Yukio Tachi

• Doctor of Engineering
• Japan Atomic Energy Agency

Palladium-107 is one of the selected radionuclides in the safety assessment of geological disposal of radioactive waste. Although isosaccharinic acid (ISA) forms strong complexes with many elements and enhances element solubility, the thermodynamic evaluation of the complex of Pd with ISA has not been conducted. In this study, the solubility of Pd(...

Palladium-107 is one of the selected radionuclides in the safety assessment of geological disposal of radioactive waste. Although isosaccharinic acid (ISA) forms strong complexes with many elements and enhances element solubility, the thermodynamic evaluation of the complex of Pd with ISA has not been conducted. In this study, the solubility of Pd(...

The REPRO-TDE test was performed at a depth of about 400 m in the ONKALO underground research facility in Finland. Synthetic groundwater containing radionuclide tracers (HTO, Cl-36, Na-22, Ba-133, Cs-134) was circulated for about four years in a packed-off interval of the injection borehole. Tracer activities were additionally monitored in two obse...

Structural determination of adsorbed atoms on layered structures such as clay minerals is a complex subject. Radioactive cesium (Cs) is an important element for environmental conservation, so it is vital to understand its adsorption structure on clay. The nuclear magnetic resonance (NMR) parameters of 133Cs, which can be determined from solid-state...

The understanding of the swelling phenomenon of montmorillonite is essential to predict the physical and chemical behavior of clay-based barriers in radioactive waste disposal systems. This study investigated the key factors controlling crystalline swelling behavior of montmorillonite with different interlayer counter-ions by molecular dynamics (MD...

Fine-grained and clay-rich mudrocks play an important role in the long-term performance of a geological repository for storing high-level nuclear wastes and petroleum production in shale formations. However, low-permeability mudrocks whose pores are poorly interconnected are known to have anomalous diffusion properties that strongly impact long-ter...

This study shows a comparison and analysis of results from a modelling exercise concerning a field experiment involving the transport and retention of different radionuclide tracers in crystalline rock. This exercise was performed within the SKB Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS). Task 9B of the...

Mineral and organic matter compositions & pore structures of fine-grained shale influence reservoir properties. To improve the understanding of spatial heterogeneity in core-sized samples, methods of microscale X-ray fluorescence (μXRF) mapping, (ultra-) small-angle x-ray scattering [(U)SAXS] and wide-angle X-ray scattering (WAXS) have been used to...

The construction of a repository for the geological disposal of radioactive waste will include the use of cement-based materials. Following closure, groundwater will saturate the repository, and the extensive use of cement will result in the development of a highly alkaline porewater, pH > 12.5; this fluid will migrate into and react with the host...

The SKB GroundWater Flow and Transport of Solutes Task Force is an international forum in the area of conceptual and numerical modeling of groundwater flow and solute transport in fractured rocks relevant for the deep geological disposal of radioactive waste. Two in situ matrix diffusion experiments in crystalline rock (gneiss) were performed at PO...

The sufficient retardation of radionuclide release by sorption is a central issue in assessing the safety of radioactive waste disposal systems and risks posed by contaminated sites. Various types of radioactive wastes and natural environments contain organic substances that can stabilize aqueous radionuclide complexes and therefore lead to a decre...

Dissolved inorganic carbon (DIC) present in groundwaters can affect both the aqueous and surface species of hexavalent uranium (U(VI)) owing to its strong complexation ability with U(VI). However, the effect of DIC on U(VI) sorption on illite, which is a critical component of argillaceous rocks, is still unclear. In this study, we investigated the...

The in‐situ long‐term sorption and diffusion (LTDE‐SD) experiment performed at the Äspö Hard Rock Laboratory in Sweden aims at increasing knowledge of sorption and diffusion processes of radionuclides in natural fractures and granodiorite matrix under in‐situ conditions. This paper presents a comprehensive modeling exercise focusing on the scaling...

The construction of a repository for geological disposal of radioactive waste will include the use of cement-based materials. Following closure, groundwater will saturate the repository and the extensive use of cement will result in the development of a highly alkaline porewater, pH > 12.5; this fluid will migrate into and react with the host rock....

Mechanistic understanding and predictive modeling of radionuclide diffusion in Na- and Ca-montmorillonite are essential to evaluate the long-term evolution of the bentonite barrier and their impact on radionuclide migration during geological disposal of radioactive wastes. Thus, the diffusion behavior of ¹³⁷Cs⁺, ¹²⁵I⁻, and tritiated water (HTO) in...

The construction of a repository for the geological disposal of radioactive waste will utilize cement-based materials. Following closure, resaturation will result in the development of a highly alkaline porewater. The alkaline fluid will migrate and react with host rock, producing a chemically disturbed zone (CDZ) around the repository. To understa...

Several repository concepts have been proposed for the disposal of radioactive wastes, some of which include argillaceous (clay-rich) host rocks and cementitious engineered barriers. The presence of hyperalkaline cement pore-fluid results in the destabilization of primary minerals in argillaceous rocks, leading to alteration at the interface betwee...

The understanding of the swelling phenomenon of montmorillonite is essential to predict the physical and chemical behavior of clay-based barriers in geoenvironmental engineering. This study investigated the crystalline swelling behavior of montmorillonite with different interlayer cations including monovalent Na, K, and Cs and divalent Ca and Sr by...

A new method of evaluating colloid diffusion and filtration in compacted bentonites using dendrimers was developed. Polyamidoamine (PAMAM) dendrimers with sizes of 5.7 and 7.2 nm were chosen as diffusion probes owing to their high monodispersity and well-defined molecular structure. The diffusion and filtration behaviors were investigated by a thro...

This study investigated the pore distributions in water-saturated Ca-montmorillonite using ¹H nuclear magnetic resonance (NMR) measurements for various dry densities (0.8–1.6 g/cm3) and porewater salinity conditions (deionized water, 0.1 and 1 M CaCl2) at a temperature range of 233–303 K. The volume fractions of the interlayer pore and the non-inte...

In the geological disposal system of high-level radioactive and transuranic wastes, an increase in Ca concentration (arising from the alteration of cementitious materials) could affect the retention of other radionuclides due to competitive sorption. Batch sorption experiments were performed to investigate the sorption behavior of Ca and its compet...

Immobilization of Se(IV) by adsorption on iron oxides exerts a key control on its mobility in a number of natural and engineered settings; however, the presence of other coexisting ligands may influence the extent and stability of adsorption. In this work, we investigated the impacts of Si, which is an abundant component of many natural waters, on...

Safety functions for the clay buffer in a repository for high-level radioactive waste (HLW) are fulfilled if the presence of montmorillonite with high swelling capacity and low permeability is maintained in the long-term. The transformation of montmorillonite to the non-swelling mineral illite is addressed in most safety assessments by using simple...

Batch sorption experiments were performed to investigate the sorption mechanism of Se on montmorillonite under reducing conditions in deep geological environments. Based on Eh–pH diagrams and ultraviolet–visible spectra, Se was dissolved as selenide (Se(–II)) anions under the experimental conditions. The distribution coefficients (Kd; m3 kg−1) of S...

We systematically investigated the sorption and fixation behaviors of radiocesium (¹³⁷Cs) for sediments taken from the rivers of Ukedo and Odaka around the Fukushima Daiichi Nuclear Power Plant. By comparing the Cs sorption and sequential desorption results at various Cs concentrations, across a range of sediment properties, we were able to underst...

In order to elucidate the radiocesium transport behaviors in natural environment, we systematically investigated sediments from the highly contaminated rivers of Ukedo and Odaka around the Fukushima Daiichi Nuclear Power Plant. We focused on determining the key factors controlling the radiocesium sorption and fixation, such as variations in the par...

The interaction of FeII and Si is at the heart of many critical geochemical processes in diverse natural and engineered environments. The resulting FeII-silicate phases play important roles in regulating the concentrations and bioavailability of FeII and Si, as well as serve as sinks for trace and hazardous elements. Therefore, a detailed understan...

Natural systems data can provide important constraints upon the performance and longevity of montmorillonite in clay engineered barriers in a geological disposal facility for radioactive wastes. Here, natural systems evidence for the effects and impacts of temperature and the activity of aqueous silica upon montmorillonite stability and behaviour i...

Microstructural and mass transport properties of compacted Na- and Cs-montmorillonites (Na-MMT and Cs-MMT) with different swelling properties at dry densities of 0.8 and 1.25 Mg m−3 were investigated by combining three-dimensional microstructure analysis using nanofocus X-ray computed tomography (CT), X-ray diffractometry (XRD), and through diffusi...

Fine-scale alterations found on fracture surfaces and between fracture surfaces and rock matrix are key uncertainties that need to be considered when developing solute transport models for fractured crystalline rocks. This paper presents a comprehensive approach developed for coupling laboratory tests, microscopic observations, and modeling in orde...

The adsorption sites of Cs on montmorillonite clays were investigated by theoretical 133Cs chemical shift calculations, 133Cs magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and X-ray diffraction under controlled relative humidity. The theoretical calculations were carried out for structures with three stacking variations in...

At the Fukushima Daiichi nuclear power station, spent zeolite used to remove radionuclides from contaminated water will be disposed of as the secondary wastes of water treatment. To estimate radionuclides inventory in the spent zeolite, sorption ability of radionuclides needs to be investigated. In this study, distribution coefficients (Kd) of U an...

The clean-up activities related to the accident at the Fukushima nuclear power plant give rise to several types of wastes containing cementitious materials as well as substances that can influence radionuclide migration, especially organic complexing substances, boron, and chloride salts. The present study focuses on a methodology, based on availab...

Various types of post-accident radioactive waste have been generated from cleanup and decommissioning activities at the Fukushima Daiichi nuclear power plant. Perturbation effects resulting from co-existing substances may influence radionuclide sorption. For the disposal of these wastes, perturbation effects on sorption parameters must be criticall...

Due to the accident of Fukushima Daiichi Nuclear Power Plant (NPP), a large amount of contaminated water has been generated in such as the reactor building. Concrete materials of the building have been in contact with contaminated water for a long time, therefore radionuclides in contaminated water may be penetrating into these concrete materials....

Concrete debris contaminated with radioactive cesium and other nuclides has been generated by the accident in the Fukushima Dai-ichi nuclear power plant. Moreover, there is concern that a large amount of radioactive concrete waste will be generated by the decommissioning of nuclear power plants in the future. Although conventional techniques are ef...

Sorption of radionuclides on host rocks is a key process in the safe geological disposal of radioactive waste. For performance assessment (PA) calculations, the magnitude of sorption, expressed normally by a distribution coefficient (Kd), needs to be determined taking into account the rock types and geochemical conditions and associated uncertainty...

The reactivity and stability of edge faces of swelling clay minerals can be altered by the layer charge and stacking structure; however, these effects are poorly understood owing to experimental limitations. The structure and stability of montmorillonite edge faces {110}, {010}, {100}, and {130} with layer charges y = 0.50 and y = 0.33 (e−/Si4O10)...

The Japanese geological disposal program has started researching disposal of spent nuclear fuel (SF) in deep geological strata (hereafter “direct disposal of SF”) as an alternative management option other reprocessing followed by vitrification and geological disposal of high-level radioactive waste. We conducted literature survey of dissolution rat...

Although spent nuclear fuel is planned to be disposed after reprocessing and vitrification of high-level radioactive waste (HLW), feasibility study on direct disposal of spent nuclear fuel (SF) has been started as one of the alternative disposal options to flexibly apply change of future energy situation in Japan. Radionuclide inventories and their...

The estimation of water population in water-saturated compacted clay was studied with ¹H NMR relaxometry and freezing point depression. The clay samples and saturated condition were Na-montmorillonite at 0.8 and 1.4 g/cm³ saturated by three salt concentrations (deionized water, 0.1 mol/L and 1.0 mol/L). The water-saturated compacted samples were pr...

Molecular dynamics (MD) simulations were performed to study the diffusion and adsorption behaviors of aqueous uranyl species in interlayer space of clay minerals, which are key processes in the safety assessment for geological disposal of radioactive waste. The diffusion behaviors of UO22+, K+, CO32− and Cl− and H2O were evaluated for the aqueous s...

This CMS Workshop Lecture Series (WLS) volume is intended to give a summary of the current state-of-the-art of different spectroscopy and microscopy methods, as presented during a workshop held in conjunction with the EUROCLAY 2015 conference in Edinburgh, UK, on the 5th of July 2015. This workshop was initiated by the NEA Clay Club, The Clay Miner...

This CMS Workshop Lecture Series (WLS) volume is intended to give a summary of the current state-of-the-art of different spectroscopy and microscopy methods, as presented during a workshop held in conjunction with the EUROCLAY 2015 conference in Edinburgh, UK, on the 5th of July 2015. This workshop was initiated by the NEA Clay Club, The Clay Miner...

The influence of pH and concentrations of Eu3+ and NaNO3 on the sorption of Eu3+ to Na-montmorillonite were investigated through batch sorption measurements and time-resolved laser fluorescence spectroscopy. The pH had little effect on the distribution coefficients (Kd) in the range of pH 4–7 at 0.01 M NaNO3, which indicates that the cation exchang...

An in-situ diffusion experiment was performed at the Grimsel Test Site (Switzerland). Several tracers (3H as HTO, 22Na+, 134Cs+, 131I- with stable I- as carrier) were continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of about two years. Subsequently, the bo...

Matrix diffusion and sorption are important processes controlling radionuclide transport in crystalline rocks. Such processes are typically studied in the laboratory using borehole core samples however there is still much uncertainty on the changes to rock transport properties during coring and decompression. It is therefore important to show how s...

The use of generic sorption data in PA requires the transfer of the data to the PA-specific conditions. A site-specific Kd setting approach for PA calculations was tested, comparing two data transfer procedures. First transfer of sorption data can be done through semi-quantitative estimation procedures, by considering differences between experiment...

The potential effect of high pH plume caused by cementitious materials must be evaluated in the performance assessment for HLW geological disposal. Alkaline plume would lead to change sorption properties of host rock by primary mineral dissolution, secondary mineral precipitation and sequential change of pore water chemistry. In this study, the eff...

To predict the long-term migration of radionuclides (RNs) under variable conditions within the framework of safety analyses for geological disposal, thermodynamic sorption models are very powerful tools. The integrated sorption and diffusion (ISD) model for compacted bentonite was developed to achieve a consistent combination of clay–water interact...

It is important to understand the coupled processes of sorption and diffusion of radionuclides (RNs) in compacted bentonite, and to develop mechanistic models that can aid in the prediction of the long-term performance of geological disposal systems of radioactive waste. The integrated sorption and diffusion (ISD) model was developed based on the c...

The Fukushima Dai-ichi Nuclear Power Plant accident has resulted in a large amount of radioactively contaminated concrete. The aim of this study is to investigate the possible application of the pulsed power discharge to reduce the amount of contaminated concrete as radioactive waste. An applicability of pulsed power discharge as a decontamination...

Montmorillonite clay is an important component of barrier materials such as bentonites and argillaceous rocks for the safe geological disposal of radioactive waste. The diffusion and sorption behaviors of cationic Cs⁺ and Na⁺, anionic I⁻ and neutral tritiated water (HTO) tracers in compacted montmorillonite were investigated as a function of porewa...

The integrated sorption and diffusion (ISD) model has been developed to quantify radionuclide transport in compacted bentonite. The current ISD model, based on averaged pore aperture and the Gouy-Chapman electric double layer (EDL) theory can quantitatively account for diffusion of monovalent cations and anions under a wide range of conditions (e.g...

Matrix diffusion is a key process for radionuclide retention in crystalline rocks. Within the LTD project (Long-Term Diffusion), an in-situ diffusion experiment in unaltered non-fractured granite was performed at the Grimsel Test Site (www.grimsel.com, Switzerland). The tracers included 3H as HTO, 22Na+, 134Cs+ and 131I- with stable I- as carrier.T...

Thermodynamic sorption models (TSMs) offer the potential to improve the incorporation of sorption in environmental modelling of contaminant migration. One specific application is safety cases for radioactive waste repositories, in which radionuclide sorption on mineral surfaces is usually described using distribution coefficients (Kd values). TSMs...

JAEA and NUMO have conducted a collaborative research work which is designed to enhance the methodology of repository design and performance assessment in preliminary investigation phase. The topics and the conducted research are follows; (1) Study on selection of host rock: in terms of hydraulic properties, items for assessing rock property, and a...

Diffusion and sorption behaviors of cationic Cs+, anionic I− and neutral HTO in samples of the Wakkanai Formation from the Horonobe underground research laboratory (URL), Japan, were investigated as a function of ionic strength (I) of synthetic groundwater by through-diffusion and batch sorption experiments and mechanistic modeling. The effective d...

In high-level waste repository, a carbon steel overpack will be corroded after closure. This will create a reducing environment in the vicinity of the repository. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as uranium. The apparent diffusion coefficients of uranium were measured in compacted benton...

Influence of operation factors in diffusion test of compacted bentonite (such as agitation of test solution in the reservoir, feed rate of the test solution and mass transfer resistance in the filter) on the diffusion data was examined by reservoir depletion (RD) test method using Cs+. The influence of these factors on the diffusion data was also a...

In a high-level waste repository, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. This will create a reducing environment in the vicinity of repository. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as uranium. The apparent diffusivities of uraniu...

Sorption and diffusion behaviors of Eu in sedimentary rock in the presence of humic substance were investigated. The sedimentary rock collected from 500 m depth of HDB-6 bore hole at horonobe URL site of Japan and Aldrich humic acid (HA) were used in the present study. Sorption behaviors of Eu and the HA on the sedimentary rocks with and without th...

Diffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective di...

The research and development plan for geological investigation, engineering technology and safety assessment during the drilling of a shaft down to intermediate depth are summarized according to the Midterm Plan till 2009 Fiscal year of JAEA. This report describes subject, current status and programme in the "Phase 2: Construction phase" (investiga...

Diffusion and sorption of cesium (Cs) and iodine (I) were investigated in a purified and moderately compacted sodium montmorillonite (dry density of 800 kg m-3) saturated with 0.01, 0.1 and 0.5M NaCl solutions. The effective diffusivity (De) and capacity factor (α) for Cs and I were measured by through-diffusion experiments, coupled with multiple c...

Sorption and diffusion of Cs in the sedimentary rock from the Horonobe generic URL were studied from the viewpoints of reliability of experimental evaluation and model prediction, focusing on its applicability to intact systems. The distribution coefficient, Kd, for Cs was measured under the same chemical conditions by both batch sorption and diffu...

The influence of humic acid (HA) on solubility of radioactive elements [Se(IV), Th(IV)] and on their retardation parameters in sedimentary rock under groundwater conditions was studied using sedimentary rocks obtained from Horonobe URL site and commercially supplied humic acid as a model organic substance. The retardation parameters were evaluated...

High-level radioactive waste (HLW) from the reprocessing of spent nuclear fuel is to be disposed of deeper than 300m below the ground surface in Japan and the operation of disposal is planed to start in 2033 to 2037. The JNC's program and examples of the recent progress on the R&D of geological disposal of the HLW are described in this paper. JNC w...

The sorption and desorption behavior of radium on bentonite and purified smectite was investigated as a function of pH, ionic strength and liquid to solid ratio by batch experiments. The distribution coefficients (Kd) were in the range of 10(2) to > 10(4) ml g-1 and depended on ionic strength and pH. Most of sorbed Ra was desorbed by 1 M KCl. The r...

Sorption and diffusion behavior of Se in tuff was studied by batch sorption and through-diffusion experiments. Tuff samples used in the experiments were obtained from the Toki Lignite-Bearing Formation, Tono area, Gifu, central Japan. The experiments were carried out in a nitrogen glove-box with an oxygen level less than 1 ppm at room temperature....

In high-level waste repositories, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. Iron corrosion products are expected to interfere with migration of radionuclides by filling the pore in bentonite and sorbing radionuclides. In this study the apparent diffusion coefficients of cesium and strontiu...