| 摘要: | 核能發電為各國主要電力來源之一，但由於核能發電所產生之用過核子燃料具有高放射性和具衰變熱 (heat decay)，而混凝土與緩衝材料介面之交互作用和未來受到地下水入侵，皆會造成內部結構及障壁功能的改變。目前國際間對於用過核子燃料之最終處置方式一致採「深地層處置」(Deep Geologic Disposal) 概念，並以工程障壁系統(Engineered Barrier System, EBS)阻止放射性核種遷移，主要受到四大因素影響，包含T熱學(Thermal)、H水力(Hydraulic)、M力(Mechanical)、C化學(Chemical)因素，稱為T-H-M-C耦合效應，將影響最終處置場之預期性能。
由於混凝土於高放處置場為封塞材料，長期受到地下水入侵，會使地下水中的鎂離子和混凝土中的鈣離子產生離子交換的情形，造成混凝土溶出鈣離子產生失鈣現象，而形成pH值較高的鹼性環境。研究結果顯示，當緩衝材料接觸pH值大於13之NaOH溶液，會使膨潤土回脹壓力下降，影響其安全性。
本研究以熱-水-力學耦合行為進行小型實驗以及數值模擬進行分析比對，以瞭解利用有限元素法模擬處置場緩衝材料之可行性。以加熱後進水之方式較符合處置場實際情形，及符合受鹼性環境影響緩衝材料回脹效果之耦合效應，使用pH值13之氫氧化鈉溶液進行小型耦合實驗，再以自來水做為入滲液體為小型耦合實驗對照組，皆以時域反射法(time domain reflectometry, TDR)做為實驗期間之含水量量測。而高塑性土壤並不適用於Topp et al.(1980)所建之一般土壤體積含水量計算方法，因此本研究建立膨潤土視介電常數-溫度-體積含水量三向圖供後續實驗計算，最後透過COMSOL有限元素程式模擬膨潤土受水分入侵之情形。由於三向圖換算體積含水量時主要假設乾密度固定狀態下，因此換算體積含水量產生誤差。本研究透過數值模擬考慮回脹應變設定和不同溫度下的吸力反應，了解緩衝材料於處置場中受到的物理變化，透過模擬乾密度結果配合試體拆卸後所量測得重量含水量，計算體積含水量與TDR量測結果比對，驗證數值模擬與TDR量測法於深地層處置應用之可行性。
;Nuclear power generation is one of the main power sources for many countries, but the spent nuclear fuel it produces is highly radioactive and has heat decay. Interactions at the interface of concrete and buffer materials, as well as future groundwater intrusion, can cause changes in the internal structure and barrier function. Currently, the international consensus on the final disposal method for spent nuclear fuel is the concept of "Deep Geologic Disposal", using an Engineered Barrier System (EBS) to prevent the migration of radioactive species. This is primarily influenced by four major factors: Thermal (T), Hydraulic (H), Mechanical (M), and Chemical (C), collectively referred to as the THMC coupling effect, which will affect the expected performance of the final disposal site.
Due to the use of concrete as a sealant in high-level waste disposal sites, will undergo ion exchange due to long-term groundwater intrusion, resulting in calcium ions in the concrete dissolving, causing decalcification and creating a high pH alkaline environment. Research results show that when buffer material comes in contact with NaOH solution with pH greater than 13, the swelling pressure of the bentonite decreases, impacting its safety.
This study conducts a small-scale experiment and numerical simulation analysis to understand the feasibility of using the finite element method to simulate the buffering material in the disposal field. It was found that introducing water post-heating closely mirrors the actual scenario in the disposal field and corresponds to the swelling effect of the buffering material affected by an alkaline environment. A small-scale coupled experiment was conducted using a sodium hydroxide solution with a pH value of 13, with tap water used as the infiltrating fluid for the control group. All experiments employed the time domain reflectometry (TDR) method to measure water content during the experiment.The general soil water content calculation method proposed by Topp et al. (1980) is not applicable to high-plasticity soils. Therefore, this study established a three-way diagram of bentonite dielectric constant-temperature-volume water content for subsequent experimental calculations. Finally, the situation of bentonite being invaded by moisture was simulated using the COMSOL finite element program. When converting the volume water content from the three-way diagram, the main assumption was a fixed dry density state, which could lead to conversion errors. This study considered swelling strain settings and the response of suction at different temperatures through numerical simulation, understanding the physical changes that the buffering material undergoes in the disposal field. By comparing the volume water content calculated from the dry density results obtained from disassembling the specimen with the water content measured by TDR, the applicability of numerical simulation and TDR measurement method in deep geological disposal was validated. |
