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6 Conclusions 
The most significant source of man-made 
radioactivity in the Baltic Sea is fallout 
from the accident at the Chernobyl nuclear 
power plant in 1986. The most important 
radionuclides in the fallout were 137 Cs and 
134 Cs. The total input of 137 Cs from Chernobyl 
to the Baltic Sea has been estimated at 4,700 
TBq, and the post-Chernobyl river discharges 
of 137 Cs to the Baltic Sea were estimated at 
300 TBq comprising 6-7% of the total fallout. 
The second most important source is global 
fallout from atmospheric nuclear weapons 
tests carried out during the late 1950s and 
early 1960s. The predominant radionuclides 
in the global fallout were 137 Cs and 90 Sr, in 
an activity ratio of about 1.6. During the 
late 1990s the decay-corrected amounts of 
weapons-test 137 Cs and 90 Sr in the Baltic Sea 
have been evaluated at 800 and 500 TBq, 
respectively. 
The corresponding decay-corrected total 
inputs to the Baltic Sea of 137 Cs and 90 Sr 
originating from nuclear reprocessing plants 
in Western Europe have been estimated at 
250 and 40 TBq, respectively. These sources 
are now only of minor importance, due to 
significant reductions in discharges in recent 
years. 
The predominant radionuclide in discharges 
from the nuclear power plants and research 
reactors in the Baltic Sea region is 3 H. Total 
discharges of 3 H from these local sources 
have amounted to 3,200 TBq, and those of 
other beta-gamma emitting radionuclides 
amounted to about 24 TBq by the end of 
2006. The total discharges of alpha emitting 
radionuclides have been 0.005 TBq. 
For 137 Cs in the Baltic Sea, the main source 
is fallout from Chernobyl (82%), followed by 
nuclear weapons test fallout (14%). For "Sr, 
the main source of contamination is fallout 
from nuclear weapons tests (81%), while the 
proportion from Chernobyl fallout was smaller 
( 13 %). 
Today, 137 Cs is the main indicator of man 
made radioactivity in Baltic seawater. The 
highest concentrations observed in seawater 
during the period 1999-2006 were found in 
the Baltic Proper and the Bothnian Sea. The 
general trend is steadily decreasing. It is 
estimated that the target value of 15 Bq/m 3 , 
corresponding to pre-Chernobyl levels, will 
be reached between 2020 and 2030. First 
estimates of effective half-lives for different 
parts of the Baltic Sea have been calculated. 
The inventory of 137 Cs in the Baltic seawater in 
2006 is estimated at 870 TBq. 
The results of the Sediment Baseline 
Study carried out by the MORS-PRO 
during the reporting period showed that 
the concentrations of naturally occurring 
radionuclides in Baltic Sea sediments 
remain at background levels. However, the 
concentrations of man-made radionuclides 
are still higher than the target specified 
in HELCOM’s ecological objective of 
“radioactivity at pre-Chernobyl level”. This is 
particularly true for the Bothnian Sea and the 
Gulf of Finland, which received the largest 
amounts of Chernobyl fallout in the Baltic 
Sea. The total inventory of 137 Cs in the Baltic 
Sea sediments was estimated at 2,100-2,400 
TBq at the beginning of the 2000s, while 
the inventories of the naturally occurring 
radionuclides 40 K and 226 Ra were estimated 
to be about 8,500 and 420 TBq, respectively, 
in the uppermost 0-10 cm sediment layer. 
Hence, the concentrations of man-made 
radionuclides in sediments were generally 
at or below the concentrations of naturally 
occurring radionuclides. 
It remains essential to monitor radioactive 
substances in the Baltic Sea to provide 
a basis for evaluating time trends and 
for understanding the state of the Baltic 
marine environment and various aspects of 
radioactivity. However, there are still gaps 
in our knowledge of radioactivity in the 
sediments of the Baltic Sea.