Influence of operational conditions on retardation parameters measured by diffusion experiment in compacted bentonite.
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1265-AA06-08
Influence of operational conditions on retardation parameters measured by diffusion experiment in compacted bentonite. Yasuo Ishii, Yoshimi Seida, Yukio Tachi and Hideki Yoshikawa Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1194, JAPAN ABSTRACT Influence of operation factors in diffusion test of compacted bentonite (such as agitation of test solution in the reservoir, feed rate of the test solution and mass transfer resistance in the filter) on the diffusion data was examined by reservoir depletion (RD) test method using Cs+. The influence of these factors on the diffusion data was also analyzed based on the mathematical sorption-diffusion model which considered the feed of test solution and mass transfer resistance in the filter as well. The reservoir depletion data showed some remarkable influences of these operational conditions, especially in the system with low ionic strength. Change in mass transfer resistance at filter-compacted bentonite due to the operational conditions was found to be potential factor which disturb the diffusion data. The influence was reduced in the system with high ionic strength of solution. INTRODUCTION Sorption and diffusion of radionuclides in deep geological environment are the key processes of radionuclide migration in the geological disposal of high level radioactive waste. To obtain reliable retardation parameters (effective diffusion coefficient, De and distribution coefficient, Kd) of radionuclides in the engineered barrier/compacted bentonite for the safety assessment, it is important to establish a reliable test method for the sorption and diffusion data aquisition. Reservoir Depletion (RD) test method, which is used as a general diffusion test method for the diffusion and sorption data acquisition in compacted clay system, has some potential negative factors that influence the diffusion and sorption data due to boundary effects. These factors will be caused by operational conditions such as agitation of reservoir solution, feed rate of test solution which will affect mass transfer resistance at solution-compacted clay boundary and change in boundary concentration under some certain conditions. Influence of various operational conditions on the retardation parameters should be evaluated in advance to keep the reliability of obtained data. However, it has not been well identified in compacted bentonite system. Thus, it is necessary to understand these influences on the retardation parameters on order to reduce uncertainties and to develop reliable test method. In the present study, influence of agitation of test solution in the reservoir, feed rate of the test solution and mass transfer resistance in the filters on the data acquisition in diffusion test method with compacted bentonite were carried out as a function of NaCl concentration in the solution. RD test method was applied to the evaluation of influence of these operational conditions using Cs+. The influence of these uncertain factors on the s
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