Study on advection–dispersion behavior for simulation of HTO and Se transport in crushed granite

Enhancing Europium Adsorption Effect of Fe on Several Geological Materials by Applying XANES, EXAFS, and Wavelet Transform Techniques

This study conducted adsorption experiments using Europium (Eu(III)) on geological materials collected from Taiwan. Batch tests on argillite, basalt, granite, and biotite showed that argillite and basalt exhibited strong adsorption reactions with Eu. X-ray diffraction (XRD) analysis also clearly indicated differences before and after adsorption. By combining X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and wavelet transform (WT) analyses, we observed that the Fe2O3 content significantly affects the Eu-Fe distance in the inner-sphere layer during the Eu adsorption process. The wavelet transform analysis for two-dimensional information helps differentiate two distances of Eu-O, which are difficult to analyze, with hydrated outer-sphere Eu-O distances ranging from 2.42 to 2.52 Å and inner-sphere Eu-O distances from 2.27 to 2.32 Å. The EXAFS results for Fe2O3 and SiO2 in argillite and basalt reveal different adsorption mechanisms. Fe2O3 exhibits inner-sphere surface complexation in the order of basalt, argillite, and granite, while SiO2 forms outer-sphere ion exchange with basalt and argillite. Wavelet transform analysis also highlights the differences among these materials.

Advection-Dispersion Behavior for Simulation of H-3 and Pu-238 Transport in Undisturbed Argillaceous Shale of a Near-Surface Repository

In this study, a column experiment was employed to evaluate the nuclide migration behavior in the surrounding rock medium of a near-surface disposal site in China and to investigate the advection-dispersion behavior of tritium (H-3) and plutonium-238 (Pu-238) in highly weathered argillaceous shale. A reasonable numerical model was selected to fit the experimental breakthrough curves (BTCs) and to obtain the relevant migration parameters. The results show the following: (1) the internal structure of the highly weathered argillaceous shale exhibited heterogeneity, and the nuclide migration BTC showed characteristics of a "curve peak moving forward" and a "tail curve trailing"; (2) compared with other models, the stream tube mode could better fit the BTCs and obtain the average dispersion coefficient <D>, average distribution coefficient <K_d>, and other parameters; (3) compared to the results of the batch experiment, the distribution coefficient K_d obtained from the column experiment was smaller than that obtained from the batch experiment, which is speculated to be due to the influence of contact time and the contact area between the nuclide and the medium.

An application research for near-surface repository of strontium-90 sorption kinetic model on mudrocks

In this study,90Sr was used as the test radionuclide to characterize the sorption kinetics and effects of initial 90Sr activity and remaining 90Sr in solid concentration were simulated for a near-surface repository. The study focused on the sorption characteristics of radionuclides in unsaturated groundwater environment (or vadose zone) is the important information for investigating the near-surface disposal of intermediate and low-level radioactive waste (ILLW). Moreover, the 90Sr sorption experiments reached equilibrium within 56 h, which fit to the first order sorption kinetic model, and the remaining 90Sr in mudrock samples showed obvious sorption equilibrium hysteresis, which fit to the second order sorption kinetic model. Before reaching the maximum sorption capacity, the sorption rate constant increases with 90Sr increasing; the distribution coefficient (Kd) of 56 h decreases with the remaining 90Sr decreasing. In addition, it showed that the slow sorption process dominated before the sorption reaches equilibrium. In fact, a reliable safety assessment methodology for on-going near-surface repository required a lot of the radionuclides parameters with local environment including the radionuclides sorption/desorption rate constant and maximum sorption capacity.

An Improved Speciation Method Combining IC with ICPOES and Its Application to Iodide and Iodate Diffusion Behavior in Compacted Bentonite Clay

An accurate and effective method combining ion chromatography (IC) and inductively coupled plasma optical emission spectrometry (ICP-OES) was applied in this work to qualitatively and quantitatively analyze individual and co-existing iodide (I⁻) and iodate (IO₃⁻) at various concentrations. More specifically, a very strong linear relationship for the peak area for the co-existing I⁻ and IO₃⁻ ions was reached, and a high resolution value between two peaks was observed, which proves the effectiveness of our combined IC-ICP-OES method at analyzing iodine species. We observed lower accessible porosity for the diffusion of both I⁻ and IO₃⁻ in samples of bentonite clay using IC-ICP-OES detection methods, where the effective diffusion coefficient varied based on the anion exclusion effect and the size of the diffusing molecules. In fact, the distribution coefficients (K_d) of both I⁻ and IO₃⁻ were close to 0, which indicates that there was no adsorption on bentonite clay. This finding can be explained by the fact that no change in speciation took place during the diffusion of I⁻ and IO₃⁻ ions in bentonite clay. Our IC-ICP-OES method can be used to estimate the diffusion coefficients of various iodine species in natural environments.

Experimental investigation on buffer/backfill materials for radioactive waste repository downward facing sorption additivity of cesium, strontium and cobalt with different concentrations

Buffer/backfill materials for radioactive waste disposal sites consist of pure bentonite or bentonite-rock mixtures. In this study, the batch test method was used to obtain the sorption characteristics of important radionuclides such as Cs, Sr and Co on buffer/backfill materials; i. e., mixing Wyoming MX-80 bentonite or local Taiwanese Zhi-Shin bentonite with possible host rock (argillite and granite) in different proportions (0∼100%). The distribution coefficients (Kd) for Cs, Sr and Co were obtained from the experiments. The distribution coefficient for the bentonite-rock mixtures were found, with more than 50% of mixing proportion of bentonite to argillite or granite, to have very similar values to that of pure bentonite. Furthermore, it was clearly found that the sorption of Cs, Sr and Co to bentonite-rock mixtures is decreased as ionic strength of the liquid phase is increased from 0.001M to 1M for NaCl solutions. According to the experimental results, in synthetic groundwater, it is quite convenient and helpful to assess the distribution coefficients (Kd) of Cs, Sr and Co for buffer/backfill materials using batch sorption experiments with bentonite-rock mixtures of fixed mixing proportions.