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Journal Articles

Modelling concrete degradation by coupled non-linear processes

Oda, Chie; Kawama, Daisuke*; Shimizu, Hiroyuki*; Benbow, S. J.*; Hirano, Fumio; Takayama, Yusuke; Takase, Hiroyasu*; Mihara, Morihiro; Honda, Akira

Journal of Advanced Concrete Technology, 19(10), p.1075 - 1087, 2021/10

Concrete in a transuranic (TRU) waste repository is considered a suitable material to ensure safety, provide structural integrity and retard radionuclide migration after the waste containers fail. In the current study, coupling between chemical, mass-transport and mechanical, so-called non-linear processes that control concrete degradation and crack development were investigated by coupled numerical models. Application of such coupled numerical models allows identification of the dominant non-linear processes that will control long-term concrete degradation and crack development in a TRU waste repository.

Journal Articles

Observation of collapse behavior of bentonite during swelling pressure test using X-ray CT measurement

Takayama, Yusuke; Kikuchi, Hirohito*

Doboku Gakkai Rombunshu, C (Chiken Kogaku) (Internet), 77(3), p.302 - 313, 2021/09

Numerous swelling pressure tests have been conducted to understand the swelling properties of bentonite which is planned to be used as a buffer material in repositories for the geological disposal of radioactive waste. In this study, in order to clarify the cause of the decrease in swelling pressure during the swell-in pressure test period, the change in wet density distribution inside the specimen during the swelling pressure test was observed by X-ray CT measurement. It was supposed that this phenomenon was caused by the collapse inside the specimen. Furthermore, in order to confirm that collapse is generated by water absorption, the swelling deformation test was carried out under various load conditions. As a result, it was confirmed that collapse occurs even under the load conditions that are equal to or slightly smaller than the swelling pressure. These test data are expected to be used for validation of coupled analysis codes for evaluating the mechanical behavior of disposal facilities during re-saturation period.

Journal Articles

HM and THM interactions in bentonite engineered barriers for nuclear waste disposal

Gens, A.*; Alcoverro, J.*; Blaheta, R.*; Hasal, M.*; Michalec, Z.*; Takayama, Yusuke; Lee, C.*; Lee, J.*; Kim, G. Y.*; Kuo, C.-W.*; et al.

International Journal of Rock Mechanics and Mining Sciences, 137, p.104572_1 - 104572_19, 2021/01

 Times Cited Count:3 Percentile:96.18(Engineering, Geological)

Bentonite-based engineered barriers are a key component of many repository designs for the confinement of high-level radioactive waste and spent fuel. Given the complexity and interaction of the phenomena affecting the barrier, coupled hydro-mechanical (HM) and thermo-hydro-mechanical (THM) numerical analyses are a potentially useful tool for a better understanding of their behaviour. In this context, a Task (INBEB) was undertaken to study, using numerical analyses, the hydro-mechanical and thermohydro-mechanical Interactions in Bentonite Engineered Barriers within the international cooperative project DECOVALEX 2019. Two large scale tests, largely complementary, were selected for modelling: EB and FEBEX. The EB experiment was carried out under isothermal conditions and artificial hydration and it was dismantled after 10.7 years. The FEBEX test was a temperature-controlled non-isothermal test combined with natural hydration that underwent two dismantling operations, a partial one after 5 years of heating and a final one after a total of 18.4 years of heating. Direct observation of the state of the barriers was possible during the dismantling operations. Four teams performed the HM and THM numerical analyses using a variety of computer codes, formulations and constitutive laws. For each experiment, the basic features of the analyses are described and the comparison between calculations and field observations are presented and discussed. Comparisons involve measurements performed during the performance of the test and data gathered during dismantling. A final evaluation of the performance of the modelling closes the paper.

JAEA Reports

Impact assessment of density change on the buffer material on the coupled thermal-hydraulic and mechanical (THM) behavior in the near-field (Contract research)

Suzuki, Hideaki*; Takayama, Yusuke

JAEA-Research 2020-015, 52 Pages, 2020/12

JAEA-Research-2020-015.pdf:3.83MB

It is anticipated that the coupled thermal hydraulic and mechanical (THM) processes will occur, involving an interactive process with radioactive decay heat arising from the vitrified waste, infiltration of groundwater from the host rock into the buffer material, swelling pressure of buffer material due to its saturation in the near-field of a geological disposal system for high-level radioactive waste repository. In order to evaluate these phenomena in the near-field, the THM model has been developed. In this report, the density dependence of thermal, hydraulic and mechanical properties of the buffer material was investigated to evaluate the near-field environment. These density dependence schemes were added to the coupled THM model. The THM analysis of the in-situ experiment of engineered barrier system (EBS) experiment at the Horonobe Underground Research Laboratory was carried out. As a result, the effect of the density change of the buffer material on the temperature and infiltration behavior of buffer material was found. A case analysis on the long-term behavior of the near field was also carried out. Then, the behavior that the buffer material swelled out toward the backfill material and the density of the buffer material decreasing was shown.

Journal Articles

Impact of non-linear elastic behavior on bentonite density evolution at the FEBEX

Takayama, Yusuke

International Journal of Rock Mechanics and Mining Sciences, 136, p.104538_1 - 104538_8, 2020/12

 Times Cited Count:1 Percentile:35.39(Engineering, Geological)

It is necessary to apply reliable numerical simulation techniques to assess the mechanical behaviour of repositories for the geological disposal of radioactive waste over a long period of time. Having a constitutive model that can describe the mechanical behaviour of bentonite is key to such numerical simulations. In this study, the applicability of linear elastic constitutive model with swelling term is examined focusing on the density heterogeneity by applying to the in situ Full-scale Engineered Barrier Experiment (FEBEX) in Grimsel. It was confirmed that this constitutive model underestimates the density change. Instead, mechanical behaviour was re-simulated by unsaturated elasto-plastic constitutive model using the changes in degree of saturation as input data. Although no feedback effects of mechanical behaviour on hydraulic and thermal behaviour were taken into account, two-dimensional stress and density could be reproduced.

JAEA Reports

Development of technique for acquiring fracture void structure data for a single fracture and acquisition of hydraulic/mass transport properties in a fracture

Sato, Hisashi*; Sawada, Atsushi; Takayama, Yusuke

JAEA-Research 2020-012, 37 Pages, 2020/11

JAEA-Research-2020-012.pdf:2.66MB
JAEA-Research-2020-012-appendix(DVD-ROM).zip:468.23MB

In the safety assessment of the geological disposal of high-level radioactive waste, it is important to fully understand the permeability and mass transport properties of actual fractures when setting parameters for permeability and mass transport properties of fractures. Permeability and mass transport properties of a fracture are affected by the void structure of the fracture. Therefore, it is necessary not only to conduct a hydraulic test but also to evaluate hydraulic properties based on the void structure of the fracture. Therefore, the purpose of this study is to develop a technique to acquire a three-dimensional void structure of a fracture from fracture surface shape data, and to obtain a dataset of three- dimensional void structure data and hydraulic data of the fracture. Specifically, we made a transparent replica sample of fracture and measure the fracture surface shape data in the exact coordinates. The three-dimensional void structure data was constituted from fracture surface shape data in the same coordinates. In addition, we conducted a permeability test and tracer test for the same sample to obtain a permeability and mass transport properties of the fracture. To assess the validity of the acquired data, we compared it with the apertures evaluated based on different methods. As a result, the average aperture from the fracture void structure was almost same as the average aperture acquired by different methods, from the above that the test result was validated.

Journal Articles

DECOVALEX-2019 Task D: INBEB Final Report

Gens, A.*; Alcoverro, J.*; Blaheta, R.*; Hasal, M.*; Michalec, Z.*; Takayama, Yusuke; Lee, C.*; Lee, J.*; Kim, G. Y.*; Kuo, C.-W.*; et al.

LBNL-2001267 (Internet), 210 Pages, 2020/10

This document is the final report of Task D of the DECOVALEX-2019 project, presenting the definitions of the problems studied, approaches applied, achievements made and outstanding issues identified for future research. Task D of the DECOVALEX 2019 project is devoted to the study of the hydro-mechanical and thermo-hydro-mechanical Interactions in Bentonite Engineered Barriers. The Task is structured around two large scale in situ experiments that were subjected to well managed dismantling operations that provided direct observations of the state of the barrier after long test periods. Four teams carried out the modelling of the two experiments: Institute of Geonics, of the Czech Academy of Sciences (IGN), supported by SURAO, Czech Republic, Japan Atomic Energy Agency (JAEA), Korea Atomic Energy Research Institute (KAERI) and National Central University of Taiwan (NCU), supported by the Taipower.

Journal Articles

A Coupled modeling simulator for near-field processes in cement engineered barrier systems for radioactive waste disposal

Benbow, S. J.*; Kawama, Daisuke*; Takase, Hiroyasu*; Shimizu, Hiroyuki*; Oda, Chie; Hirano, Fumio; Takayama, Yusuke; Mihara, Morihiro; Honda, Akira

Crystals (Internet), 10(9), p.767_1 - 767_33, 2020/09

 Times Cited Count:1 Percentile:37.3(Crystallography)

Details are presented of the development of a coupled modeling simulator for assessing the evolution in the near-field of a geological repository for radioactive waste disposal where concrete is used as a backfill. The simulator uses OpenMI, a standard for exchanging data between simulation software programs at run-time, to form a coupled chemical-mechanical-hydrogeological model of the system. The approach combines a tunnel scale stress analysis finite element model, a discrete element model for accurately modeling the patterns of emerging cracks in the concrete, and a finite element and finite volume model of the chemical processes and alteration in the porous matrix and cracks in the concrete, to produce a fully coupled model of the system. Combining existing detailed simulation software in this way with OpenMI has the benefit of not relying on simplifications that might be necessary to combine all of the modeled processes in a single piece of software.

Journal Articles

A Study on applicability of elasto-plastic constitutive model to mechanical behavior of buffer material in salt water conditions

Takayama, Yusuke; Kikuchi, Hirohito*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 27(1), p.12 - 21, 2020/06

In this study, an applicability of the modified Cam clay model to the buffer material under saltwater conditions was examined. First, consolidated-undrained triaxial test was conducted using NaCl solution and artificial seawater. Based on the consolidated-undrained triaxial compression test results and the existing consolidation test results, the difference in the mechanical behavior of the buffer material under distilled water and saltwater condition was clarified. In particular, there was a difference in the unloading behavior in the consolidation test. Through reproducibility analysis of these experimental data, it was confirmed that the mechanical behavior of the buffer material can be roughly reproduced by setting the swelling index according to the salt concentration.

Journal Articles

Development of evaluation technique for long-term mechanical behavior of near-field

Takayama, Yusuke

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 25(2), p.103 - 106, 2018/12

At the 34th Back-end Summer Seminar, I introduce the evaluation technique for long-term mechanical behavior of near field. This report summarizes the presentation at the seminar.

Journal Articles

Simulation of saturation process in a transuranium disposal facility

Takayama, Yusuke; Iizuka, Atsushi*; Kawai, Katsuyuki*

Environmental Geotechnics (Internet), 4(5), p.339 - 352, 2017/10

 Times Cited Count:1 Percentile:9.96(Engineering, Geological)

Due to its low permeability and excellent expansion characteristics, bentonite materials are considered to be the material of choice for engineered barriers in Trans-Uranium (TRU) disposal facilities. It is necessity to predict the mechanical behaviour of bentonite using numerical simulation to evaluate the long-term performance of a TRU disposal facility. In this paper, a comprehensive summary of the saturation process of bentonite and its mathematical modelling was explained. Subsequently, a series of numerical simulations were carried out. First, a one-dimensional swelling pressure test simulation was carried out in order to examine the swelling characteristics of buffer and backfill. Next, we simulated the saturation process in a TRU disposal facility from an initially unsaturated state. The mechanical behaviour of bentonite buffer and backfill in a TRU disposal facility during the saturation process was examined.

JAEA Reports

Backfilling test in drilling pits as part of Groundwater REcovery Experiment in Tunnel (GREET) Project

Takayasu, Kentaro; Onuki, Kenji*; Kawamoto, Koji*; Takayama, Yusuke; Mikake, Shinichiro; Sato, Toshinori; Onoe, Hironori; Takeuchi, Ryuji

JAEA-Technology 2017-011, 61 Pages, 2017/06

JAEA-Technology-2017-011.pdf:9.15MB

The Groundwater REcovery Experiment in Tunnel (GREET) was put into effect as development of drift backfilling technologies. This test was conducted by making the Closure Test Drift (CTD) recovered with water after carrying out a plug around 40m distance from northern edge face of horizontal tunnel of depth 500m, for the purpose of investigation of recovering process of rock mass and groundwater under the influence of excavation of tunnel. This report presents the efforts of backfilling investigation using bentonite composite soil and execution of backfilling into borehole pits excavated in the CTD which were carried out in fiscal 2014 as a part of GREET, and succeeding observation results inside pits from September 2014 to March 2016.

Journal Articles

Long-term mechanical analysis code considering chemical alteration for a TRU waste geological repository

Mihara, Morihiro; Hirano, Fumio; Takayama, Yusuke; Kyokawa, Hiroyuki*; Ono, Shintaro*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 24(1), p.15 - 25, 2017/06

A computer program MACBECE has been developed to provide rigorous calculations of the long-term mechanical behavior of a TRU waste geological repository. Consideration is given to the expected chemical alteration of repository components, including cementitious materials and bentonite, and the mechanical interactions between repository and host rock. The long-term mechanical behavior of a TRU waste repository was evaluated in a deep soft rock site, where creep deformation is likely to occur from the initial construction phase to 10$$^{5}$$ years after repository closure. It was found that the stress didn't fall into a singularity of yield surface of the EC model applied to the mechanical behavior model of bentonite. The calculated displacement of the inner diameter of the repository, considering mechanical interaction between repository and host rock, was about half that of a result in 2nd progress report on R&D for TRU waste disposal in Japan.

Journal Articles

Constitutive modeling for compacted bentonite buffer materials as unsaturated and saturated porous media

Takayama, Yusuke; Tachibana, Shinya*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Kobayashi, Ichizo*

Soils and Foundations, 57(1), p.80 - 91, 2017/02

 Times Cited Count:7 Percentile:51.03(Engineering, Geological)

Bentonite has remarkable swelling characteristics and low permeability that makes it a suitable buffer material in a purpose built repository for the geological disposal of radioactive waste. It is necessary to use reliable numerical simulation techniques to demonstrate that the repository is safe and mechanically stable for tens of thousands of years. Constitutive model that describes the mechanical behavior of bentonite is a key technique in such numerical simulations. The current paper proposes a constitutive model for bentonite, which treats bentonite as an unsaturated elasto-plastic material that changes its mechanical properties as it becomes saturated. In the proposed model, the swelling index and an expression formula for negative dilatancy are treated as functions of degree-of-saturation. Montmorillonite content is used as an input parameter in the proposed model and so is applicable to a variety of bentonite based materials of varying montmorillonite content. Experimental results from swelling volume and swelling pressure tests taken from the literature are shown to be satisfactorily predicted by the proposed model.

JAEA Reports

Study on effects of coupled phenomenon on long-term behavior for crystalline rock; FY2014 (Contract research)

Ichikawa, Yasuaki*; Kimoto, Kazushi*; Sato, Toshinori; Kuwabara, Kazumichi; Takayama, Yusuke

JAEA-Research 2015-025, 31 Pages, 2016/03

JAEA-Research-2015-025.pdf:13.0MB

It is important to evaluate the stability of a repository for high-level radioactive waste not only during the design, construction and operation phases, but also during the post-closure period, for time frames likely exceeding several millennia or longer. The rock mass around the tunnels could be deformed through time in response to time dependent behavior. On the other hand, it was revealed that the chemical reaction of groundwater in a rock had an influence on the long-term behavior. An evaluation of the microcracks to have an influence on these mechanical and chemical coupled phenomena should be worked on chiefly. In fiscal year 2014, this study performed numerical analysis to examine the supersonic scattering attenuation decrement behavior in the crystalline rock and a measurement sequentially last year. The measurement of the head and surface waves were carried out. As a result, group speed was provided. On the other hand, the spread scattering analysis of the elastic wave by the FDTD (Finite Difference Time-Domain) method made a numerical analysis. However, a laboratory finding is different from expectation of the simulation, and crystal anisotropic influence of a microcrack and rock-forming minerals is thought about as a cause of this estrangement. Therefore it was revealed that it was necessary to examine these two points of influence more in future.

JAEA Reports

Study on crystalline rock aiming at evaluation method of long-term behavior of rock mass; FY2014 (Contract research)

Fukui, Katsunori*; Hashiba, Kimihiro*; Sato, Toshinori; Kuwabara, Kazumichi; Takayama, Yusuke

JAEA-Research 2015-015, 61 Pages, 2015/11

JAEA-Research-2015-015.pdf:5.52MB

With respect to high-level radioactive waste disposal, knowledge of the long-term mechanical stability of shafts and galleries excavated in rock is required, not only during construction and operation but also over a period of thousands of years after closure. On the other hand, it is known that rock and the rock mass surrounding the disposal gallery shows time dependent behavior such as creep or the stress-relaxation. It becomes the issue in the stability evaluation of the disposal gallery to grasp the behavior. About this issue, we pushed forward research development. In the fiscal year of 2014, the creep test was continuously conducted and the total testing time exceeded 17 years. The testing equipment and procedure were examined to investigate the deformation, failure and time-dependency of rock under wet conditions and between room temperature and 100$$^{circ}$$C. The long-term strength of rock under triaxial stress state was researched with the aid of laboratory testing results and in situ stress measurement.

JAEA Reports

Mizunami Underground Research Laboratory Project; Rock mechanical investigations at the -500m stage

Kuwabara, Kazumichi; Sato, Toshinori; Sanada, Hiroyuki; Takayama, Yusuke

JAEA-Research 2015-005, 378 Pages, 2015/07

JAEA-Research-2015-005.pdf:125.5MB
JAEA-Research-2015-005.zip:0.53MB

This report presents the results of following rock mechanical investigations conducted at the -500m Stage. (1) Laboratory tests using cores and block samples obtained at the -500m Stage. (2) In-situ stress measurement using Compact Conical-ended Borehole Overcoring (CCBO) method at the -500m Stage. (3) In-situ stress measurements using Differential Strain Curve Analysis(DSCA) method at the -500m Stage. (4) Development of rock mechanical model.

JAEA Reports

Three-dimensional excavation analysis based on crack tensor model at the Mizunami Underground Research Laboratory

Takayama, Yusuke; Sato, Toshinori; Sanada, Hiroyuki; Tada, Hiroyuki*; Kumasaka, Hiroo*; Fukuda, Tsuyoshi*; Kobayashi, Shinji*

JAEA-Research 2015-003, 102 Pages, 2015/07

JAEA-Research-2015-003.pdf:20.21MB

In the rock mechanical study in the Mizunami Underground Research Laboratory Project, the development of the evaluation method for the excavation damaged zone due to excavation of shafts and research galleries is one of the important issues. In this study, three-dimensional excavation analysis is conducted. Crack tensor used in these analysis were measured using the data collected in wall surface observation in study tunnels such as the horizontal tunnels to a depth of 500 m. The results of analysis were compared with existing measurements at a depth of 500 m obtained using in-situ strain meters. The excavation analysis using crack tensor and initial stress obtained from the Phase I and Phase II were carried out to examine the influence of crack tensor and initial stress on the results of analysis.

Journal Articles

Groundwater recovery experiment in Mizunami Underground Research Laboratory; Numerical simulation of H-M coupled behavior of rock and backfill materials to evaluate the influence on the surrounding rock

Takayama, Yusuke; Sato, Toshinori; Onoe, Hironori; Iwatsuki, Teruki; Saegusa, Hiromitsu; Onuki, Kenji

Dai-43-Kai Gamban Rikigaku Ni Kansuru Shimpojiumu Koenshu (CD-ROM), p.313 - 318, 2015/01

In the Mizunami Underground Research Laboratory, groundwater recovery experiment is being conducted to construct the method to understand the transition of geological environment due to groundwater recovery at the -500m access and research gallery-north. As a part of this experiment, backfill test is planned using drilling pits filled with artificial materials (clay and concrete) to evaluate the influence on the surrounding rock mass due to the interaction of rock and artificial materials. In this study, numerical simulation of the backfill test has been carried out to predict the qualitative hydro-mechanical behavior.

Journal Articles

Numerical evaluation of a multiple soil barrier system preventing water infiltration

Takayama, Yusuke*; Ikuta, Yuki*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Taki, Tomihiro; Sakao, Ryota; Ichikawa, Yasuaki*

Unsaturated Soils; Research & Applications, p.659 - 665, 2014/06

In this paper, a multiple unsaturated soil barrier consisting of gravel, sand and bentonite-mixed soil layers is considered as a hardly permeable covering to prevent rainwater from infiltrating into waste deposits. Not only the quite low permeability of bentonite-mixed soil layer but also a function of capillary barrier between granular layers having different water-retention capabilities is expected to prevent water infiltration. A series of numerical simulation is carried out to evaluate performance of the multiple soil barrier using saturated/unsaturated soil/water coupled FE computation technique. The multiple unsaturated soil barrier system considered here is 10m long and 1.5 m thick consisting of sandy soil surface, sand filter, gravel layer, sand filter and bentonite-mixed clayey layer. And the soil barrier system is assumed to be placed as a covering with a 5% inclination on the waste deposit. The performance of the multiple soil barrier system is shown as a management chart to maintain impermeable ability associated with possible rainfall intensity. Particularly, in this paper, actual rainfall record in Okayama area is employed to determine the rainfall intensity which is imposed as a boundary condition in the computation. According to the simulation results, it is found that the capillary barrier is a key issue in evaluating the long term performance of the barrier system. And, it is confirmed that the barrier system considered in this paper can work well against the maximum intensity of the rainfall in the past record.

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