Tamotsu Kozaki

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.

Activation energy for diffusion of chloride ions in compacted sodium montmorillonite

Abstract Diffusion of chloride ions in compacted montmorillonite was studied for a performance assessment as buffer material in geological disposal of high-level radioactive waste. The apparent diffusion coefficients were from 6.8×10 −12 to 2.2×10 −10 m 2 s −1 for sodium montmorillonite at dry densities from 1.0 to 1.8 Mg m −3 at diffusion temperatures from 278 to 323 K. The activation energy increased from 14 kJ mol −1 (lower than that in free water, 17.4 kJ mol −1 ) to 25 kJ mol −1 (higher than that in free water), as the dry density increased from 1.0 to 1.8 Mg m −3 . These facts can be attributed to changes in the predominant diffusion process as dry density increases. The dry density dependence of the activation energy cannot be explained by conventional diffusion models, such as the pore water diffusion model.

Am(III) and Eu(III) uptake on hematite in the presence of humic acid

Summary The uptake of Am(III) and Eu(III) on hematite (1.0 g/L) in the presence of humic acid (HA; 4.7×10-5 M) was investigated at ionic strengths of 0.05 to 0.5 M (NaClO4) in the pH range of 4 to 10. The sorption behavior of Am(III) was similar to that of Eu(III). The Am(III) and Eu(III) exhibited 100% retention on the solid in the entire pH region studied at high ionic strength (0.5 M). As the ionic strength was decreased below 0.1 M, Am(III) and Eu(III) sorption which was complete at low pH values decreased stepwise with increasing pH from 5 to 7. In order to test the additivity and to understand the solid–water interface reactions for Am(III) sorption in ternary systems, binary interactions between Am(III), HA, and hematite were also examined under identical conditions. The additive results reproduced the stepwise decreases of sorption up to a pH of about 6 for lower ionic strengths, which can be ascribed to HA adsorption behavior. However, inconsistencies in additivity from experimental results were found in the pH regions above 7 at low ionic strengths and in the entire pH region at an ionic strength of 0.5 M. Some non-additive effects are discussed in this paper.

Effect of Dry Density on Activation Energy for Diffusion of Strontium in Compacted Sodium Montmorillonite

For performance assessments of geological disposal of high-level radioactive waste, activation energies for the diffusion of strontium ions and the basal spacings of compacted sodium montmorillonite in the water-saturated state were determined. Basal spacings determined by XRD indicated changes in the interlamellar space from a three-water layer hydrate state to a two-water layer hydrate state as the dry density of the montmorillonite increased from 1.0 to 1.8 Mg/m{sup 3}. Activation energies from 17.3 to 30.8 kJ/mol for the apparent diffusion coefficients of strontium ions were obtained. The lower activation energies than for diffusion of strontium ions in free water were determined for montmorillonite specimens of lower dry density (1.2 Mg/m{sup 3} and below), while the higher activation energies were at higher dry densities (1.4 Mg/m{sup 3} and above). These findings cannot be explained by changes in only the geometric parameters, which the pore water diffusion model is based upon. Possible explanations for the dry density dependence of the activation energy are the changes of the temperature dependence of the distribution coefficients and/or of the diffusion process with increasing dry density.

Tip artifact in atomic force microscopy observations of InAs quantum dots grown in Stranski–Krastanow mode

The tip artifact in atomic force microscopy (AFM) observations of InAs islands was evaluated quantitatively. The islands were grown in the Stranski–Krastanow mode of molecular beam epitaxy. The width and height of the islands were determined using transmission electron microscopy (TEM) and AFM. The average [1¯10] in-plane width and height determined using TEM excluding native oxide were 22 and 7nm, respectively; those determined using AFM including the oxide were 35 and 8nm, respectively. The difference in width was due to the oxide and the tip artifact. The sizes including the oxide were deduced from TEM observations to be a width of 27nm and a height of 6nm with correction for the thickness of the oxide. The residual difference of 8nm between the width determined using AFM and that determined using TEM including the oxide was ascribed to the tip artifact. The results enable us to determine the actual size of the islands from their AFM images.

Effect of silica sand on activation energy for diffusion of sodium ions in montmorillonite and silica sand mixture

The effect of silica sand on the diffusion of sodium ions in mixtures of montmorillonite and silica sand was studied by measuring the apparent diffusion coefficients, activation energies for diffusion, and the basal spacing of the mixed samples. These diffusion experiments suggest that the apparent diffusion coefficients of sodium ions in the mixed samples were almost the same as those of pure montmorillonite samples having the same partial dry densities of montmorillonite. The activation energy dependence for diffusion of sodium ions on the partial dry density was different between the mixed samples and the pure montmorillonite samples. The activation energy increased by adding silica sand at the partial dry density of 1.0 Mg m(-3), and decreased by adding silica sand at the partial dry densities higher than 1.2 Mg m(-3). A change in the XRD profile was observed after adding silica sand at the partial dry density of 1.6 Mg m(-3). Here, a three-water-layer hydrate state of montmorillonite was found in the mixed sample whereas only a two-water-layer hydrate state was observed in the pure montmorillonite sample. These experimental results suggest that silica sand changed the montmorillonite microstructure in the mixed samples, which then altered the sodium-ion diffusion process.

Characterization of Fe-montmorillonite: A Simulant of Buffer Materials Accommodating Overpack Corrosion Product

Candidate materials as engineered barriers for high-level radioactive waste disposal include carbon steel for the overpack and bentonite for the buffer material. If these materials are selected, corrosion products from the overpack will migrate into the compacted bentonite. 1–3) Potential diffusing species of the corrosion products include Fe 2+ ions. The Fe2+ ions will be sorbed on montmorillonite, which is the major mineral component of bentonite. Some Fe 2+ ions may form more stable hydroxides and oxides. Radioactive nuclides leached from waste forms will migrate in such altered montmorillonite. An anticipated shortcoming of the alteration of montmorillonite is degradation of swelling property, which is liable to cause acceleration of nuclide migration. On the other hand, an expected advantage may be the immobilization of nuclides with high sorption affinities for iron compounds. However, the characteristics of such altered montmorillonite including retentivity for radioactive nuclides have yet to be clarified. Here we briefly report characterization of homoionic Fe 2+montmorillonite (Fe-montmorillonite) and its oxidized product that do retain Se(VI) for which natural montmorillonite such as Na +-type and Ca 2+-type has little retentivity. We used Fe-montmorillonite as a simulant of the buffer material in which Fe2+ ions diffused.

Observation of Microstructures of Compacted Bentonite by Microfocus X-Ray Computerized Tomography (Micro-CT)

In this study, nondestructive, three-dimensional images of the internal microstructures of compacted bentonite samples in dry and water-saturated states were examined with the micro-CT.

A New Method for Fe(II)-montmorillonite Preparation Using Fe(II)-nitrilotriacetate Complex

Bentonite clay is a candidate engineered barrier material for a high-level nuclear waste geological repository. Montmorillonite is the major clay mineral of bentonite. The experimental procedure of the new method and the results of the chemical analysis of the prepared Fe(II)-montmorollonite are described in this report

Apparent diffusion coefficients and chemical species of neptunium (V) in compacted Na-montmorillonite.

Diffusion of neptunium (V) in compacted Na-montmorillonite was studied through the non-steady state diffusion method. In this study, two experimental attempts were carried out to understand the diffusion mechanism of neptunium. One was to establish the diffusion activation energy, which was then used to determine the diffusion process in the montmorillonite. The other was the measurement of the distribution of neptunium in the montmorillonite by a sequential batch extraction. The apparent diffusion coefficients of neptunium in the montmorillonite at a dry density of 1.0 Mg m-3 were from 3.7 x 10(-12) m2 s-1 at 288 K to 9.2 x 10(-12) m2 s-1 at 323 K. At a dry density of 1.6 Mg m-3, the apparent diffusion coefficients ranged between 1.5 x 10(-13) m2 s-1 at 288 K and 8.7 x 10(-13) m2 s-1 at 323 K. The activation energy for the diffusion of neptunium at a dry density of 1.0 Mg m-3 was 17.5 +/- 1.9 kJ mol-1. This value is similar to those reported for diffusion of other ions in free water, e.g., 18.4 and 17.4 kJ mol-1 for Na+ and Cl-, respectively. At a dry density of 1.6 Mg.m-3, the activation energy was 39.8 +/- 1.9 kJ mol-1. The change in the activation energy suggests that the diffusion process changes depending on the dry density of the compacted montmorillonite. A characteristic distribution profile was obtained by the sequential extraction procedure for neptunium diffused in compacted montmorillonite. The estimated fraction of neptunium in the pore water was between 3% and 11% at a dry density of 1.6 Mg m-3 and at a temperature of 313 K. The major fraction of the neptunium in the montmorillonite was identified as neptunyl ions sorbed on the outer surface of the montmorillonite. These findings suggested that the activation energy for diffusion and the distribution profile of the involved nuclides could become powerful parameters in understanding the diffusion mechanism.