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EFFECTS OF SUPERSATURATED SILICIC ACID CONCENTRATION ON DEPOSITION RATE AROUND GEOLOGICAL DISPOSAL SYSTEM

[+] Author and Article Information
Tsuyoshi Sasagawa

Dept. of Quantum Science & Energy Engineering Graduate School of Engineering, Tohoku University, Aramaki-Aza-Aoba 6-6-01-2, Aoba-ku, Sendai 980-8579, JAPAN
sasagawa314@michiru.qse.tohoku.ac.jp

Taiji Chida

Dept. of Quantum Science & Energy Engineering Graduate School of Engineering, Tohoku University, Aramaki-Aza-Aoba 6-6-01-2, Aoba-ku, Sendai 980-8579, JAPAN
taiji.chida@qse.tohoku.ac.jp

Yuichi Niibori

Dept. of Quantum Science & Energy Engineering Graduate School of Engineering, Tohoku University, Aramaki-Aza-Aoba 6-6-01-2, Aoba-ku, Sendai 980-8579, JAPAN
yuichi.niibori@qse.tohoku.ac.jp

1Corresponding author.

ASME doi:10.1115/1.4037163 History: Received September 05, 2015; Revised June 20, 2017

Abstract

Cementitious materials for the construction of a geological repository of radioactive waste alter the pH of groundwater to a highly alkaline condition (pH?13). While this alkaline groundwater dissolves silicate minerals, the soluble silicic acid polymerizes or deposits on the surface of rock with the decrease in pH by mixing with the surrounding groundwater (pH=8). In particular, the deposition of silicic acid leads to a clogging effect in flow-paths, which retards the migration of radionuclides. This study estimated the clogging of silicic acid in flow-paths with the one-dimensional advection-dispersion model considering the deposition rate constants evaluated in our previous study. As some of the most important parameters, these estimations focused on the initial supersaturated concentration of silicic acid and the density of deposited minerals. As a result, the aperture of flow-paths (initial width: 0.1 mm, flow-rate: 5 m/year, initial supersaturated concentration: 0.01, 0.1 and 1.0 mM) was almost clogged within about 200 years by the deposition of silicic acid. The period for the clogging became shorter under the conditions of higher initial supersaturated concentration and lower density of deposited minerals. In other words, the use of cementitious materials for constructing the repository might produce a retardation effect of radionuclide migration by the deposition/clogging processes of the supersaturated silicic acid.

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