Compacted Na-bentonite, of which the major mineral is montmorillonite, is a candidate buffer material for the geological disposal of high-level radioactive waste. A potential alteration of the bentonite in a repository is the partial replacement of the exchangeable cations of Na However, the apparent diffusion coefficient of Ca 2+ and the activation energy for diffusion at the same dry density were independent of the ionic equivalent fraction of Ca 2+ ions. These findings suggest that unlike HTO, which can be postulated to diffuse mainly in pore water, Ca 2+ ion diffusion could occur predominantly in interlayer spaces, of which the basal spacing was determined to be constant by the XRD technique.
Since the Fukushima nuclear power station accident, large quantities of radiocesium-contaminated soil generated from decontamination activities have been stored in Fukushima prefecture. To complete the final disposal of decontamination soil, reducing the disposal volume through recycling can prove effective. The Ministry of the Environment of Japan has developed a policy of handling low-activity decontamination soil as recycled materials under the management of public authority. The recycling is limited to civil engineering structures in public projects, such as road embankments and coastal levees. However, there has been no practical review or safety assessment of decontamination soil recycling. In this study, to contribute to guideline development for decontamination soil recycling by the Ministry of the Environment, dose estimation was considered as a way of ensuring that the use of recycled decontamination soil for road embankments was safe. First, based on Japanese construction standards, additional doses to workers and the public in construction and service (e.g., use of a road embankment) scenarios were evaluated. From the result, the radioactive cesium concentration level of recycled materials that would result in all additional doses meeting the radiation criterion of 1 mSv y was derived to be 6,000 Bq kg. Then, construction conditions were reviewed to reduce additional doses to the public in a service scenario. To confine doses to the public to below 10 μSv y based on the derived radioactivity level, an additional layer of soil slope protection of 40 cm or more was needed. Finally, additional doses expected in a disaster scenario were confirmed to be below 1 mSv y based on the derived radioactivity level, an additional layer of soil slope protection of 40 cm or more was needed. Finally, additional doses expected in a disaster scenario were confirmed to be below 1 mSv y based on the derived radioactivity level.
Highly alkaline environments induced by cementitious materials in radioactive waste repositories are likely to alter montmorillonite, the main constituent of bentonite buffer materials, and are likely to cause the physical and/or chemical properties of the buffer materials to deteriorate. The deterioration may cause variation in hydraulic conductivity of the buffer. However, empirical data on the variation of hydraulic conductivity are scarce, mainly because the alteration of compacted buffer materials, sand-bentonite mixtures, is extremely slow. In this study, laboratory experiments were performed to observe changes in hydraulic conductivity of sand-bentonite mixtures, accompanied by their alkaline alteration, using NaOH-based solutions at 80–90°C. Series-1 multi-step alteration/water conduction experiments resulted in an increase in the hydraulic conductivity by one order of magnitude over a 200 day period. Series-2 single step alteration/water conduction experiments revealed a decrease in the montmorillonite contents with time and a resulting increase in the hydraulic conductivity by 30 times over the 67 day period. Series-3 simultaneous alteration/water conduction experiment also demonstrated an increase in the hydraulic conductivity by 30 times over the 150 day period. The results proved that the alkaline alteration of the bentonite buffer can increase the hydraulic conductivity. The data obtained in this study are useful for verification of the code that will be used for assessing the alteration.
The dissolution rate for montmorillonite under compacted condition was studied in order to evaluate long-term alteration behavior of bentonite buffer materials by highly alkaline groundwater. The dissolution rate of compacted montmorillonite was found to be larger than that of montmorillonite in compacted sand-bentonite mixtures at 130 o C, which revealed that the dissolution of montmorillonite was inhibited by decreasing the activity of hydroxide ions (a OH-) in the compacted mixtures including accessory minerals such as silica. In order to provide reliability for the analysis of bentonite alteration using dissolution rate of montmorillonite, it is important to quantify the decrease of a OH-in the compacted mixtures and to formulate the dissolution rate of compacted montmorillonite.
AB ST R ACT : We studied the diffusive transport of Cs, Np, Am and Co in compacted sandbentonite mixtures by using the through-diffusion method. The experiments for Cs were performed under various aqueous compositions. Effective diffusivity (D e ) values of 4.7610 À10 to 5.9610 À9 m 2 s À1 were obtained with a somewhat large variation. Apparent diffusivity (D a ) values, on the other hand, showed less variation, ranging from 2.0610 À12 to 6.2610 À12 m 2 s À1 . The results indicated that diffusive flux was proportional to the concentration gradient on the basis of the amount of Cs in the unit volume of the compacted sand-bentonite mixtures rather than the Cs concentration gradient in pore water. Because the former concentration gradient in the mixtures was nearly equal to that of adsorbed Cs, the diffusion of Cs in the mixtures was probably dominated by the concentration gradient of the Cs adsorbed on the mixtures. In addition, the effective/apparent diffusivity of 237 Np(IV) and apparent diffusivity of 241 Am(III) and 60 Co(II) in the mixtures were determined in 0.3/0.03 mol l À1 (NH 4 ) 2 CO 3 /Na 2 S 2 O 4 solution.
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