42 
Figure 1: 
Map of the Baltic Sea 
showing water sub- 
regions applied in the 
model.Sub-regions no. 
69, 73, 77 and 79 refer 
to bottom waters in sub- 
regions with stratification 
The remaining numbers 
refer to surface waters 
and sub-regions without 
stratification. 
of 90 Srand 137 Cs in Baltic seawater from 
inventories calculated by the model expressed 
in terms of effective half-lives. The values for 
137 Cs compare well with corresponding values 
derived from observed inventories, showing 
values in the range of 9 to 15 years, as 
described in Chapter 3A on seawater. 
Important aspects to incorporate in further 
modelling work include river run-off in 
agreement with available observed data, and 
the transfer of radioactivity from contaminated 
seabed sediments to the water column. Both 
of these processes contribute to the fact that 
the concentrations of 137 Cs in Baltic seawater 
remain relatively high more than twenty years 
after the Chernobyl accident in 1986. 
4.2 Dose calculations 
Estimates of radiation doses accumulated 
until the year 2000 by human individuals and 
populations from radioactivity in the Baltic Sea 
were made by the MORS Group in HELCOM 
(2003). These estimates were based on 
model calculations and included a range of 
exposure pathways including ingestion of 
fish, crustaceans and molluscs, inhalation, 
and external exposure. Doses to individuals 
were based on human habits assumed to be 
characteristic for a critical group expected 
to receive the largest radiation dose. The 
dominating exposure was found to be due to 
137 Cs and the ingestion offish. 
The concentrations of the dominating man 
made radionuclides in the Baltic Sea, 90 Sr 
and 137 Cs, have been declining since 2000. 
The only man-made radionuclides showing 
increasing trends in Baltic seawater are the 
small quantities of radionuclides originating 
from discharges from Sellafield and La Hague 
("Tc and 129 l), but in terms of radiation doses 
to humans these are insignificant. 
For the reporting period covered by the 
present report, we may estimate an upper 
limit for individual doses from man-made 
radionuclides in the Baltic Sea. During the 
period 1999-2006 the concentrations of 
137 Cs in fish from the Baltic Sea have been 
below 20 Bq/kg. For an individual having a 
high-rate consumption of 90 kg fish per year 
this concentration corresponds to an annual 
radiation dose of 20 pSv. This dose is well 
below the safety limit for the annual radiation 
dose to a member of the public of 1000 pSv 
(EC 1996). The corresponding annual dose 
from naturally occurring radionuclides in fish 
is about 100 pSv, of which the dominating 
contribution is from polonium-210. The 
annual dose contribution from tritium ( 3 H) is 
insignificant by comparison, less than 0.01 
pSvfrom natural and man-made sources 
combined. 
The annual radiation doses calculated for the 
period 1955-2005 for individuals consuming 
9 kg fish flesh per year are shown in Figure 
4. The calculations show that the annual