EVALUATION OF THE IMPORTANCE OF RESERVOIR SEDIMENTS AS SINKS FOR REACTOR-DERIVED RADIONUCLIDES IN RIVERINE SYSTEMS

Low activities of Co-60 and Cs-137, discharged by Swiss nuclear reactors, have been used to study migration and adsorption phenomena of anthropogenic radionuclides. gamma-measurements on water samples, suspended particles and sediments downstream of the Muhleberg nuclear reactor help to quantify the transfer of dissolved and particulate Co-60 to natural suspended particles and to understand their fate in a riverine system. A 5 km river section, dammed up for hydroelectric purposes has been selected for this study. Dated sediment cores, where radionuclide activities vary as a function of locality and depth (maximum activities of Co-60 and Cs-137 of 38 and 47 Bq/kg, respectively), in comparison with known radionuclide input functions helped to demonstrate the minor importance of reservoir sediments as radionuclide sinks (<10%). The similarity between modelled breakthrough curves, which neglect radionuclide sedimentation and measured breakthrough curves for two low water discharge situations yielded the same overall conclusion. These findings were supported by the determination of 'dissolved' and particulate radionuclide proportions. More than 75% of the total discharged Co-60 is found 'in solution' (or adsorbed to colloidal particles <0.2 mu m), the remaining portion being adsorbed to particles in the grain-size range 10-40 mu m. Particles in this range remain suspended in the principal zones of the river section studied. Adsorption differences encountered in our natural system were more a function of grain-size rather than mineralogy.