Consistency of the strontium transport parameters in Boom Clay obtained from different types of experiments: accounting
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Consistency of the strontium transport parameters in Boom Clay obtained from different types of experiments: accounting for the filter plates Marc Aertsens, Joan Govaerts, Norbert Maes, Liesbeth Van Laer Waste & Disposal Department, SCK•CEN, Boeretang 200, B-2400 Mol, Belgium ABSTRACT In a previous MRS paper, the consistency of migration parameters for strontium (Sr) in Boom Clay, obtained by different types of experiments, was examined. No consistent value could be obtained for the product KR of the diffusion accessible porosity K and the retardation factor R. Furthermore the nearly flat concentration profile measured in one of the through diffusion experiments could not be explained by the traditional through diffusion model. A reason is that the filter plates confining the clay sample have not been taken into account. Therefore, for Sr and tritiated water (HTO), the apparent diffusion coefficient and the product KR in the filters are measured in through diffusion experiments on filter plates. Taking into account the filter plates, the outlet fluxes and the Sr profiles in the clay of both Sr through diffusion experiments, are described well with (i) the previously estimated Sr apparent diffusion coefficient in the clay of 7 × 10-12 m2/s, (ii) an apparent filter diffusion coefficient in the range 2 × 10-12 m2/s to 5 × 10-11 m2/s (vs. 1 × 10-11 m2/s measured in the filter through diffusion experiments), (iii) a clay capacity factor KR in the range between 5000 and 22000, and (iv) a filter capacity factor between 0.3 and 0.6 (in agreement with the filter through diffusion measurements). However, using the above parameters, the evolution at the inlet could not be described. So although inconsistency diminished, some inconsistency remains. INTRODUCTION For performance assessment evaluations of Boom Clay as a reference host rock for geological disposal of radioactive waste, reliable values for the transport parameters of the radionuclides are needed. This requires that different types of experiments lead to similar values of the transport parameters. Consistency between the optimal values fitted from different types of experiments is obtained for tritiated water [1], dissolved silica [1] and cesium [2]. For strontium (Sr), an in-situ percolation experiment, electromigration experiments (using an electrical field as driving force), a lab percolation test and two through diffusion tests (on clay cores of different lengths), all lead to a robust value for the apparent diffusion coefficient of Sr between 5 10-12 m2/s and 1 10-11 m2/s [3]. No reliable value could be obtained for the product KR of the diffusion accessible porosity K and the retardation factor R. In one of the through diffusion experiments, a nearly flat concentration profile was measured [3], which cannot be explained by the traditional mathematical description of a through diffusion experiment. A reason for the discrepancy is that for cation transport through clays, the filter plates confining the sample have to be taken into account [4, 5]. Therefore separa
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