Apparent and Simple Diffusion Coefficients in Compacted Bentonite

by J. L. Conca, Washington State Univ Tri-Cities, Richland, United States,
T. Ashida, Washington State Univ Tri-Cities, Richland, United States,
H. Sato, Washington State Univ Tri-Cities, Richland, United States,



Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1991

Abstract: Diffusion coefficients were experimentally determined in bentonite to evaluate the effectiveness of bentonite layers as a diffusion barrier to ionic transport away from high-level radioactive waste packages. The electrical conductivity was measured using a 1 kHz conductivity bridge on bentonite-NaCL solution samples packed into conductivity cells. The diffusion coefficient was calculated from the electrical conductivity value using the Nernst-Einstein equation. These results were compared to apparent diffusion coefficients obtained using long-term, tracer diffusion tests in the same compacted bentonite with 3H, 129I, 99Tc, 241Am, 90Sr, 137Cs and 237Np. As expected, the retardation effects were much lower than predicted from batch tests, and the retardation behavior of all species changed with increasing compaction density. 90Sr, 137Cs and 237Np behave especially non-linearly at high compaction densities.

Subject Headings: Diffusion (porous media) | Bentonite | Diffusion (chemical) | Radioactive wastes | Electrical conductivity | Compaction (material) | Recycling | Bridges

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