Mass Transfer & Porous Media (MTPM) - Andra

P/MTPM/11n t (mol)as received8 10 -6 from kaofrom Na-momfrom Na-iIl6 10 -64 10 -62 10 -600 2 4 6 8 10 12Time (h)Figure 1: Accumulated mass of Sr, n t , diffusedacross stainless steel filter discs as a function oftime. The filter discs were previously used indiffusion experiments with compacted clays(~1950 kg m -3 ) according to the legend.infiltrating the filter pores leading to a reducedgeometry for diffusion (viz. reduced constrictivity ortortuosity). The second interpretation is rathersupported by the experimental findings, because Namomand kao differ largely in their swellingbehaviour. Further it was observed that the increasein diffusive resistance of the filter was almostindependent on the clay thickness used in thepreceding diffusion experiment with clay, whichalso supports the second interpretation.D f values measured for the diffusion of HTO, 22 Na,Sr and Cs in fresh and used stainless steel filterdiscs correlated fairly well with the respectivemolecular diffusion coefficients in bulk water.Although such correlations are inherently associatedwith some uncertainties, they generally allow for areasonable estimate to be made for diffusants, forwhich no D f values are available. The presentcontribution further shows how the overalluncertainty of an effective diffusion coefficient incompacted clay may be assessed from theindividual sources of uncertainty. Such considerations are helpful to decide, whether, for a givenexperimental setup, the uncertainty of D f may be critical for the overall uncertainty of diffusion in clay. Ifthis sensitivity is relatively small, it is sufficient to rely on correlations with bulk water diffusioncoefficients, which are readily available in the literature (e.g. Li and Gregory, 1974). Time-consumingmeasurements of the diffusive resistance of the filter discs may thereby be avoided.References:Glaus, M.A., Baeyens, B., Bradbury, M.H., Jakob, A., Van Loon, L.R., Yaroshchuk, A. (2007). Diffusionof 22 Na and 85 Sr in montmorillonite: Evidence of interlayer diffusion being the dominant pathway athigh compaction. Environ. Sci. Technol. 41: 478–485.Li, Y.H., Gregory, S. (1974). Diffusion of ions in sea water and in deep-sea sediments. Geochim.Cosmochim. Acta 38: 703–714.Page 444INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCECLAYS IN NATURAL & ENGINEERED BARRIERSFOR RADIOACTIVE WASTE CONFINEMENT