Abstract. The disposal of spent nuclear fuels and high-level radioactive wastes in deep
geological repositories represents one of the greatest scientific-technical
and societal challenges of our times. Most disposal concepts rely on a
multibarrier system, consisting of a combination of engineered materials,
geotechnical and geological barriers to provide a safe containment of the
radioactive waste to protect humans and the environment against dangers
arising from ionizing radiation. A reliable safety assessment of a deep
geological repository over assessment time scales of several 100 000 years
requires a profound and comprehensive understanding of the complex coupled
physical (thermal, hydraulic, mechanical), chemical and biogeochemical
(THM/CB) processes that govern the long-term evolution of the repository
system. As a result of thermal and chemical gradients at the interfaces of different
components and materials of the multi-barrier system (e.g. interfaces between
metallic waste containers and bentonite backfill or between structural
concrete and clay host rock), mineral dissolution and precipitation reactions
are promoted; thus the (local) porosity, the volume filled with gas and/or
water, can increase or decrease leading to changes in the macroscopic
transport properties of the respective media. Although a reduction of the
porosity (porosity clogging) appears to be desirable to inhibit radionuclide
migration, it can also be detrimental, particularly in the case of gas
pressure build-up due to canister corrosion or bacterial activity. So far, porosity clogging at barrier interfaces and associated consequences on
solute or gas transport remain poorly understood; currently used mathematical
descriptions of porosity clogging in reactive transport codes usually fail to
capture respective experimental observations (Chagneau et al., 2015; Deng et
al., 2021). In this context, we are developing a “lab-on-a-chip” set-up,
which combines time lapse optical microscopy imaging and in operando Raman
spectroscopy (Poonoosamy et al., 2019, 2020) to determine
(i) whether complete clogging is possible and permanent, (ii) which parameters
control the porosity clogging and (iii) which changes in transport properties
of porous media are induced due to porosity clogging. Our approach comprises
micronized counterdiffusion experiments with in situ visualization and
monitoring of the evolution of mineralogy and microstructure/pore architecture
with time. Complementary pore scale modelling will be used to derive key
relationships that describe changes in transport properties due to mineral
precipitation-induced porosity clogging. This approach will help to improve
reactive transport codes and their predictive capabilities thus enhancing
confidence and reduce uncertainties in long-term predictions, leading to more
realistic descriptions of the evolution of complex repository systems.