EUROISMAR 2019

Geological disposal of high-level radioactive waste usually includes bentonite buffer materials. However, the long-term performance of the bentonites for the containment of the nuclear waste is being increasingly questioned due to specific issues associated with “homogenisation” occurring in these materials. Homogenisation is driven by erosion, variations in density and development of swelling pressure. However, the role of pore water pressure, it's spatial development and impact on swelling potential, self-sealing and permeability have yet to be adequately explored. We have developed novel ²H micro-MRI methods to demonstrate that these governing hydration processes are indeed both complex and non-uniform in bentonites and natural materials homogenised in the laboratory setting and used for containment of radioactive waste in many European countries.
Bentonites and natural materials used in this study were homogenized with D₂O. ²H NMR imaging, combined with ²H spin-echo T₂ spectroscopy was used to map out T₂ relaxation in these samples. Surface relaxivity of deuterium oxide was determined using beads with different diameters soaked in D2O.
We have shown that T₂s in these materials are non-uniform and the distributions change as deuterium oxide progress toward the end of a core during homogenization. Obtained values of T₂ were indicative of high structural heterogeneity. We further converted T₂ maps to pore size distribution maps to probe length scales of structural heterogeneities that occur in bentonites and natural materials during homogenization. The maps demonstrated that the method captures structural irregularities that are lower than 2500nm. These structural irregularities are not visible in high resolution X-CT images of these cores.
We conclude that ²H MRI has a great potential to assess heterogeneity in man-made and natural clays. The information is vital for a proper choice of materials used in containment of the nuclear waste.