Effects of supersaturated silicic acid concentration on deposition rate around geological disposal system

Cementitious materials for the construction of the geological repository of radioactive wastes alter the pH of groundwater to the 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). Especially, the deposition of silicic acid leads a clogging effect in flow-paths, which retards the migration of radionuclide. This study estimated the clogging of silicic acid in flow-paths with one-dimensional advection-dispersion model considering the deposition rate constants evaluated in the authors' previous study. In these estimations, the initial supersaturated concentration of silicic acid and the density of deposited minerals were focused as some of the most important parameters. 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 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 bring a retardation effect of radionuclide migration by the deposition/clogging processes of the supersaturated silicic acid.