Host laboratory: Research laboratory for migration and interactions in the geosphere (LETIS)
Beginning of the thesis: October 2014
Agustin Molinero GuerraSubject 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.
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
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.
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.
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.