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4 Modelling and Dose Calculations
Günter Kanisch 1 and Sven Nielsen 2
1) vTI, Johann Heinrich von Thünen-Institute, Germany
2) Rise DTU, National Laboratory for Sustainable Energy, Denmark
4.1 Model work
Environmental modelling of radioactive
substances in the Baltic Sea is used by
the MORS Group to support and interpret
environmental data, and as a tool for
radiological assessments.
The Group has used a model (HELCOM
to simulate levels of 90 Srand 137 Cs in Baltic
seawater based on known inputs from the
dominating sources, i.e. atmospheric fallout
from nuclear weapons testing, atmospheric
fallout from the Chernobyl accident, and
discharges into the sea from the European
reprocessing facilities at Sellafield and La
Hague. The model also includes the runoff
Environmental
compartment
137 Cs
“Sr
P/O geometric SD of No. of
ratio P/O values
P/O . . „ . No. of
.. geometric SD of P/O
ratio values
Seawater
1.05
1.16
396
0.99
1.10
367
Fish
1.03
1.29
362
0.97
1.30
178
Fucus vesiculosus
1.01
1.23
150
0.98
1.27
39
2003) that was implemented in software
which does not run on the latest computers.
Work is presently in progress to implement
the model in Windows compatible software.
The model uses first-order kinetics to
simulate the transfer of radioactivity between
compartments comprising water regions and
underlying sediments.
The model has been developed to assess
the radiological consequences of releases
of radioactive material into the marine
environment, covering European coastal waters
including the Baltic Sea. The model simulates
the dispersion of radioactive substances in
the water due to advective transport, including
mixing from wind and tidal forces. The
association of radionuclides with suspended
sediment material is taken into consideration,
in addition to any subsequent transfer into
sediments through particle scavenging. Starting
with specified inputs of radioactivity into the
marine environment, the model calculates time-
dependent concentrations in seawater and
sediments. This data may be used to calculate
doses to man from a range of exposure
pathways. The locations of the water boxes
defined for these studies are shown in Figure
1 .
The new implementation of the model
(Kanisch et al. 2000) has made it possible
of radioactivity from land to sea via rivers.
Figures 2a and 2b show comparisons
between calculated (red lines and squares)
and observed (black circles) concentrations
of 137 Cs in seawater in various sub-regions.
The observed concentrations are the
Range of
Effective half-life (years)
years
137 Cs
90 Sr
1988- 1996
9.0
15.2
1999-2006
12.8
14.5
1993-2006
11.5
14.6
annual average values of data collected by
MORS-PRO. The vertical bars represent the
variability of the observed concentrations
within a single year.
The reliability of the model calculations is
illustrated in Figures 3a and 3b showing
scatter plots of observed and calculated
seawater concentrations of 90 Srand 137 Cs
across all regions of the Baltic Sea for
the years 1965-2006. The data points are
distributed both above and below the line of
the 1:1 relationship thus indicating the model’s
overall unbiased quality. Table 1 shows
the corresponding results of the statistical
evaluation of the ratios predicted/observed;
with the results for fish and Fucus vesiculosus
included. Table 2 shows the rates of reduction
Table 1:
Summary of comparisons
between model predictions
(P) and observations (O) in
environmental compartments
of the Baltic Sea. Geometric
means and geometric
standard deviations were
calculated for the P/O
ratios. The concentration
factors considered for fish,
differentiated by marine and
freshwater fish, round fish and
flat fish, fillets and edible parts,
as well as for the bladder wrack
Fucus vesiculosus, were taken
from Chapter 3.
Table 2:
Rates of reduction of “Srand
,37 Cs in the Baltic Sea, in terms
of effective half-lives derived
from calculated total inventories
during different time periods.
Baltic Sea Environment Proceedings No. 117
