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 reacto rs, have been used to study migration and adsorption phenomena of anth ropogenic radionuclides. gamma-measurements on water samples, suspende d particles and sediments downstream of the Muhleberg nuclear reactor help to quantify the transfer of dissolved and particulate Co-60 to na tural suspended particles and to understand their fate in a riverine s ystem. 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 activiti es of Co-60 and Cs-137 of 38 and 47 Bq/kg, respectively), in compariso n with known radionuclide input functions helped to demonstrate the mi nor importance of reservoir sediments as radionuclide sinks (<10%). Th e similarity between modelled breakthrough curves, which neglect radio nuclide sedimentation and measured breakthrough curves for two low wat er discharge situations yielded the same overall conclusion. These fin dings were supported by the determination of 'dissolved' and particula te 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-siz e range 10-40 mu m. Particles in this range remain suspended in the pr incipal zones of the river section studied. Adsorption differences enc ountered in our natural system were more a function of grain-size rath er than mineralogy.