膨潤土在熱水力耦合下之緩衝回填材料性能

Abstract

目前各國多以地下處置場為放射性廢棄物的最終處置建議，地下處置場為利用天然障蔽與人工障蔽為主要設計概念，利用多重障蔽將放射性廢棄物固定於處置場中以確保其長期穩定性及避免汙染物外釋，其中緩衝回填材料作為多重障壁的一環主要目的為防止地下水的入滲並避免放射性核種滲漏，膨潤土為具有膨脹性能與較佳阻水性能的一種黏土礦物，其能夠吸附輻射物質，可被當作緩衝回填材料使用。

本研究針對壓密之膨潤土進行水熱環境與乾熱環境之膨脹性能試驗，以了解作為緩衝回填材料之膨潤土熱水力偶合作用下其材料性能的變化。 本研究使用日本進口的 KUNIGEL-V1 與美國進口的 MX-80 純膨潤土試體，在乾密度為1.6 g/cm^3下於去離子水中進行水熱環境與乾熱環境下之材料性能試驗。KUNIGEL-V1與 MX-80同為鈉型膨潤土，而M土有較高的蒙脫石含量。 從試驗結果，MX-80具有較好的膨脹指數、束制膨脹壓力與較低的水力傳導度，在水熱環境下，最大膨脹率與最大膨脹壓力與常溫下相比都有提升，對於緩衝回填材料而言是有益的。在乾熱環境下，105℃與300℃乾燥加熱後之試體並沒有表現出與常溫下之試體明顯的差異，而溫度提高到500℃時可以觀察出其膨脹性能已大幅度降低，水力傳導係數度大幅度的提升，不適合用於緩衝回填材料。 鈉型膨潤土所受溫度小於300℃時，膨脹性能受影響不大，受500℃的高溫烘乾後，其內部結構發生不可逆反應，不可作為緩衝回填材料使用，

Underground disposal facilities for the disposal of radioactive waste are recommended by many countries. The facilities primarily rely on natural and engineered barriers as the main design concept. Multiple barriers are used to control radioactive waste within the disposal site, ensuring long-term stability and preventing the release of contamination. Buffer and backfill materials are one of the components of the multiple barriers, aimed at preventing the infiltration of groundwater and the leakage of radioactive nuclides. Bentonite, a clay mineral with swelling properties and low water resistance, can adsorb radioactive substances and be used as buffer and backfill materials. This study conducted tests on compacted bentonite under hydrothermal and dry-heating environments to study the variation of its swelling properties as buffer and backfill materials under thermo-hydraulic coupling effect. The test specimens used were KUNIGEL-V1 from Japan and MX-80 from the United States. The tests on material properties were conducted in deionized water, with a dry density of 1.6g/cm^3. Both KUNIGEL-V1 and MX-80 are sodium-type bentonites, in which MX-80 possesses a higher montmorillonite content. According to the test results, MX-80 exhibited better swelling index, restrained swelling pressure, and lower hydraulic conductivity. Under the hydrothermal environment, both the maximum swelling ratio and maximum swelling pressure increased compared to those at room temperature conditions. This outcome sounds promising for buffer backfill materials. Under the dry-heating environment, the specimens stoved at 105℃ and 300℃ did not show significant differences comparing to room temperature specimens. However, stoving at 500℃, a significant reduction in swelling performance and a significant increase in hydraulic conductivity were observed, making it inappropriate to serve as buffer and backfill materials. For sodium-type bentonite, when the temperature is below 300℃, it exhibits reversible behavior with minimal impact on swelling performance. However, when the temperature goes as high as 500℃, it undergoes irreversible changes in its internal structure and cannot be used as buffer and backfill materials.