[H-CG20] 原子力と地球惑星科学

[HCG20-P06] Sorption and desorption characteristics of cesium and strontium on the bentonite as a buffer material in the deep geological SNF repository

*Soyoung Park¹、Soyoung Jeon¹、Daehyun Shin¹、Jungtaek Kim²、Minhee Lee³ (1.Major of Earth and Environmental Sciences, Division of Earth Environmental System Science, Pukyong National University, Republic of Korea、2.Chemical Analysis Center of Korea Research of Chemical Technology, Republic of Korea、3.Major of Environmental Geosciences, Division of Earth Environmental System Science, Pukyong National University, Republic of Korea)

キーワード：Bentonite, Buffer, Deep Geological Repository, Radionuclide, Sorption, Spent Nuclear Fuel

The deep geological repository (DGR) for the permanent disposal of spent nuclear fuel (SNF) involves depositing it approximately 500 m below the surface within bedrock to prevent the spread of high-level radioactive waste (HLW). Buffer is a critical component in the engineered barrier system (EBS), playing a role in preventing the external leakage of radionuclides and the inflow of groundwater into the SNF canister. This study aims to evaluate the sorption capacity of the bentonite (Bentonil WRK: Waste Repository Korea from CLARIANT Korea) for Cs⁺ and Sr²⁺, by conducting sorption/desorption batch experiments under various conditions. Additionally, sorption model studies and mineralogical-structural analyses (XRD, XRF, SEM/EDS and XPS) were conducted for the identification of the sorption mechanisms and for the quantitative evaluation of the adsorption capacity. Batch sorption tests were carried out at various conditions such as the solid-liquid ratio (2, 5, 10 and 20 g/L), reaction time (10 to 1,440 minutes), the initial concentration (0.1 to 100 mg/L), the pH (3 to 11), the competitive cation (K⁺, Ca²⁺, Mg²⁺ and Na⁺) and temperature (20 to 50 ℃) and the reaction with gases (CO₂ and H₂S). Desorption experiments were also conducted over 24 hours at the pH values of 5, 7 and 9.
Results in batch experiments showed that as the solid-liquid ratio increased, the sorption efficiency both of Cs⁺ and Sr²⁺ was proportionally increased, and the sorption equilibrium was reached within a short period (< 30 minutes) with the high sorption efficiency (> 90% for both Cs⁺ and Sr²⁺) at 1 mg/L of initial concentration. The Cs⁺ exhibited the high sorption efficiency (>80%) for the bentonite across wide range of pH conditions, whereas the Sr²⁺ showed a 75% efficiency at pH 3, increasing to over 88% under other conditions. The addition of K⁺ ions in solution resulted in the decrease in the sorption efficiency for Cs⁺, whereas the existence of Ca²⁺ and Mg²⁺ ions in solution led to the decrease in the Sr²⁺ sorption efficiency. Desorption rates of the bentonite were 6% and 12% for the Cs⁺ and the Sr²⁺, respectively. The sorption model studies supported that the sorption of Cs⁺ and Sr²⁺ onto the bentonite was better described by the Pseudo second-order model and the Freundlich model, indicating a predominance of the chemical sorption and multilayer sorption processes. Results in this study suggested that the bentonite can efficiently adsorb the radionuclides, Cs⁺ and Sr²⁺, potentially delaying their migration in the buffer and it may be applied as the buffer material for the long-term safety of SNF disposal sites.

* 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 work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00409497).