Seichi Sato

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.

Effects of humic acid on the sorption of Am(III) and Cm(III) on kaolinite

Sorption coefficients of Am(III) and Cm(III) on kaolinite were determined at pH from 3.5 to 10 and humic acid (HA) concentrations of 0 to 20 ppm at ionic strength of 0.1 M (NaClO4) at 298 K with N2 bubbling. The sorption ratio of HA on kaolinite was also measured under the same experimental conditions. The sorption coefficients of Am(III) and Cm(III) in the absence of HA increased with pH over the whole pH range. The presence of HA enhanced sorption coefficients at pH values up to around 5, above which the sorption coefficients were lowered by introducing HA. The effects of HA on sorption coefficients were interpreted by the use of a linear additive model, modified for metal-humate complex formation in the aqueous phase. The model was found to be in fairly good agreement with the experimental data.

Effects of humic acid on the sorption of neptunium(V) on kaolinite

Abstract The sorption coefficient of Np(V), K d , on kaolinite was measured at pH 6 to 11 and in humic acid concentrations of 0 to 40 mg dm −3 at the ionic strength of 0.1 M. The K d value increased with pH both with and without humic acid. The K d value increased slightly with increasing concentration of humic acid in the pH range below 8 and decreased not more than an order of magnitude with increasing concentrations of humic acid in the pH region above 8. Below pH 8, Np(V) sorption is considered to be enhanced by the sorption of humic acid on the kaolinite to form Np(V)-humate complexes. Above 8, it is probable that the desorption of humic acid and formation of Np(V)-humate in solution result in the decrease of K d . The behavior of Np(V) sorption on kaolinite with humic acid is described by a simple model.

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.

E.m.f. measurements of molten oxide mixtures III. Sodium oxide + silicon dioxide

Abstract The relative partial molar quantities of binary molten mixtures of Na2O + SiO2 were evaluated from e.m.f. measurements of the melts containing from 45 to 77.5 moles per cent of SiO2. In addition to a steep change of the activity, a substantial inflection of the partial molar enthalpy and entropy at the composition of 60 moles per cent of SiO2 was observed.

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.

Thermodynamic properties of water in compacted sodium montmorillonite

Compacted bentonite is a promising material as an engineering barrier to enclose nuclear waste. The migration of nuclides occurs in the water of bentonite, where the major mineral is sodium montmor...

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.