Sorption of radionuclides on rocks is an important factor for safety assessments of geologic disposal of radioactive wastes. Batch sorption tests were conducted to investigate effects of concentrations of NaNO₃ and NaCl on sorption distribution coefficients (K_d) of selenium on a sandy mudstone under reducing environment of underground. The rock and groundwater used in the tests were sampled from depths of 129-156 m with special care minimizing contact with air. Selenium was spiked as HSe^-to the test solutions and kept reducing conditions (Eh, pH) of the solutions throughout the test periods. Obtained log K_d (m³ kg^-1) values were in a range of -1.84 to -1.44, which were as high as those for Cs.

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  Shallow groundwater samples were collected from 0 m to 45 m depth from surface in a borehole by multi-packer system. Fluorescent characteristics of the dissolved organic materials were analyzed by three-dimensional fluorescence spectroscopy without any pretreatments. The shallow groundwaters have fluorescent peaks of fulvic-like materials at an excitation wavelength of 285-325 nm and an emission wavelength of 400-425 nm. The organic carbon concentration of fulvic-like materials were estimated for 0.01-0.13 mg/l on the basis of relationship between fluorescence intensity and organic carbon content of isolated groundwater fulvic acid. The fulvic-like materials in groundwaters from sedimentary rock layers have 12 nm higher peak position on an excitation wavelength than those of the granitic formation. These results indicate that fulvic-like materials is transported from different sources to groundwaters in the granitic formation and/or their characteristics vary through the transport due to sorption. The association of fulvic-like materials with uranium may occur in these groundwaters on the basis of a positive correlation in concentrations between uranium and fulvic-like materials

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  To assess the effects of groundwater humic substances on migration behavior of radionuclides underground environments, we have to characterize humic substances (humic and fulvic acids) in groundwaters. This study reports characteristics of humic and fulvic acids isolated with DEAE-cellulose resin from groundwaters at sedimentary rock (depth of ca. 160 m from the ground surface) and granitic formation (ca. 180 m depth) in the Tono area, Gifu Prefecture, Japan. The Tono groundwater humic acids have the nitrogen content of about 8%, 75% of molecular size above 10,000 Da and E4/E6 ratio ranging from 6.1 to 7.5. The percentage of sedimentary groundwater humic acid with molecular size above 10,000 Da is 20% higher than that of granitic groundwater. On the other hand, the Tono groundwater fulvic acids consist mainly of molecular size less than 5,000 Da (60-68 %) and exhibit similar structural features. These results indicate that differences in features of groundwater humic acids from sedimentary rock and granitic formations, may reflect the differences in sorption ability of groundwater humic acids during the transport of humic acids from sedimentary rock to granitic formation. The similarity features for both groundwater fulvic acids are considered to be due to low sorption of the groundfwater fulvic acids onto geologic media.

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  A safety assessment methodology especially for radioactive waste disposal facility with multiple engineered barrier system is proposed in this paper. This proposed method is based on the concept of migration rate which had been used for the safety assessment for existing radioactive waste disposal facilities in Japan. The advantages of this proposed methodology are to calculate nuclides out fluxes from Engineered Barrier System (EBS) considering the effect of leaching rates of wastes without using complex numerical simulation, and to provide migration rates composed of three barrier performance indicators: retardation, impermeability and diffusivity. Additionally, several sensitivity analyses for radionuclides mass flux from a facility are carried out using this method and the phase diagrams of migration rate, leaching rate and maximum out flux from a facility are shown in this paper.

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  In order to demonstrate the robustness of the design of a high-level waste (HLW) disposal system, an important measure is to preliminarily set parameter values, which take into account safety margins for an acceptable dose. In evaluating the safety margin, it is necessary to estimate a range of parameters for an acceptable dose by a sensitivity analysis based on the appropriate conditions for each scenario. In this study, the comprehensive sensitivity analysis method was applied to performance assessment of a HLW disposal system. This paper presents the approach to assess a safety margin for parameters. As a result, safety margins for the parameters of the engineered barrier system (glass dissolution rate, overpack failure time, buffer thickness) were illustrated under conservative conditions of the natural barrier. Such an approach will provide useful information for scenario analysis and offer insights into the design and construction of a robust system for HLW disposal concept.

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  In the performance assessment of the geological disposal of radioactive wastes, the retardation of radionuclide migration is generally evaluated by using the sorption distribution coefficient (Kd). The Kd values would, however, be influenced by the colloidal substance in aqueous phase because these colloids may have the potential abilities to sorb the radionuclides. In this study, Cs-137, bentonite colloids and quartz sands were used for the batch-type sorption tests in order to investigate the influence of colloids on the sorption behavior of radionuclides. The Kds of Cs-137 onto quartz sands (Kd₁) and onto bentonite colloids (Kd₂) were determined from the distributed amounts among solid phase, colloidal phase and aqueous phase, which were distinguished by the pore size of filters used for separations of the experimental solutions. Under these experimental conditions, distribution of Cs-137 in the coexistence of quartz sands and bentonite colloids was able to be estimated from Kd₁ and Kd₂, which were obtained from other sorption tests carried out in the presence of sands and colloids, respectively. Thus selecting a relevant pore size for filtration would be available for evaluation of colloidal influence on radionuclide sorption onto the solids.

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  In high-level radioactive wastes geological disposal, an engineered barrier by bentonite which is called buffer around a waste package is built. When the buffer suffers high temperature by generation of heat from wastes for long term, the heat may deteriorate barrier performance of the buffer material. In order to evaluate suitable heat influence for design of the repository, behavior of buffer under high temperature needs to be clarified. In this research, dry shrinkage examinations of bentonite-based buffer were carried out, and the tendency of the relations between specification of buffer, temperature and water ratio depression, dry shrinkage were clarified.

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