Radionuclide migration experiments under laboratory conditions

Arguments are given in favour of performing non-conservative radionuclide migration experiments in natural and artificial fractures under laboratory conditions. Results are presented from a series of migration experiments performed in a natural fracture in a quarried block of granite with overall dimensions of 81 × 90 × 75 cm. These experiments were conducted in a dedicated experimental facility and were designed to complement field migration experiments, to develop and test methodologies and to evaluate transport models. The flow path in the fracture was selected from the hydraulic characterization of the fracture using borehole to borehole pumping tests. Following migration experiments with stable, conservative tracers Br− and uranine, migration experiments were performed with a number of radionuclides at a flow rate of 3 mL/h. At the completion of the migration experiments, the fracture surfaces were scanned using a Ge(Li) gamma detector and an alpha probe to determine the sorbed radioisotope distributions on the fracture surfaces. The results from these experiments show that the behaviour of radioisotopes is usually more complex than can be represented by a simple, reversible sorption coefficient (Ka). However, the transport models that use this simple Ka tend to give conservative estimates of contaminant transport in environmental and safety assessment calculations.