Abstract
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.
Highlights
In Finland, the spent nuclear fuel from the currently operating nuclear power reactors will be disposed of in a bedrock repository at a depth of about 400 m
We propose the upscaling factors for converting the distribution coefficients obtained by batch sorption experiments with crushed rock into ones for intact rock
There was no correlation found between the determined porosities and rock type that is typical for crystalline rock samples taken from the same site
Summary
In Finland, the spent nuclear fuel from the currently operating nuclear power reactors will be disposed of in a bedrock repository at a depth of about 400 m. In the final disposal of spent nuclear fuel in geological formations, such as bedrock in Finland, the rock above the waste acts as the last barrier against the release of radionuclides into the biosphere. This has been taken into account in the safety assessment of the final disposal concept by the evaluation of the transport properties of radionuclides in the bedrock. In the safety assessment effective porosity, diffusion coefficient and distribution coefficient (Kd) define the transport properties of the rock, and the magnitude of matrix diffusion and sorption. The diffusion coefficients, on the other hand, are typically measured using long-lasting and tedious experiments
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