放射性廢棄物最終處置場多重障壁系統中之緩衝回填材料，多以膨潤土與砂混合壓實而成，因膨潤土具有良好的吸水回脹與吸附核種之特性，可膨脹自癒材料間之孔隙與降低水力傳導係數，以減少核種藉由地下水擴散外滲之可能性。在最終處置場的運行期間，緩衝回填材料需克服外在環境的變遷，如地下水位之變化而可能使膨潤土材料由飽和狀轉為不飽和狀態，並對緩衝回填材料之性質產生影響。有鑑於此，本研究以日產膨潤土Kunigel-V1RW與石英砂依不同配比進行混合製成試體，進行單向度膨脹變形率、束制膨脹壓力與水力傳導度等試驗，並針對單向度膨脹變形率試驗與束制膨脹壓力試驗之試體遭受乾化-溼化循環作用後，其膨脹性能之變化進行探討。
試驗結果顯示，緩衝回填材料歷經第一次溼化與第二次溼化後，其膨脹性能皆產生明顯變化，膨脹性能變化率會隨著乾化階段結束時之含水量的增加而呈線性下降之趨勢，並發現當膨脹性能變化率為零時，所對應之含水量恰接近縮性限度。此現象產生之原因推估為，試體歷經乾化-溼化作用時，黏土顆粒間的組構會產生破壞與重組，使各溼化階段之膨脹性能產生相異之表現，當試體乾化含水量高於縮性限度時，顆粒排列傾向分散結構，使再濕化之膨脹性能變化率下降，而當試體乾化含水量低於縮性限度時，顆粒組成偏向更低度定向性之絮凝結構，使再濕化之膨脹性能變化率上升。

A mixture of bentonite and sand is considered as a buffer and backfill materials in the multi-barrier system of the radioactive waste disposal facility, because of their high swelling potential which enables to seal the pores within the materials and low hydraulic conductivity which reduces the possibility of nuclear extravasation by groundwater. During the operation of the disposal facility, the buffer and backfill materials need to overcome the changes of the environment, such as the migration of groundwater level which may transform the materials from saturated condition into unsaturated, and the properties of buffer and backfill materials will be affected.
A mixture of bentonite Kunigel-V1RW, produced in Japan, and quartz sand has been used in this study. The specimens were made from bentonite and quartz sand in various proportions. The laboratory tests including swelling deformation, swelling pressure, and hydraulic conductivity were performed. The influence of drying-wetting cycle on the swelling deformation and swelling pressure of specimens were examined.
The results showed that the buffer and backfill materials have a marked change on the swelling behavior under the action of drying and wetting cycle. The ratio of change of swelling performance after wetting will decrease linearly with the increment of water content at the end of the drying stage. In addition, when the ratio of change of swelling performance is zero, the corresponding water content is close to the shrinkage limit. The reason causing these results was surmised that the process of drying and wetting leads to the destruction and the reconstruction of the particle structure. When the drying water content of the specimen is higher than the shrinkage limit, the arrangement of particle tends to dispersed structure and makes the re-wetting swelling performance decrease, on the other hand, when the drying water content of the specimen is lower than the shrinkage limit, the arrangement of particle tends to flocculated structure with less orientation and makes the re-wetting swelling performance increase.

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