Migration Behavior of Bentonite Colloids through a Fractured Rock
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1124-Q07-14
Migration Behavior of Bentonite Colloids through a Fractured Rock Yoshio Kuno and Hiroshi Sasamoto Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, Tokai-mura, Ibaraki, 319-1194, JAPAN
ABSTRACT Bentonite colloids released into groundwater from the buffer in a geological repository for high-level radioactive waste may influence the migration of radioactive elements in fractures of the host rock. In the present study, column experiments were carried out to investigate the migration behavior of bentonite colloids in artificially fractured granite. The objective of the study is to determine whether the colloids are filtered by the fracture, considering the effects of the fracture length and the ionic strength of transporting solutions. Results indicate that bentonite colloids are not filtered if the transporting solution is dilute (i.e., distilled water or 10-4 M NaCl solution). This is evidenced by the observed tendency for normalized colloid concentrations in the column effluent (C/C0) to rapidly approach 1. An initial increase in the breakthrough curves is also observed at early stages of migration experiments involving more concentrated solutions (10-3 M NaCl solution), but the curves later evolve to an approximate steady state with C/C0 < 1. The amount of filtered colloids tends to increase with increasing fracture length. These results suggest that bentonite colloids can be filtered by interactions between the colloids and fracture surfaces, and that these interactions are affected by the ionic strength of the transporting solution. This migration behavior is simulated by a model of colloid migration, taking into consideration the effects of colloid filtration by fracture surfaces. The information on the filtration of bentonite colloids would contribute to a more realistic assessment of colloid effects on radionuclide migration. INTRODUCTION Bentonite is planned for use as a buffer material in many geological repository concepts for the permanent disposal of high-level radioactive waste (HLW). The buffer is expected to swell into open fractures in the repository’s host rock, which raises the possibility that colloidal particles of bentonite could form at the intruding front and be migrated away from the repository in flowing groundwater [1]. Such clay colloids are known to be stable in solutions of low ionic strength [2,3], and to be strong sorbents for many radionuclides [4,5]. Colloidal migration of sorbed radionuclides could enhance the migration of these elements from a HLW repository. In order to investigate these colloid effects, in-situ experiments were carried out at the Grimsel Test Site in Switzerland [6]. The results indicate that the migration of bentonite colloids in fractured crystalline rock is not retarded if the groundwater flow rate is high (i.e., much higher than is expected in a deep underground repository for HLW). On the other hand, significant amounts of bentonite colloids have been observed to be filtered in similar laboratory experiments
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