Diffusion of Sodium and Copper in Compacted Sodium Bentonite at Room Temperature
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DIFFUSION OF SODIUM AND COPPER IN COMPACTED SODIUM BENTONITE AT ROOM TEMPERATURE
Arto Muurinen, Markus Olin and Kari Uusheimo Technical Research Centre of Finland, Reactor Laboratory, Otakaari 3 A, SF02150 ESPOO, FINLAND ABSTRACT Diffusion studies were carried out with sodium and copper in sodium at room temperature and densities of bentonite. The experiments were performed the samples varied from 0.8 to 1.8 g/cm3 . This paper describes the experimental methods used for the diffusion tests and gives the obtained measurement results. The evaluation of the diffusion mechanisms is at this stage preliminary, however. Diffusion of sodium seems to follow similar mechanisms as has been observed for cesium and strontium in several previous studies. The phenomena measured could be explained by some kind of diffusion of 12sorbed ions.12 The to 300x10- m2/s and the from 50x10apparent diffusivities of sodium varied 2 9 2 effective diffusivities from 80xIO-1 to 1.6x10 - m /s depending on the density of bentonite and the salt concentration of water solution. Low solubility of copper caused precipitation thus interfering the diffusion experiments. However, the part of copper which did diffuse into as sodium. The measured bentonite seemed to follow the same type of mechanism apparent diffusivities of copper varied from 5x10]1z to 50xIO-1 2 m2/s. The interaction between the cations in the circulating solutions and the sodium ion in bentonite were essential in this study. INTRODUCTION Compacted sodium bentonite has been planned for buffer material for final disposal of spent fuel in many countries. For safety analysis the migration mechanisms of substances in bentonite and the corresponding parameters must be known. The hydraulic conductivity of compacted bentonite is so low that migration of substances occurs mainly by diffusion. However, different species The seem to follow different diffusion mechanisms [1,2,3,4,5,6,7]. diffusivities of different ions especially anions have been found to be strongly dependent also on the bulk density of bentonite and on the concentration of dissolved ions in the porewater. Future modelling of the transport of corrodants and canister corrosion products as well as radionuclides requires that different alterations between bentonite and the materials present in the repository are known. Also the effects on the microstructural pattern of bentonite and on the migration behavior have to be predicted. The cations are expected to have more significant interactions with bentonite than anions. The selected ions for the diffusion tests were sodium and copper. Sodium simulates the migration of a simple cation and the effects of strongly increased salinity of groundwater in the repository. Copper in high concentrations is expected to behave as calcium causing very limited swelling capacity in bentonite. EXPERIMENTAL ARRANGEMENTS The experimental method used in this study was a steady-state diffusion test, outlined in Fig. 1. Mat. Res. Soc. Symp. Proc. Vol. 17e. V1990 Materials Research Society
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