Ion exchange kinetics of fission products between molten salt and zeolite-A

Experimentation, data analysis, and modeling of ion exchange kinetics between fission products (cesium and strontium) and zeolite-A beads in molten LiCl–KCl have been performed to support optimization of an electrochemical process to treat used nuclear fuel. Models based on pseudo-first- and pseudo-second-order sorption as well as diffusion have been adapted and compared to experimental data to assess their validity in describing the system. Individual experiments were performed with different concentrations of CsCl or SrCl2 in the LiCl–KCl salt. Zeolite beads were removed from the molten salt at prescribed intervals of time and prepared for analysis by inductively coupled plasma-mass spectrometry. Results indicate maximum cesium (Cs) and strontium (Sr) loading occurred at approximately 31 min and 104 min of contact, respectively. The rate of loading and maximum loading were found to increase with increasing initial concentration of Cs or Sr. Data analysis included determination of rate constants and diffusion coefficients of the proposed models for each experimental condition. Results reveal that the diffusion model provides the best fit to the experimental data with average diffusion coefficients of 2.0 × 10−10 m2 s−1 for Cs and 6.3 × 10−11 m2 s−1 for Sr. This suggests that chemical diffusion is the dominant mechanism of mass transfer.

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► Ion exchange kinetics tests were run for Cs+ or Sr2+ uptake by zeolite in molten salt.
► Equilibrium was reached in approximately 30 min for Cs+ and 100 min for Sr2+.
► Four models were compared to the experimental data.
► The diffusion-limited model was found to best fit the experimental data.
► Diffusion coefficients were 2.0 × 10−10 m2/s for Cs+ and 6.3 × 10−11 m2/s for Sr2+.