Fundamental Experiments on Permeability Change of Flow-path by Highly Alkaline Plume

In the geological disposal system, natural barrier contains many selective flow-paths. Since cement used for the repository construction alters the condition of groundwater to a highly alkaline pH of about 13, such hyperalkaline plume would affect permeabilities of the flow-paths. To obtain more reliable estimate on the migration of radionuclides released from the repository, we must consider the changes in flow-paths with time and/or in space. In this study, the influence of highly alkaline plume on the permeability has been examined, considering also the direction of flow. In order to simulate the flow-paths, the amorphous silica particles were packed in the column, and the NaOH solution (0.1 M) was injected continuously at a constant flow-rate into the column at room temperature. The change in the permeability was traced, and the concentration of silicic acid in the eluted solution was measured by using the silicomolybdenum-yellow method. It was confirmed that the difference of pH values at the inlet and outlet of the column was negligibly small (pH=13.0). The experimental results showed that the change in fraction dissolved with time strongly depended on a flow-rate and a flow-direction. However, in the relation between the permeability and the fraction dissolved, the permeability did not change in the range of up to 0.35 in fraction dissolved. The SEM images of particle surface showed that the inner pores of particle increased in size. This suggested that, in this range of fraction dissolved, the porosity between particles is almost retained, while each particle dissolves mainly through its inner pores. Moreover, the dissolution rate in the column flow system was considered as being remarkably limited by diffusion process, in comparison with that estimated from the batch test.