Evaluation of temporal changes in fracture transmissivity in an excavation damaged zone after backfilling a gallery excavated in mudstone

Aoyagi, Kazuhei; Ishii, Eiichi

Environmental Earth Sciences, 83(3), p.98_1 - 98_15, 2024/02

https://doi.org/10.1007/s12665-023-11416-x

The long-term geological disposal of high-level radioactive waste relies on predictions of future changes in a disposal facility's hydro-mechanical characteristics to assess potential leakage through fractures in the excavation damaged zone (EDZ) after backfilling the facility. This study evaluated the transmissivity of EDZ fractures using in situ hydraulic tests around the area of a full-scale, experimental, engineered barrier system in the Horonobe Underground Research Laboratory, Hokkaido, Japan. After their installation, the buffer blocks swelled, altering the stresses within the EDZ fractures. The effects of these changing stresses on the fractures' transmissivity were assessed over a period of 4 years. The transmissivity continuously decreased in this period to about 41% of its value measured prior to the swelling. Using the Barton-Bandis normal-stress-dependent fracture-closure model, the decrease in transmissivity is quantitatively attributed to closure of the EDZ fractures, which was caused by the swelling pressure increasing up to 0.88 MPa. Evidence of fracture closure came from seismic tomography surveying, which revealed a slight increase in seismic velocity in the study area with increasing swelling pressure. The results show that EDZ fractures were closed by swelling of the full-scale buffer material. They also demonstrate the applicability of the Barton-Bandis model to preliminary estimation of the long-term transmissivity of EDZ fractures in facilities for the geological disposal of radioactive waste.

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.

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

https://doi.org/10.1016/j.sandf.2017.01.006

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.

Polyelectrolyte membranes based on crosslinked polytetrafluoroethylene

Yamaki, Tetsuya; Asano, Masaharu; Yoshida, Masaru

Kogyo Zairyo, 51(4), p.39 - 42, 2003/04

https://ci.nii.ac.jp/naid/80015805714

no abstracts in English

Crosslinked fluoropolymer electrolyte membranes prepared by ionizing radiation

Yamaki, Tetsuya; Yoshida, Masaru

Zukai, Nenryo Denchi no Subete, p.117 - 119, 2003/00

no abstracts in English

Effects of irradiation temperature on swelling and shrinking kinetics of thermo-responsive gels prepared by radiation-induced polymerization

Hiroki, Akihiro*; Maekawa, Yasunari; Yoshida, Masaru; Katakai, Ryoichi*

Polymer, 42(15), p.6403 - 6408, 2001/07

https://doi.org/10.1016/S0032-3861(01)00112-4

no abstracts in English

Evaluation of Coupled Thermo-Hydro-Mechanical Phenomena in the Near Field for Geological Disposal of High-Level Radioactive waste

Chijimatsu, Masakazu*; Fujita, Tomoo; Sugita, Yutaka; Taniguchi, Wataru

JNC TN8400 2000-008, 339 Pages, 2000/01

JNC-TN8400-2000-008.pdf:30.96MB

Geological disposal of high-level radioactive waste (HLW) in Japan is based on a multibarrier system composed of engineered and natural barriers. The engineered barriers are composed of vitrified waste confined within a canister, overpack and buffer material. Highly compacted bentonite clay is considered one of the most promising candidate buffer material mainly because of its low hydraulic conductivity and high adsorption capacity of radionuclides. In a repository for HLW, complex thermal, hydraulic and mechanical (T-H-M) phenomena will take place, involving the interactive processes between radioactive decay heat from the vitrified waste, infiltration of ground water and stress generation due to the earth pressure, the thermal loading and the swelling pressure of the buffer material. In order to evaluate the performance of the buffer material, the coupled T-H-M behaviors within the compacted bentonite have to be modelled. Before establishing a fully coupled T-H-M model, the mechanism of each single Phenomenon or partially coupled phenomena should be identified. Furthermore, in order to evaluate the coupled T-H-M phenomena, the analysis model was developed physically and numerically and the adequacy and the applicability was tested though the engineered scale laboratory test and in-situ test. In this report, the investigative results for the development of coupled T-H-M model were described. This report consists of eight chapters. In Chapter l, the necessity of coupled T-H-M model in the geological disposal project of the high-level radioactive waste was described. In Chapter 2, the laboratory test results of the rock sample and the buffer material for the coupled T-H-M analysis were shown. The rock samples were obtained from the in-situ experimental site at Kamaishi mine. As the buffer material, bentonite clay (Kunigel V1 and Kunigel OT-9607) and bentonite-sand mixture were used. In Chapter 3, in-situ tests to obtain the rock property were shown. As ...

Evaluation of long-term mechanical stability of near field

Takachi, Kazuhiko; Sugino, Hiroyuki

JNC TN8400 99-043, 52 Pages, 1999/11

JNC-TN8400-99-043.pdf:5.2MB

In the near field, as tunnels and pits are excavated, a redistribution of stresses in the surrounding rock will occur. For a long period of time after the emplacement of waste packages various events will take place, such as the swelling of the buffer, sinking of the overpack under its own weight, deformation arising from expansion of overpack corrosion products and the creep deformation of the rock mass. The evaluation of what effects these changes in the stress-state will have on the buffer and rock mass is a major issue from the viewpoint of safety assessment. Therefore, rock creep analysis, overpack corrosion expansion analysis and overpack sinking analysis have been made in order to examine the longterm mechanical stability of the near field and the interaction of various events that may affect the stability of the near field over a long period of time. As the results, rock creep behavior, the variations of the stress-state and the range of the influence zone differ from the rock strength, strength of buffer in the tunnel and side pressure coefficient etc. about the hard rock system and soft rock system established as basic cases. And the magnitude of the stress variations for buffer by the overpack sinking and rock creep deformation is negligible compared with it by the overpack corrosion expansion. Furthermore, though very limited zone of buffer around the overpack is close to the critical state by the overpack corrosion expansion, the engineered barrier system attains a comparatively stable state for a long period of time.

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Fukazawa, E.*; Tanaka, M.*; Yamamoto, H.*; Taira, K.*; Yamamoto, M.*; Okutsu, Kazuo*; Hane, Koji*; Aoyagi, Takayoshi*; Morikawa, Seiji*; Furuichi, Mitsuaki*

PNC TJ1100 98-006, 434 Pages, 1998/02

PNC-TJ1100-98-006.pdf:59.14MB

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PNC TJ1201 97-001, 264 Pages, 1997/03

PNC-TJ1201-97-001.pdf:23.64MB

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