Deposition rate of supersaturated silicic acid on Na-type bentonite as a backfilled material in the geological disposal

When constructing geological disposal systems for radioactive waste, saline groundwater, originating from fractured rock matrices surrounding disposal tunnels, may reduce bentonite swelling properties and accelerate radionuclide migration. In contrast, high alkaline groundwater, caused by the use of cementitious materials, induces supersaturated conditions with regard to silicic acid. The resulting deposition of silicic acid may decrease the hydraulic conductivity of flow paths. Therefore, this study experimentally examined the apparent deposition rate constant, k (m/s), by considering the supersaturated concentration of silicic acid and deep-underground temperatures (288–323 K). As a result, the apparent deposition rate constants, obtained experimentally, were not affected by an initial supersaturated concentration and were only slightly affected by temperature. We estimated the apparent activation energy at 8.9 kJ/mol. Furthermore, because the Damköhler number, described by the ratio between the apparent deposition rate constant and the groundwater flow rate, had a large value, exceeding 10³, the deposition of supersaturated silicic acid may cause the narrowing flow paths. These results suggest that silicic acid deposition contributes to retardation effects via the decrease of the hydraulic conductivity in backfilled tunnels.