| dc.description.abstract | Both concrete and buffer serve as engineered barriers for isolation of low-level radioactive wastes in a repository. 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. The proposed study aims at simulating the long-term scenario of engineered barrier materials and the corresponding effects of the scenario on the expected function of the 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 buffer material, was made using Black Hills bentonite from Wyoming mixed with Taitung area argillite to produce different ratios of sand-bentonite mixture as buffer. And the barrier concrete mixes were proportioned according to traditional American Concrete Institute (ACI) mix design method and Reactive Powder Concrete (RPC) with steel fiber.
Also, concrete barrier and buffer are both subjected to extended unsaturated situation as well as saturated situation for further evaluation. High-low relative humidity cycling test was designed to investigate the extended unsaturated situation. Finally, the buffer was treated with water permeated through the concrete after the migration test so as to simulate the sequence of unsaturated and saturated scenarios to be experienced. And the buffer was then tested for the change accompanied by the saturation process.
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. And the shorter the distance to the interface, the more the increases in the ratio of calcium to sodium content of buffer material.
In case that the disposal vault remains unsaturated for extended periods, the absorption/loss of moisture causes changes in volume and weight of buffer material, which in turn result in some cracking and localized scaling off at the outer edge of compacted buffer materials. The use of crushed Taitung area argillite will decrease the change in volume and weight of buffer during the wet-dry cycling. However, due to the reactivity nature of Taitung area argillite, traces of alkali-aggregate reaction were noticed on surfaces of buffer mixed with crushed argillite as a result of long-term exposure to wet environment.
Upon saturation of the disposal vault, the leaching of hydroxyl ions from concrete to the buffer may cause (1) dissolution of montmorillonite, and (2) precipitation of mineral such as NaAlSi2O6. Both tend to decrease the swelling capacity of the buffer.
The addition of crushed Taitung argillite aggregate to bentonite showed less damaging effects, resulting from interactions between the contacting barriers, than pure bentonite on the buffer itself. Also, buffer material with 30% crushed aggregate can reduce the change in volume and weight of buffer subjected to high-low relative humidity cycling. On the other hand, the effect of leaching from reactive powder concrete (RPC) on the buffer is less detrimental than that from traditional concrete. Thus, RPC is considered more appropriate as barrier concrete for the final disposal of low-level radioactive wastes.
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