| dc.description.abstract | Both concrete and backfill serve as engineered barriers for isolation of low-level radioactive wastes in a repository for radioactive wastes. As the disposal site is expected to serve a very long time, the interactions between the two barriers need to be evaluated under the potential unsaturated/saturated situations. This study aims at simulating the long-term scenario of engineered barrier materials and the corresponding effects of the scenario on the expected function of barrier materials in a final disposal site for low-level radioactive wastes.
In this research, a migration technique was applied to accelerate the migration of calcium ions from the pore solution of concrete so as to investigate the alteration of compacted bentonite in contact with the concrete. The backfill material was prepared by mixing 30-50% of Black Hills bentonite from Wyoming with 50-70% of Taitung area argillite to produce different ratios of sand-bentonite mixture as backfill. And the barrier concrete is a low-pH concrete having a binder comprised of 60% cement and 40% silica fume. After the migration test, backfill material was tested for swelling pressure to evaluate the effects of long-term contact. Finally, concrete barrier and backfill were both subjected to extended unsaturated situation as well as saturated situation for further evaluation. The backfill was treated with groundwater permeated through the concrete after the migration test so as to simulate the sequence of unsaturated and saturated scenarios that are expected to be experienced. Also, concrete barrier and backfill were both subjected to extended unsaturated situation as well as saturated situation for further evaluation. The backfill was treated with groundwater chemistry, pH environment after the migration test so as to simulate the sequence of unsaturated and saturated scenarios that are expected to be experienced. Results from the study show:(1) It was found from the accelerated migration test that the release of calcium from concrete in unsaturated situation results in reduction of swelling capacity of the contacting buffer. The shorter the distance to the interface, the more the increases in the ratio of calcium to sodium content in the backfill material, and the swelling pressure decreased as well. (2) in NaCl and CaCl2 solution, the swelling pressure of bentonite decreases with increasing cation concentration, and the effect of NaCl on swelling pressure is more pronounced; (3) at solutions with pH < 13, no significant reduction on swelling pressure was observed; (4) as illustrated by ICP analysis, backfill in NaOH [1.0M] solution shows decreases in montmorillonite and quartz minerals, while the Si4+ ions exhibits obvious increase.
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