國際間核能先進國家對於高放射性廢棄物處置，皆以多重障壁深地層處置的概念為主。深地層處置為高放射性廢棄物處置場建於距地表下數百公尺之穩定岩層中，而緩衝材料屬多重障壁包覆廢棄物之主要安全貢獻因子。由於處置場位於地表下數百公尺處，處置完成封閉以後，長期必然會受到地下水入侵之問題。緩衝材料之功能在維持廢棄物包件的完整性並延緩核種釋出的速率，故多採用高塑性之膨潤土在氣乾條件下壓製而成。本計畫擬利用未飽和土壤吸力之觀念，掌握地下水入侵時緩衝材料之反應，以便模擬處置場中緩衝材料之再飽和行為，並進行必要的安全性評估。影響緩衝材料再飽和行為的主要因素，為緩衝材之水力傳導度及處置場中的高溫環境條件。目前推求未飽和土壤之水力傳導度模式，所採用之試驗土壤多為一般工程或農業應用上常遭遇之土壤，將其套用於極高塑性膨潤土所製成之緩衝材料是否適用，目前尚未明確。故本研究亦將探討未飽和土壤之水力傳導度多種推估模式，應用於美國 Black Hills 膨潤土及國產日興土之適用性。由於土壤-水分特性曲線(Soil-Water Characteristic Curve, SWCC)可表示土壤中孔隙狀態及通透性，故多數未飽和土壤水力傳導度之推估模式，多於土壤-水分特性曲線之基礎下計算。本研究將採用較為穩定的含鹽溶液法控制封閉系統中的飽和蒸氣壓，以量測土壤於不同含水量下之吸力值，建立膨潤土之 SWCC，並利用不同模式，推估所用土壤之完整 SWCC，以探討各模式之適用性並檢視試驗值之精確度。此外，並將考慮處置場可能遭遇的高溫環境，量測不同溫度環境下膨潤土之土壤-水分特性曲線。然後，以所得不同溫度下之 SWCC 套用至不同未飽和水力傳導度推估模式中，以建立適用於推估未飽和膨潤土水力傳導度之適當模式。最後，將透過膨潤土試體攝取水實驗及飽和過程的溫度監測，對於緩衝材再飽和行為的推估模式加以驗證。 Clay-based materials serve as buffer material in an engineered barrier system for isolation of high-level radioactive wastes in a repository. The safety of a deep geological repository depends upon the stability of engineering barriers. Being a major component in the barrier system, buffer material is expected to create an impermeable zone around the high level waste canisters. Compacted bentonites have been considered by many countries as the prime candidate for buffer material due to their sealing capacity. This study investigates the unsaturated hydraulic conductivity and resaturation behavior of buffer material, with emphasis on the effects of soil suction on the groundwater intrusion processes. Groundwater intrusion is taken as an important scenario in the performance assessment of a geological repository. The resaturation of the buffer is considered a hydro-process occurring at elevated temperatures in the near-field of a repository. The hydraulic conductivity and temperature scenario to be exposed for buffer material are the major concerns in the resaturation processes of radwaste repository. Prediction models for hydraulic conductivity of unsaturated soils have been developed for typical engineering soils and/or agricultural soils. The suitability of these models for highly plastic bentonites remains uncertain. Soil-water characteristic curve (SWCC) provides an insight into the pore sizes and connectivity. Thus, most prediction models used were developed based on SWCC. Soil suction of clay specimens will be determined using salt solutions method on clay specimens so as to determine the soil water characteristic curves of Zhisin clay at different temperatures. Using the soil water characteristic curve, a few models will be introduced to estimate the hydraulic conductivity of unsaturated clay. This study will address the hydraulic and temperature features that might affect flow in the near-field of a waste repository. Two bentonites will be characterized for its hydraulic conductivity in unsaturated states using the relationships between soil suction and the degree of saturation of the clay. With emphasis on the unsaturated soil behavior and swelling characteristics of the bentonite, simulations of the resaturation processes of buffer material upon groundwater intrusion in the near-field will be carried out numerically in this study. 研究期間 : 9808 ~ 9907
