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Ishidera, Takamitsu; Kurosawa, Seiichi*; Kibe, Satoshi*; Ouchi, Yuji*
Physics and Chemistry of the Earth, 65, p.60 - 65, 2013/00
Times Cited Count：1 Percentile：5.36(Geosciences, Multidisciplinary)The sorption behaviour of radionuclides depends on the kind of minerals contained in host rock. However, conventional investigations such as X-ray diffraction analysis cannot avoid large errors in quantification of minerals. In addition, the differences of sorption behaviour have been reported even among the same kind of minerals. For these reasons, it has been difficult to predict the sorption behaviour of radionuclides based on the quantification of mineral contents. The radionuclides sorption is caused by the uptake on several sorption sites on minerals. Therefore, each sorption site capacity should be evaluated for accurate estimation of radionuclides sorption behaviour. In this study, sorption behaviour of Cs on sedimentary rocks obtained from the underground around Horonobe Underground Research Center, JAEA, was investigated. The sorption sites capacities in the sedimentary rock of Horonobe area were estimated from Cs sorption isotherms under KCl and NaCl background electrolytes.
ions in Na- and Ca-montmorillonite mixturesKozaki, Tamotsu*; Sawaguchi, Takuma; Fujishima, Atsushi; Sato, Seichi*
Physics and Chemistry of the Earth, 35(6-8), p.254 - 258, 2010/00
Times Cited Count：25 Percentile：56.56(Geosciences, Multidisciplinary)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
with Ca. The Ca cations could be released from cementitious materials and diffuse into the buffer material in the repository. In this study, to evaluate the alteration that could reduce the performance of the bentonite buffer, the apparent diffusion coefficients of HTO and Ca ions were determined from non-steady, one-dimensional diffusion experiments using Na- and Ca-montmorillonite mixtures with different ionic equivalent fractions of Ca ions. The apparent diffusion coefficient of HTO at a dry density of 1.0 Mg m
slightly increased with an increase in the ionic equivalent fraction of Ca ions. However, the apparent diffusion coefficient of Ca and the activation energy for diffusion at the same dry density were independent of the ionic equivalent fraction of Ca ions. These findings suggest that unlike HTO, which can be postulated to diffuse mainly in pore water, Ca ion diffusion could occur predominantly in interlayer spaces.
Ishidera, Takamitsu; Ueno, Kenichi; Kurosawa, Seiichi*; Suyama, Tadahiro*
Physics and Chemistry of the Earth, 33(Suppl.1), p.S269 - S275, 2008/00
Times Cited Count：24 Percentile：52.24(Geosciences, Multidisciplinary)Carbon steel was placed into compacted bentonite under low-oxygen condition for about ten years at 80
C. Swelling characteristics of the bentonite were observed by X-ray diffractometry (XRD) using oriented samples from clay fractions of the bentonite, before and after treatment with ethylene glycol. Furthermore, the corrosion products migrating into the compacted bentonite were investigated by the selective dissolution analysis, which can estimate the crystallinity of Fe-bearing compounds. The valence of Fe in corrosion products was spectrophotometrically determined.
O and applying electric potential gradientTanaka, Shingo*; Noda, Natsuko*; Higashihara, Tomohiro*; Sato, Seichi*; Kozaki, Tamotsu*; Sato, Haruo; Hatanaka, Koichiro
Physics and Chemistry of the Earth, 33(Suppl.1), p.S163 - S168, 2008/00
In order to identify mass transport pathway in compacted bentonite, water transport behavior in compacted montmorillonite which is the major clay mineral constituent of the bentonite was studied. Back-to-back diffusion and electro-osmosis experiments for HO were carried out at montmorillonite densities of 1.0, 1.2 and 1.4 Mg/m
using HO as a tracer. Apparent diffusivities from the diffusion experiments and advection velosities and hydraulic dispersities from the electro-osmosis experiments were determined. The mass transport pathways were discussed by comparing with concentration profiles and peak positions of He, Na and Cl which were reported in the past. The hydraulic dispersities decreased in the order of He, HO, Cl and Na, and these differences were considered to be due to that transport pathway depended on species and hydraulic dispersity for each species also depended on transport pathway.
Yamaguchi, Tetsuji; Yamada, Fumika; Negishi, Kumi*; Hoshino, Seiichi; Mukai, Masayuki; Tanaka, Tadao; Nakayama, Shinichi
Physics and Chemistry of the Earth, 33(Suppl.1), p.S285 - S294, 2008/00
It is important to assess long-term alteration of engineered barrier composed of bentonite and cement for the safety assessment of radioactive waste disposal. In order to promote our development of the assessment methodologies, we developed a secondary mineral formation model for cement based materials and a hydraulic conductivity model for bentonite buffer materials. These models were verified by comparing with experimental observations. We also calculated changes in mineralogy of bentonite buffer materials and accompanying changes in the hydraulic conductivity over 10,000 y. We identified the temperature as an important factor dominating the alteration of the buffer. We also identified that the alteration is limited by slow kinetics of the dissolution of montmorillonite and by the diffusive mass transfer. Our calculation showed that the mineralogical change proceeds rather fast during the initial 1,000 y and slows down afterwards, and that salinity of the groundwater has both positive and negative effects on the hydraulic conductivity.
Sato, Haruo
Physics and Chemistry of the Earth, 33(Suppl.1), p.S538 - S543, 2008/00
Swelling pressure of bentonite was estimated based on the thermodynamic data of water on the surface of smectite which is a major clay mineral constituent of the bentonite composing engineered barriers and backfill materials. The activity and the relative partial molar Gibbs free energy (dEa) of water on the surface of smectite as a function of water content in a density range of 0.6-0.9 Mg/m. The activity and dG of water decreased with decreasing water content and showed similar trend to data of Kunipia-F bentonite (almost 100wt.% smectite) reported in the past. Swelling presure was estimated from the difference in the chemical potential balance of water in equilibrium state between free water and misturized smectite, and compared to data measured for various kinds of bentonites. The calculated swelling pressures were in good agreement with the measured data. This indicates that swelling pressure of bentonite can be calculated based on the thermodynamic data of water.
Sugita, Yutaka; Fujita, Tomoo; Takahashi, Yoshiaki*; Kawakami, Susumu; Umeki, Hiroyuki; Yui, Mikazu; Uragami, Manabu*; Kitayama, Kazumi*
Physics and Chemistry of the Earth, 32(1-7), p.32 - 41, 2007/00
Times Cited Count：6 Percentile：19.38(Geosciences, Multidisciplinary)The H12 repository concept for vitrified high-level radioactive waste was developed based on a multi-barrier system with the emphasis on robust engineered barrier performance to ensure its feasibility for a wide range of geological conditions typically observed in Japan. The buffer is clay-based and plays a very important role in the engineered barrier system (EBS). The decision to use a volunteer siting process requires maximum flexibility of the repository concept to allow it to be adapted to potential sites and hence a wide range of variants of the basic H12 repository design has been developed. In order to evaluate the feasibility and the applicability of different repository options to specific siting environments, NUMO has established a set of "design factors" which classify the aspects that need to be considered when evaluating the pros and cons of different repository options. A Na-type bentonite from Japan is used as the reference material for all clay-based repository components (buffer, backfill, clay plug, etc.). The characteristics of this bentonite (thermal, mechanical, chemical, hydraulic) have been examined with consideration of various practical constraints (limitation on the repository footprint, the influence of saline water, the interaction of hyperalkaline leachates and practical working environment, etc.). Clay-based seals, which close off the tunnels after emplacement of the EBS, may also be key components for assessment of the repository. Full analyses considering all engineered barrier components (buffer, backfill, clay plug, concrete lining, tunnel, concrete plug, host rock) that may be used in a repository will be an essential future task. As a first step towards this goal, a numerical analysis focusing on hydraulic behaviour at the intersections of the disposal tunnels and the main tunnel is presented to illustrate how the design requirements of clay-based seals can be determined.
Yamaguchi, Tetsuji; Sakamoto, Yoshifumi; Akai, Masanobu; Takazawa, Mayumi; Iida, Yoshihisa; Tanaka, Tadao; Nakayama, Shinichi
Physics and Chemistry of the Earth, 32(1-7), p.298 - 310, 2007/00
Times Cited Count：49 Percentile：74.23(Geosciences, Multidisciplinary)Dissolution rate of montmorillonite, diffusivity of hydroxide ion and permeability coefficient in compacted sand-bentonite mixtures were experimentally determined and formulated. A coupled mass-transport/chemical-reaction code was developed to predict variation in permeability of engineered bentonite barrier with alkaline fluid by using the formulae.
Ochs, M.*; Lothenbach, B.*; Shibata, Masahiro; Yui, Mikazu
Physics and Chemistry of the Earth, 29(1), p.129 - 136, 2004/01
Times Cited Count：24 Percentile：49.20(Geosciences, Multidisciplinary)Compacted bentonite is foreseen as the buffer material in the engineered barrier system of the geological disposal system. To derive 
d values and diffusion coefficients of radionuclides for performance assessment, it is critical to be able to describe the chemistry of the pore solution and to understand how it is influenced by different factors. This paper presents a sensitivity analysis on the influence of important parameters on porewater chemistry in compacted bentonite. The principal parameters varied were the fractions of calcite, gypsum, and NaCl dissolving from bentonite, and pCO. Model calculations treated ion exchange reactions of smectite and protonation/deprotonation at the edge of smectite using the diffuse layer model. Some calculations consider electric double layer effects in the porespace. The model results show that two powerful pH buffer systems are operative in compacted bentonite: Amphoteric edge SOH sites and the carbonate buffer system. If pCO is imposed externally, the resulting porewater pH is mainly controlled by the carbonate buffer. If bentonite is treated as a closed system, the buffering action of the SOH sites becomes more important. In both cases, the dissolution of calcite and gypsum from the bentonite is important. If the development of an electric double layer in the bentonite pores is considered, entire porespace may be occupied with diffuse layers, leaving no space for free porewater, at higher dry densities (1200 kg/m).
