Effect of collapse behavior on changes in swelling pressure of bentonite during swelling pressure test

Takayama, Yusuke; Sato, Daisuke*; Kikuchi, Hirohito*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 29(2), p.101 - 111, 2022/12

https://nuce.aesj.or.jp/jnuce/vol29/jnuce-vol29-2-p101-111.pdf

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. It has been reported that the swelling pressure obtained by swelling pressure test increases monotonically or temporarily decreases after increasing with the passage of the time depending on the initial water content. In this study, swelling pressure tests were conducted under several different initial water content conditions, and the change in wet density distribution inside the specimen during the swelling pressure tests was observed by X-ray CT measurement. It was confirmed that the presence or absence of collapse behavior and its magnitude affect the shape of the change in swelling pressure. When the collapse deformation was large, the amount of temporary decrease in swelling pressure was large. On the other hand, swelling pressure increased monotonically when there was no deformation due to collapse.

Study of natural fracture topography in 50cm scale granodiorite rock block including natural fracture intersection measured by precision grinder

Tetsu, Keiichi*; Takayama, Yusuke; Sawada, Atsushi

JAEA-Research 2022-005, 84 Pages, 2022/08

JAEA-Research-2022-005.pdf:10.32MB
JAEA-Research-2022-005-appendix(CD-ROM).zip:35.68MB

Nuclide migration analyses are conducted for the safety assessment in geological disposal system of HLW. Nuclide migration evaluation of fractured host rock mainly uses a model that approximates the fracture with parallel flat plates. However, the actual fractures in the host rock are different from the parallel flat plates, the fractures have complex characteristics such as roughness of fracture surface and the fillings in the fracture. In approximating a fracture model, the methodology development how to set parameter values such as the transmissivity coefficient and the fracture aperture is an issue. One of the issues is to investigate the geometrical features of actual fractures in the host rock. In this study, for the purpose of understanding the geometrical features of fractures including fracture intersection, the internal fracture shape was measured in detail using the method of surface grinding on 50cm scale granodiorite with natural fracture intersection. Thus, the fracture width, fracture aperture, and the shape of the fracture surfaces were obtained. From the obtain data, characteristics such as the average value of the fracture width, the roughness of the fracture surfaces, and the distribution of the fracture aperture were evaluated. As a result, it was confirmed that one of the fractures near the fracture intersection has a particularly large fracture width and fracture aperture as compared with the other part of fractures. Thus, in the granodiorite rock block used in this study, it was inferred that the most permeable path is not the fracture intersection itself, but the particularly large fracture aperture in the vicinity of the fracture intersection.

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

https://doi.org/10.3151/jact.19.1075

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.

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

https://doi.org/10.1016/j.ijrmms.2020.104572

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.

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

https://doi.org/10.1016/j.ijrmms.2020.104538

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.

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.

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

https://doi.org/10.3327/jnuce.27.1_12

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.

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

https://doi.org/10.1680/jenge.14.00041

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.

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

https://doi.org/10.3327/jnuce.24.1_15

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 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.

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.