7 Batteile Pacific Northwest Laboratories, ... Pripyat river and to the Dnieper river and reservoirs, were used in the other scenario .... TRM = Tennessee River Mile.
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MODELLING OF RADIONUCLIDE TRANSFER IN RIVERS AND RESERVOIRS Validation study performed within the IAEA/CEC VAMP programme M. ZHELEZNYAK 1 , G. BLAYLOCK 2 , J. GARNIER-LAPLACE 3 , G. GONTIER 3 , A.V. KONOPLEV 4 , A. MARINETS 1 ' 5 , L. MONTE 6 , Y. ONISHI 7 , K.-L. SJOEBLOM 8 , M. TSCHURLOVITS 5 , O. VOJTSEKHOVICH 9 , V. BERKOVSKY 10 , A. BULGAKOV 4 , P. TKALICH 1 ' 1 , N. TKACHENKO 11 , G. WINKLER 5
1
Institute of Mathematical Machines and Systems, Cybernetics Centre (CC), Ukrainian Academy of Sciences, Kiev, Ukraine
2
SENES Oak Ridge Inc., Oak Ridge, Tennessee, United States of America
3
Institut de protection et de sûreté nucléaire (IPSN-CEA), Commissariat à l'énergie atomique, Centre d'études de Cadarache, Saint-Paul-lez-Durance, France
4
Scientific Production Association "Typhoon", Institute of Experimental Meteorology, Obninsk, Russian Federation
5
Atominstitut der Österreichischen Universitäten (AI), Vienna, Austria
6
Settore AMB-ANV, ENEA, CRE Casaccia, Rome, Italy
7
Batteile Pacific Northwest Laboratories, Richland, Washington, United States of America
8
Division of Nuclear Fuel Cycle and Waste Management, International Atomic Energy Agency, Vienna
9
Ukrainian Hydrometeorological Institute (UHI), Kiev, Ukraine 355
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Ukrainian Centre of Radiation Medicine (UCRM), Kiev, Ukraine
11
Hygiene Centre (HC), Ukrainian Ministry of Public Health, Kiev, Ukraine
Abstract M O D E L L I N G OF R A D I O N U C L I D E T R A N S F E R IN RIVERS A N D
RESERVOIRS:
V A L I D A T I O N S T U D Y P E R F O R M E D WITHIN T H E I A E A / C E C V A M P P R O G R A M M E . The comprehensive data obtained for different rivers after the Chernobyl accident, primarily the data f r o m the heavily contaminated Pripyat river-Dnieper
river-reservoir
system, provide a unique basis for validations and improvements of transport models for aquatic radionuclides. These data were used in two scenarios by the River Sub-Group of the Aquatic Working Group of the I A E A / C E C co-ordinated research programme Validation of Environmental Model Predictions (VAMP). For one model validation scenario, the Clinch River-Tennessee River system, contaminated by releases from the Oak Ridge National Laboratory, was chosen. The data collected during the post-Chernobyl period, related to the Pripyat river and to the Dnieper river and reservoirs, were used in the other scenario. Different kinds of models were applied, and the data f r o m the two scenarios were used to simulate the dynamics of radionuclides in water, suspended sediments and bottom sediments. Analytical models, two kinds of box models and two kinds of one-dimensional (crosssectionally averaged) models were used for the simulation of radionuclide concentrations in water and sediments. These models represent different levels of model complexity for the description of interactions between radionuclides, suspended sediments and
sediments
deposited on the bottom layer. The vertical profiles of radionuclides in bottom sediments were simulated for the Tennessee River scenario by the multilayer bottom model and by two kinds of vertical diffusion models. For this scenario, the modelling of radionuclide distribution in the aquatic food-chain, mainly in fish, was based on static approaches. For the other scenario, the temporal changes in
137
Cs concentrations in bream, trench and Carpió carpius
in the
Kiev reservoir (Dnieper river) were simulated by the dynamic model. The doses to the population via aquatic pathways were calculated on the basis of simulated concentrations in an environmental compartment and were used in analyses of the role of aquatic pathways in the formation of the total radiation dose after large releases.
1.
INTRODUCTION
Studies of the environmental impact of radionuclide releases demonstrate that, after the initial fallout, large river systems are the main pathways for radionuclide transport from the point of discharge to distances of hundreds to thousands of kilometres. Some of these modelling methods have been reviewed by the IAEA [1] and by different authors (see, e.g., Refs [2, 3]). The spread of computer technology
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during the last decade and the urgent need to increase the predictability of models in order to provide adequate information for decision making concerning remedial measures in the most contaminated water bodies after the Chernobyl accident have led to an intensive development of river-reservoir modelling. The comprehensive data obtained for different river-reservoir systems after the Chernobyl accident provide an excellent basis for model validation and improvement. To provide a mechanism for validation of assessment models by using environmental data on transfer of radionuclides from the Chernobyl accident was the main aim of the IAEA/CEC co-ordinated research programme Validation of Environmental Model Predictions (VAMP) [4], which was established in 1988 and finished in 1994. In the framework of the VAMP Aquatic Working Group, a Sub-Group for Rivers and Reservoirs was established in 1992 [5]. This paper gives a brief description of its activities.
2.
SCENARIOS
The data collected during the post-Chernobyl period relating to the Pripyat river and the Dnieper river and reservoirs, for water, fish and irrigated products, were used in scenario R2, prepared by the Ukrainian Hydrometeorological Institute (UHI), Kiev. The Clinch River-Tennessee River system, contaminated by releases from Oak Ridge National Laboratory, was chosen for the model validation scenario RI (developed by SENES Oak Ridge Inc.) for a comparison of the predictability of models in different hydrological and physico-chemical conditions. The validation study could not be done on the basis of direct dose measurements; however, intercalibration of the dose simulation approaches could be used for analysing the role of aquatic pathways in the formation of the total radiation dose after large releases. 2.1. Clinch River-Tennessee River (Scenario RI) The Oak Ridge National Laboratory, located in the south-eastern part of the United States of America, has discharged radionuclides into the White Oak Creek since 1943. Significant releases took place from 1949 to 1968. Maximum annual amounts of selected radionuclides were discharged in the following years: 137Cs: 6290 GBq in 1956; 106Ru: 74 000 GBq in 1961; 90Sr: 5550 GBq in 1949; 60Co: 2849 GBq in 1956. The White Oak Dam, which is located 1 km upstream of the mouth of the creek, controls the release of radionuclides into the Clinch River at kilometre CRK 33.5. The Clinch River joins the Tennessee River at kilometre TRK 913.4. The Clinch River-Tennessee River system is a highly regulated riverreservoir system (Fig. 1) that is used for hydroelectric power generation, flood control, navigation and recreation.
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K-25Emory River
C R M
U
CRM " K ^ C C
Poplar Creek
Y-\c
5
X_!P
WATTS BARLAKE,
F MELTON HILL LAKE f* F O R T
^ ^ V r f l H ? LOUDON * LAKE TRM
530
Jf VTRM 5 0 2
'TRM 4 9 0 CHICKAMAUGA LAKE ^ . T R M
465
'CHATTANOOGA
10 '
km
20 '
30 i
X-10 = Oak Ridge National Laboratory CRM = Clinch River Mile TRM = Tennessee River Mile
FIG. 1. Clinch River-Tennessee River system.
For model testing, the following tasks for scenario R1 have been proposed [5]: (1)
(2)
(3)
Given the concentrations of 137Cs and 90 Sr in water at the White Oak Dam, to predict the monthly concentrations of these radionuclides from December 1960 until November 1962 at the following locations: Centers Ferry, CRK 8.8; Watts Bar Dam, TRK 529.9; and Chickamauga Dam, TRK 471.0. To predict the profile of a sediment core taken from the Tennessee River just below its confluence with the Clinch River at TRK 567.5, and to provide the concentration of 137Cs for each 4 cm section of the core to a depth of 96 cm, representing the years 1949 to 1986. To predict the annual average concentrations of 137 Cs, 106Ru and 90 Sr in the water column and in fish in the Clinch River at CRK 23.3 and in the Tennessee River at TRK 758.0 for 1961-1963. Using the predictions for these two loca-
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tions, to estimate the radiation dose to an individual from the aquatic pathway, i.e. from swimming in the sea and consumption of fish. 2.2. Dnieper reservoirs (scenario R2) The watershed of the Dnieper river in Ukraine, Russia and Belarus was heavily contaminated by radionuclides accidentally released from reactor Unit No.4 of the Chernobyl nuclear power plant (NPP) in Ukraine on 26 April 1986. The Chernobyl NPP is located on the west bank of the Pripyat river, approximately 30 km from its mouth at the Kiev reservoir on the Dnieper river. A significant portion of the radionuclides released were deposited on the watershed of the Pripyat river in Ukraine and Belarus. Other areas that were heavily contaminated by l37 Cs are in the upper Dnieper watershed in Russia and Belarus. As a result of this surface contamination, there was a long term influx of 137Cs and 90Sr into the Dnieper river, which passes through reservoirs before discharging into the Black Sea (Fig.2). The modern Dnieper river system downstream of the mouth of the Pripyat river includes six large reservoirs: the Kiev, Kanev, Kremenchug, Dnieprodzerzhinsk, Zaporozhie and Kakhovka reservoirs. The average capacity of the reservoirs varies from 2.45 km 3 (Dnieprodzerzhinsk reservoir) to 18.2 km 3 (Kakhovka reservoir). The population of the Dnieper basin region is 32.5 million. Water from the Dnieper river that is used in the municipal tap water supply is consumed by more than 8.1 million people in ten regions and in the Crimean Republic. The Dnieper reservoirs are also used for commercial fishing. Water from the Dnieper river is used for irrigation of more than 1.8 million hectares in Ukraine, for 1.3 million hectares of which (72%) water from the Kakhovka reservoir is used. The modelling scenario was prepared on the basis of monthly averaged data from May to December 1986 and ten-day averaged data during 1987-1993 on water discharge, suspended sediment discharge and influx of radionuclides ( 137 Cs and 90 Sr) in solute and suspended sediments into the Kiev reservoir through crosssections of the Pripyat river at Chernobyl and the Dnieper river near the village of Nedanchichi (points 1 and 2, respectively, in Fig. 2) [5, 6]. To validate the models of water, sediment and fish contamination, the 137Cs and 90Sr data for the Dnieper reservoir were used as a starting point for modelling (1 January 1987). The initial version of this scenario includes data on radionuclide fallout on the surface of the Kiev reservoir and estimated data on the monthly averaged concentrations of 137Cs and 90Sr in the Dnieper tributaries in the year 1986. It was proposed that the starting point of modelling should be May 1986. However, later, more detailed analyses of the situation were performed, which demonstrated that the uncertainties in the fallout data and especially in the data on the contamination of the Pripyat river in 1986 were so large that they had an impact on the water transport models and deformed the simulated results. Therefore, the data for the period until 1 January 1987 were made available for modelling.
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FIG. 2. Schematic diagram of the Dnieper reservoirs.
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The main tasks for Scenario R2 are as follows: (1)
(2) (3)
3.
To predict the monthly and annual average concentrations of 137Cs and 90 Sr in water, suspended particles and bottom sediments of all six Dnieper reservoirs over the period 1987-1993; and to provide uncertainty estimates for the predictions. The 1986 data were used as a starting point for the simulations. To estimate the 137Cs concentrations in commercial species of fish during the post-accident period. To estimate the radiation dose for individuals and for the population due to consumption of drinking water and fish, and of products from areas irrigated with water from the Dnieper reservoirs.
MODELLING OF RADIONUCLIDE CONCENTRATIONS IN WATER SEDIMENTS AND FISH
Modelling of radionuclide dispersion in rivers and reservoirs has some peculiarities compared with modelling of dispersion in lakes. The radionuclide dispersion in rivers and reservoirs is affected by different flow velocities, short retention times and large variability in water discharge during the year. As a result, there are strong temporal variations in the sedimentation-resuspension rates. Suspended and bottom sediments interact with water, whereby radionuclides are exchanged by physical, chemical and biological processes. The main physical exchange mechanisms are sedimentation of contaminated suspended matter in the river bed and resuspension of the bottom sediment. These mechanisms are controlled by hydraulic factors (e.g. river flow and sediment transport) and are strongly dependent on the sediment size fractions (e.g. clay, silt, sand, gravel). Radionuclide diffusion through interstitial water is the physical process that accounts for migration phenomena which are not related to sediment transport. Adsorption and desorption of radionuclides by surface bed sediments are the main chemical processes. They are not always completely reversible and are controlled by geochemical reactions of dissolved radionuclides with sediment. Uptake and subsequent excretion of radionuclides by aquatic biota and, in general, perturbation of the sediment due to the action of living organisms represent biological processes that are responsible for the exchange of radionuclides between water and bottom sediment. Different levels of complexity of transfer processes of radionuclides between the water phase and the solid phase have been described and different averaging scale models have been used in the validation study. The following models have been used: (a)
The analytical model describing contamination in water and sediments (AI, HC [5]);
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(b)
A box-type (compartmentally averaged) model which does not simulate the suspended sediment dynamics, but which accounts for three bottom layers: the first, thin top layer in which radionuclides are in chemical equilibrium with the overlying water; the second layer (just below the first one) in which radionuclides mix with overlying water through the top layer; and the deep layer which acts as an ultimate radionuclide sink [7];
(c)
Box-type and one-dimensional (cross-sectionally averaged) models simulating the dynamics of the concentrations of suspended sediments of one dominant grain size, and the concentrations of radionuclides in dilution, on suspended sediments and in the upper part of the bottom deposition layer (CC-WATOX and CC-RIVTOX models [8]);
(d)
A one-dimensional model describing the dynamics of the concentrations of sand, silt and clay in water and simulating the radionuclide dynamics for each of these three typical grain sizes (PNL-TODAM model [3]).
The AI model, the analytical HC model and the RIVTOX and TODAM models were applied on the basis of data on releases from Oak Ridge and hydrological data for the Clinch River-Tennessee River scenario, to simulate radionuclide concentrations in water and in the upper part of the bottom deposition layer in the Clinch River and the Tennessee River. All models exept HC were used for the Dnieper river scenario. Some results of simulations of the 137Cs and 90 Sr concentrations for scenario R2 are presented in Figs 3 and 4. The distributions were analysed for each of the listed models for each of the six Dnieper reservoirs. The simulated data for scenario R1 were validated with measured data averaged over a much larger time-scale. Sensitivity and uncertainty analyses were performed for ENEA and for the CC-WATOX and CC-RIVTOX models on the basis of data for scenario R2. The vertical profiles of 137Cs in bottom sediments in the Clinch RiverTennessee River system were simulated with the TODAM model using a multiple bed layer approximation; with the TYPHOON DIFF model [9], which explicitly accounts for exchangeable and non-exchangeable processes of radionuclide sorption by sediments and which solves two sets of one-dimensional mass balance equations for the exchangeable and non-exchangeable radionuclide components; and with the CC vertical diffusion model BOTOX, which describes only one component — the total 137Cs concentration in interstitial water [5]. It was demonstrated that estimates of the key model parameter Kd — the distribution coefficient for 137Cs — using data from the literature and from the two scenarios, allowed good agreement to be achieved between calculated and experimental results. The vertical distribution of 137 Cs in bottom sediments under non-uniform entry of radionuclides into the river is determined by the type of the relationship between sedimentation rate and time. Thus, in this ease, the use of complicated physical and chemical models of vertical migration does not lead to significant improvement in predictions.
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Days (rom 1.1.1987 ,37
FIG. 3. Concentrations of
Cs in the Kiev reservoir (scenario R2).
Days from 1.1.1987
FIG. 4. Concentrations of
90
Sr in the Kakhovka reservoir (scenario R2).
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Modelling of radionuclide distribution in the aquatic food-chain, mainly fish (tasks of scenario RI), was based on static approaches, which are characterized by mathematical relations independent of time, using the concentration factor (HC, AI and CC models). Comparison with measured data shows an overprediction of fish contamination, as a result of which the estimated dose to people would be too high. The dynamic model TRANSAQUA [10, 11], which describes the migration of radionuclides through the compartments of the food-chain using 'transfer rates', was applied to simulations of temporal changes in 137Cs concentrations in bream and roach in the Kiev and Kakhovka reservoirs. On the basis of annual comparisons, TRANSAQUA gives quite good results, only taking into account a simple pelagic food-chain.
4.
DOSE CALCULATIONS
Doses from the aquatic pathways were calculated on the basis of modelled concentrations in an environmental compartment (by teams from AI, HC, UCRM, UHI and CC) in order to analyse the role of aquatic pathways in the formation of the total radiation dose after large releases. Pathways such as consumption of drinking water and fish, irrigation, swimming in the sea and external irradiation on beaches were considered for both scenarios. The assessment of the dose to the population via Dnieper water intake is not of purely scientific interest but is also a problem of great practical significance. Dose calculations were made on the basis of experimental data for 1986-1993 (which could be simulated with reasonable accuracy, as demonstrated [5]) and on the basis of simulations of 90Sr concentrations in water, obtained with the WATOX model for stochastically simulated hydrological data until the year 2056. The dose calculations were done at UCRM, using input data from UHI, HC and CC. The radiation dose which the public could have potentially received as a result of intake of radionuclides from Dnieper water was calculated in terms of the annual committed effective dose (ACED, in Sievert) — the dose resulting from intake over one year, committed to age 70; and the collective cumulative (for 70 years) committed effective dose (CCCED70, in man-Sievert) — the age dependent ACED, integrated over 70 years of intake and over the age structure of the population. A very important characteristic of dose formation is the ACED to the maximally exposed member of the general public (MEMGP). This dose was calculated by assuming an 'average' diet for a particular region, which includes only products from irrigated land and tap water from the Dnieper river. The consumption rate of Dnieper fish is determined by the commercial catch. The basis of the software module, which simulates irrigation and radionuclide transport in food-chains, is the ECOSYS-87 radioecological model. The simulation of the metabolic processes in the human body under chronic intake of radionuclides
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is based on the metabolic models of caesium and strontium from ICRP-56. Nuclear decay data from ORNL and the Specific Absorbed Fraction files of the ICRP recommendations (1990) are used. For dose calculation the specially adopted multipurpose computer software internal dosimetry support system (IDSS) [12] is used, which has been developed specifically for the needs of the ICRP. In the simulation of long term processes of dose formation, only doses due to the intake of 137Cs and 90Sr were considered. The collective dose in the Kiev region in 1986 exceeded the current dose level by factors of five to seven. A diametrically opposite effect has been observed in the Crimean Republic. This is due to the low level of contamination of the water of the lower Dnieper river from primary fallout. For 1986, the ACEDs to the MEMGP due to the use of water in the Kiev region are 1.7 x 10~5 Sv and 2.7 x 10~5 Sv for 90Sr and 137Cs, respectively. The assessible contributions of irrigation with Dnieper water, of its use in the municipal tap water supply and of fish consumption to the CCCED 70 are, respectively, 18%, 43%, 39% in the Kiev region, 8%, 25%, 67% in the Poltava region, and 50%, 50%, 0% in the Crimean Republic (no consumption of fish from the Dnieper river). The predicted average contribution of 90 Sr to the CCCED 70 to the population of the Dnieper regions resulting from the use of water is 80%. The CCCED 70 to the population of the Dnieper regions (32.5 million people) resulting from the use of water is about 3000 man-Sv.
5.
CONCLUSIONS
The main output of the study was the practical validation of the applicability of different radionuclide dispersion models for simulating large scale processes in river-reservoir systems. It was demonstrated that even simple models could be tuned (calibrated) for the specific case studies. However, it was also shown that only models the structure of which reflects the main hydrological, physical and chemical processes are valid for different situations with different sets of generic parameters. The modelling results confirm the important influence of water body processes such as reversible and non-reversible kinetics of radionuclide exchange in the system water-suspended sediments-bottom depositions, radionuclide transfer through algae as an amplification factor for oscillations of radionuclide concentrations during summer, and the governing role of sedimentation in the formation of bottom sediment profiles. The similar results obtained for different models with a more or less complicated structure (one-layer bottom or multilayer bottom, one or three grain sizes of typical suspended sediments) were helpful in finding a balance between the complexity of the models and their applicability. The study has decreased the uncertainty range for the main model parameters (distribution coefficient, time-scales of the transfer rate) for 90 Sr and l37 Cs in large river systems.
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The study has demonstrated the significant contribution of aquatic pathways connected with the use of water from the Dnieper river (mainly through drinking water and irrigation of land) to the doses received by the population of southern Ukraine from the Chernobyl accident. Taking into account that the 90Sr contamination of the Dnieper river continues to be a problem that is not only of scientific interest, a very important output of the study is the increasing predictive possibilities of modelling, which can be used for estimating the radioactive contamination of the Dnieper river water in cases of future high floods.
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