Fracture flow and radionuclide transport in block-scale laboratory experiments

Block-scale migration experiments were introduced to evaluate the simplified radionuclide transport concept used in assessing the safety of underground spent nuclear fuel repositories. The experiments were aimed to demonstrate visually the fracture flow, and to determine the hydraulic characteristics of a natural planar fracture and the transport behaviour of non-sorbing and sorbing radionuclides. For drill holes orthogonal to the fracture and equipped with injection or sealing packers flow rates in this study were measured as a function of hydraulic head. The outflow positions of water at each four side of the block were determined using uranine dye tracer. Tracer tests were performed using uranine, 99mTc and 22Na. Transport of a non-sorbing tracer through one of the flow channels was interpreted using an advection-dispersion model that on the generalised taylor dispersion. Characterisation of the hydraulic properties of the fracture indicated that some drill holes were located in the region where the fracture was open and water conductive. No water conductivity was observed in two drill holes indicating closure of the fracture. Reasonably low flow rates obtained from three drill holes indicated their suitability for further radionuclide transport experiments. Elution times of technetium and uranine were fairly similar. Sodium was slightly retarded and was spread over a wider area than uranine and technetium. High water flow rates suggest that advective flow field dominated tracer transport. Experimental and calculated elution curves substantiate the suitability of our experimental set-up for further radionuclide transport experiments. (orig.)