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 | Radioactive wastes | Diffusion (chemical) | Compaction (material) | Electrical conductivity | Waste disposal

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