| 摘要: | 對於放射性廢棄物最終處置場的設計，國內主要採用坑道處置及深地層處置概念，以防止放射性核種外洩造成生物與環境危害。然而，處置母岩本身與受力產生之裂隙，為放射性核種外釋的主要通道。由於地層中的裂隙可能受到壓溶作用(pressure dissolution)與自由面溶解作用(free surface dissolution)的影響，使得裂隙內寬(aperture)與滲透率(permeability)發生變化，進而影響核種傳輸特性。因此，瞭解裂隙內寬與滲透率受到外在條件影響而隨時間變化特性，有助於評估放射性核種外釋可能性，進而確保放射性廢棄物處置場之安全性。本研究採用岩體裂隙幾何運算方式，藉多重物理量耦合分析軟體(COMSOL Multiphysics®)建置二維水力-力學雙孔隙介質機制模型，將水-力數值模擬結果引入商業數學軟體(MATLAB®)中進行裂隙幾何變化計算，再將裂隙演變結果代回數值模式中進行迭代運算，以模擬岩體裂隙在受到壓溶作用與自由面溶解下，裂隙內寬隨時間變化的情形，並針對其結果計算等效水力傳導係數(equivalent hydraulic conductivity)以評估該區域之流體傳輸特性。接著，透過相對敏感度(relative sensitivity)之分析以了解各參數對於等效水力傳導係數影響程度。在本研究設定條件下，以每1小時為時間間隔進行1500小時之模擬中，分別以地下水(pH ≈ 8)及蒸餾水(pH ≈ 6)之水化學環境進行模擬。研究顯示，因裂隙接觸點受到較高的局部應力影響，導致壓溶作用較自由面溶解作用顯著，造成裂隙內寬逐漸減小。其中在地下水環境中，由於較低之溶解速率以至於裂隙內寬減小緩慢，等效水力傳導係數下降約1.3%；在蒸餾水環境中，因反應較前者顯著，故有較快的溶解速率，造成裂隙內寬快速減小，並且於模擬前期裂隙內寬快速下降後逐漸趨於平緩。其原因可能是裂隙系統中造成裂隙內寬減小與增大之壓溶作用，與自由面溶解作用影響相當而達成平衡所致，等效水力傳導係數下降約10.2%。參數敏感度分析結果顯示，總應力變化對等效水力傳導係數影響最大，其值變化不僅影響等效水力傳導係數之變化速率，更影響等效水力傳導係數達平衡時的最終值。最後，嘗試將裂隙幾何演變模式應用於現地場址特性，以評估硬頁岩裂隙在長期受壓溶作用與自由面溶解下流體傳輸情形。結果顯示，模型在進行時間為一百萬年之模擬後，等效水力傳導係數僅下降約0.2%。可能原因為：(1)現地硬頁岩裂隙在酸鹼值接近中性的環境下，受化學作用之影響不顯著；(2)由於單一裂隙內寬占整體岩體比例過於小，因此對等效水力傳導係數值影響不大。本研究採用雙孔隙介質水-力耦合模型，成功發展岩體單一裂隙在水化學作用下之演化數值模擬技術。然而，由於目前幾何模型相對簡化，仍有部分因素尚未完全考量。未來研究可考量不同岩體、化學環境、裂隙接觸面建置三維度裂隙幾何模型，並探討條件與參數之不確定性，以了解裂隙演化之行為，以利於將來應用到現地處置場的安全評估工作中。;For the final disposal of radioactive waste, Taiwan primarily adopts the concepts of tunnel disposal and deep geological disposal to prevent the release of radioactive nuclides. However, fractures in the bedrock caused by stress are main pathways for radionuclide migration. These fractures may be influenced by pressure dissolution and free surface dissolution, leading to changes in fracture aperture and permeability, thereby affecting radionuclide transport characteristics. This study employs the fracture geometry simulation method proposed in literature and utilizes the multiphysical coupling analysis software (COMSOL Multiphysics®) to establish a coupled two-dimensional hydro-mechanical model in dual-porosity media. Numerical simulation results for hydro-mechanical interactions are imported into the commercial mathematical software (MATLAB®) to calculate the geometric changes in fractures under stress. The results of fracture evolution are then iteratively fed back into the numerical model to simulate the temporal changes in fracture aperture under the combined effects of pressure dissolution and free surface dissolution. The equivalent hydraulic conductivity is calculated to evaluate the fluid flow characteristics of the rock mass. Subsequently, a relative sensitivity analysis was conducted to evaluate the influence of various parameters on the equivalent hydraulic conductivity (Keq). Under the conditions set in this study, simulations were carried out over a period of 1500 hours with an hourly time step, using two hydrochemical environments: groundwater (pH ≈ 8) and distilled water (pH ≈ 6). The results indicate that, because fracture asperity contacts experience higher local stress, pressure dissolution dominates over free surface dissolution, leading to a gradual reduction in fracture aperture. In the groundwater environment, the lower dissolution rate causes slow aperture reduction, and the Keq decreases in approximately 1.3%. In the distilled water environment, the reaction is more significant, producing a faster dissolution rate and rapid aperture reduction in the early stages of the simulation; this decrease then gradually levels off, likely due to a balance being reached between pressure dissolution and free surface dissolution effects, and the Keq decreases in approximately 10.2%. The results of the parameter sensitivity analysis show that stress-related parameters exert the most significant influence on the Keq. Variations in these parameters affect not only the rate of change in Keq but also the final equilibrium value it reaches. Finally, this study combines the indoor water flow test model setting with field parameters and applies the developed fracture geometry evolution model to the actual field to evaluate the fluid transport behavior of fractured argillite rock mass under long-term pressure dissolution and free surface dissolution. After simulating for one million years, the Keq decreased by about 0.2 %, which is attributed to two factors: 1) under near-neutral pH conditions, chemical dissolution effects for argillite fractures are not significant; 2) the fracture aperture occupies a small fraction of the overall rock volume, its reduction has minimal impact on the Keq. This study employed a dual-porosity hydro-mechanical coupling model under the influence of hydrochemical processes to successfully develop a numerical simulation technique for single-fracture evolution in rock. However, due to the relatively simplified geometric model, some factors have not been fully considered. Future research can consider different rock masses, chemical environments, and fracture contact surfaces to build a three-dimensional fracture geometry model, and explore the uncertainty of conditions and parameters to understand the behavior of fracture evolution, so as to facilitate future applications in safety assessment work at on-site disposal sites. |
