Papers by Author: Qing Chun Yang

Abstract: Safety assessment of nuclear waste disposal in a deep geological repository requires understanding and quantifying radionuclide transport through the hosting geological formation. Radionuclide diffusion is the main transport mechanism in clay formations since they usually have small hydraulic conductivities. Thus, understanding diffusion and determining diffusion parameters under real conditions is crucial for the performance assessment of a deep geological repository. In this paper, a comparative analysis is performed which focus on the dimensions of the packed-off section where tracers are injected and the packer between the intervals, diffusion of neutral (HTO), anionic (I) and sorbing cationic tracers with different distribution coefficients (²²Na and ⁸⁵Sr) has been simulated considering the anisotropy effect. The results indicate that The expected anisotropy has been clearly measurable for the sake of a short injection interval, in the final geometric configuration, the length of injection interval is larger than the transport distance, so the anisotropy effect is not as clearly measurable as in the preliminary because practically no tracer breakthrough from one interval to the other is expected if diffusion anisotropy is confirmed. The tracer depletion in the final design is larger than in the preliminary design.

Abstract: Deep geological disposal (DGD) is selected for the long-term confinement of high-level radioactive waste (HLW) by many countries. Safety assessment of nuclear waste disposal in a deep geological repository requires understanding and quantifying radionuclide behavior through the hosting geological formation. This paper presents a numerical model to deal with the pore water composition in bentonite barrier in the evolving geochemical environment which includes bentonite, concrete and clay in a high level radioactive waste repository designed in clay formation, the model considers the following processes: advection, diffusion, aqueous complexation, mineral dissolution/precipitation and cation exchange. Bentonite porosity changes caused by mineral dissolution/precipitation reactions are taken into account in the model. The numerical sensitivity analysis to the accessible porosity of bentonite are performed, the results illustrate that the pH in bentonite is sensitive to changes in accessible porosity of bentonite, the concentrations of aqueous species are sensitive to bentonite porosity before 1000 years, and the effective diffusion coefficient of bentonite controls the extent of high pH plume in bentonite.

Abstract: Safety assessment of nuclear waste disposal in a deep geological repository requires understanding and quantifying radionuclide transport through the hosting geological formation. Determining diffusion parameters under real conditions is necessary for the performance assessment of a deep geological repository where high level wastes are placed for safety disposal. The in situ diffusion and retention (DR) experiments are designed to study the transport and retention properties of the Opalinus clay formation. In this paper, a scoping numerical simulation is performed in Opalinus Clay, The simulated results for all the traces illustrate that the maximum transport distance perpendicular to the bedding is larger in the isotropic case and those along the bedding is larger in the anisotropic case. Tracer depletion in the isotropic case is a little larger than in the anisotropic case. Deuterium and iodide can be detected in the other interval but strontium can’t. Since the length of injection interval is shorter than the transport distance, the anisotropy effect is clearly measurable. This numerical simulation of diffusion experiment aims at contributing to the optimum design of the experiment. The results of this experiment will provide additional insight into the role of diffusion anisotropy and sorption parameters for radionuclides in clays.