Technischer Bericht NTB 85-42

Hydraulische Leitfähigkeit, Porosität und Uranrückhaltung von Kristallin und MergelBohrkern-Infiltrationsversuche

The hydraulic conductivity and the dynamic porosity under mechanical stress have been measured on intact or fissure bearing crystalline rock cores from the Nagra (National Cooperative for the Storage of Radioactive Waste) drilling programme in the central part of northern Switzerland along with marl cores from Oberbauenstock (Canton of Obwalden). Breakthrough profiles of uranium-233 on four of the crystalline samples have been established and in addition the immobilized activity was registered by α-autoradiography of sliced core samples.

For the intact crystalline samples hydraulic conductivities were in the range of 1E-13 to 1E-11 m/s, where in the marl displayed much smaller values of approx. 1E-15 m/s. The dynamic porosity in the crystalline cores is some 10 to 90 % of the total porosity, falling to 5 % in the marl sample. Calculations of apertures and surfaces of the water conducting paths as well as an estimation of the coefficient of surface sorption were made by using the definition of the hydraulic radius and the relation between the overall (measured) retardation and the sorption coefficient, R = 1 + af Ka (assuming a linear sorption isotherm), respectively.

All four breakthrough profiles have been used in a separate publication to model radionuclide transport using the radionuclide transport model RANCHMD for two flow geometries. The measured and calculated profiles agree well; the extracted parameters are consistent with values from the literature and independent measurements. Here few parameters (apertures, surface and sorption coefficients) resulting from these calculations are compared with the values derived from the hydraulic radius concept. α-autoradiographs are shown to be helpful in typifying flow path geometries.