STAMBERG, D.VOPALKA, J.SKRKAL, P.BENES, K.CHALUPSKA Czech Technical University, Department of Nuclear Chemistry, Bfehovfi 7, 11519 Prague Czech Republic, [email protected] ABSTRACT The transport of six U(VI) species, HCO 3'- and CO3,2 through the bentonite bed was modelled as diffusion in pore water combined with sorption/desorption on the surface of bentonite particles. The studied system consisted of the Czech commercial sodium bentonite SABENYL and synthetic granitic water spiked with . 3.U(VI), where equilibrium distribution of 233U was experimentally determined at first. The transport was simulated by means of eight dynamic, I D- partial differential equations while the equilibrium sorption/desorption was assumed. Three types of surface complexation models, namely the Constant Capacitance Model (CCM), Diffuse Double Layer Model (DLM) and the so-called Chemical Equilibrium Model (CEM), were used for the description of sorption/desorption interaction processes. The characteristic parameters of the individual models were obtained from evaluation of experimental data. The results of the simulation were expressed as path-length dependent concentrations of total U(VI), total carbonates and of individual migration species, as path-length dependent pH and uranium K, values at a given time and as time dependent concentrations of the total U(VI) at a given migration distance. It has been found that the transport of U(VI) through the bentonite bed is significantly influenced first of all by the value of pH and by total carbonates concentration. The important length dependent changes of the pH and Kd values were determined. INTRODUCTION
Computer simulations of the radionuclide transport processes in barrier system, consisting of contaminated liquid phase and solid barrier material, are used for the prediction of the behaviour and properties of such system. The transport is usually modelled by means of a dynamic, one-
dimensional (ID) partial differential equation comprising three or four types of processes, namely the diffusion, advection, interaction with the solid phase and radioactive decay. The interaction,
i.e. sorption and desorption, plays an important role in retarding radionuclide migration. Therefore it is necessary to describe the interaction as well as possible. In safety analyses of the repositories and other practical applications, sorption/desorption is often described by the linear isotherm (so-called Kdmodel) where equilibrium distribution of radionuclide is expressed by distribution coefficient, Kd, which is considered to be constant. However, in reality the equilibrium distribution depends on many factors, e.g., on pH, concentration of ligands, number of migrating radionuclides etc. For many systems and conditions the equilibrium isotherm is nonlinear, i.e. the Kd model is invalid. At present, the so-called surface complexation models (SCM) are considered as the best type of equilibrium models, by means of which it is possible to describe the influence of the sorption interactions of the ind
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