Development of evaluation for swelling and sealing characteristics of heated bentonite based buffer in high-temperature conditions.

核电站排放的高放射性废物具有很高的放射性水平，需要很长的时间才能变成对人类生存环境没有影响的废物。因此，高活度废弃物的长期安全储存和管理是非常困难的。国际上普遍认为地质处置是最好的处置方式。在日本，高活度核废料也储存了大约30-50年，并计划处置到地下300米以上的地质层。认为在地质处置时，采用工程屏障和天然屏障组成的多屏障系统，可实现高活性废物的长期安全隔离。该工程屏障由玻璃化废物、包层和缓冲材料组成。利用膨润土作为缓冲材料的材料具有溶胀自密封的特性，是很有前景的。另一方面，高活度废弃物通过放射性的衰变，产生了大量的放射性衰变热。缓冲材料内部由于放射性衰变热的作用而变得高温。地下处置高活性核废料后，缓冲材料的温度达到几十年来的最高水平。它随着时间的推移而逐渐减少。然而，这种高温状态预计将持续至少数百年。膨润土基缓冲材料的自密封性是通过膨胀来证明的，但由于膨胀前的放射性衰变热，其自密封性会降低。为了解决上述问题，本研究通过实验研究了几种热史对膨润土材料膨胀的影响。本研究采用A、B、C、e四种膨润土，热温度分别为60、90、110、130℃。本研究进行了膨胀压力实验和膨胀变形实验。结果表明，膨润土A、B、E的热历史对膨润土膨胀压力特性的影响较小。另外，当围应力为1000kPa时，热历史对膨胀变形性能的影响较小。围压1000kPa时，膨润土C的热历史对膨胀变形性能影响不大。高初始干密度条件下，膨润土C的热历史对膨胀压力特性影响不大。当围压为1000kPa，加热温度为60℃时，热历史对膨胀变形性能的影响较小。为探讨热历史对膨润土溶胀特性的影响机理，本研究对膨润土进行了实验前后的x射线衍射分析、围水化学分析、亚甲基蓝吸收测试和阳离子交换容量测试。从上述实验结果来看，A膨润土溶胀特性的降低可以认为是蒙脱土对阳离子吸附能力的降低。膨润土C溶胀特性的降低可以认为是蒙脱土含量的降低。在上述实验结果的基础上，本研究提出了“考虑热历史影响的膨胀行为评价方程”。在该方程中，认为蒙脱土的阳离子吸附能力随热历史的变化而降低。根据亚甲基蓝吸附量的测定结果，计算了阳离子交换量和阳离子交换容量。将上述方程的计算结果与实验结果进行了比较。结果表明，本文提出的方程是有效的。少

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 … More 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