7 
In addition to the above, no other sources of 
radioactivity noticeably contributed to atmos 
pheric fallout prior to the Chernobyl accident, 
except, e.g., the Windscale reactor fire in 1957, 
which mainly had a local impact. However, 
aquatic discharges from the Western European 
nuclear reprocessing plants (Sellafield, UK, and 
La Hague, France) have influenced the concen 
trations of Cs-137 and some other radionuclides 
in the marine environment of the southern Baltic 
Sea via the inflow of saline waters through the 
Danish Straits. 
The Baltic Sea was the marine area most 
affected by the Chernobyl accident because the 
first radioactive clouds from Chernobyl travelled 
north and caused strong deposition in the Baltic 
Sea region (Povinec et al., 1996). The deposition 
was greater there than, e.g., in the Black Sea, 
the Mediterranean Sea or any other marine or 
brackish water area. 
The International Atomic Energy Agency, IAEA, 
(1986) estimated that the total amount of radio 
active substances released to the atmosphere 
as a result of the Chernobyl accident was 1-2x 
10 18 Bq. Most of the radionuclides released were 
short-lived, and their impact on the environ 
ment was negligible. Among the longer-lived 
radionuclides, Cs-137 was the most important 
owing to its relatively long half-life (30 years) 
and its relevance with respect to radiation doses 
to man. The total amount of Cs-137 released 
to the environment has been estimated to vary 
between 38 000 TBq ± 50% and 167 000 TBq, 
with the most frequently used value 70 000 TBq. 
The total input of Cs-137 from the Chernobyl 
accident into the Baltic Sea area has been 
estimated at 4 100-5 100 TBq (decay corrected 
to 1991). 
During the acute fallout situation, Chernobyl- 
derived caesium was dispersed directly onto the 
surface of the sea and, with a delay, throughout 
the entire drainage area. In the course of time, 
the Baltic Sea has received caesium from the 
surrounding terrestrial and adjacent coastal areas 
as a result of runoff, river discharges, and coastal 
currents. A large part of this caesium has been 
deposited in the sediments, but a significant 
amount of Chernobyl caesium has also been 
transported by sea currents from the Gulf of 
Bothnia and the Gulf of Finland into the Baltic 
Proper and further out from the Baltic Sea through 
the Danish Straits. Currently, the proportion of 
Chernobyl-derived caesium is clearly dominant in 
the Baltic Sea environment, while that originating 
from other sources is decreasing, and it is often 
difficult to distinguish the ’’old” caesium from that 
of Chernobyl. 
The distribution pattern of Chernobyl-derived Cs- 
137 in the catchment area of the Baltic Sea was 
very patchy, with the highest deposition values 
occurring in the areas surrounding the Gulf of 
Bothnia and the Gulf of Finland (Figure 1). The 
highest Cs-137 concentrations and total amounts 
(per square metre) in bottom sediments also 
occur in these gulfs, but the patchy distribution 
has been further emphasized as a consequence 
of river discharges, sea currents, and varying 
sedimentation rates on hard (erosion) and soft 
(sedimentation) bottoms. 
Radioactivity 
Natural phenomenon which is a property of 
many elements. It consists of a spontaneous 
emission of charged particles or radiation. 
After this event the nucleus has changed, 
which results in a different element or a differ 
ent state of the same element. Elements with 
this property are called radionuclides. The 
‘‘half-life” of a radionuclide describes the time 
after which half of the material has undergone 
a radioactive transformation. Radioactivity is 
measured in becquerels (Bq), which is defined 
as the activity of a quantity of radioactive mate 
rial in which one nucleus decays per second. 
Units used in this report include terabecquerel 
(TBq), or 10 12 Bq, and millibecquerel (mBq), or 
10- 3 Bq. 
Natural and artificial radionuclides 
Some radionuclides have been present from 
the beginning of the universe, while others 
are produced by natural processes; both are 
called natural radionuclides. Other radionu 
clides are produced by human activities such 
as in the operation of nuclear power plants 
or explosion of nuclear weapons. These are 
called artificial radionuclides. 
Radionuclides of interest 
As for other types of contaminants in the 
sea, relevant properties for the definition of a 
‘‘nuclide of interest” are abundance, longev 
ity and affinity of accumulation. Owing to 
the slow transport processes in the marine 
environment, only certain radionuclides are of 
interest. 
Long-lived radionuclides in the seabed of the Baltic Sea