The effect of model mineralized solution on diffusion of 137Cs and 85Sr in the clay from “Markovskoe” deposit when used in near-surface radioactive waste disposal

 The migration of radionuclides ¹³⁷Cs and ⁹⁰Sr in a compacted clay sample from the “Markovskoye” deposit of the Gomel region depending on the mineralization of its pore solution has been studied by the in-diffusion method. Distilled water and model mineralized solution were added to clay; the mineralized solution was modeling the chemical composition of pore water of the clay sample in case of atmospheric precipitation permeation into the radioactive waste disposal site and its consecutive passage through the materials of the physical barriers: concrete – Na-bentonite – concrete. The apparent diffusion coefficients (D_a) of ¹³⁷Cs and ⁹⁰Sr in the clay sample were determined by the results of experiments: (1.5 ± 0.1) · 10^–12 m²/s for ¹³⁷Cs and (3.8 ± 0.2) · 10^–11 m²/s for ⁸⁵Sr in distilled water, (1.6 ± 0.1) · 10^–12 m²/s for ¹³⁷Cs and (6.3 ± 0.3) · 10^–11 m²/s for ⁸⁵Sr in mineralized solution. It was found that with increasing mineralization of the pore solution of the clay sample Da ⁹⁰Sr increases by 1,7 times, and D_a ¹³⁷Cs does not change within the limits of the error of the experiments. This change of ⁹⁰Sr diffusion coefficient in clay should be taken into account in case of its use as a part of the underlying screen of the disposal site of low- and medium-active waste of NPP, as it will increase the probability of ⁹⁰Sr migration beyond its limits. There is a necessity to come up with technological solutions to prevent or decrease water permeation into the radioactive waste disposal as well as increase the diffusion time by raising the thickness of the clay barrier. The results of the experiments in this study confirm that clay from the “Markovskoye” deposit of the Gomel region is promising for use as a compacted clay barrier in the radioactive waste disposal facility. 

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