5:15 PM - 7:15 PM
[HCG20-P05] Evaluation of the diffusion coefficients of cesium (Cs+) and iodine (I-) in the bentonite, a buffer material for Spent Nuclear Fuel (SNF) repository
Keywords:Bentonite, Buffer, Deep Geological Repository, Diffusion coefficient, Radionuclide, Spent Nuclear Fuel
To isolate and permanently dispose of the spent nuclear fuel (SNF) from ecosystem and human being, the most extensively studied method both domestically and internationally is the deep geological repository (DGR), and its design involves engineered barriers within natural barriers at depths of 500 ~ 1,000 meters to prevent the surface release of radioactive nuclides. Over time, groundwater from the natural barrier infiltrates and fills in pores of the buffer, one of engineered barriers. In the absence of external forces causing damage, the primary mechanism for the migration of radioactive nuclides within the buffer is expected to be diffusion driven by SNF concentration gradients in pore water. This study aims to evaluate the diffusion coefficients of Cs+ and I-, two main radioactive nuclides for the bentonite, assessing the safety assessment of the SNF disposal. The bentonite (Bentonil-WRK; a candidate domestic buffer material from CLARIANT Korea) was used for this study and its physical and mineralogical properties were identified and the diffusion experiment for Cs+ and I- was conducted over 7 months to obtain their diffusivity values for the bentonite.
Using a compacted bentonite core (density of 1.6 g/cm3, height of 3 cm) and a 35 mM of CsI solution, the diffusion phenomena from the bottom to the top of the core was maintained under constant confining pressure. After the diffusion experiment, the core was sectioned in the 0.5*0.5*0.5 cm3 size and Cs+ and I- in the section were extracted using acetic acid and distilled water. Each extract was centrifuged and its concentration was analyzed using an inductively coupled plasma-mass spectrometer (ICP-MS for Cs+) and an ion chromatography (IC for I-) to calculate the diffusion coefficients (De) of both Cs+ and I-. The Bentonil-WRK, the Ca-type bentonite, primarily composed of montmorillonite with a 2:1 dioctahedral structure, showed high specific surface area and excellent cation exchange capacity. These properties make it a promising candidate for limiting the radionuclide migration in the SNF repository site. Experimental results confirmed that Cs+ and I- concentrations decreased with increased core height, supporting the diffusion-driven transport. Fick’s second law was applied to calculate the De, assuming the effective diffusion coefficients for Cs+ and I-, and they were determined to be 2.0602*10-12 m2/sec and 6.6455*10-11 m2/sec, respectively. Comparison with the result from previous research, including Cho et al., (1994), which reported similar De values for Cs+ and I- in compacted bentonites, indicates that the Bentonil-WRK has comparable diffusion properties. Results in this study provides important data for designing barriers to ensure the long-term stability and safety of the SNF repository site.
Acknowledgements
This research was supported by the Institute for Korea Spent Nuclear Fuel (IKSNF) and National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT, MSIT) (2021M2E1A1085202).
This study was conducted with the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (No. 2021R1A6C101A415).
Using a compacted bentonite core (density of 1.6 g/cm3, height of 3 cm) and a 35 mM of CsI solution, the diffusion phenomena from the bottom to the top of the core was maintained under constant confining pressure. After the diffusion experiment, the core was sectioned in the 0.5*0.5*0.5 cm3 size and Cs+ and I- in the section were extracted using acetic acid and distilled water. Each extract was centrifuged and its concentration was analyzed using an inductively coupled plasma-mass spectrometer (ICP-MS for Cs+) and an ion chromatography (IC for I-) to calculate the diffusion coefficients (De) of both Cs+ and I-. The Bentonil-WRK, the Ca-type bentonite, primarily composed of montmorillonite with a 2:1 dioctahedral structure, showed high specific surface area and excellent cation exchange capacity. These properties make it a promising candidate for limiting the radionuclide migration in the SNF repository site. Experimental results confirmed that Cs+ and I- concentrations decreased with increased core height, supporting the diffusion-driven transport. Fick’s second law was applied to calculate the De, assuming the effective diffusion coefficients for Cs+ and I-, and they were determined to be 2.0602*10-12 m2/sec and 6.6455*10-11 m2/sec, respectively. Comparison with the result from previous research, including Cho et al., (1994), which reported similar De values for Cs+ and I- in compacted bentonites, indicates that the Bentonil-WRK has comparable diffusion properties. Results in this study provides important data for designing barriers to ensure the long-term stability and safety of the SNF repository site.
Acknowledgements
This research was supported by the Institute for Korea Spent Nuclear Fuel (IKSNF) and National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT, MSIT) (2021M2E1A1085202).
This study was conducted with the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (No. 2021R1A6C101A415).