Diffusion Measurements in Compacted Bentonite
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DIFFUSION MEASUREMENTS IN COMPACTED BENTONITE B. TORSTENFELT, K. ANDERSSON, H. KIPATSI, B. ALLARD, and U. OLOFSSON Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96, G6teborg, Sweden ABSTRACT The diffusion of Cs, Sr, Am and Tc in compacted bentonite clay has been studied, using a diffusion cell where diffusion takes place axially from the center of a cylinder of the clay. The effect of addition of potential radionuclide getters to the clay (apatite for Am, pyrite, olivine, and Fe(s) for Tc) as well as using complexed (acetate) or colloidal Am has been studied. The fff sivities obtai?2d lor Cs and Sr were very high 9.0xlO m /s and 2.6xi0m /s, considering that if the effect of sorption on the clay 9 is 2 substracted th 2 diffusivities would be 1.8x10- m /s and 3.1xi0- m /s. These diffusivities are, however, maximum values as the concentration profiles do not follow the theoretical form for these nuclides. A small amount of the ions seems to migrate very fast, while the rest moves slower. For Tc, that may be expected I b2 anionic and thus poorly sorbed, a diffusivity of 5.3xi0m /s was obtained. Addition of Fe(s) decreases the diffusivity by one order of magnitude while olivine and pyrite have no measurable influence during the present contact times (2-3 months). Americium is practically immobile. INTRODUCTION Compacted bentonite clay has been proposed as a suitable backfill material in underground storage of high-level waste [1]. Since this material would serve as a barrier preventing water flow through the repository, the only possible mechanism for radionuclide transport would be diffusion. The diffusion is expected to be slow, since the porosity of the clay is low and many radionuclides are strongly sorbed on the clay. The diffusion of Cs, Sr, Am and Tc has been studied, using a diffusion cell, where diffusion takes place axially from the center of a cylinder of compacted bentonite. EXPERIMENTAL Diffusion cell A diffusion cell for studies of radionuclide diffusion in compacted clay has been constructed, Figure 1. The pressing of the clay is performed in three stages: (1)a clay body of half the desired length is pressed in the cylinder (2) the radionuclide is put on the top surface (3) the rest of the clay body is pressed over the radionuclide Then the end surfaces of the cylinder are covered with metal frits and the whole cell is placed in a synthetic groundwater preequilibrated with the clay.
296 After sufficient contact time (2-3 months) the cell is opened and a cylindrical core of the clay is punched out. The core is cut in 0.2 mm thick slices. The radioactivity is determined in a Nal well-type detector (for Cs, Sr and Am) or a liquid scintillation counter (Tc). The distribution coefficient (K ) between clay and water was determined by a batch technique described elsewherg 12]. Studied systems The studied systems are given in Table I. TABLE I Nuclides and solid phases. Radionuclide 85Sr + 137
Solid phase + 241Am
85Sr + 137Cs 24 1 Am 24 1AmC) 24
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