| Summary: | 博士 === 國立中央大學 === 土木工程研究所 === 93 === Bentonites serve as buffer material in an engineered barrier system for isolation of high-level radioactive wastes because of their high swelling potential. In the current proposal for deep geological disposal of the high-level wastes in Taiwan, compacted bentonite is used to contain the metallic waste canisters and separate the waste from the host rock and backfilled materials. Two bentonite samples are used in this study, namely Zhisin clay and BH bentonite. Zhisin clay was identified as a Ca-bentonite, and BH bentonite is a Na-bentonite.
Several near field conditions including decay heat, radioactivity, and groundwater intrusion in a deep geological repository are simulated in this work. Free swelling tests were performed in accordance with the ISRM suggested procedures.
The maximum swelling strain of Zhisin clay decreases with increasing hydrothermal temperature. On the other hand, the maximum swelling strain of BH bentonite increases with increasing hydrothermal temperature. Lattice contraction and osmotic pressure enhancement are judged to be the dominating mechanisms affecting the swelling of Zhisin clay and BH bentonite, respectively.
Thermal treatment of clayey soils was found to result in marked decrease in CEC (cation exchange capacity). Reductions in CEC resulted from thermal treatment caused the swelling of bentonites to decrease.
The maximum swelling strain of bentonites decreases in salt solutions. The amount of reduction in swelling strain is affected by the type of electrolyte and the concentration of solution. The swelling behavior in a NaCl solution conforms to the diffuse double layer theory, so the swelling strain decreases with increasing concentration. Due to the formation of quasi-crystals in the presence of calcium ions, the swelling strains of bentonites in CaCl2 solutions, regardless of the concentration, are much lower.
To enhance the swelling potential of Zhisin clay, a cation exchange process by the addition of Na2CO3 powder is introduced in this research. Experimental results show that Na2CO3-activated Zhisin clay is superior in swelling potential to untreated Zhisin clay. Due to the ion exchange hysteresis, activated bentonite shows a different type of time-swell curve than the traditional sigmoid-shaped curve. The optimal amount of Na2CO3 addition was found to be 1%, and the maximum swelling strain was 3 times as much as that of untreated Zhisin clay. The Na2CO3-activated Zhisin clay exhibits improved resistance to thermal environments and behaves similar to the Na-type bentonites under different hydrothermal temperatures.
|
|---|
