In the safety assessment of the underground disposal of radioactive wastes, present models describing nuclide migration estimate the retardation effect of solid materials mainly by using sorption distribution coefficient K_d. However, since the sorption mechanisms are not well understood, some requirements should be met in using K_d in the transport models. The problem or difficulty arises from the fact that sorption is the heterogeneous reaction occurring at or near the solid-water interface and that geological media for radionuclide transport are heterogeneous in their physical and chemical properties. Based on the understanding that sorption occurs at or near the solid-water interface, this paper discusses the effectiveness of K_d in describing sorption phenomena and the validity and applicability of K_d to predict in situ radionuclide migration.

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  As a part of study on leaching behavior of solidified dry Low-Level Radioactive Waste(LLW), effects of the mix proportions of mortar raw materials on mortar fluidity, compressive strength, bleeding and shrinkage were examined. The relation between mortar fluidity and other properties, and the mix proportion of mortar raw materials for solidified dry LLW were also examined. Better mortar fluidity, low compressive strength, high bleeding and high shrinkage showed when water/cement ratio was high. Large effects of water/blast furnace cement ratio on these properties were clarified. This difference between blast furnace cement and ordinary portland cement results from the existence of the fine blast furnace particles and small contents of cement in blast furnace cement. Mortar fluidity was also improved as chemical agent/cement ratio increased up to a certain value. Shrinkage decreases with increasing fine aggregate/cement ratio. This must be caused by the volume reduction while cement hydration. Furthermore, production of bleeding water, segregation and drastic decrease of compressive strength were recognized when mortar fluidity of blast furnace cement was good. The mix proportion of mortar raw materials for solidified dry LLW was proposed in consideration of mortar fluidity, compressive strength, bleeding and shrinkage.

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  Dry Low-Level Radioactive Waste(LLW), incombustible solid LLW, generated from nuclear power stations is planed to be solidified with cement backfill in drums. The solidified dry LLW will be buried to shallow underground at Rokkasyo LLW Disposal Center. It is well known that corrosion of aluminum and hydrogen gas evolution occur in high pH environments such as mortar. Gas evolution from aluminum is likely to affect the leachability of solidified dry LLW with mortar. Though aluminum removal from dry LLW is planed, a small amount of aluminum will be actually included in dry LLW. Large effects of pH and temperature on corrosion rate of aluminum and gas evolution were recognized in our previous study. It was also found that 1.5 mole hydrogen gas evolves while 1 mole aluminum corrodes under 60°C. Actually aluminum in drums is likely to contact with carbon steel of which main element is iron. The gas evolution behavior of aluminum is expected to be affected by its direct contact with iron. Therefore, effect of direct contact with iron on gas evolution behavior of aluminum was studied. The corrosion rate of aluminum increased by contacting it with iron in simulating mortar environments. The amount of gas evolution from aluminum was reduced by contacting with iron. The reduction in gas evolution was considered to result from the change of cathode reaction from hydrogen evolution to oxygen reduction. When aluminum contacts with iron, the corrosion and gas evolution behavior of aluminum is significantly affected oxygen in environment.

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  In high level radioactive waste disposal project, the workstages after the selection and the investigation of the disposal site can be divided into the three phases of construction, operation and sealing. Among these phases, the purpose of sealing is the securing of long term safety for the repository to prevent the radionuclide migration speed increasing at shaft, tunnel and disturbed zone of host rock , by means of backfill, plugs and grout after waste has been positioned in the repository.
  This report proposes a design method of sealing system.

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  Groundwater colloids which adsorb actinide ions play an important role in actinide migration. The existence of colloids causes an effect of facilitating the migration, while it also causes a possibility of retardation of colloid transport due to filtration effect. The filtration phenomenon in porous media is dominated by deposition of colloidal particles onto stationary grain surfaces from flowing suspensions. The deposition behavior depends on the chemical characteristics of the particle surfaces and grain surfaces and on the solution chemistry, while the mechanisms of the behavior are not understood enough under the subsurface aquatic environment. In this study, we carried out column and batch experiments to investigate effects of flow field and deposited particles on the filtration behavior in porous media. We clarified the importance of the effects in quantifying the filtration effect in the migration of colloids through porous media.

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  The behavior of actinide elements including uranium in geomedia is controlled by redox conditions. Under the oxidized conditions, uranium forms uranyl ion (UO₂²⁺) and its complexes, and dissolves in ground water. Under the reduced conditions, U(IV) has much lower solubility than uranyl ion. In the Koongarra uranium deposit, Australia, lead-bearing uraninite, uranyllead oxide and uranyl silicate minerals occur in the unweathered, primary ore zone, and uranyl phosphate minerals occur in the weathered, secondary ore zone. Between unweathered and weathered zones, the transition zone exists as a redox front. In the transition zone, graphite and sulfide minerals react as reducing agents for species dissolved in ground water. By SEM, spherical grains of uraninite were observed in veins with graphite. Pyrite had coffinite rim with crystals of uraninite. Calculation based on the ground water chemistry and hydrology at Koongarra shows that the uranium in the transition zone may be fixed from the ground water. In the Koongarra transition zone, recent mineralization of uranium by reduction takes place. Mineralization is much stronger fixation mechanism than adsorption on clay minerals. Pyrite in the buffer materials of possible radioactive waste repositories can fix radionuclides in oxidized ground water by mineralization with reducing reactions.

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  By using the semi-empirical molecular orbital calculation code MOPAC, the structure of ringed silicate cluster such as the bond distance and bond angle of Si-O-Si, the formation free energy and infrared spectra were evaluated as a function of the number of Si. Infrared spectra of amorphous silica were experimentally observed at different temperatures, and compared with the evaluated ones by MOPAC. It was suggested that MOPAC is a useful tool for structure estimation of amorphous silica.

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  Low-level dry active wastes of nuclear power plants are grouted with cement mortal in a container and planned to disposed into the shallow land disposal site. The characteristics of radionuclides contained in dry active wastes are same as homogeneous solidified wastes. In the previous report[1], we reported the applicability of the radioactivity evaluation methods established for homogeneous solidified wastes to pre-packed concrete packages. This report outlines the developed radioactivity evaluation methods for pre-packed concrete packages based upon recent data.
  Since the characteristics of dry active wastes depend upon the plant system in which dry active wastes originate and the types of contamination, sampling of wastes and activity measurement were executed to derive scaling factors. The radioactivity measurement methods were also verified. The applicability of non-destructive methods to measure radioactivity concentration of pre-packed concrete packages was examined by computer simulation. It is concluded that those methods are accurate enough to measure actual waste packages.

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