Analytical Model for Rapidly Simulating Migration of Radionuclide Decay Chain in a Fractured Rock and Its Application to Safety Assessment of Deep Geological Disposal

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.