The study of scale-dependent diffusion coefficient of radionuclide transport in porous media

Abstract

Radionuclide behavior in disposal repositories is complex because of the spatial heterogeneity of porous media coupled flow-transport mechanisms and multiple chemical reaction processes The assessment of radioactive species behavior requires research on diffusion mechanism and backfill material characteristic with the aim to protect the leakage of nuclide to biosphere Thence buffer materials such as bentonite are vital for absorbing radionuclide leakage and retarding migration from radioactive waste canisters Discrepancies in the diffusion behavior of a non-sorbing tracer (HTO) and a reactive tracer (137Cs) in porous media have long been recognized but are not yet fully understood which hinders effective assessment of the capabilities of buffer materials This dissertation was dedicated to exploring and explaining the discrepancies in the transport behavior of non-sorbing and reactive tracers through laboratory experiments and an investigation of contributing mechanisms A novel parameter identification process based on an iterative and analytical method (PIPIAM) is proposed here to obtain the target parameters of bentonite using concentration data The results of PIPIAM and the graphical method are compared through an error analysis of concentration The results show that PIPIAM outperforms the graphical method in terms of the error reduction of the concentration According to the experimental observations and inverse results for a bentonite sample of the same thickness 137Cs has smaller apparent and less effective diffusion coefficients than those for HTO These discrepancies can be attributed to the viscoelectric effects atomic properties and chemical reactions In the case of bentonite samples with different thicknesses the apparent and effective diffusion coefficients show an increasing trend with bentonite thickness and the geometrical factor shows a decreasing trend According to the experimental data and inverse results the diffusion coefficients and geometrical factor are highly related to bentonite thickness Thus scale effects on transport parameters were proposed to explain the results which were attributed to the nonuniform distribution of the pore space in the bentonite sample The scale effect behavior of tracers was quantified through a regression analysis The results can be used to improve buffer designs for radionuclides This study is contributing to clarify the transport mechanisms related to radionuclide behavior in bentonite and the provides alternative method for acquiring transport parameters for the use in safety assessments of nuclear waste repositories The results can be used to improve buffer designs for radionuclides

Date of Award	2021
Original language	English
Supervisor	Kuo-Chin Hsu (Supervisor)