| Research Abstract |
High-level radioactive waste (HLW) exhausted from the nuclear plant has very high radioactivity level, so it will be required the very long term of years to become no influence to human's living environment. Therefore, it is very difficult to keep storing and to manage high activity waste for a long term safely. In the international society, it is considered that the geological disposal is the best. The high activity waste also in Japan is stored for about 30-50 years, and it is scheduled to dispose to geologic layer deeper than 300m in the underground. It is thought that the high activity waste can be safely isolated for a long term by adopting the multibarrier system that consists of the engineered barrier and the natural barrier when geological disposal. The engineered barrier is composed of the vitrified waste, overpack, and buffer material. The use of the bentonite that has the self sealing by the swelling for the material of the buffer material is promising. On the other hand, th
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e high activity waste evolves a large amount of radioactive decay heat by decay of the radioactivity. The inside of the buffer material becomes a high temperature by the radioactive decay heat. The temperature in the buffer material becomes the highest in tens of years after disposing the high activity waster in underground. It decreases gradually with the passage of time. However, the state of the high temperature has expected to continue for at least many hundreds of year. The-self-sealing expected of a bentonite-based buffer material is demonstrated by swelling However, the decrease in the self-sealing is afraid by the radioactive decay heat before the swelling.
To solve the problem mentioned above, this study investigated the influence of some thermal histories to swelling of bentonite materials by experimental works. This study used four kinds of bentonite named A, B, C, and E. The thermal temperatures were 60, and 90, 110, 130℃. This study conducted the swelling pressure experiment and the swelling deformation experiment. From the results, the influence of the thermal history of bentonite A, B, and E, was a little in the swelling pressure characteristics. In addition, the influence of the thermal history was a little in the swelling deformation property in case of confining stress 1000kPa. The influence of the thermal history of bentonite C was a little in the swelling deformation property in case of cofining stress 1000kPa. The influence of the thermal history of bentonite C was a little in the swelling pressure characteristics of condition with a high initial dry density. Moreover, the influence of the thermal history was a little in the swelling deformation property in case of cofining stress 1000kPa of the heating temperature 60℃.
To discuss the mechanism on influences of thermal history to swelling characteristics of the bentonite, this study conducted the X-ray diffraction analysis, surround water chemical analysis after experiment, and the methylene blue absorption tests and the cation exchange capacity of bentonite before and after experiments. From the above experimental results, the decrease in the swelling characteristics of bentonite A was considered as the decrease in the adsorption ability of the cation of montmorillonite. The decrease in the swelling characteristics of bentonite C was considered as the decrease of content of montmorillonite. Furthermore, this study proposed "Equations for evaluating the swelling behavior that considered the influence of the thermal history" on the basis of the above experimental results and discussions of mechanism. In this proposed equations, it was considered that the cation adsorption ability of montmorillonite decreased by the thermal history. The exchangeable cation amount and the cation exchange capacity were calculated based on the measurement result of the amount of the methylene blue adsorption. The calculated results by the above proposed equations were compared with the experimental results. From the results, the validity of proposed equations was clarified. Less
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