Speaker
Description
Nuclear energy remains strategic for the decarbonisation of the energy sector. However, despite its high efficiency in producing low-carbon electricity, the radioactive waste generated by nuclear power plants requires strict long-term disposal solutions. Deep geological disposal in expansive clayey host rocks is widely envisaged, as clay acts as a natural barrier capable of delaying and mitigating radionuclide migration. During disposal operations, the clay surrounding excavated galleries is initially subjected to drying, followed by progressive resaturation after facility closure. Drying-induced shrinkage alters the clay microstructure, pore connectivity and stress state, thereby directly affecting water distribution and transport properties.
The properties and distribution of pore water in clays remain insufficiently characterised, particularly under drying conditions. The distinction between adsorbed water, strongly interacting with clay mineral surfaces and often assumed to have limited mobility, and free (bulk-like) water is still poorly constrained. As a result, clay behaviour models remain largely phenomenological and do not explicitly account for the impact of microstructural changes on water mobility—closely related to radionuclide transport—or on the hydromechanical response of the clay.
Among emerging experimental techniques addressing this gap, low-field Nuclear Magnetic Resonance (NMR) relaxometry provides non-invasive, molecular-scale information on water dynamics through the analysis of hydrogen proton relaxation times. Recent studies (Eizaguirre, 2025) tracked the dynamic evolution of adsorbed and free water populations during clay hydration, showing an increase of free water at high water contents. Relaxation times are also sensitive to mechanical changes, including variations in dry density and volumetric deformation associated with shrinkage. However, previous studies investigating the influence of dry density (Ohkubo et al., 2008, 2016; Eizaguirre et al., 2023) were limited to fully saturated samples, making it difficult to isolate the role of dry density from that of water content.
We present a new experimental campaign on compacted reconstituted Boom Clay at three initial dry densities, focusing on the evolution of NMR relaxation times during drying. Preliminary tests performed at varying water contents and uncontrolled dry densities show a promising linear relationship between the transverse relaxation time $T_2$ and water content. Ongoing experiments combine NMR relaxometry with X-ray microtomography to quantify drying-induced shrinkage and associated microstructural changes at controlled water contents. This combined approach provides an integrated view of the hydromechanical factors governing relaxation times under mechanically unloaded conditions. When coupled with the methodology proposed by Eizaguirre (2025) to quantify adsorbed and free water populations, the results offer new insights into the evolution of water populations and clay–water interactions under repository-relevant drying path. Complementary measurements of suction (chilled-mirror potentiometry) and pore size distribution (mercury intrusion porosimetry) further support the interpretation of water distribution within the clay microstructure.
| References | Eizaguirre (2025) https://theses.fr/2024ENPC0033 // Eizaguirre et al. (2023) https://doi.org/10.1051/e3sconf/202338214003 // Ohkubo et al. (2008) https://doi.org/10.1016/j.pce.2008.10.042 // Ohkubo et al., (2016) https://doi.org/10.1016/j.clay.2016.01.014 // Fleury et al. (2022) https://doi.org/10.1007/s42860-022-00195-4 |
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| Country | The Netherlands |
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