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 diffusivities obtained for Cs and Sr were very high − 9.0×10−13 m2/s and 2.6×10−12 m2/s, considering that if the effect of sorption on the clay is substracted the diffusivities would be 1.8×10−9 m2/s and 3.1×10−8 m2/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 to be anionic and thus poorly sorbed, a diffusivity of 5.3×10−11 m2/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.
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Torstenfelt, B., Andersson, K., Kipatsi, H. et al. Diffusion Measurements in Compacted Bentonite. MRS Online Proceedings Library 6, 295–302 (1981). https://doi.org/10.1557/PROC-6-295