Abstract
Cement-based materials used in the construction of the repository for high/low level radioactive wastes may produce a highly alkaline calcium-rich groundwater (plume). The Ca ions react with soluble silicic acid, depositing calcium-silicate-hydrate (CSH) gel on the surfaces of the groundwater flow-paths and decreasing the permeability of the bedrock. Such a decrement of permeability may play a role in retarding the migration of radionuclides. In this study, the deposition behavior in a fracture was experimentally examined by using a micro flow-cell consisting of silicon plate (including a slit (60 mm×5 mm, or 60 mm×2 mm)) and granite-chip. The initial equivalent-aperture based on the square law was estimated in the range of 26 µm to 45 µm from the flow test of pure water.
In the experiments, a Ca(OH)2 solution of 6.36 mM (pH: 12.2 to12.5, including NaOH) was continuously injected into the flow system at a constant flow rate of 1 or 2 ml/h. The solution flowed on the surface of the granite-chip. In this study, we prepared two kinds of chips that differed in the treatment of the surface. One chip was roughly ground with #2000 sandpaper (hereinafter referred to as rough surface) and another was polished to mirror-like surface. As a result, on the rough surface the deposits of CSH gel appeared along flow-channels across mineral grain-boundaries, while the deposits on the mirror-like surface were relatively uniform. Furthermore, the permeability in the case of rough surface became smaller than that in the case of mirror-like surface, showing the repeats of rapid decrement and increment due to the relatively large roughness of the surface. In order to estimate the decrement degrees of permeability, a simple, one-dimensional mathematical model is proposed in this study.
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Acknowledgments
This study was supported by Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B) No. 21360460. The authors are grateful to Mr. Okuyama (HITACHI Ltd) for his instructive support of the micro flow-cell and to the anonymous reviewers for their appropriate comments, which greatly improved this study.
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Niibori, Y., Komatsu, K. & Mimura, H. Deposition of Calcium-Silicate-Hydrate Gel on Rough Surface of Granite from Calcium-rich Highly Alkaline Plume. MRS Online Proceedings Library 1475, 349–354 (2012). https://doi.org/10.1557/opl.2012.598
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DOI: https://doi.org/10.1557/opl.2012.598