Abstract
Cembinder, Eka EXP36, and MEYCO MP320 are three colloidal silica materials that have been proposed for post-excavation grouting of deep tunnels in a radioactive waste repository. In this study, samples of these colloidal silicas were tested for their particle size distribution, gel induction time (tG), gel time (TG), and physical erosion, under mildly saline groundwater flow conditions. In order to achieve a desired gel time range, from 15 to 50 min, it is recommended that the colloidal silica is mixed with a NaCl accelerator at a 5:1 volume ratio. At 20 °C, the concentration range for the NaCl solution should be 1.5 to 1.7 M for MEYCO, 1.23 to 1.38 M for Eka EXP36, and 1.3 to 1.47 M for Cembinder. The physical erosion of the set silicas remained steady during a 10 h flow cell experiment, when grouts were subjected to 0.05 M NaCl at a superficial velocity of 2.2 × 10−5 m/s. For these test conditions, the results show that MEYCO has the highest average erosion rate (0.85 mg/h) of the three grout materials, as well as the greatest variability in this rate. Cembinder performed best with the lowest silica removal rate. Extrapolation of the measured erosion rates suggests that grout fracture dilation would not be significant under natural quiescent groundwater flow conditions, but would be high if there was hydraulic communication between the geosphere and the repository.
Highlights
High-level waste (HLW) and spent fuel (SF) are produced as part of nuclear power plant operation [1]
The physical erosion of the set silicas remained steady during a 10 h flow cell experiment, when grouts were subjected to 0.05 M NaCl at a superficial velocity of 2.2 × 10−5 m/s
Amongst the colloidal silica selection criteria published by the Chalmers University of Technology, is a recommendation that the gel time should be controlled, so that it lies between 15 and 50 min [5]
Summary
High-level waste (HLW) and spent fuel (SF) are produced as part of nuclear power plant operation [1]. Geological disposal sites for these wastes have been established across Europe, Asia, and North America. As part of the “Engineered Barrier System”, these wastes are disposed in tunnels excavated at approximately 650 m below the earth’s surface, and deep tunnel grouting may be required as a component of the engineered barrier system. Grouting is proposed as a post-excavation activity, i.e., after tunnel excavation. Grouts are injected under pressure, until fractures are effectively sealed, the permeability of the rock mass is reduced [2], and the seepage is within specification limits. Rock fractures vary in the opening width and lateral extent; different grouts have been selected, depending on their penetrative ability. It is not permitted to use normal
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