Transport of radionuclides in natural fractures: some aspects of laboratory migration experiments

Results are reported for a series of migration experiments performed in a hydraulically characterized, single, natural fracture in a block of granite with overall dimensions of 1 × 1 × 0.7 m (all approximate), using the conservative, poorly sorbing and strongly sorbing radionuclides ³H₂0, ¹³¹I, ²²Na, ⁸⁵Sr, ¹³⁷Cs, ⁶⁰Co, ^154,155Eu, ²³⁷Np, and ²³⁸Pu. The volumetric flow velocity of the transport solution was 3 ml h⁻¹, giving a residence time in the fracture of approximately 50 h. Elution profiles were obtained for ³H₂0, ¹³¹I, ²²Na, ⁸⁵Sr and ¹³⁷Cs but no evidence of the other radionuclides was observed in the eluent. Results from supporting static sorption measurements on crushed geological materials and granite coupons showed in general higher sorption on alteration minerals than on granite. Sorption was lowest for ²²Na and ⁸⁵Sr.The migration of ¹³¹I, ²²Na, ⁸⁵Sr through the fracture in real time was followed using end-window Geiger-Müller probes located in unused boreholes. Additional information, obtained by alpha and gamma scanning of the fracture surfaces after separating the block along the fracture, confirmed that transport had occurred along the flow path predicted from the hydraulic characterization of the fracture and that, over a 5.5 month period, the bulk of the injected ¹³⁷Cs had migrated only 75 cm along the flow path. The ⁶⁰Co, the rare earths and the actinides had not moved beyond the location of the injection borehole, suggesting that fracture infilling minerals played a major role in retarding radionuclide transport. Additional confirmation of the role of secondary minerals in radionuclide retardation was obtained using selective sequential extraction on the fracture surfaces. These observations support the inclusion of sorption data for fracture infilling minerals in the sorption database developed for the geosphere model for the Canadian Nuclear Fuel Waste Management Program.