The micro flow channel method was utilized for the in-situ determination of both effective diffusion and distribution coefficients by attaching of the tinny flow channel on the borehole wall excavated in a granite formation. Three different solutes such as deuterium oxide D₂O, uranine and potassium ion K⁺ were injected simultaneously into the rock matrix, then the breakthrough curves had been obtained and analyzed by numerical means. The effective diffusion coefficients D_e of non-sorbing solutes (D₂O, uranine) as well the distribution coefficient of sorbing solute (K⁺) were obtained at the intact surface were in good accordance with the reported values.On the other hand, the observation at the surface with a single crack gave different breakthrough patterns and the Des for the non-sorbing solutes were greater than those at the intact surface about one order of magnitude. Even if the crack opening was small, less than 0.5 mm, the tin crack might cause the overestimation of the matrix diffusivity. Through the in-situ experiments, the method enables us to quick and simultaneous estimates of the important parameters for the nuclide migration analysis.

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  Generally speaking, a vast, advanced and unfamiliar science and technology is unacceptable by public for the fear of its unknown nature. The siting problem for the disposal of high level radioactive wastes is one of the typical examples. This study examined the desirable research scheme for geological disposal of the high level radioactive wastes seen from social acceptance of science and technology in the deductive approach by the technical essence described in the first report. As the safety is the most prioritized concern in the research of radioactive waste disposal, the correlation between safety and cost has not been argued. Therefore, the argument has only turned in seeking for limitless safety without paying attention to an economic principle.
  In this report, as the examination evolving from a social acceptability, the author tries to explain the safety together with cost information in accordance with the principle of the risk benefit.
  As a result, it turned out that the relation between the existing Japanese default proposal (H12 report) and overseas examples can be figured out to explain their relative position by the safety and cost correlation diagram made in accordance with the risk benefit principle. The nuclear field specialist can discuss a relative location of the Japanese disposal concept by this correlation diagram with another field specialist.

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  The standard potential of Se (IV) /Se (VI) couple has been obtained by cyclic voltametry in the relatively pure system.
  The formal potentials of the selenium redox reaction have been determined in NaClO₄ medium of various ionic strength (I (mol·kg^-1) =0.500, 1.00, 1.50, 2.00) at room temperature. These data have been interpreted by using the Specific Ion Interaction Theory to give the following standard potential. In addition, the reactive chemical species have been identified by the dependence of peak potential of oxidation wave on Se concentration. As a result, the following standard potential was determined:
            HSeO₃^- + H₂O = SeO₄^2- + 3H⁺ + 2e^-, E⁰ = 0.821±0.167 V vs. SHE.
  This standard potential is consistent with the other experimental study which confirmed the existence of Se (VI) in the potential region which is inconsistent with the selected value by OECD/NEA.

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  It is anticipated that thermal - hydraulic - mechanical - chemical (THMC) processes will be coupled in the bentonite buffer material of a high-level radioactive waste repository. The main contributors to these processes are heat arising from the radioactive decay of the vitrified waste, infiltration of groundwater from the host rock and/or leachate from the cementitious component of the repository, and the consequent increase in swelling pressure and chemical reactions. In order to evaluate these coupled processes in the bentonite buffer material, it is necessary to take steps towards the development of a credible and robust THMC model.
  The current paper describes the measured data of an engineering-scale coupled THC process experiment and the calculated results of a THC model undergoing development. The coupled experiment used an electric heater, bentonite blocks and a mortar block, subjected to infiltrating water to simulate a high-alkaline porewater derived from the concrete tunnel support seeping into the bentonite buffer material under a thermal gradient provided by the vitrified waste.
  Temperature and water content of the bentonite buffer material were measured by several sensors continuously for several months. After this time, the buffer material was sampled. The results of mineral analysis of the samples suggested that the precipitate of amorphous hydrate with silica was found in the buffer material in contact with the mortar. The developing THC model simulated C-S-H gel precipitation as a secondary mineral in the exact same locality because of the solution being saturated with respect to portlandite and chalcedony, thereby providing some confidence in the chemical feature of the developing THC model.
  Some important issues in the future development of the model were also identified, including the concentration of porewater being influenced by vapor movement in the bentonite buffer material due to heating from the vitrified waste and geochemical reactions being affected by the water retention feature of the bentonite buffer material.

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  In high-level radioactive wastes disposal, it may be decided to free and retrieve the emplaced overpack for some reasons. We have been paying attention to the method of slurrying bentonite buffer around overpack with fluid (salt solution) as an option method for freeing overpack. Some laboratory tests performed for finding out the fiesibility of this method. The results from the tests indicate that the buffer material, constituted of bentonite and quartz sand, can easily be dissolved in NaCl 3 or 4wt% solution, and dissolved material is deposited soon in the slurry. The results from the flushing experiments applied to a small specimen of buffer material suggest that flushing NaCl solution is effective for speedy stripping of buffer material.

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