Browsing by Subject "Distribution coefficient"

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  • Lehto, Jukka; Puukko, Esa; Lindberg, Antero; Voutilainen, Mikko (2019)
    The distribution coefficient (Kd) of radionuclides on bedrock is one of the key parameters used in the safety analysis of spent nuclear fuel repositories. Typically, distribution coefficients have been determined using crushed rock. However, recent studies have shown that crushing of the rock increases considerably the distribution coefficient compared with the values of intact rock. This study aimed to test if batch sorption experiments using different grain sizes (i.e. mean diameter of grains) can be used to evaluate the Kd of strontium (Sr) and cesium (Cs) on intact crystalline rock, which would decrease the needed experimental time compared with transport experiments. Here we report the results of the batch sorption experiments with crushed rocks and compare the results with those from a recent study performed using electromigration experiments with intact drill core samples (Puukko et al., 2018). The batch sorption experiments were done for rock samples from Olkiluoto, Finland, as a function of grain size and of Cs and Sr concentration. Furthermore, the specific surface areas of the same rock samples with different grain sizes were determined. It was shown that Cs distribution coefficients correlate with specific surface areas of the studied rocks and biotite, the correlation coefficient being 0.95. The Cs distribution coefficient was highest for biotite at about 0.1 m3/kg at 10−4 M cesium concentration and increased systematically to about 1 m3/kg at 10−8 M. Distribution coefficients for rocks were up to about two orders of magnitude lower, being lowest with the rock with the lowest biotite content (3.3%). The distribution coefficient of Sr varied from 0.04 m3/kg to 0.007 m3/kg and behaved in a different manner: it remained constant in two out of three studied rocks in the concentration range of 10−8-10−4 M and only in the case of one rock a decreasing trend was seen at the higher concentration range. It was also shown that batch sorption experiments overestimate the distribution coefficient in respect to intact rock. The decrease of the distribution coefficient as a function of grain size can be estimated using a power law function. It was also shown that estimation of distribution coefficients of Cs and Sr for intact rock by extrapolation of distribution coefficients determined for different grain sizes is not possible without increasing grain size, but in that case diffusion into the grains would also affect the results. A new method was developed for estimating the fraction of the inner surface area of the total surface area of crushed grains. For the mean grain sizes of 0.25 mm and 0.75 mm the fraction of the inner surface was found to be 35–70% and 60–90%, respectively. The inner specific surface area was highest with biotite at 1.2 m2/g and lowest with the rock with lowest biotite content (3.3%) at 0.07 m2/g. The surface area analysis revealed that crushing creates and/or allows access to additional inner surface area that is not measured in intact rock. Furthermore, it was demonstrated that sorption of Cs on crushed rock was dominated by mica minerals in multiple concentrations while the effect of mica minerals on the Kd of Sr was not as straightforward.
  • Puukko, Esa; Lehto, Jukka; Lindberg, Antero; Voutilainen, Mikko (2018)
    This study aims to determine upscaling factors for the radionuclides' distribution coefficients (Kd) on crushed rocks to intact rock for the safety analysis of radionuclide migration from spent nuclear fuel in bedrock towards biosphere. Here we report the distribution coefficients for intact rock determined by electromigration sorption experiments and compare the results with those from recently performed batch sorption experiments. In total 34 rock samples, representing three typical rock types from Olkiluoto Finland, were studied in order to determine distribution coefficients, effective diffusion coefficients and porosities using the electromigration sorption experiments, formation factor experiments and porosity measurement. The parameters determined represent the three main parameters of geosphere used in the safety assessment of spent nuclear fuel disposal. The distribution coefficients of cesium and strontium on intact rock varied between (0.12–26.2) × 10−3 m3/kg and (1.4–13.3) × 10−3 m3/kg, respectively, whereas recent results for crushed rock varied between (2–57) × 10−3 m3/kg and (17–40) × 10−3 m3/kg, respectively. This implies that crushing increases the distribution coefficient significantly and upscaling factors from 3 to 33 were determined for scaling the distribution coefficients of crushed rock to ones of intact rock. The determined distribution coefficients of cesium and strontium for intact rock can be directly applied in the safety assessment whereas the upscaling factors can be used to convert distribution coefficients determined for crushed rock into ones for intact rock. Based on the results for porosities and effective diffusion coefficients it was concluded that they do not seem to correlate with sorption parameters. However, an alteration state, heterogeneity and mineral content seem to be important factors affecting the distribution coefficients and upscaling factors.