Diffusion of 60 Co, 137 Cs and 152 Eu in Opalinus Clay
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1124-Q07-06
Diffusion of 60Co, 137Cs and 152Eu in Opalinus Clay Miguel García-Gutiérrez1, José L. Cormenzana2, Tiziana Missana1, Manuel Mingarro1, Ursula Alonso1 1 CIEMAT, Departamento de Medioambiente, Av. Complutense 22, 28040 Madrid, SPAIN 2 Empresarios Agrupados, Magallanes 3, 28015 Madrid, SPAIN ABSTRACT This study addresses the diffusion of representative sorbing elements, cobalt, cesium and europium in the Opalinus Clay (OPA). The methodology used here to determine diffusion coefficients is the “instantaneous planar source” method. In this setup, a paper filter impregnated with tracer is introduced between two clay samples, avoiding contact between the tracer and the experimental vessels. The apparent diffusion coefficients (Da) perpendicular to the bedding plane, obtained with this experimental method and fitting the experimental results with an analytical solution, were Da(Co) = (2.4-3.5)·10-14 m2/s, Da(Cs) = (5.9-8.0)·10-14 m2/s, and Da(Eu) = (1.0-2.1)·10-15 m2/s. With cobalt and cesium, classical in-diffusion experiments were also performed for comparison, and similar Da values were obtained but with a large dispersion. To analyze the possible effects of the paper filter impregnated with the tracer on the determinations of Da with the analytical solution, one experiment was also analyzed using a detailed stochastic model of the setup. The good agreement between the two modeling approaches confirms the validity of this experimental setup and the analytical model fitting procedure. INTRODUCTION Clay formations are being considered as potential host rocks for radioactive waste disposal in many countries. In these materials, which have low hydraulic conductivity, diffusion is the main transport mechanism for radionuclides accidentally released from the canisters. The Mont Terri Underground Research Laboratory (URL) is located in a service gallery of the Mont Terri motorway tunnel, near St. Ursanne in north-western Switzerland, and is excavated in the Opalinus Clay (OPA). The OPA is currently under investigation to demonstrate the basic feasibility of disposing of spent fuel, vitrified high-level waste, and long-lived intermediate-level waste in Switzerland [1]. Diffusion is a process by which mass is transported from one part of a system to another by random molecular motion. The mathematical description of diffusion is based on Fick’s laws, which have to be modified for porous media such as clays, because several interactions between solid phases and solutes exist. The nature of the ions and the porous medium, as well as the specific geochemical conditions, are all elements influencing the diffusion process [2]. Several studies of diffusion behavior of neutral (HTO), anionic (Br-, I-, Cl-), and weakly sorbing cationic elements (Na+, Sr2+) on clay formations exist [3-5], but only a few studies are
available for medium sorbing elements such as Cs [6], and no studies for Eu, a highly sorbing element. Diffusion studies with sorbing elements are not straightforward to carry out, because it is necessary to take
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