Radionuclides in sediments of the Aare and Rhine river system: Fallouts, discharges, depth-age relations, mass accumulation rates and transport along the river.

The Aare-Rhine river system with its four nuclear power plants on the banks of these rivers and with its intermediate lakes and reservoirs provide the unique chance to analyze the input of radioactivity into the system thereby furnishing information on the sources, to analyze the transport within the sediment and along the rivers, and to refine unsupported Pb dating validated by known discharge maxima. At three locations (Lake Biel, Klingnau Reservoir, old branch of the Rhine) in the Aare and Rhine rivers system downstream of the older nuclear power plants (NPPs) Mühleberg and Beznau, the vertical distributions of Cs, Pb, Pb, Bi, K, Be, Pu, Pu, Am, and Np in sediment cores were determined. Depth-age relations using the excess 210 Pb were established with the raw and with the piecewise Constant Rate Supply (CRS) models. A comparison of the piecewise CRS method with the imprints of known discharges showed differences of up to two years. Besides typical Cs signals (about 100 Bq∙kg) from the atmospheric nuclear weapons testing (NWT) and the Chernobyl fallouts, imprints of known Cs discharges (10-70 Bq∙kg) from the NPPs were found in the sediments. The Np distributions (6-10 Bq∙kg) essentially follow the Cs NWT distributions. In the sediment downstream the NPP Mühleberg (Lake Biel) aPu distribution (<3 Bq∙kg) was found, which was solely due to the NWT fallout. Downstream the NPP Beznau (Klingnau Reservoir and an old branch of the Rhine), besides the NWT distribution, also imprints of Pu discharges (up to 7 Bq∙kg) were found within the time interval 1963 to 1986.Pu/Pu ratios revealed that the burn-up times of the nuclear fuel in the NPP (U enrichment of 3.5%), from which the discharges stem, should be about 1 year or less. A comparison between the calculated and the measured Cs/Pu ratio revealed no large discrepancies for the Lake Biel and Rhine positions, but in the Klingnau distribution, the calculated Cs/Pu ratio is one order of magnitude larger than the measured one. The reason could be either a natural uranium research reactor as the source, or strong, short-range Pu precipitation after the discharge from the Beznau NPP. The largest Pu peak in the Rhine sediment (1968/70) corresponds to no major peak in the Klingnau sediment. For the NPP Mühleberg discharge of 1982 the ratio of the Cs deposition in sediments from Lake Biel, Klingnau Reservoir and the Rhine river is about 1 : 0.5 : 2.9. For the 1977/78 Pu deposition the ratio is 1 : 0.02, for the Klingnau Reservoir and the Rhine sediments, respectively. These numbers indicate a long-range transport of Cs and a rather short-range transport of Pu.