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Research Papers

Effects of Supersaturated Silicic Acid Concentration on Deposition Rate Around Geological Disposal System

[+] Author and Article Information
Tsuyoshi Sasagawa

Department of Quantum Science and Energy
Engineering,
Graduate School of Engineering,
Tohoku University,
Aramaki-Aza-Aoba 6-6-01-2, Aoba-ku,
Sendai 980-8579, Japan
e-mail: sasagawa314@michiru.qse.tohoku.ac.jp

Taiji Chida

Department of Quantum Science and
Energy Engineering,
Graduate School of Engineering,
Tohoku University,
Aramaki-Aza-Aoba 6-6-01-2,
Aoba-ku, Sendai 980-8579, Japan
e-mail: taiji.chida@qse.tohoku.ac.jp

Yuichi Niibori

Department of Quantum Science and
Energy Engineering,
Graduate School of Engineering,
Tohoku University,
Aramaki-Aza-Aoba 6-6-01-2, Aoba-ku,
Sendai 980-8579, Japan
e-mail: yuichi.niibori@qse.tohoku.ac.jp

1Corresponding author.

Manuscript received September 5, 2015; final manuscript received June 20, 2017; published online July 31, 2017. Assoc. Editor: Brian Ikeda.

ASME J of Nuclear Rad Sci 3(4), 041010 (Jul 31, 2017) (6 pages) Paper No: NERS-15-1189; doi: 10.1115/1.4037163 History: Received September 05, 2015; Revised June 20, 2017

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/y, initial supersaturated concentration of silicic acid: 0.01, 0.1 and 1.0 mM) was almost clogged within about 200 y 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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Figures

Grahic Jump Location
Fig. 1

One-dimensional parallel flat board

Grahic Jump Location
Fig. 2

Schematic of the one-dimensional mass balance

Grahic Jump Location
Fig. 3

Dependencies of initial supersaturated concentration on the deposition of silicic acid: (a) 2 mM, (b) 4 mM, (c) 6 mM, and (d) 8 mM, amount of solid phase: 1.0 g

Grahic Jump Location
Fig. 4

Calculation of deposition rate-constant k (m/s): (a) the calculation of rini (1/s) and (b) the relation of rini and specific surface area A (1/s))

Grahic Jump Location
Fig. 5

The clogging behavior of flow path: (a) α-quartz, k = 3.67 × 10−11 m/s, (b) α-quartz, k = 2.10 × 10−10 m/s, (c) amorphous silica, k = 3.67 × 10−11 m/s, (d) amorphous silica, k = 2.10 × 10−10 m/s, (e) silicic acid, k = 3.67 × 10−11 m/s, and (f) silicic acid, k = 2.10 × 10−10 m/s

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