由於結晶岩體構造之穩定性、低滲透性以及具高度吸附放射性核種的能力，因此國際上普遍接受安置高階放射性核廢料於深層結晶岩體中是一個相當合適的選擇。由於處置設施可能發生放射性核種外釋，而進入裂隙系統中並隨地下水流動的遷移而到達生物圈，因此全面性的了解相關之基本機制對於放射性核種在裂隙介質層中傳輸之影響是非常重要的工作。 由於長期性的實際觀測核種從處置場外釋後進入到裂隙地質中的傳輸行為是很難在實驗示或現地進行，因此地下水傳輸模式結合現地場址的實驗、量測以及觀測提供了非常有效的方法來計算預測外釋到地下水中的核種放射性濃度。放射性核種會衰變成其他放射性產物或是相對穩定之物種分別稱為母物種及子物種，因此考慮放射性核種之衰變過程像是錒系和超鈾元素對於利用模式進行傳輸模擬就顯得特別重要。 本計畫提出一個在裂隙與母岩介質系統的放射性核種衰變鏈傳輸的解析解模式。所發展的解析解模式主要根據兩組互相耦合的傳輸方程式：分別是描述在裂隙中以及在母岩介質中的傳輸。考量的傳輸機制包含在裂隙中的移流、延散、放射性衰變反應、表面之吸附作用以及在母岩中的擴散、吸附作用。有關於處置場址外釋的放射性核種來源項，模式中將會考慮雙組成滲出(two-component leach)的來源模式。本研究將提出一求解耦合偏微分方程式之新方法並求得全解析解並驗證所發展之解析解模式的準確性。所發展之解析解模式將會用於了解傳輸過程或是機制對於放射性核種在裂隙岩層介質衰變鏈傳輸之影響。此外，也會探討不同位置之核種劑量隨著時間的變化以及探討對於公眾場所之潛在輻射影響。所發展的解析解模式預期成為核廢料深層地質處置場址安全評估之有效工具。 ;Disposal of high-level radioactive waste in deep crystalline rock formation is considered one of preferred options because its properties such as stability, low permeability and high capacity for sorbing radionuclides. A comprehensive understanding of the fundamental mechanisms affecting radionuclide transport in fractured rock formation is of paramount importance because that the radionuclides may escape from repository, move along with groundwater flowing mainly through the factures and could ultimately reach the biosphere. Groundwater transport analytical models are efficient tools for understanding of the fundamental mechanism affecting radionuclide transport in fractured porous media. Actual observation of long-term transport behavior of radionuclides in fractured geological media after their release from the waste disposal areas cannot be performed however. Groundwater transport models along with experiments, measurements and observations performed in the field provides efficient means to calculate the expected radioactivity concentration of radionuclides following their release into the groundwater. Radionuclides decay into other radioactive products or stable species, called daughter species or progeny. The chain decay processes of radionuclides are particularly important for modeling the transport of actinides and transuranics. This project is thus designed to develop a novel analytical model for studying problem of radionuclide decay chain during transport through a discrete fracture located in porous rock matrix. The analytical model is developed in terms of two coupled transport equations, one for the fracture and the other for the porous rock matrix. The processes include advection, dispersion radioactive decay and sorption on the surface of the fracture and diffusion, radioactive decay and adsorption in the microfissures of the host rock. A source model based on two-component model leach flux concept will be considered for the release of radionuclides from the repository. A strategy for solving coupled partial differential equations will be presented to obtain exact analytical solutions. The accuracy of the developed analytical model is evaluated. The developed analytical model will be applied to understand the transport processes or mechanisms affecting radionuclide transport in fractured porous media. Moreover, the time histories of the radiological doses at different locations are presented to understand the potential radiological impact on general public. The developed analytical model is expected to be efficient tool for safety assessment of nuclear waste deep geologic disposal.
