Hiroyasu Kato

Effect of particle size on the diffusion behavior of some radionuclides in compacted bentonite

Abstract For performance assessment of bentonite buffer material in geological disposal of high-level radioactive waste, the particle size of bentonite and its effect on the diffusion behavior of radionuclides in compacted bentonite were studied. Bentonite samples with different particle sizes were prepared, and characterized by the BET and EGME methods for specific surface areas, by the laser diffraction/scattering particle size analysis for particle size distribution, and by SEM observations. Apparent and effective diffusion coefficients of tritiated water (HTO), Cl − ions and Cs + ions in compacted bentonite were also determined using bentonite samples with different particle sizes. With HTO and Cl − ions, there were higher diffusion coefficients in fine grained samples, but opposite particle size effects were observed with Cs + ions. These findings cannot be explained by the conventional pore water diffusion model, and suggest a different diffusion process for Cs + ions and also at higher dry density region.

Iodine Immobilization by Sodalite Waste Form

Leachabilities and solubilities of the synthesized iodide sodalite and natural (chloride) sodalite were measured by leach test. The iodide sodalite was synthesized in nitrogen gas flow at 800°C for 2 hours. The crystalline structure of the product was certified by XRD analysis. The natural sodalite containing chlorine was obtained at Bancroft, Canada. The several types of solution were used to evaluate the influence of the solubility of sodalite that included the varied pH and the chemical compositions. The solubilities of chloride sodalite were calculated by the thermodynamics data. The solubility measured for the synthesized iodide sodalite was compared with that calculated. The solubility of the synthesized iodide sodalite was approximately 2 x 10-4 mol/L, which shows a good agreement with the calculated one of the natural sodalite.

A novel technique of in situ phase-shift interferometry applied for faint dissolution of bulky montmorillonite in alkaline solution

The effect of alkaline pH on the dissolution rate of bulky aggregated montmorillonite samples at 23 °C was investigated for the first time by using an enhanced phase-shift interferometry technique combined with an internal refraction interferometry method developed for this study. This technique was applied to provide a molecular resolution during the optical observation of the dissolution phenomena in real time and in situ while remaining noninvasive. A theoretical normal resolution limit of this technique was 0.78 nm in water for opaque material, but was limited to 6.6 nm for montmorillonite due to the transparency of the montmorillonite crystal. Normal dissolution velocities as low as 1 × 10−4 to 1 × 10−3 nm/s were obtained directly by using the measured temporal change in height of montmorillonite samples set in a reaction cell. The molar dissolution fluxes of montmorillonite obtained in this study gave considerably faster dissolution rates in comparison to those obtained in previous investigations by solution analysis methods. The pH dependence of montmorillonite dissolution rate determined in this study was qualitatively in good agreement with those reported in the previous investigations. The dissolution rates to be used in safety assessments of geological repositories for radioactive wastes should be obtained for bulky samples. This goal has been difficult to achieve using conventional powder experiment technique and solution analysis method, but has been shown to be feasible using the enhanced phase-shift interferometry.

Interlayer dissolution of montmorillonite observed by internal refraction interferometry

Dissolution behavior of a clay mineral such as montmorillonite is one of the most important phenomena for a long-term safety of high-level radioactive waste disposal. Dissolution rates of aggregated montmorillonite samples in basic solutions at room temperature were investigated in flow-through experiments by using internal refraction interferometry with an enhanced phase-shift interferometry. Conventional solution analysis methods cannot measure the effects of dissolution occurring within the interlayer of montmorillonite. Internal refraction interferometry can measure the crystal dissolution of montmorillonite, including both the dissolution of the outer surface and the interlayer of montmorillonite. The dissolution rate of montmorillonite in the interlayer was first observed. It was slower than the dissolution rate of outer surface. As the number of the montmorillonite crystal laminations increased, the montmorillonite dissolution rate in the interlayer decreased. Montmorillonite dissolution rates showed limited dependence on pH in the alkaline solutions. This can be explained by the effect of the laminated structure of montmorillonite crystal on the dissolution rate, especially in highly alkaline solution.