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IRSN, Institut de radioprotection et de sûreté nucléaire

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Enhancing Nuclear Safety


Research

Theses in progress

Experimental and digital characterizations of the hydro-mechanical behavior of a heterogeneous powder/pellet bentonite material


Host laboratory: ​​Research laboratory for migration and interactions in the geosphere (LETIS)

Beginning of the thesis: October 2014

Student name: Agustin Molinero Guerra



Subject description
Deep geological repository has been considered for high-level radioactive waste (HLW) in several countries. The safety of this disposal concept is based on the multi-barrier system consisting on the geological barrier and engineered barriers (waste container, buffer and sealing elements). In the French concept, once the waste containers will be installed, galleries and access shafts will be closed and sealed by swelling clay based cores.

In situ compacted MX80 powder/pellets mixture is one of the candidate sealing materials, not only because of its low permability, high swelling capacity and hight radionuclide retardation properties but also for operational advantages (lower compaction effort, reduced gaps between the rock and the seal). Once installed in the repository, theses sealing materials will be subject to coupled hydro-mechanical loadings: hydration due to the infiltration of pore water from the natural barrier and mechanical confinement resulting from the engineered barriers.

In this context, the French Institute of Radiation Protection and Nuclear Safety (IRSN) has launched the SEALEX project (SEALing performance Experiments) to which this work is closely related. The current work focuses on the mixture of MX80 bentonite powder and pellets with a proportion of 20/80 in dry mass used in the last two SEALEX in situ tests. This material in characterized by the multimodal nature of its porous networks which influences most of its properties. The objectives of this thesis are (i) to caracterize the hydromechanical behavior of the mixture at the micro and macro-structural levels and (ii) to proposer a double structure constitutive model to describe the complex behavior of this material.

At the microstructure scale, one single pellet at two different suctions has already been examined by microfocus X-ray computed tomography (microCT); this allowed a qualitative analysis of the evolution of the microstructure of the material. MicroCT observations will also be performed on the mixture during hydration. Mercury intrusion propsimetry tests are being performed to characterize the pore distribution of pellets and powder of bentonite.

At the macrostructure scale, water retention of the materials have been obtained using different techniques. Moreover, swelling pressure tests under constant volume conditions, suction controlled oedometer tests and two small scale (1/10) mock up tests of the SEALEX in situ experiments will be conducted.


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