1 Introduction 
Sven Nielsen 
Ris 0 DTU, National Laboratory for Sustainable Energy, Denmark 
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. Examples of the former include 
isotopes of uranium, thorium and potassium, 
which have half-lives comparable to the age 
of the Earth, i.e. billions of years. Examples 
of the latter include tritium ( 3 H, super heavy 
hydrogen) and carbon-14 ( 14 C), with half lives 
of 12 years and 5,700 years, respectively. 
The long-lived isotopes 235 U, 238 U and 232 Th 
are transformed by radioactive decay into 
a series of decay products, which are also 
themselves radioactive, thus adding to the 
number of radioactive substances in the 
environment. Examples include 226 Ra and 
210 Po, with half lives of 1,600 years and 140 
days, respectively. 
Therefore, the marine environment contains 
naturally occurring radionuclides. One cubic 
metre of seawater typically contains 1000 
Bq 3 H, 4 Bq 14 C, 40 Bq 238 U, 4 Bq 226 Ra, 4 Bq 
210 Po and 12,000 Bq 40 K (National Academy of 
Sciences, 1971). 
The development and use of nuclear power 
for military and peaceful purposes have 
resulted in the production of a number of man 
made radioactive substances. Explosions 
of nuclear weapons in the atmosphere 
distribute radioactive substances in the 
environment, while underground nuclear 
explosions release little or no radiation into 
the environment. The routine operations 
of nuclear power plants give rise to small 
controlled discharges of radioactive 
substances, but accidents at nuclear power 
plants can cause releases of considerable 
amounts of radioactivity into the environment. 
Man-made radionuclides of particular concern 
to man and the environment are 90 Sr and 
137 Cs, which are both formed by nuclear 
fission. Both of these radioisotopes have half 
lives of about 30 years, so when released 
into the environment they remain there for 
many years. Furthermore, 90 Sr and 137 Cs 
are readily transported through food chains, 
since strontium and caesium have chemical 
similarities to calcium and potassium, which 
means that they may contaminate food and 
expose humans to radioactivity through 
ingestion. Other man-made radionuclides of 
concern are 239 Pu and "Tc, with half lives of 
24,000 years and 210,000 years, respectively. 
The occurrence of man-made radioactive 
substances in the Baltic Sea has four main 
causes: 
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 of spent nuclear fuel, at 
Sellafield 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 
Photo by Visvaldis GRAVERIS