Determination of the minimal sample thickness for diffusion experiments with strong sorbing tracers


Determination of the minimal sample thickness for diffusion experiments with strong sorbing tracers

Mibus, J.; Trepte, P.; Brendler, V.

The long-term risk assessment of underground repositories for radioactive waste requires among others diffusion coefficients of actinides in bentonite to account for the steady state diffusion process after exhaustion of the limited sorption capacity of the bentonite sealing. The determination of effective diffusion parameters of strong sorbing tracers such as actinides in laboratory diffusion experiments is very time consuming. For that, the sample thickness has to be reduced to a minimum. However, falling below a critical distance may lead to a distur-bance of the clay texture and to inadequate boundary conditions and thus, to a misinterpreta-tion of the diffusion data. Our work focuses on the dependence of the diffusion parameters of tritiated water (HTO) in bentonite on the sample thickness.
We conclude, that the clay plugs are mechanically stable and fulfill the hydraulic sealing function. From the relationship between sample thickness and De we see that the filter plates exert a significant influence on the boundary condition as soon as sample and filter plate are of the same thickness. Filter plates should be of about 1 to 2 mm thickness to resist the swell-ing pressure. Thus, a sample thickness in this dimension is possible to work with strong sorb-ing tracers minimizing the duration of the diffusion experiment and providing representative results.
Based on literature values for MX-80 pore water chemistry and sorption data of Uranium on montmorillonite a distribution coefficient KD was estimated. This approach utilized the sur-face complexation model and a respective data base (RES³T). Computations were performed with the FITEQL 3.2 code. The calculated KD’s range from 30 to 300 ml g-1. This enables a blind prediction of the accumulated diffused activity for different sample thickness. A time frame of at least one year will be necessary to reach steady state. This will be checked by re-spective long lasting diffusion experiments.

Keywords: Clay; Uranium; Diffusion; Sorption

  • Contribution to proceedings
    2nd International Meeting, Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, ANDRA, March 14 – 18, 2005, Tours, France
  • Poster
    2nd Int. Meeting, Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, 14.-18.03.2005, Tours, France

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