Effects of Calcium Ion in Highly Alkaline Plume on Permeability Change of Flow-Path
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Effects of Calcium Ion in Highly Alkaline Plume on Permeability Change of Flow-Path Hideo Usui, Yuichi Niibori, Koichi Tanaka, Osamu Tochiyama, Hitoshi Mimura Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, JAPAN, [email protected] ABSTRACT For the construction of the geological disposal system of radioactive wastes, a large amount of cement is used. Such a material alters the pH of groundwater to highly alkaline region [1-3]. The highly alkaline plume contains rich Ca ion compared to the surrounding environment, and the Ca ions reacts with soluble silicic acid. Its product would deposit on the surface of flow-paths in the natural barrier and decrease the permeability. In this study, the influence of Ca ion in highly alkaline plume on flow-paths has been examined by using packed bed column. The column was packed with the amorphous silica particles of 75-150 µm in diameter. The Ca(OH)2 solution (0.78 mM, 4.37 mM, and 8.48 mM, pH:12.2~12.3) was continuously injected into the column at constant flow rate (1.1 mm/s), and the change of permeability was continuously monitored. At the same time, [Ca]total and [Si] in the eluted solution was measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES). The Ca(OH)2 solutions were prepared with CO2-free pure water, and filtrated through 0.22 µm and 0.45 µm filter. The permeability was normalized by the initial permeability value. The experimental results showed that the permeability change and the chemical reactions in the packed bed depended on the concentration of Ca(OH)2 in the feed solution. The SEM images showed that the feed solution of 8.48 mM in [Ca]total apparently altered the particle surface with the products with Ca (deposition). Moreover, from the distribution of Ca on the cross section of packed particle (by EPMA), it was found that Ca deposited also in the inner pores of the particle. These results suggest that the deposition does not always change the permeability in the early stage of precipitation reaction. In the eluted solution, the concentrations of Ca and Si dramatically changed, depending on the concentration of Ca in the feed solution. INTRODUCTION Cement used for the construction of geological repository system of radioactive wastes would alter the groundwater to highly alkaline condition and high calcium concentration. Such an altered groundwater is called “highly alkaline plume”. The highly alkaline plume may affect
the chemical and physical properties of rocks surrounding the repository. Since the surrounding rocks play an important role to delay the migration rate of radionuclides released from the repository in the future, it is a key issue to predict the effect of alkaline plume on the solid surface. In other words, the rocks exposed to the highly alkaline plume may change its nature such as permeability and the radionuclide-adsorption property, and as a result, the retardation effect may decrease. The surrounding rocks, whether they are
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