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Hydro-mechanical behaviour of bentonite-sand mixture used as sealing materials in radioactive waste disposal galleries

​Simona Saba has defended her thesis on the 9th December 2013.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PRP-DGE/SRTG/LETIS

Authors > SABA Simona

Publication Date > 09/12/2013

Summary

​In order to verify the effectiveness of the geological high-level radioactive waste disposal, the
French Institute for Radiation protection and Nuclear Safety (IRSN) has implemented the
SEALEX project to control the long-term performance of swelling clay-based sealing
systems, and to which this work is closely related. Within this project, In-situ tests are carried
out on compacted bentonite-sand mixture in natural conditions and in a representative scale.
This material is one of the most appropriate sealing materials because of its low permeability
and good swelling capacity. Once installed, this material will be hydrated by water from the
host-rock and start swelling to close all gaps in the system, in particular the internal pores,
rock fractures and technological voids. Afterwards, swelling pressure will develop. In the
present work, laboratory experiments were performed to investigate the sealing properties
under these complex hydro-mechanical conditions taking into consideration the effect of
technological voids. The microstructure of the material in its initial state was first examined
by microfocus X-ray computed tomography (μCT). This allowed identification of the
distribution of grains of sand and bentonite as well as the pores in the sample. Macro-pores
are found concentrated at the periphery of the sample and between the grains of sand, which
could affect in the short term the permeability. The hydration of the same material in limited
swelling conditions was then observed by 2D photography and 3D μCT. The swelling
mechanism with bentonite gel production, the swelling kinetics, the density decrease and the
homogenisation of the material were analyzed. The hydration in the conditions of prevented
swelling was also studied by swelling pressure tests with radial and axial measurements of
swelling pressure. The difference found between the axial and radial swelling pressures
suggested the presence of an anisotropic microstructure. Mock-up tests at a 1/10 scale of the
in situ SEALEX tests were carried out for the study of the recovery capacity of the mixture in
case of the failure of the confining structures. Local measurements of swelling pressures
along the sample allowed analysis of the density gradient evolution during axial swelling.
Finally, a comparison between the laboratory results and those from an in-situ test was done,
showing a good fitting in the relative humidity curves for the same infiltration length while
considering the saturation effect from the technological void. The swelling pressure
comparison resulted in a time factor of 100 between the laboratory (1/10) and in situ tests.

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