ABSTRACT
Heterogeneity and tightness of the barrier demand particular sensitive and long-term measuring methods, as well as spatial resolution of small details. For this reason, we developed a guard-ring surface-packer for process tomography of brine migration into barrier material with positron-emission-tomography (PET). Based on radiolabeling, this method is extremely sensitive, non-destructive and without retroaction. As an example for transport in the engineered barrier, we investigated injection of [22Na] NaClsat – solution from a fluid reservoir with a diameter of 20 mm into the contact zone of adjacent MgO-shotcrete layers. The driving pressure was 0.2 MPa, which caused intrusion of 1 mL of solution over a period of 260 d. The overlay of the temporal sequence of PET images and the structural CT image shows that deeper penetration (> 10 mm) occurred predominantly along one single pathway that was predetermined by the pore network structure. Second, we observed a slowly propagating diffuse tracer front that encompassed only a small portion of the injected fluid. Although the permeability of the material is very low (1e-20 m2), the major portion of the brine propagates through a strongly localized migration channel and thus may reach a penetration depth of 50 mm, beyond the predictions based on assumed homogeneous flow through the material.
