​Radioactive substances occur naturally in the environment - mainly from the  substances of which the solar system and the Earth were originally formed and partly from the Earth’s atmosphere by the slowing down of particles from the sun. The marine environment thus contains naturally
occurring radionuclides. The development and use of nuclear power for military and peaceful purposes have however, resulted in the production of a number of man-made radioactive substances.

Man-made radionuclides of particular concern to man and the environment are 90Sr and 137Cs, which are both formed by nuclear fission. These radioisotopes have half-lives of about 30 years and remain in the environment for many years once released. Furthermore, 90Sr and
137Cs are readily transported through food chains, since strontium and caesium have chemical similarities to calcium and potassium. These radionucleids may contaminate food and expose humans to radioactivity through ingestion.

The occurrence of man-made radioactive substances found in the Baltic Sea is due to four main events:

  1. During 1950-1980, the United States and the Soviet Union carried out atmospheric nuclear weapons tests which peaked in the 1960s causing radioactive fallout throughout the northern hemisphere. This pollution is still noticeable in the seas and on land (UNSCEAR, 2000).
  2. The accident at the Chernobyl nuclear power plant in 1986 caused heavy pollution in the vicinity of the power plant and also considerable
    fallout over the Baltic Sea.
  3. The two European facilities for reprocessing spent nuclear fuel - Sellafi eld in the UK and La Hague in France - have both discharged radioactive substances into the sea. Some of this radioactivity has been transported by sea currents to the North Sea, from where a small proportion has entered the Baltic Sea. 
  4. Authorised discharges of radioactivity into the sea occurring during the routine operation of nuclear installations in the Baltic Sea region (nuclear power plants and nuclear research reactors) have also contributed.