Radioactivity in the Baltic Sea 1992-1998

Since 1984, the Contracting Parties to the Helsinki Convention have collected monitoring data on radionuclides in the Baltic Sea. The data covers concentrations of radioactivity in the Baltic marine environment and discharges from nuclear installations (nuclear power plants and nuclear research facilities) in the catchment area of the Baltic Sea. Additionally, important sources of anthropogenic radionuclides in the Baltic Sea have been considered - namely atmospheric fallout from nuclear weapons testing and the Chernobyl accident, and discharges into sea from European reprocessing facilities at Sellafield in the UK and La Hague in France. Due to the relatively slow exchange of water between the Baltic Sea and the North Sea, contaminants such as anthropogenic radionuclides have a prolonged residence time in the Baltic Sea. Levels of 90Sr and 137Cs are consequently still high in the Baltic Sea compared with other water bodies around the world. Strontium-90 contamination originates from atmospheric nuclear weapons testing, which peaked in the 1960's and led to direct input to the Baltic Sea from atmospheric fallout, and to delayed input via rivers. Caesium-137 was also released during atmospheric nuclear weapons tests, but the related input was small compared to the direct input from the Chernobyl accident in 1986. The delayed input of 137Cs to the Baltic Sea from rivers is smaller than the direct atmospheric fallout of 137Cs, and also much smaller than delayed inputs of 90Sr, since due to its chemical properties, caesium is less mobile in the environment than strontium. The anthropogenic radionuclides present in the Baltic Sea originate from several sources and modes of input. Direct atmospheric fallout has accounted for the main inputs, from atmospheric nuclear weapons tests and from the Chernobyl accident. Run-off from the land into sea via rivers has also contributed significantly for 90Sr, and to a lesser extent for 137Cs. Hydrodynamic transport has been significant for radionuclides from European nuclear reprocessing facilities, even though these are located beyond the North Sea, and far away from the Baltic Sea. Only a small proprotion of the total radioactive discharges into the sea from these facilities is estimated to reach the Baltic Sea, but on a relative scale this input is significant for the Baltic Sea. Finally, direct discharges into the Baltic Sea occur in coastal waters from the routine operations of nuclear facilities along the shores of the Baltic Sea itself. These discharges are very low, however, and are authorised by national regulatory authorities. Other anthropogenic radionuclides are present in the Baltic Sea, but 137Cs is by far the most important radionuclide with respect to radiation doses to humans. Since the Chernobyl accident in 1986, the levels of anthropogenic radionuclides in the Baltic Sea have generally declined mainly due to radioactive decay and the gradual outflow of water through the Belt Sea and the Kattegat. However, increased discharges of radionuclides into the Irish sea from Sellafield since 1994 have affected European coastal waters including the Baltic Sea, which receives small but measurable amounts of technetium-99 in inflow from the North Sea. This report describes the work carried out by the MORS group during the period 1992-1998 concerning radioactive substances in the Baltic Sea. Chapter 2 lists the sources of anthropogenic radionuclides in the Baltic Sea. Chapter 3 describes the monitoring network which forms the backbone of the HELCOM Recommendation 18/1 on monitoring activities. Chapter 4 describes the work carried out on the analytical quality of the monitoring data. Chapters 5, 6 and 7 describe the levels of radionuclides in seawater, sediments and biota, respectively. Chapter 8 describes the modelling of radioactivity in the Baltic Sea, and Chapter 9 covers the assessment of radiation doses to humans from radioactivity in the Baltic Sea. (orig.)