高放射性廢棄物深層地質處置緩衝材料之回脹行為研究; The study on swelling behavior of buffer material in a deep geological repository

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题名:	高放射性廢棄物深層地質處置緩衝材料之回脹行為研究;The study on swelling behavior of buffer material in a deep geological repository
作者:	陳文泉;Wen-Chuan Chen
贡献者:	土木工程研究所
关键词:	緩衝材料;深地層處置;回脹;膨潤土;buffer material;deep geological repository;swelling behavior;bentonit
日期:	2004-12-18
上传时间:	2009-09-18 17:14:46 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	高放射性廢棄物最終處置是核能發電國家關切的環保議題，目前各國都致力於深層地質處置系統的研究發展。本論文以深層地質處置系統中，緩衝材料在處置場近場環境回脹成效為研究軸心，以具潛力的台灣日興土與美國懷俄明州膨潤土（BH膨潤土）為試驗材料，探索膨潤土在深層地質處置場特殊環境下的回脹行為。試驗結果顯示日興土的主要黏土組成為鈣型鋁蒙脫石，材料成分中含高量鐵化合物，其性質與法國FoCa黏土相近。BH膨潤土則以鈉型蒙脫石為主要組成礦物。熱重分析結果顯示BH膨潤土比日興土具較佳的抗熱性。高放射性廢棄物處置後，其衰變熱與放射性會持續釋出而影響近場環境，而海水入侵也是瀕海處置場首要分析的重要情節。本研究模擬近場環境對緩衝材料的回脹性質影響，結果發現：（1）水熱溫度的提昇，將使日興土因熱擾動而使表面吸附水層減少，引發回脹量下降。而BH膨潤土則因滲透壓力增加而回脹量增加。（2）經高溫熱處理後的膨潤土，其可交換陽離子會移入黏土結構層間或緊密嵌在表面之上，補償結構負電荷，可交換陽離子容量隨之降低，回脹量也因而降低。（3）膨潤土在NaCl及CaCl2溶液系統中形成不同結構，在NaCl溶液系統中的結構趨向分散狀態，致使回脹行為與擴散雙層理論一致；在CaCl2溶液系統中的膨潤土則形成擬結晶結構，回脹行為無法適用擴散雙層理論。日興土的自由回脹量約為BH膨潤土的七分之一，藉由添加Na2CO3使其於溶液相中引發離子交換程序，可使日興土改質為鈉型膨潤土，提升其回脹量。由於離子交換的改質程序具遲滯性，故改質土的回脹歷時曲線呈現後期上升的曲線型態。經離子交換程序後的改質日興土，最大回脹量可提升3倍。改質土對輻射衰變熱的耐受性良好，對於200℃乾燥加熱與90℃的水熱環境條件下，都能發揮其回脹潛能。 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.
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