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A box model for calculation of collective dose commitment from radioactive waterborne releases to the baltic sea
J. Environ. Radioactivi~ 2 (1985) 41-57
A Box Model for Calculation of Collective Dose Commitment from Radioactive Waterborne Releases to the Baltic Sea
Sverker Evans Studsvik Energiteknik AB, Nuclear Division, Environmental Protection, S-6ll 82 Nyk6ping, Sweden
ABSTRACT To meet the new regulations of maximum yearly releases from Swedish nuclear power plants issued by the Swedish National Radiation Protection Institute, and to make realistic calculations of collective dose commitment derived from radionuclides discharged to the Baltic Sea, a box model A Q U A P A T H - B A L T I C has been developed. The model calculates radionuclide concentrations in 25 boxes for discharges into given compartments and takes into consideration the dispersion rates of the water masses, the sediment-water interaction and radioactive decay. The calculated concentrations in water and sediment at steady state are then used to evaluate individual and collective intakes of activity and external exposures. The radiation doses to the global population following discharges to the Baltic Sea are also calculated.
INTRODUCTION
Estuaries and coastal waters constitute the primary recipient of releases to the aquatic environment from different parts of the nuclear power fuel cycle. The radiation doses assessed from aquatic discharges make a significant contribution to the dose burden to the critical group and populations. The Baltic Sea is surrounded by "highly industrialized countries where a number of nuclear power plants are in operation or under construction. The worldwide fallout from nuclear weapon tests is 41
J. Environ. Radioactivity 0265-931X/85/$03.30 (~) Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
' "
,-e_
Sverker
Ecans
still the main source of man-made radionuclides present in the Baltic Sea. These fallout products reach the sea by direct precipitation on the water surface and by runoff from surrounding land areas. Some additional radionuclides originating from nuclear power plants and industrial, medical or scientific sources can also be released to the environment and enter the aquatic ecosystem. At present Baltic Sea water contains about 4 x 105 GBq (1 x 10* Ci) of both 9°Sr and t37Cs, relating to an average concentration of about 2 × 10 -2 Bq liter -t (0"5 pCi liter -t) for both isotopes and about 1 x 10 -s Bq liter- 1 (0-3 fCi liter- 1) of 239,240pu" The wide range of concentrations in different types of sediments and their non-uniform regional distribution make it almost impossible to calculate the total inventory of radionuclides in Baltic Sea sediments. This figure is therefore unknown. Very few data are available on the distribution of other relevant artificial radionuclides in the Baltic. Nor is the radionuclide content of the total biomass quantified for the Baltic Sea region. To restrict pollution in the Baltic Sea such that the influence of anthropogenic effects on the sea is maintained within acceptable bounds, detailed and effective assessment models are required. This study represents a further advance in such development with the aim of forecasting exposures from long-term turnover of radioactive effluents discharged to the Baltic Sea ecosystem. A box model AQUAPATHBALTIC divides the Baltic Sea into a number of compartments and takes into consideration sediment-water interactions. It therefore provides a more detailed picture of radionuclide dispersion and a more realistic assessment of dose burdens. The model also provides the opportunity to superimpose discharges from several outlets, thus yielding an integrated contribution to calculated dose rates to the different populations around the Baltic Sea. WATER E X C H A N G E OF T H E BALTIC SEA The large-scale circulation pattern and water exchange of the different sectors of the Baltic have been calculated by several authors (Knudsen, 1900; Brogmus, 1952; Fedsov & Zaitsev, 1959; Soskin, 1963; Fonselius, 1969, 1971; Bolin, 1972; Svansson, 1972, 1980; Falkenmark & Mikulski, 1975; Stigebrandt, 1983). The magnitude of water exchange through the Danish straits is determined by the fresh water supply and by the intensity of mixing in the Baltic. The latter is primarily governed by
,4 Baltic box model for dose calculations of ~'aterborne radioactirity
43
winds which give rise to an inflow of water of oceanic origin to the Baltic Sea. This saline water of high density accumulates in deeper parts of the central Baltic but is gradually transferred to the surface by wind influence. It is then mixed with fresh water and again discharged to the North Sea. The outflow from the Baltic is primarily governed by the surplus of fresh water which causes the sea level to rise relative to that in the adjacent seas, thus creating a horizontal pressure towards the open ocean. The inflow of water from the North Sea can be separated into two components: a continuous deep water current generated by the horizontal pressure gradient and a much more intense inflow of high density water driven by long-lasting westerly winds. The primary halocline, which is permanently developed at 60-80 m depth in the Baltic proper, separates overlying surface water from deep water and restricts vertical exchange between the two water masses. In the Baltic the mean circulation is cyclonic. This causes a slightly increased salinity in the eastern sectors of the Baltic proper.
SEDIM ENT-WATER I N T E R A C T I O N The ability of suspended matter and bottom sediments to absorb and adsorb radioactive nuclides can result in a significant depletion of activity from the water column. Unfortunately, the state of the art in evaluating sediment-related effects is less advanced, even though the latter may be critical in determining the magnitude of the exposure pathway to man. The fraction of activity retained in sediments for a given nuclide has been estimated from the concentration factor (kd) between sediment and water, k a is defined as the ratio of the concentration of the radionuclide in sediment (Bq g-~ dry wt) to the concentration in overlying water (Bq g-l). Depletion by sediments is thus greater for those nuclides with higher values of kd; it is also greatest in those areas with high suspended sediment loads and high rates of sediment deposition. The removal of activity into bottom sediments was evaluated using a particle scavenging model by Clark & Webb (1980). Here the removal of a radionuclide from solution is determined by two main factors: - - t h e k d value - - t h e rate of settling of particulate matter
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S v e r k e r Evans
The model assumes that the radioactivity is uniformly distributed as a function of water depth, a reasonable assumption in most continental shelf waters. The depth distributions of the Baltic Sea subareas were calculated from Ehlin and Mattison (1976). The removal rate of activity from the water to sediments, kwh, is given by kws-
kdS (year- 1) hm(1 + kdSS )
where S = sedimentation rate (g m - 2 y e a r - 1), hm = mean water depth (m), and S S = t h e a m o u n t of suspended material in the water column (g liter- t). Knowing S, SS and hm, the rate of loss to sediments can be estimated for each nuclide in each compartment. The return of activity from sediments remains a major topic requiring further work. Such an effect, however, would only be of importance for long-lived nuclides which are removed to a large extent into and from bottom sediments. For nuclides with half-lives of the order of a few years or less, the return of activity from sediments is probably insignificant. THE AQUAPATH-BALTIC MODEL The A Q U A P A T H - B A L T I C model calculates water concentrations for discharges into given reservoirs, taking into consideration the dispersion of the water masses, the interaction of nuclides with sediments, and radioactive decay. The calculated concentrations in water and sediment are then used in association with concentration factors for edible aquatic species to evaluate collective intakes of activity by the different populations around the Baltic Sea and by the world population. C o m p a r t m e n t analysis is used to model the movements of water and the associated activity between connected reservoirs. This technique assumes instantaneous uniform mixing of each compartment, the transfer rates between compartments being described by transfer coefficients (year-I). The differential equation which describes the variation of the activity in compartment i is of the form: dA i
i=.
-- ~', k j i A ~ - kijAi - kiAi - ),Ai + Qi dt i=i
where Ai = the activity present at time t in compartment i (Bq), kia = the
A Baltic box model for dose calculations o/waterborne radioactit'ity
45
transfer rate from compartment/to compartment j (year-1), ki = the rate of loss from compartment i by sedimentation etc. (year-t), ).=the rate of loss from compartment i by radioactive decay (year-1), Qi = the rate of discharge into compartment i (Bq year-t), and n = t h e number of compartments in the system. The compartment model is displayed in Fig. 1. The longitudinal 8othn~n Bay
Bothnlan Sea
/
.J
fish
Dl
200
mussel algae
200
Gulf of Finland
K~ W. Baltic proper
mussel 30 algae 50
fish 200 mussel 30 algae i00 ~fish I00 mussel 30 algae 50
Gulf of Riga
~CK
I
Belt Sea
1
E
Baltic proper
I
t
Fig. 1. The Baltic Sea compartment model, the location of the Swedish nuclear power plant sites, and 137Cs concen'tration ['actors for fish, mussel and algae.
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Sverker Evans
separation of the Baltic proper into two sections is a concession to the mathematical treatment of the model system; the division essentially induces a delay of dispersion in the east-west direction. Because of the heavy density stratification of the water masses in the Baltic proper, these boxes are also separated vertically into two layers (Fig. 2). The calculated water exchange between the connected compartments of the model forms the carrier system for the released activity. Runoff is the best assessed parameter because these figures are based on direct measurements. The remaining exchange rates naturally have considerable associated uncertainty. The compartment volumes and their supplies of fresh water are displayed in Table 1 and the water exchange rates between compartments in Table 2. Sediment reservoirs are connected to each subarea and nuclide specific transfer coefficients describe the exchange of radionuclides through the sediment-water interface. The upper 10 centimeters of sediment is assumed to take an active part in the exchange process. For the great majority of the nuclides considered, the possibility of exchange with the atmosphere is very limited and it is reasonable to restrict the model to the aquatic environment alone. The exceptions are
Fig. 2.
The vertical division of the model.
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47
TABLE 1 The Compartment Volumes and their Fresh Water Supplies (= run-off+ A {precipitation-evaporation))
Compartment
Kattegat Belt Sea, the Sound West Baltic East Baltic Gulf of Riga Gulf of Finland Bothnian Sea Bothnian Bay Reference
Volume km 3
< 80 > 80 < 80 > 80
m m m m
515 285 3 785 770 6 965 1 525 405 1 100 4 890 1 480 Ehlin & Mattison (1976)
Fresh water supply km 3 year25 11 92 32 125 90 100 Fedsov & Zaitsev (1959) Mikulski (1970) Dahlin (1976)
3H, 1'~C and 129I, for which, as a result of their chemical properties and their long radioactive half-lives, a significant fraction of activity released in liquid form can be transferred to the atmosphere by evaporation. The values of the concentration factors and sedimentation rates have been simplified so as not to be valid for a specific site but for a geographical situation. Thus, for example, only one concentration factor for fish is utilized for each radionuclide, ignoring possible distinctions due to species and habits. The concentration of a given radionuclide in the edible parts of aquatic organisms is calculated by multiplying its activity in water by the appropriate concentration factor.
EXPOSURE PATHWAYS A N D RADIATION DOSE CALCULATIONS
For the Baltic Sea, fish consumption is the dominant source of internal exposure. In the Kattegat and Belt Sea areas, a certain amount of crustaceans (edible crab, lobster, pandalid shrimps) and molluscs (blue mussel, oyster) are harvested for human consumption. The yearly consumption rates are based on the quantities of marine food products
48
Sverker Evans TABLE 2 Water Exchange Rates Between the C o m p a r t m e n t s (year- t)
year - 1 K a t t e g a t - B e l t Sea K a t t e g a t - N o r t h Sea Belt S e a - K a t t e g a t Belt S e a - E a s t Baltic < 80 m Belt S e a - E a s t Baltic > 80 m Gulf of R i g a - E a s t Baltic < 80 m West Baltic < 80 m-Belt Sea West Baltic < 80 m - W e s t Baltic > 80 m West Baltic < 80 m - E a s t Baltic < 80 m West Baltic> 80 m-Belt Sea West Baltic > 80 m - W e s t Baltic < 80 m West Baltic> 80 m - E a s t Baltic > 80 m East Baltic < 80 m - G u l f of Riga East Baltic < 80 m - W e s t Baltic < 80 m East Baltic < 80 m - E a s t Baltic > 80 m East Baltic < 80 m - G u l f of Finland East Baltic < 80 m - B o t h n i a n Sea East Baltic> 80 m - W e s t Baltic > 80 m East Baltic > 80 m - E a s t Baltic < 80 m Gulf of F i n l a n d - E a s t Baltic < 80 m Bothnian S e a - W e s t Baltic < 80 m Bothnian S e a - B o t h n i a n Bay Bothnian B a y - B o t h n i a n Sea
1'41 3"96 4"11 9"5x 7"7x 8"5x 1-9x 3-0x 1"84 2"9x 1.4x 2'9x 4-0x l'0
10 -1 10 - t I0 - t 10 - t 10-" 10 - I 10 - t 10 - t 10 -2
3 . 0 x 10 . 2 90x
10 -2
8"0x 2"9x l'4x 6"5x 1.5x 4"Ox l'9x
10 -2 10 - t 10 -1 10 - I 10 - I I0 - z I0 - l
harvested in each reservoir together with the assumed consumption rate per capita. The quantities of food-stuffs used for human consumption, assumed consumption rates and population sizes are displayed in Tables 3 and 4. The pathways for external exposure to radioactive nuclides--bathing, visiting beaches, and handling fishing tackle contaminated with bottom sediments--and adherent dose calculations are similar to those used by Bergman et al. (1979). The model also takes into consideration the impact on the global population from releases of radioactive effluents to the Baltic Sea. The global marine ecosystem comprises two compartments, an upper surface layer down to about 100 m depth, and a compartment containing the deep sea basins. The uppermost sediment reservoir encircles the continents and amounts to about 4% of the total sediment area. The same
A Baltic box modelfi~r dose calculations o f waterborne radioactivity
49
TABLE 3 L a n d i n g s of F i s h (1979), C r u s t a c e a n s a n d M u s s e l s ( 1 9 7 7 ) i n t h e Baltic Sea A r e a s a n d t h e E s t i m a t e d A m o u n t s U s e d for H u m a n C o n s u m p t i o n
,4rea
Fish Amount landed (tonnes)
Kattegat Belt Sea, the Sound West Baltic East Baltic Gulf of Finland Bothnian Sea Bothnian Bay Reference
Crustaceans Amount used Jbr human consumption (%)
Amount landed (tonnes)
1'07 x 105 5"8 x t0 "t
50 50
1'99 x 105 4"31 x 105 6"7 x 10"*
50 50 50
Mussels
Amount used for human consumption (%;
Amount landed /tonnes)
2 x 103 1 x 10-'
50 50
4 x 102 8 × 103
4 x lO t i x lO t
50 50
2-7 x I0 "~ 80 1-1 x 10"~ 80 Anon. (1980) Fishery Board of Sweden (pets. comm.)
Anon. (1980) Fishery Board of Sweden (pets. comm.)
Amount used for human consumption ,'%J 50 50
Anon. (1980) Fishery Board of Sweden (pets. comm.)
TABLE 4 C a l c u l a t e d P o p u l a t i o n Sizes of t h e Baltic S u b a r e a s B a s e d o n t h e P a r t of F i s h L a n d i n g s in E a c h A r e a U s e d for H u m a n C o n s u m p t i o n , a n d a Yearly I n t a k e o f 15 k g y e a r -
Area
Kattegat Belt Sea W e s t Baltic E a s t Baltic G u l f of F i n l a n d B o t h n i a n Sea B o t h n i a n Bay
Fish yield (kg y e a r - 1) 2-7 x 1.5 x 5.0 x 1.1 x 1"7 x 1.1 x 4-4 x
10 v 10 v 10 v l0 s l0 T 10 v 106
Population size 1.8 1.0 3.3 7.3 l-1 7-3 2-9
x x x x x x x
106 106 106 106 106 105 105
paths of exposure have been estimated for the world population as for the Baltic Sea area. It is assumed that 1% of the world population lives in coastal regions with an exposure situation similar to that in the Baltic. The A Q U A P A T H - B A L T I C program calculates the dose contri-
50
Scerker Evans
butions from different exposure pathways to the individuals and populations around the Baltic as follows: --the --the --the --the
dose rates for the different exposure pathways (Sv year-t) collective dose rate (manSv year-1) dose commitment for the different exposure pathways (Sv) collective dose commitment (manSv) ACCURACY OF M O D E L FORECASTS
To verify the reliability of the model forecasts, a series of simulations was performed. Starting with the Baltic as a fresh water lake, simulations of salt intrusions from the North Sea into the Baltic were run on the computer. The exchange rates shown in Table 2 were used and the predicted steady state levels were compared with the observed distribution of salinity in the different compartments. A small timestep of 0"02 year was necessary due to short-term factors involved in the Kattegat and Belt Sea areas. The buildup of salinity in the compartment system following a discharge of 34%o saline water to the Kattegat compartment is displayed in Fig. 3. The modelled salinity values show close agreement with the observed long-term averages (Bock, 1971). To investigate the impact of different turnover values on the salt balance in the Baltic proper, a variation analysis was performed. The assumed water exchange between the surface bodies of the eastern and western Baltic proper was varied by multiples from 0"25 to 5, while exchange of the underlying eastern and western sections was varied by 2-4 times. The effect of varying the vertical mixing rate through the halocline by 0-5-2 times was also investigated. In fact, varying the transfer factors between the compartments of the Baltic proper generated only a small change in the resultant salinity distributions. The system is thus rather insensitive to perturbations in flow rates. The stable salinity regime observed in the Baltic underlines the assumed steady state conditions. M O D E L APPLICATIONS: D I S T R I B U T I O N OF C O L L E C T I V E DOSES F R O M 13VCs RELEASED TO T H E BALTIC SEA Two scenarios were studied using the AQUAPATH-BALTIC code. Firstly, the program was run to obtain the distribution of the collective
A Baltic box model for dose calculations of aarerborne raJioactitiry
51
Satinity Kattegat
%. 25
/
20
BeR Sea 15
E. Bottic ~80m
10
~
/ S/
10
2.'.~
Battic ~80m
WBo[tic ( 80 m .
.
E. Baltic ( 80m . .
--
- =" = ~ o° -
~.-%~ i 0
Fig. 3.
/ / /
.
//////-1~Bothnian
//
o
W
20
Sea
. . . . . . .
,
,
1
,
,
,
,
30
40
50
60
70
80
90
!___ 100
--
I 300 yr
The build-up of salinity in the c o m p a r t m e n t system following a discharge of 34%
saline water to the Kattegat compartment.
dose c o m m i t m e n t from t3VCs released to one c o m p a r t m e n t of the Baltic. 3-7 x 10 l° Bq (1 Ci) t37Cs y e a r - t was c o n t i n u o u s l y discharged into the K a t t e g a t and western Baltic < 80 m c o m p a r t m e n t s , respectively, and the water concentrations, truncated after 500 years, were used to calculate the collective radiation doses. Secondly, the corresponding dose rates derived from the actual yearly releases of w a t e r b o r n e 137Cs discharged from the four Swedish nuclear power plant sites were calculated. Some nuclide-specific d a t a were attached to the model. The s e d i m e n t - w a t e r c o n c e n t r a t i o n factor for cesium is salinity dependent, ranging from 5 x 102 in sea water to 3 x 10"* in fresh water. Assuming that the s e d i m e n t a t i o n rate and suspended m a t t e r load are both constant (5 x 102 g m -2 y e a r - ~ a n d 1 x 10 -3 g liter- t respectively), kws becomes a
52
Sverker Et,ans
function of salinity and depth (Fig. 4). In the Baltic, k,,.~ rates around (.1-6) x I0 - t year - t were calculated for 137Cs (Koivuletho etal., 1979; Ramberg & Lampe, 1980), which were far beyond the values 1 × 10 - ~ 1 x 10 -3 year -1 used by Bergman et al. (1977). The mobile character of cesium is also emphasized by recent studies (Santschi et al., 1983;
10 -I
kws , y r - 1
10-2
10-3 0
J
I
'
i
!jjz
Depth , m 200
300
~.jj/To
sa,ioi~y. °joo
0
Fig. 4. The transfer between water and sediment (kws) for cesium, expressed as a function of salinity and depth.
,4 Baltic box model for dose calculations of waterborne radioactivity
53
Sholkovitz et al., 1983). The depth distribution and salinity-dependent concentration factor characteristic of each subarea were used to calculate kws (Table 5). The reflux of 137Cs from the sediment back to the water phase is still a matter for further research. A rate of 1 × 10- 3 y e a r - x was assumed to be valid for the Baltic area. The resulting collective dose commitments for the Baltic Sea subareas and the global population following a discharge of 3-7 x 101° Bq (1 Ci) 137Cs for 500 years within and outside the Baltic Sea entrance, respectively, are shown in Table 6. A collective dose commitment of 2 x 10-2 manSv (2 manrem) was obtained. For the corresponding discharge situation on the Swedish west coast, the dose became a factor of ten lower. The results from the A Q U A P A T H - B A L T I C predictions can be compared to those from the North Sea model by Clark and Webb (1980). The collective dose commitment following a release of 1 Bq s- 1 for one year into the Baltic amounted to 7 x 10- 1~ manSv Bq- 1. However, as the fish concentration factor used was about four times too low, the values should rather be 3 x 1 0 - 1 3 manSv Bq -1. A corresponding value of 5 x 10 - 1 3 manSv Bq -1 _was predicted by the A Q U A P A T H - B A L T I C model, which is in close agreement with that by Clark and Webb (1980). The predicted concentrations of 1 3 7 C s in water and sediment based on the actual present day releases from the four Swedish nuclear power sites TABLE 5 E x c h a n g e R a t e s o f t3VCs ( y e a r - ~) B e t w e e n W a t e r a n d S e d i m e n t for t h e Baltic S u b a r e a s and Global Compartments
Kattegat Belt Sea West Baltic<80 m West Baltic>80 m E a s t B a l t i c < 8 0 rn East Baltic>80 m Gulf of Riga Gulf of Finland B o t h n i a n Sea Bothnian Bay Global surface water Global deep water
6-3 x 1.6x 3-3 x 4-4x 3"2x 6.8x 7.3x 4.0 x 3"0 x 5-1 x 1-0 x 1.2 x
10 - 2 10 -* 10-t 10 . 2 10 -1 10 - 2 10 -1 I0- 1 10-, I0 -1 10-'* 10 - 6
1x l x I x 1x I x 1x 1x 1x 1x 1x 1x 1x
10 - 3 10 - 3 10 - 3 10 - 3 10 - 3 10 . 3 10 - 3 10 -3 10 - 3 10 . 3 10 - 6 10 - 6
54
Sverker Evans
TABLE 6 Collective Dose C o m m i t m e n t (manSv) After a Release of 37 × I0 t° Bq (i Ci) t3VCs y e a r - t to the K a t t e g a t and West Baltic Sea, Respectively, Truncated After 500 Years
Collective dose commitment (manSv)
Compartment
Kattegat 1 2 3 4 5 6 7 8
Kattegat Belt Sea West B a l t i c < 8 0 m East B a l t i c < 80 m Gulf of Finland Bothnian Sea Bothnian Bay Global surface
3.4 x 2.1 x 4"0 x 9.3 x 1"2 x 5-8 x 1.4 x 1.4 x
Y 2-7
West Baltic
10 -~ 10 -~ 10 -'~ 10 -'t 1.0 -~ 10-5 10 -5 10-5
2"0 x l0 -~ 5-2 x I0 -'~ 5 2 x 10 -3 1.08 x 10 -2 1-4 x 10- 3 6.7 x 10 -¢ 1-6 x 10 -~ 8.3 x 10-6
l ' 7 x 10 .3
l ' 9 x 10 -2
are displayed in Fig. 5. A cumulative discharge of 120-3 GBq 137Cs year- t (3.25 Ci year- 1) results in a collective dose commitment of 3 x 10 -2 manSv (3 manrem) to the Baltic populations. This value can be compared to the new regulations of maximum permissible yearly releases from Swedish nuclear power plants which have been issued by the Swedish Bq/kg
Bq/l
'
10 "4
I
[
i~
i
10 -1
W. Baltic (above halocline) ~
Kattegat
10-5
10-2
10-6 I~/ l/ f~" 10-7
/
// t
,
Release GBq/yr 0.2 Simpevarp 100.0 Barsebiick 0.1 ,Ringhals. 20.0 r
0
Fig. 5.
10
20
30
10-3
L
I
40 yr
10-4
,
50
C o n c e n t r a t i o n s of 137Csin water ( , Bq liter- t) and sediment ( , B q k g -t) following superimposed discharges from Swedish nuclear power plants.
,4 Baltic box model for dose calculations of ~acerborne radioactivicy
55
National Radiation Protection Institute. These regulations came into operation in January 1981 and limit the sum of the weighted organ dose equivalents to the critical group to 0-1 mSv year- ~ and the collective dose equivalent commitment to 5 manSv G W - 1 installed electric power. CONCLUSIONS For substances with long half-lives, where the internal circulation time is short relative to the half-life, a model consisting of a simple compartment may be used. In this situation, the concentration in the water body is fairly homogeneous, satisfying 'the completely mixed' limitation of the model. However, in describing the dispersion of a nuclide during shorter lengths of time, several communicating compartments give better resolution of the resulting concentration gradient. The AQUAPATHBALTIC model predictions are of course subject to uncertainties arising from the inherent approximations of the model. However, the close correspondence between the predicted and observed salinity distributions, and between the two estimates of the dose commitment to the Baltic population, implies that the predictions are realistic, if at order-ofmagnitude level. The model also illustrates the as yet unresolved problems associated with defining nuclide transfer parameters and has therefore identified the areas in which more detailed studies are required. In particular, different exchange rates for radionuclide transfer between water and sediment will significantly affect the value of the ultimate dose. This dependence emphasizes the importance of the sediments as a governing factor controlling the turnover of radionuclides in the water column. The model can thus be used as a screening tool to study controlling mechanisms for the dispersal of radioactive materials in the marine environment. A C K N O W L E D G E M ENTS The study was financed by the Swedish National Institute of Radiation Protection and this support is gratefully acknowledged. REFERENCES Anon. (1980). Bulletin statistique des p~ches maritimes (1980). Cons. Int. Explor. Mer., 6fi, 1-103.
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Sverker Ecans
Bergman, R., Bergstr6m, U., Evans, S. & Lampe, S. (1977). Compartment model for turnover of waterborne releases to brackish water systems. (In Swedish.) STUDSVIK S-549. Study commissioned by the Swedish Industrial Department, 26 pp. Bergman, R., Bergstr6m, U. & Evans, S. (1979). Dose and dose commitment from groundwater-borne radioactive elements in the final storage of spent nuclear fiwl. STUDSVIK/K2-79/92, KBS 100. Study commissioned by the Swedish Industrial Department, 199 pp. Bock, K.-H. (1971). Monatskarten des Salzgehattes der Ostsee. Dt. Hydrogr. Z. Erg. -H. B, 12, 1-147. Bolin, B. (1972). Model studies of the Baltic Sea. In Ambio Special Report No. 1, 115-19. Brogmus, W. (1952). Einer revision des Wasserhaushaltes der Ostsee. Kieler Meeresforsch, 9, 15-43. Clark, M. J. & Webb, G. A. M. (1980). A model to assess exposure from releases of radioactivity into the seas of northern Europe. In Impacts ofradionuclide releases into the marine environment. Proc. IAEA Syrup., Vienna, 6-10 October 1980, 629-48. Dahlin, H. (1976). Hydrochemical balance of Bothnian Sea and Bothnian Bay. (In Swedish.) Vannet i Norden, 1, 62-73. Ehlin, U. & Mattison, J. (1976). Volumes and areas in the Baltic Sea area. (In Swedish.) Vannet i Norden, 1, 16-20. Falkenmark, M. & Mikulski, Z. (1975). The Baltic Sea--a semi-enclosed sea, as seen by the hydrologist. Nordic Hydrology, 6, 115-36. Fedsov, M. V. & Zaitsev, G. N. (1959). Water balance and chemical regime of the Baltic Sea and its gulfs. Cons. Int. Explor. Mer. CM Hydrographical Committee No. 66, 8 pp. Fonselius, S. H. (1969). Hydrography of the Baltic deep basins, III. Fishery Board of Sweden, Ser. Hydrography, Rep. No. 23. Fonselius, S. H. (197l). On the hydrography of the Baltic Sea, with special reference to the Bothnian Bay. (In Swedish.) Vatten, 3, 309-24. Knudsen, M. (1900). Ein hydrographischer Lehrsats. Ann. d. Hydr. u. Marit. Meteorol., 28, 316-20. Koivuletho, M., SaxOn, R. & Tuomainen, K. (1979). Radionuclides in aquatic environments. In Studies on environmental radioactivity in Finland, 1978. Annual Rep. Inst. Radiation Protection, Helsinki. Mikulski, Z. (1970). Inflow of river water to the Baltic Sea in the period 1951-60. Nordic Hydrology, 4, 216-27. Ramberg, L. & Lampe, S. (1980). Sedimentation of cobalt-60 and cesium-137 in Tuiiren Bay 1964-1980. STUDSVIK/K2-80/432. Study commissioned by the Swedish National Institute of Radiation Protection, 29 pp. Santschi, P. H., Li, Y.-H., Adler, D. M., Amdurer, M., Bell, J. & Nyffeler, U. P. (1983). The relative mobility of natural (Th, Pb and Po) and fallout (Pu, Am, Cs) radionuclides in the coastal marine environment: results from model ecosystems (MERL) and Narragansett Bay. Geochim. Cosmochim. Acta, 47, 201-10.
.4 Baltic box modelfor dose calculation.s o/~~z:erhorne radioacti~,it)"
57
Shotkovitz, E. R., Cochran, J. K. &Carev, A. E. (1983). Laboratory studies of the diagenesis and mobility of _,39. _,-tdpu and L~-Cs in nearshore sediments. Geochim. Cosmochim. Acta, 47, [369-79. Soskin, J. (19631. Long term changes of the hydrographic characteristics oj the Baltic. Ministerial Board for Hydro-Meteorology, Leningrad. Stigebrandt, A. (1983). A model for the exchange of water and salt between the Baltic and the Skagerrak. J. Physical. Oceanogr., 13, 411-27. Svansson, A. (I972). The water exchange of the Baltic. In Ambio Special Report No. 1, 15-19. Svansson, A. (1980). Exchange of water and salt in the Baltic and adjacent seas. Oceanologica Acta, 3, 431-40.
A Box Model for Calculation of Collective Dose Commitment from Radioactive Waterborne Releases to the Baltic Sea
Sverker Evans Studsvik Energiteknik AB, Nuclear Division, Environmental Protection, S-6ll 82 Nyk6ping, Sweden
ABSTRACT To meet the new regulations of maximum yearly releases from Swedish nuclear power plants issued by the Swedish National Radiation Protection Institute, and to make realistic calculations of collective dose commitment derived from radionuclides discharged to the Baltic Sea, a box model A Q U A P A T H - B A L T I C has been developed. The model calculates radionuclide concentrations in 25 boxes for discharges into given compartments and takes into consideration the dispersion rates of the water masses, the sediment-water interaction and radioactive decay. The calculated concentrations in water and sediment at steady state are then used to evaluate individual and collective intakes of activity and external exposures. The radiation doses to the global population following discharges to the Baltic Sea are also calculated.
INTRODUCTION
Estuaries and coastal waters constitute the primary recipient of releases to the aquatic environment from different parts of the nuclear power fuel cycle. The radiation doses assessed from aquatic discharges make a significant contribution to the dose burden to the critical group and populations. The Baltic Sea is surrounded by "highly industrialized countries where a number of nuclear power plants are in operation or under construction. The worldwide fallout from nuclear weapon tests is 41
J. Environ. Radioactivity 0265-931X/85/$03.30 (~) Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
' "
,-e_
Sverker
Ecans
still the main source of man-made radionuclides present in the Baltic Sea. These fallout products reach the sea by direct precipitation on the water surface and by runoff from surrounding land areas. Some additional radionuclides originating from nuclear power plants and industrial, medical or scientific sources can also be released to the environment and enter the aquatic ecosystem. At present Baltic Sea water contains about 4 x 105 GBq (1 x 10* Ci) of both 9°Sr and t37Cs, relating to an average concentration of about 2 × 10 -2 Bq liter -t (0"5 pCi liter -t) for both isotopes and about 1 x 10 -s Bq liter- 1 (0-3 fCi liter- 1) of 239,240pu" The wide range of concentrations in different types of sediments and their non-uniform regional distribution make it almost impossible to calculate the total inventory of radionuclides in Baltic Sea sediments. This figure is therefore unknown. Very few data are available on the distribution of other relevant artificial radionuclides in the Baltic. Nor is the radionuclide content of the total biomass quantified for the Baltic Sea region. To restrict pollution in the Baltic Sea such that the influence of anthropogenic effects on the sea is maintained within acceptable bounds, detailed and effective assessment models are required. This study represents a further advance in such development with the aim of forecasting exposures from long-term turnover of radioactive effluents discharged to the Baltic Sea ecosystem. A box model AQUAPATHBALTIC divides the Baltic Sea into a number of compartments and takes into consideration sediment-water interactions. It therefore provides a more detailed picture of radionuclide dispersion and a more realistic assessment of dose burdens. The model also provides the opportunity to superimpose discharges from several outlets, thus yielding an integrated contribution to calculated dose rates to the different populations around the Baltic Sea. WATER E X C H A N G E OF T H E BALTIC SEA The large-scale circulation pattern and water exchange of the different sectors of the Baltic have been calculated by several authors (Knudsen, 1900; Brogmus, 1952; Fedsov & Zaitsev, 1959; Soskin, 1963; Fonselius, 1969, 1971; Bolin, 1972; Svansson, 1972, 1980; Falkenmark & Mikulski, 1975; Stigebrandt, 1983). The magnitude of water exchange through the Danish straits is determined by the fresh water supply and by the intensity of mixing in the Baltic. The latter is primarily governed by
,4 Baltic box model for dose calculations of ~'aterborne radioactirity
43
winds which give rise to an inflow of water of oceanic origin to the Baltic Sea. This saline water of high density accumulates in deeper parts of the central Baltic but is gradually transferred to the surface by wind influence. It is then mixed with fresh water and again discharged to the North Sea. The outflow from the Baltic is primarily governed by the surplus of fresh water which causes the sea level to rise relative to that in the adjacent seas, thus creating a horizontal pressure towards the open ocean. The inflow of water from the North Sea can be separated into two components: a continuous deep water current generated by the horizontal pressure gradient and a much more intense inflow of high density water driven by long-lasting westerly winds. The primary halocline, which is permanently developed at 60-80 m depth in the Baltic proper, separates overlying surface water from deep water and restricts vertical exchange between the two water masses. In the Baltic the mean circulation is cyclonic. This causes a slightly increased salinity in the eastern sectors of the Baltic proper.
SEDIM ENT-WATER I N T E R A C T I O N The ability of suspended matter and bottom sediments to absorb and adsorb radioactive nuclides can result in a significant depletion of activity from the water column. Unfortunately, the state of the art in evaluating sediment-related effects is less advanced, even though the latter may be critical in determining the magnitude of the exposure pathway to man. The fraction of activity retained in sediments for a given nuclide has been estimated from the concentration factor (kd) between sediment and water, k a is defined as the ratio of the concentration of the radionuclide in sediment (Bq g-~ dry wt) to the concentration in overlying water (Bq g-l). Depletion by sediments is thus greater for those nuclides with higher values of kd; it is also greatest in those areas with high suspended sediment loads and high rates of sediment deposition. The removal of activity into bottom sediments was evaluated using a particle scavenging model by Clark & Webb (1980). Here the removal of a radionuclide from solution is determined by two main factors: - - t h e k d value - - t h e rate of settling of particulate matter
44
S v e r k e r Evans
The model assumes that the radioactivity is uniformly distributed as a function of water depth, a reasonable assumption in most continental shelf waters. The depth distributions of the Baltic Sea subareas were calculated from Ehlin and Mattison (1976). The removal rate of activity from the water to sediments, kwh, is given by kws-
kdS (year- 1) hm(1 + kdSS )
where S = sedimentation rate (g m - 2 y e a r - 1), hm = mean water depth (m), and S S = t h e a m o u n t of suspended material in the water column (g liter- t). Knowing S, SS and hm, the rate of loss to sediments can be estimated for each nuclide in each compartment. The return of activity from sediments remains a major topic requiring further work. Such an effect, however, would only be of importance for long-lived nuclides which are removed to a large extent into and from bottom sediments. For nuclides with half-lives of the order of a few years or less, the return of activity from sediments is probably insignificant. THE AQUAPATH-BALTIC MODEL The A Q U A P A T H - B A L T I C model calculates water concentrations for discharges into given reservoirs, taking into consideration the dispersion of the water masses, the interaction of nuclides with sediments, and radioactive decay. The calculated concentrations in water and sediment are then used in association with concentration factors for edible aquatic species to evaluate collective intakes of activity by the different populations around the Baltic Sea and by the world population. C o m p a r t m e n t analysis is used to model the movements of water and the associated activity between connected reservoirs. This technique assumes instantaneous uniform mixing of each compartment, the transfer rates between compartments being described by transfer coefficients (year-I). The differential equation which describes the variation of the activity in compartment i is of the form: dA i
i=.
-- ~', k j i A ~ - kijAi - kiAi - ),Ai + Qi dt i=i
where Ai = the activity present at time t in compartment i (Bq), kia = the
A Baltic box model for dose calculations o/waterborne radioactit'ity
45
transfer rate from compartment/to compartment j (year-1), ki = the rate of loss from compartment i by sedimentation etc. (year-t), ).=the rate of loss from compartment i by radioactive decay (year-1), Qi = the rate of discharge into compartment i (Bq year-t), and n = t h e number of compartments in the system. The compartment model is displayed in Fig. 1. The longitudinal 8othn~n Bay
Bothnlan Sea
/
.J
fish
Dl
200
mussel algae
200
Gulf of Finland
K~ W. Baltic proper
mussel 30 algae 50
fish 200 mussel 30 algae i00 ~fish I00 mussel 30 algae 50
Gulf of Riga
~CK
I
Belt Sea
1
E
Baltic proper
I
t
Fig. 1. The Baltic Sea compartment model, the location of the Swedish nuclear power plant sites, and 137Cs concen'tration ['actors for fish, mussel and algae.
46
Sverker Evans
separation of the Baltic proper into two sections is a concession to the mathematical treatment of the model system; the division essentially induces a delay of dispersion in the east-west direction. Because of the heavy density stratification of the water masses in the Baltic proper, these boxes are also separated vertically into two layers (Fig. 2). The calculated water exchange between the connected compartments of the model forms the carrier system for the released activity. Runoff is the best assessed parameter because these figures are based on direct measurements. The remaining exchange rates naturally have considerable associated uncertainty. The compartment volumes and their supplies of fresh water are displayed in Table 1 and the water exchange rates between compartments in Table 2. Sediment reservoirs are connected to each subarea and nuclide specific transfer coefficients describe the exchange of radionuclides through the sediment-water interface. The upper 10 centimeters of sediment is assumed to take an active part in the exchange process. For the great majority of the nuclides considered, the possibility of exchange with the atmosphere is very limited and it is reasonable to restrict the model to the aquatic environment alone. The exceptions are
Fig. 2.
The vertical division of the model.
,4 Baltic box model for dose calculations of waterborne radioactivity
47
TABLE 1 The Compartment Volumes and their Fresh Water Supplies (= run-off+ A {precipitation-evaporation))
Compartment
Kattegat Belt Sea, the Sound West Baltic East Baltic Gulf of Riga Gulf of Finland Bothnian Sea Bothnian Bay Reference
Volume km 3
< 80 > 80 < 80 > 80
m m m m
515 285 3 785 770 6 965 1 525 405 1 100 4 890 1 480 Ehlin & Mattison (1976)
Fresh water supply km 3 year25 11 92 32 125 90 100 Fedsov & Zaitsev (1959) Mikulski (1970) Dahlin (1976)
3H, 1'~C and 129I, for which, as a result of their chemical properties and their long radioactive half-lives, a significant fraction of activity released in liquid form can be transferred to the atmosphere by evaporation. The values of the concentration factors and sedimentation rates have been simplified so as not to be valid for a specific site but for a geographical situation. Thus, for example, only one concentration factor for fish is utilized for each radionuclide, ignoring possible distinctions due to species and habits. The concentration of a given radionuclide in the edible parts of aquatic organisms is calculated by multiplying its activity in water by the appropriate concentration factor.
EXPOSURE PATHWAYS A N D RADIATION DOSE CALCULATIONS
For the Baltic Sea, fish consumption is the dominant source of internal exposure. In the Kattegat and Belt Sea areas, a certain amount of crustaceans (edible crab, lobster, pandalid shrimps) and molluscs (blue mussel, oyster) are harvested for human consumption. The yearly consumption rates are based on the quantities of marine food products
48
Sverker Evans TABLE 2 Water Exchange Rates Between the C o m p a r t m e n t s (year- t)
year - 1 K a t t e g a t - B e l t Sea K a t t e g a t - N o r t h Sea Belt S e a - K a t t e g a t Belt S e a - E a s t Baltic < 80 m Belt S e a - E a s t Baltic > 80 m Gulf of R i g a - E a s t Baltic < 80 m West Baltic < 80 m-Belt Sea West Baltic < 80 m - W e s t Baltic > 80 m West Baltic < 80 m - E a s t Baltic < 80 m West Baltic> 80 m-Belt Sea West Baltic > 80 m - W e s t Baltic < 80 m West Baltic> 80 m - E a s t Baltic > 80 m East Baltic < 80 m - G u l f of Riga East Baltic < 80 m - W e s t Baltic < 80 m East Baltic < 80 m - E a s t Baltic > 80 m East Baltic < 80 m - G u l f of Finland East Baltic < 80 m - B o t h n i a n Sea East Baltic> 80 m - W e s t Baltic > 80 m East Baltic > 80 m - E a s t Baltic < 80 m Gulf of F i n l a n d - E a s t Baltic < 80 m Bothnian S e a - W e s t Baltic < 80 m Bothnian S e a - B o t h n i a n Bay Bothnian B a y - B o t h n i a n Sea
1'41 3"96 4"11 9"5x 7"7x 8"5x 1-9x 3-0x 1"84 2"9x 1.4x 2'9x 4-0x l'0
10 -1 10 - t I0 - t 10 - t 10-" 10 - I 10 - t 10 - t 10 -2
3 . 0 x 10 . 2 90x
10 -2
8"0x 2"9x l'4x 6"5x 1.5x 4"Ox l'9x
10 -2 10 - t 10 -1 10 - I 10 - I I0 - z I0 - l
harvested in each reservoir together with the assumed consumption rate per capita. The quantities of food-stuffs used for human consumption, assumed consumption rates and population sizes are displayed in Tables 3 and 4. The pathways for external exposure to radioactive nuclides--bathing, visiting beaches, and handling fishing tackle contaminated with bottom sediments--and adherent dose calculations are similar to those used by Bergman et al. (1979). The model also takes into consideration the impact on the global population from releases of radioactive effluents to the Baltic Sea. The global marine ecosystem comprises two compartments, an upper surface layer down to about 100 m depth, and a compartment containing the deep sea basins. The uppermost sediment reservoir encircles the continents and amounts to about 4% of the total sediment area. The same
A Baltic box modelfi~r dose calculations o f waterborne radioactivity
49
TABLE 3 L a n d i n g s of F i s h (1979), C r u s t a c e a n s a n d M u s s e l s ( 1 9 7 7 ) i n t h e Baltic Sea A r e a s a n d t h e E s t i m a t e d A m o u n t s U s e d for H u m a n C o n s u m p t i o n
,4rea
Fish Amount landed (tonnes)
Kattegat Belt Sea, the Sound West Baltic East Baltic Gulf of Finland Bothnian Sea Bothnian Bay Reference
Crustaceans Amount used Jbr human consumption (%)
Amount landed (tonnes)
1'07 x 105 5"8 x t0 "t
50 50
1'99 x 105 4"31 x 105 6"7 x 10"*
50 50 50
Mussels
Amount used for human consumption (%;
Amount landed /tonnes)
2 x 103 1 x 10-'
50 50
4 x 102 8 × 103
4 x lO t i x lO t
50 50
2-7 x I0 "~ 80 1-1 x 10"~ 80 Anon. (1980) Fishery Board of Sweden (pets. comm.)
Anon. (1980) Fishery Board of Sweden (pets. comm.)
Amount used for human consumption ,'%J 50 50
Anon. (1980) Fishery Board of Sweden (pets. comm.)
TABLE 4 C a l c u l a t e d P o p u l a t i o n Sizes of t h e Baltic S u b a r e a s B a s e d o n t h e P a r t of F i s h L a n d i n g s in E a c h A r e a U s e d for H u m a n C o n s u m p t i o n , a n d a Yearly I n t a k e o f 15 k g y e a r -
Area
Kattegat Belt Sea W e s t Baltic E a s t Baltic G u l f of F i n l a n d B o t h n i a n Sea B o t h n i a n Bay
Fish yield (kg y e a r - 1) 2-7 x 1.5 x 5.0 x 1.1 x 1"7 x 1.1 x 4-4 x
10 v 10 v 10 v l0 s l0 T 10 v 106
Population size 1.8 1.0 3.3 7.3 l-1 7-3 2-9
x x x x x x x
106 106 106 106 106 105 105
paths of exposure have been estimated for the world population as for the Baltic Sea area. It is assumed that 1% of the world population lives in coastal regions with an exposure situation similar to that in the Baltic. The A Q U A P A T H - B A L T I C program calculates the dose contri-
50
Scerker Evans
butions from different exposure pathways to the individuals and populations around the Baltic as follows: --the --the --the --the
dose rates for the different exposure pathways (Sv year-t) collective dose rate (manSv year-1) dose commitment for the different exposure pathways (Sv) collective dose commitment (manSv) ACCURACY OF M O D E L FORECASTS
To verify the reliability of the model forecasts, a series of simulations was performed. Starting with the Baltic as a fresh water lake, simulations of salt intrusions from the North Sea into the Baltic were run on the computer. The exchange rates shown in Table 2 were used and the predicted steady state levels were compared with the observed distribution of salinity in the different compartments. A small timestep of 0"02 year was necessary due to short-term factors involved in the Kattegat and Belt Sea areas. The buildup of salinity in the compartment system following a discharge of 34%o saline water to the Kattegat compartment is displayed in Fig. 3. The modelled salinity values show close agreement with the observed long-term averages (Bock, 1971). To investigate the impact of different turnover values on the salt balance in the Baltic proper, a variation analysis was performed. The assumed water exchange between the surface bodies of the eastern and western Baltic proper was varied by multiples from 0"25 to 5, while exchange of the underlying eastern and western sections was varied by 2-4 times. The effect of varying the vertical mixing rate through the halocline by 0-5-2 times was also investigated. In fact, varying the transfer factors between the compartments of the Baltic proper generated only a small change in the resultant salinity distributions. The system is thus rather insensitive to perturbations in flow rates. The stable salinity regime observed in the Baltic underlines the assumed steady state conditions. M O D E L APPLICATIONS: D I S T R I B U T I O N OF C O L L E C T I V E DOSES F R O M 13VCs RELEASED TO T H E BALTIC SEA Two scenarios were studied using the AQUAPATH-BALTIC code. Firstly, the program was run to obtain the distribution of the collective
A Baltic box model for dose calculations of aarerborne raJioactitiry
51
Satinity Kattegat
%. 25
/
20
BeR Sea 15
E. Bottic ~80m
10
~
/ S/
10
2.'.~
Battic ~80m
WBo[tic ( 80 m .
.
E. Baltic ( 80m . .
--
- =" = ~ o° -
~.-%~ i 0
Fig. 3.
/ / /
.
//////-1~Bothnian
//
o
W
20
Sea
. . . . . . .
,
,
1
,
,
,
,
30
40
50
60
70
80
90
!___ 100
--
I 300 yr
The build-up of salinity in the c o m p a r t m e n t system following a discharge of 34%
saline water to the Kattegat compartment.
dose c o m m i t m e n t from t3VCs released to one c o m p a r t m e n t of the Baltic. 3-7 x 10 l° Bq (1 Ci) t37Cs y e a r - t was c o n t i n u o u s l y discharged into the K a t t e g a t and western Baltic < 80 m c o m p a r t m e n t s , respectively, and the water concentrations, truncated after 500 years, were used to calculate the collective radiation doses. Secondly, the corresponding dose rates derived from the actual yearly releases of w a t e r b o r n e 137Cs discharged from the four Swedish nuclear power plant sites were calculated. Some nuclide-specific d a t a were attached to the model. The s e d i m e n t - w a t e r c o n c e n t r a t i o n factor for cesium is salinity dependent, ranging from 5 x 102 in sea water to 3 x 10"* in fresh water. Assuming that the s e d i m e n t a t i o n rate and suspended m a t t e r load are both constant (5 x 102 g m -2 y e a r - ~ a n d 1 x 10 -3 g liter- t respectively), kws becomes a
52
Sverker Et,ans
function of salinity and depth (Fig. 4). In the Baltic, k,,.~ rates around (.1-6) x I0 - t year - t were calculated for 137Cs (Koivuletho etal., 1979; Ramberg & Lampe, 1980), which were far beyond the values 1 × 10 - ~ 1 x 10 -3 year -1 used by Bergman et al. (1977). The mobile character of cesium is also emphasized by recent studies (Santschi et al., 1983;
10 -I
kws , y r - 1
10-2
10-3 0
J
I
'
i
!jjz
Depth , m 200
300
~.jj/To
sa,ioi~y. °joo
0
Fig. 4. The transfer between water and sediment (kws) for cesium, expressed as a function of salinity and depth.
,4 Baltic box model for dose calculations of waterborne radioactivity
53
Sholkovitz et al., 1983). The depth distribution and salinity-dependent concentration factor characteristic of each subarea were used to calculate kws (Table 5). The reflux of 137Cs from the sediment back to the water phase is still a matter for further research. A rate of 1 × 10- 3 y e a r - x was assumed to be valid for the Baltic area. The resulting collective dose commitments for the Baltic Sea subareas and the global population following a discharge of 3-7 x 101° Bq (1 Ci) 137Cs for 500 years within and outside the Baltic Sea entrance, respectively, are shown in Table 6. A collective dose commitment of 2 x 10-2 manSv (2 manrem) was obtained. For the corresponding discharge situation on the Swedish west coast, the dose became a factor of ten lower. The results from the A Q U A P A T H - B A L T I C predictions can be compared to those from the North Sea model by Clark and Webb (1980). The collective dose commitment following a release of 1 Bq s- 1 for one year into the Baltic amounted to 7 x 10- 1~ manSv Bq- 1. However, as the fish concentration factor used was about four times too low, the values should rather be 3 x 1 0 - 1 3 manSv Bq -1. A corresponding value of 5 x 10 - 1 3 manSv Bq -1 _was predicted by the A Q U A P A T H - B A L T I C model, which is in close agreement with that by Clark and Webb (1980). The predicted concentrations of 1 3 7 C s in water and sediment based on the actual present day releases from the four Swedish nuclear power sites TABLE 5 E x c h a n g e R a t e s o f t3VCs ( y e a r - ~) B e t w e e n W a t e r a n d S e d i m e n t for t h e Baltic S u b a r e a s and Global Compartments
Kattegat Belt Sea West Baltic<80 m West Baltic>80 m E a s t B a l t i c < 8 0 rn East Baltic>80 m Gulf of Riga Gulf of Finland B o t h n i a n Sea Bothnian Bay Global surface water Global deep water
6-3 x 1.6x 3-3 x 4-4x 3"2x 6.8x 7.3x 4.0 x 3"0 x 5-1 x 1-0 x 1.2 x
10 - 2 10 -* 10-t 10 . 2 10 -1 10 - 2 10 -1 I0- 1 10-, I0 -1 10-'* 10 - 6
1x l x I x 1x I x 1x 1x 1x 1x 1x 1x 1x
10 - 3 10 - 3 10 - 3 10 - 3 10 - 3 10 . 3 10 - 3 10 -3 10 - 3 10 . 3 10 - 6 10 - 6
54
Sverker Evans
TABLE 6 Collective Dose C o m m i t m e n t (manSv) After a Release of 37 × I0 t° Bq (i Ci) t3VCs y e a r - t to the K a t t e g a t and West Baltic Sea, Respectively, Truncated After 500 Years
Collective dose commitment (manSv)
Compartment
Kattegat 1 2 3 4 5 6 7 8
Kattegat Belt Sea West B a l t i c < 8 0 m East B a l t i c < 80 m Gulf of Finland Bothnian Sea Bothnian Bay Global surface
3.4 x 2.1 x 4"0 x 9.3 x 1"2 x 5-8 x 1.4 x 1.4 x
Y 2-7
West Baltic
10 -~ 10 -~ 10 -'~ 10 -'t 1.0 -~ 10-5 10 -5 10-5
2"0 x l0 -~ 5-2 x I0 -'~ 5 2 x 10 -3 1.08 x 10 -2 1-4 x 10- 3 6.7 x 10 -¢ 1-6 x 10 -~ 8.3 x 10-6
l ' 7 x 10 .3
l ' 9 x 10 -2
are displayed in Fig. 5. A cumulative discharge of 120-3 GBq 137Cs year- t (3.25 Ci year- 1) results in a collective dose commitment of 3 x 10 -2 manSv (3 manrem) to the Baltic populations. This value can be compared to the new regulations of maximum permissible yearly releases from Swedish nuclear power plants which have been issued by the Swedish Bq/kg
Bq/l
'
10 "4
I
[
i~
i
10 -1
W. Baltic (above halocline) ~
Kattegat
10-5
10-2
10-6 I~/ l/ f~" 10-7
/
// t
,
Release GBq/yr 0.2 Simpevarp 100.0 Barsebiick 0.1 ,Ringhals. 20.0 r
0
Fig. 5.
10
20
30
10-3
L
I
40 yr
10-4
,
50
C o n c e n t r a t i o n s of 137Csin water ( , Bq liter- t) and sediment ( , B q k g -t) following superimposed discharges from Swedish nuclear power plants.
,4 Baltic box model for dose calculations of ~acerborne radioactivicy
55
National Radiation Protection Institute. These regulations came into operation in January 1981 and limit the sum of the weighted organ dose equivalents to the critical group to 0-1 mSv year- ~ and the collective dose equivalent commitment to 5 manSv G W - 1 installed electric power. CONCLUSIONS For substances with long half-lives, where the internal circulation time is short relative to the half-life, a model consisting of a simple compartment may be used. In this situation, the concentration in the water body is fairly homogeneous, satisfying 'the completely mixed' limitation of the model. However, in describing the dispersion of a nuclide during shorter lengths of time, several communicating compartments give better resolution of the resulting concentration gradient. The AQUAPATHBALTIC model predictions are of course subject to uncertainties arising from the inherent approximations of the model. However, the close correspondence between the predicted and observed salinity distributions, and between the two estimates of the dose commitment to the Baltic population, implies that the predictions are realistic, if at order-ofmagnitude level. The model also illustrates the as yet unresolved problems associated with defining nuclide transfer parameters and has therefore identified the areas in which more detailed studies are required. In particular, different exchange rates for radionuclide transfer between water and sediment will significantly affect the value of the ultimate dose. This dependence emphasizes the importance of the sediments as a governing factor controlling the turnover of radionuclides in the water column. The model can thus be used as a screening tool to study controlling mechanisms for the dispersal of radioactive materials in the marine environment. A C K N O W L E D G E M ENTS The study was financed by the Swedish National Institute of Radiation Protection and this support is gratefully acknowledged. REFERENCES Anon. (1980). Bulletin statistique des p~ches maritimes (1980). Cons. Int. Explor. Mer., 6fi, 1-103.
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Sverker Ecans
Bergman, R., Bergstr6m, U., Evans, S. & Lampe, S. (1977). Compartment model for turnover of waterborne releases to brackish water systems. (In Swedish.) STUDSVIK S-549. Study commissioned by the Swedish Industrial Department, 26 pp. Bergman, R., Bergstr6m, U. & Evans, S. (1979). Dose and dose commitment from groundwater-borne radioactive elements in the final storage of spent nuclear fiwl. STUDSVIK/K2-79/92, KBS 100. Study commissioned by the Swedish Industrial Department, 199 pp. Bock, K.-H. (1971). Monatskarten des Salzgehattes der Ostsee. Dt. Hydrogr. Z. Erg. -H. B, 12, 1-147. Bolin, B. (1972). Model studies of the Baltic Sea. In Ambio Special Report No. 1, 115-19. Brogmus, W. (1952). Einer revision des Wasserhaushaltes der Ostsee. Kieler Meeresforsch, 9, 15-43. Clark, M. J. & Webb, G. A. M. (1980). A model to assess exposure from releases of radioactivity into the seas of northern Europe. In Impacts ofradionuclide releases into the marine environment. Proc. IAEA Syrup., Vienna, 6-10 October 1980, 629-48. Dahlin, H. (1976). Hydrochemical balance of Bothnian Sea and Bothnian Bay. (In Swedish.) Vannet i Norden, 1, 62-73. Ehlin, U. & Mattison, J. (1976). Volumes and areas in the Baltic Sea area. (In Swedish.) Vannet i Norden, 1, 16-20. Falkenmark, M. & Mikulski, Z. (1975). The Baltic Sea--a semi-enclosed sea, as seen by the hydrologist. Nordic Hydrology, 6, 115-36. Fedsov, M. V. & Zaitsev, G. N. (1959). Water balance and chemical regime of the Baltic Sea and its gulfs. Cons. Int. Explor. Mer. CM Hydrographical Committee No. 66, 8 pp. Fonselius, S. H. (1969). Hydrography of the Baltic deep basins, III. Fishery Board of Sweden, Ser. Hydrography, Rep. No. 23. Fonselius, S. H. (197l). On the hydrography of the Baltic Sea, with special reference to the Bothnian Bay. (In Swedish.) Vatten, 3, 309-24. Knudsen, M. (1900). Ein hydrographischer Lehrsats. Ann. d. Hydr. u. Marit. Meteorol., 28, 316-20. Koivuletho, M., SaxOn, R. & Tuomainen, K. (1979). Radionuclides in aquatic environments. In Studies on environmental radioactivity in Finland, 1978. Annual Rep. Inst. Radiation Protection, Helsinki. Mikulski, Z. (1970). Inflow of river water to the Baltic Sea in the period 1951-60. Nordic Hydrology, 4, 216-27. Ramberg, L. & Lampe, S. (1980). Sedimentation of cobalt-60 and cesium-137 in Tuiiren Bay 1964-1980. STUDSVIK/K2-80/432. Study commissioned by the Swedish National Institute of Radiation Protection, 29 pp. Santschi, P. H., Li, Y.-H., Adler, D. M., Amdurer, M., Bell, J. & Nyffeler, U. P. (1983). The relative mobility of natural (Th, Pb and Po) and fallout (Pu, Am, Cs) radionuclides in the coastal marine environment: results from model ecosystems (MERL) and Narragansett Bay. Geochim. Cosmochim. Acta, 47, 201-10.
.4 Baltic box modelfor dose calculation.s o/~~z:erhorne radioacti~,it)"
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Shotkovitz, E. R., Cochran, J. K. &Carev, A. E. (1983). Laboratory studies of the diagenesis and mobility of _,39. _,-tdpu and L~-Cs in nearshore sediments. Geochim. Cosmochim. Acta, 47, [369-79. Soskin, J. (19631. Long term changes of the hydrographic characteristics oj the Baltic. Ministerial Board for Hydro-Meteorology, Leningrad. Stigebrandt, A. (1983). A model for the exchange of water and salt between the Baltic and the Skagerrak. J. Physical. Oceanogr., 13, 411-27. Svansson, A. (I972). The water exchange of the Baltic. In Ambio Special Report No. 1, 15-19. Svansson, A. (1980). Exchange of water and salt in the Baltic and adjacent seas. Oceanologica Acta, 3, 431-40.