The decommissioning and dismantling of nuclear power plants will produce a large quantity of non-active waste concrete. From the viewpoint of recycling of this waste concrete the recovery of aggregate contained in concrete at 80% and reuse of it into a new plant construction are envisioned. For these purposes we have studied the recovery process of aggregate from concrete composed of a heating step followed by a milling step onto waste concrete blocks. We have found that higher operation temperature brings a better effect for the separation of aggregate from a concrete body, however too high temperature may reversely degrade a quality of recovered aggregate itself. The most effective heating temperature which is considered not to give the damage to a quality of aggregate stays between 200～500℃. The effect of a duration at such temperature zone is relatively small. As a conclusion we have found that 300℃ of heating temperature and 30～120 minutes of a duration in a rod mill with high efficiency of rubbing work for getting coarse aggregate and an agitate mill for fine aggregate might be proper operating conditions under which we can recover both coarse and fine aggregate with the quality within JASS 5N standard.

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  Concrete used for LLW facilities is required to be highly durable. The authors evaluated concrete containing glycol ether derivatives and silica fume as admixtures. Compressive strength, diffusion coefficient of water, depth of accelerated carbonation, drying shrinkage, depth of chlorides penetration and resistance to freezing and thawing were investigated using concrete specimens. Compressive strength, depth of accelerated carbonation, diffusion coefficient of ¹³⁷Cs were investigated using mortar specimens before and after irradiation of gamma rays. Results showed that using glycol ether derivatives and silica fume was effective in improving the durability.

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  Inorganic admixtures to enhance the fluidity of cement material was developed. These admixtures turned into easy to immobilize the miscellaneous radioactive waste using cement material. It was found that the ζ potential of cement particles was directly proportional to the content of the inorganic admixtures in cement paste and the particles of cement were dispersed at the high ζ potential. The condensed sodium phosphate, which was the main component of the inorganic admixtures, retarded the dissolution of Ca²⁺ ion from the cement, and generated the colloids by incorporating dissolved Ca²⁺ ion. The cement material containing the inorganic admixtures was found to have the same mechanical strength and adsorption potential of radionuclides in comparison to normal cement materials. It was confirmed that the cement material containing the inorganic admixture was effectively filled gaps of miscellaneous radioactive waste.

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  A study of solidification of radioactive wastes by a tailored alkali activated slag cement (AASC) was carried out in both bench size tests and pilot plant tests.
The target wastes were concentrates,ion exchange resin,incinerator ash and HEPA filter in nuclear power stations and reprocessing facilities. In addition, an application of AASC was studied to be used in the cement grouting. It was confirmed that the solidified waste forms have a high content of each waste and suitable physical and chemical properties.
  The data of retention ability of radionuclides in the waste forms were obtained experimentally and it was suggested that the waste forms have the satisfactory retention required for safety operations in LLW disposal site.
  In this paper, the solidification behavior of AASC for the each waste was reported and an integrated treatment system was discussed and compared with the current solidification systems.

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  A large reduction of total radioactive waste is expected, if recycled cement from the waste concrete of decommissioned nuclear power plants would be able to be used the material for backfilling mortar among the miscellaneous waste. In this paper, we discuss the hydration, strength and consistency of recycled cement compared with normal portland cement.
  The strength of recycled cement mortar is lower than that of normal portland cement mortar on the same water to cement ratio. It is possible to obtain the required strength to reduce the water to cement ratio by using of high range water-reducing AE agent.
  According to reducing of water to cement ratio, the P-type funnel time of mortar increase with the increase of its viscosity. However, in new method of self-compactability for backfilling mortar, it became evident that there was no difference between the recycled cement and normal porland cement on the self-compactability.

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  The sorption mechanism of inorganic C-14(CO₃^2-) was investigated using batch sorption experiments, XPS analyses, and zeta potential measurements. The results suggested that C-14 was adsorbed onto the cement surface by an electrostatic force, due to the reaction between SiO₂ and CaO contained in the cementitious composition. That is, SiO₂ was originally negatively charged (SiO^-) in cement, but became positively charged (SiO-Ca⁺) through the interaction of Ca²⁺. These positive sites on the SiO₂ surface adsorbed inorganic C-14. Based on these investigations, the sorption mechanisms of Cs-134, Co-60 and Am-241 were clarified from the view point of electrostatic adsorption. The results suggested Cs was adsorbed by negative sites (SiO^-) in cement, while Co-60 and Am-241 were adsorbed by the reaction product between SiO₂ and CaO in cement.
  Ordinary Portland cement (OPC) did not contain enough SiO^- compared with its CaO content to produce sufficient numbers of adsorption sites. In order to raise the distribution coefficient (Kd) of each radionuclide, blast furnace slag, which produces enough SiO^-, was added to optimize the Ca²⁺/SiO^- ratio in cement. All Kd values were increased by adding 50 wt% blast furnace slag into OPC.

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  After closure of the radioactive waste repository, a cementitious material contacts groundwater and is gradually altered by leaching of component from itself for a long term. Leaching experiments were carried out to investigate effects of silica fume in cement on leaching behavior of cement paste. Chemical composition in leachate and solids before and after experiments were analyzed. Modelling for leaching of cement pastes were carried out by using thermodynamic model of C-S-H gel based on Berner model and the results were compared with experimental results.

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  In the radioactive waste disposal system, cement materials have been used as (1) structual materials in disposal facilities, (2) solidifying materials for enclosing radioactive nuclides, and (3) internal filling mortar for the space between the waste and the concrete. One of the important features of cement materials is its ability to establish a high pH in the water within the repository. A high pH is beneficial because in this condition the solubilities of many radionuclides, especially TRU,in aqueous solution are low. It is necessary to maintain the high degree of sorption of radionuclides based on the high pH-for the long term safety of the system. Then the important thing is to gain a reasonable evaluation of the pH within the repository. Atkinson et al. have presented a method of evaluating pH in the leaching groundwater from cement. They predicted the dissolution property of cement based on a simplified model using equilibrium data for the CaO-SiO₂-H₂O system. In the present report, we explain the Atkinson proposed method of evaluating the pH in the repository. Our studies on solubility of cement hydrate are also discussed.

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  Gas migration properties of bentonite/sand mixture in a full-scale structure are actually estimated on the basis of laboratory test results obtained by small test specimen. Water and gas migration tests are carried out for clarifying the scale effect of the mixture on migration properties.

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  Knowledge on microporous structure in rocks is important for quantitative understanding of diffusion of ions in the rock. When the Fickian diffusion theory is applied to the diffusion of ions in rocks, the pore size is assumed to be significantly larger than the diffusing ions and the pore structure is assumed to be uniform. The water saturation method and the mercury intrusion porosimetry were applied to the characterization of micropores in granite. Although these techniques have been commonly used to characterize pores in rocks, special cares are needed for precise measurement of the porosity and the pore size of low-porosity rocks. The water saturation method uses water-saturated weight, submerged weight and dry weight of the rock samples to calculate the porosity. The errors accompanied in the measurements of the water-saturated weight often bring about significant errors in determination of the porosity. Analysis of drying curves of water-saturated weight enabled precise measurements of porosity. In the mercury intrusion porosimetry, a major part of the error in the porosity determination is due to rough surfaces of the sample. For the porosity determination described in this paper, the rock sample was cut into rectangular parallelepipeds, instead of crushing, to reduce the surface roughness. The detection limit of the mercury intrusion is also a cause for the error for low-porosity samples. Five samples whose total volume was 6 ml were simultaneously put in the measurement cell of the porosimeter to increase the amount of intruding mercury. These effort made reliable characterization of pores in a granite from Inada, Ibaraki, Japan and pores in a granite from the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, Manitoba, Canada. The distribution of the pore diameter of these granites were nearly logarithmic normal. The porosity of Inada granite was determined to be (0.49±0.07) % with the modal diameter of 160 nm. The micropores of a few tens to a few hundred nanometers in width were observed by SEM. The URL granite had the porosity of (0.40±0.10) % and the modal diameter of 340 nm. The assumptions in applying the Fickian diffusion theory to the diffusion of ions in porous materials were confirmed for these granites.

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