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The Aare river system in Switzerland, with two nuclear power plants on the banks of the river, and its intermediate lakes and reservoirs, provides a unique opportunity to analyze the contribution of different sources to the radioactive contamination. Sediment cores were collected from two lakes and a reservoir, all connected by the river Aare. In order to study the influence of the Chernobyl accident, one sediment core was collected from a lake in the southern part of Switzerland. The sediment cores were sliced and analyzed with gamma ray spectrometry. Plutonium, americium, and uranium were extracted radiochemically, and their concentrations were measured with a sector field ICP-MS. The uranium isotope ratios were further measured with a multi collector ICP-MS. The maximum 137 Cs activity from the Chernobyl accident and the Pu and 137 Cs activities associated to the 1963 global fallout maximum were well identified in sediments from all three lakes. High-resolution records of plutonium isotopes in the zone of the sediments corresponding to the period of maximum fallout from the atmospheric nuclear weapon testing showed distinct fingerprints, depending on the different test activities. Pu isotope ratios could be used to detect non-global fallout plutonium. The ratio 241 Am/ 241 Pu was used to determine the age of the plutonium. Despite of very low 241 Pu and 241 Am concentrations, the calculated plutonium production dates seemed to be reasonable for the sediment layers corresponding to the NWT tests. The calculated production date of the plutonium in the upper most 15 cm of the sediment core seemed to be younger. The reason for this could be additional non-global fallout plutonium. For the lake sediments, natural ratios for 235 U/ 238 U and enriched or depleted ratios for 234 U/ 238 U were measured, depending on the lake. A small increase of the 236 U/ 238 U ratio could be recognized for the NWT zone in all three lakes and, for Lake Lugano, a further distinct increase in the Chernobyl layer., (© 2022. The Author(s).)

This paper presents the results of 137 Cs and 210 Pb dating of sediments from the northern and western basins of Lago Maggiore and the southern basin of Lago di Lugano. Vertical distributions of 137 Cs in sediment cores taken in 2017/18 were compared to those of the sediment cores taken almost two decades ago at the same positions. This comparison showed that 137 Cs peaks do not alter their shape significantly over time and 137 Cs remains a reliable time-marker for many decades. 210 Pb dating using for the first time the piecewise "constant rate supply" (CRS PW ) model constrained with 137 Cs time-markers is performed for sediment cores of Lago Maggiore and Lago di Lugano. "Event-linked" mass accumulation rates (MARs) were derived from the CRS PW model. In combination with the dry bulk density, organic matter content, 40 K and 210 Pb activity concentrations, these "event-linked" MARs were used for the identification of one or more event layers (turbidites, detrital layers) in 1-cm thick sediment samples. "Continuous" mean MARs which are independent of the event layers were estimated: in Lago Maggiore sediment cores they are 0.066-0.090 g cm -2 yr -1 ; in Lago di Lugano they vary in the range of 0.032-0.057 g cm -2 yr -1 with higher values in the time interval 1963-1986 as compared to 1986-present., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

(241)Pu was determined in slurry samples from a nuclear reactor decommissioning project at the Paul Scherrer Institute (Switzerland). To validate the results, the (241)Pu activities of five samples were determined by LSC (TriCarb and Quantulus) and ICP-MS, with each instrument at a different laboratory. In lack of certified reference materials for (241)Pu, the methods were further validated using the (241)Pu information values of two reference sediments (IAEA-300 and IAEA-384). Excellent agreement with the results was found between LSC and ICP-MS in the nuclear waste slurries and the reference sediments., (Copyright © 2011 Elsevier Ltd. All rights reserved.)