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. The disposal of spent nuclear fuels and high-level radioactive wastes in deepgeological repositories represents one of the greatest scientific-technicaland societal challenges of our times. Most disposal concepts rely on amultibarrier system, consisting of a combination of engineered materials,geotechnical and geological barriers to provide a safe containment of theradioactive waste to protect humans and the environment against dangersarising from ionizing radiation. A reliable safety assessment of a deepgeological repository over assessment time scales of several 100 000 yearsrequires a profound and comprehensive understanding of the complex coupledphysical (thermal, hydraulic, mechanical), chemical and biogeochemical(THM/CB) processes that govern the long-term evolution of the repositorysystem. As a result of thermal and chemical gradients at the interfaces of differentcomponents and materials of the multi-barrier system (e.g. interfaces betweenmetallic waste containers and bentonite backfill or between structuralconcrete and clay host rock), mineral dissolution and precipitation reactionsare promoted; thus the (local) porosity, the volume filled with gas and/orwater, can increase or decrease leading to changes in the macroscopictransport properties of the respective media. Although a reduction of theporosity (porosity clogging) appears to be desirable to inhibit radionuclidemigration, it can also be detrimental, particularly in the case of gaspressure build-up due to canister corrosion or bacterial activity. So far, porosity clogging at barrier interfaces and associated consequences onsolute or gas transport remain poorly understood; currently used mathematicaldescriptions of porosity clogging in reactive transport codes usually fail tocapture respective experimental observations (Chagneau et al., 2015; Deng etal., 2021). In this context, we are developing a “lab-on-a-chip” set-up,which combines time lapse optical microscopy imaging and in operando Ramanspectroscopy (Poonoosamy et al., 2019, 2020) to determine(i) whether complete clogging is possible and permanent, (ii) which parameterscontrol the porosity clogging and (iii) which changes in transport propertiesof porous media are induced due to porosity clogging. Our approach comprisesmicronized counterdiffusion experiments with in situ visualization andmonitoring of the evolution of mineralogy and microstructure/pore architecturewith time. Complementary pore scale modelling will be used to derive keyrelationships that describe changes in transport properties due to mineralprecipitation-induced porosity clogging. This approach will help to improvereactive transport codes and their predictive capabilities thus enhancingconfidence and reduce uncertainties in long-term predictions, leading to morerealistic descriptions of the evolution of complex repository systems.

Deciphering porosity clogging at barrier interfaces in deep geological repositories for radioactive waste