The work described in this paper was performed under contract to the Commission of the European Communities.

Transcription

1 A methodology for evaluating the radiological consequences of radioactive effluents released to the aquatic environment in normal operations G. Lawson, C. Attwood, T. Cabianca, J.R. Simmonds National Radiological Protection Board, Chilton, Didcot, ABSTRACT Low levels of radioactive waste are routinely released to the aquatic environment from nuclear sites, hospitals, industry and research centres. This paper describes a general comprehensive methodology for assessing the radiological consequences of these releases. The methodology consists of river and estuary models together with models of the seas around Europe, including the Mediterranean Sea. The models are usually generic in nature because the methodology has to be capable of being applied throughout the European Community. The models are generally of the compartmental type and include transport of radionuclides in the water column and interaction with sediments. A large number of potential exposure pathways are considered in the methodology. These include ingestion of radionuclides from drinking water or from the contamination of other foodstuffs, inhalation of radionuclides resuspended from the sea or from riverbanks or beaches, and external irradiation from, for example, beach sediments. The work described in this paper was performed under contract to the Commission of the European Communities. INTRODUCTION In 1979 the Commission of the European Communities (CEC) published a report describing a methodology for evaluating the radiological consequences of radioactive effluents released in normal operations*. Recently the CEC

2 270 Water Pollution contracted the National Radiological Protection Board (NRPB) to co-ordinate work, also involving other institutions in the European Community (EC), to produce a revised methodology^. This paper describes the revised methodology which has been developed for assessing the radiological consequences of routine releases to the aquatic environment. Other parts of the complete methodology deal with routine releases to atmosphere. The models described in the paper are necessarily generic given that the methodology needs to address the requirements of radiological assessments throughout the EC. Low-level liquid radioactive effluent may be routinely discharged to a freshwater (principallyrivers),estuarine or marine environment. Radionuclide transport within and out of the aquatic environment can lead to the exposure of people by a large number of pathways. A methodology for assessing the radiological impact of liquid discharges must be capable of predicting the movement of radionuclides in all relevant areas of the environment and the subsequent radiation exposure of people. The purpose of the methodology described in this paper is two-fold: first, to calculate the collective doses arising from a discharge of radionuclides. The collective dose is the sum of the doses received by individuals within a population, such as the UK population, and is one factor used to determine whether the discharges are as low as reasonably achievable (ALARA). The second purpose of the methodology is to estimate doses received by those people likely to receive the highest doses, that is, the critical group, in order to determine that those doses are below statutory limits or recommended constraints. The models described in this paper have been developed so as to retain sufficient flexibility to satisfy both requirements (see Figure 1). RIVERS Radionuclide transport Although complex river models based on the solution of hydraulic transport equations are available, these require considerable quantities of site-specific data. For this generic methodology a simple compartmental model has been adopted. In this model the river is divided in the downstream direction into a number of sections or water compartments, with the radionuclide source entering the first compartment. Within each compartment the radionuclide concentrations are assumed to be uniform. Transport between compartments is modelled using transport coefficients, which are the river water velocity in a compartment divided by the length of that compartment. Radionuclide interactions with suspended sediment are modelled using an element-dependent equilibrium distribution coefficient, K<j, to relate activity adsorbed on sediment to the concentration in solution. Interaction with the bed sediment uses the

3 1 Dispersion Sed mentation 1 o^ H* o' Figure 1 Aquatic pathways

4 272 Water Pollution model developed by Schaeffer*; in this model it is assumed that radionuclide concentrations in the water column decrease exponentially downstream as a result of removal to bed sediments. Transport to the bed sediments uses an element-dependent parameter to represent the net adsorption. The bed sediments are also assumed to move downstream, but at a much lower velocity than the water. Radionuclides may also be transported to the terrestrial environment by the use of river water for irrigation, the dredging of bed sediment and its emplacement on the river banks or adjacent land, and as a result of flooding. Exposure pathways The predicted concentrations of radionuclides in the river water and the bed sediment are used to estimate the intakes by man and external irradiation exposure. Unfiltered river water can be used to irrigate crops and as drinking water for animals; filtered river water can be used as drinking water for humans; radionuclide concentrations on theriverbank are taken to be the same as those in the bed sediment and can lead to external exposure of people, for example, anglers, spending time on the river bank. Radionuclide concentrations in freshwater fish are calculated from the concentrations in filtered water using an element-dependent concentration factor. The analysis of those pathways involving transport to the terrestrial environment is performed using other models of radionuclide transport in the terrestrial environment, developed as part of the overall methodology. Finally, dosimetric models are used to calculate the doses received from intakes of radionuclides and from external exposure. ESTUARIES Radionuclide transport Estuaries represent a particularly complex aquatic environment. The degree to which it is necessary to model the behaviour of radionuclides within an estuary depends on the particular application of this methodology. In particular, it depends upon the site of the discharge and the importance of exposure pathways originating within the estuary. Where a complex estuary model is not justified, then a simple interface to represent the movement of radionuclides between the freshwater and marine environments may suffice. This interface should take into account the movement of radionuclides in the water phase and those absorbed onto suspended particulates. It should also take into account the downstream movement of radionuclides within the river bed and their possible desorption when they enter the marine environment. In some cases the simple interface model will not be sufficient. The large differences between one estuary and another preclude the design of a simple generic model suitable for describing the behaviour of radionuclides in all

5 Water Pollution 273 estuaries. Compartment models have therefore been developed for some of the major estuaries in Europe, for example, the Severn*. Exposure pathways Human activity in and around estuaries may be higher than in adjacent freshwater or marine areas. At low tide exposed mud flats may be frequented by people digging for fishing bait or bird-watching. In general the pathways by which people may become exposed to radionuclides in or from estuaries are the same as those in the marine environment. The salinity of the water in estuaries generally precludes its use for irrigation purposes. MARINE ENVIRONMENT Radionuclide transport Models have been created to represent the dispersion of radionuclides in European coastal waters. For a release of radionuclides at a particular location, the models calculate the time-dependent concentrations in the various sea areas taking into account advection and diffusion, radioactive decay and the interaction of radionuclides with suspended and seabed sediments. The calculated concentrations of radionuclides in environmental materials are used in the calculation of collective and individual exposures by taking into account the appropriate habit data. For direct discharges to the marine environment, the local environmental conditions may be important in determining the maximum individual exposures and also influence the amount of activity which becomes more widely dispersed, thus exposing populations at a distance from the discharge. For this reason the model chosen to represent dispersion in the marine environment is subdivided into 'local' and 'regional' components which are interfaced. The regional model (see Figure 2) can be interfaced with a number of local models, each representing discharges from a single site, if the overall radiological impact of, say, a nuclear power generating programme, is required. Dispersion on a local scale, up to a few kilometres from the discharge point, is modelled by a single well-mixed water compartment together with associated sediment compartments. This exchanges water and suspended sediment with the adjacent regional marine model compartment. If detailed assessments of the local radiological impact of discharges from a particular site are to be undertaken, then site-specific hydrographical data will be required. Figure 2 illustrates the various regions used in the regional marine model for the continental shelf. Each of the water compartments has associated suspended sediment, and water compartments in contact with the seabed have underlying seabed sediment compartments. A compartmental model has also been developed for the Mediterranean Sea.

6 274 Water Pollution Figure 2 Compartment model of European waters, excluding the Mediterranean Sea

7 Water Pollution 275 The model describes the significant movements of water in European coastal seas by a system of interlinked compartments. The adsorption of radionuclides by suspended sediment is modelled using a concentration factor or sediment distribution coefficient, K<,, relating the activity on the sediment to that in solution. Different values of the K^ for different elements and for both coastal and deep sea sediments have been published^. The removal of activity to bottom sediments is evaluated using a particle scavenging model which assumes that the removal of a radionuclide to the ocean floor is determined by the K^ and the rate of settling of paniculate matter. The ocean bed within each sea area is modelled using two compartments, representing the upper 0.1 m of the seabed and the underlying 1.9 m. The model allows for the return of radionuclides to the water column and also their migration within the seabed. Exposure pathways Usually the most important pathway in terms of dose is the ingestion of seafood. Radionuclide concentrations in the edible parts of seafood are obtained from the concentrations in the filtrate fraction of seawater using a concentration factor^. Doses to individuals from the consumption of seafood are obtained from the calculated radionuclide concentrations in the food and information about which marine areas have supplied the individual's intake and in what proportion. Collective doses from the ingestion of seafood are obtained from the recorded catches of fish, crustaceans, molluscs and seaweed as published by the International Council for the Exploration of the Sea (ICES)'. Radionuclide concentrations on beach material are calculated from the predicted concentrations in the top layer of bed sediment in the adjacent marine compartment. People may inadvertently inhale or ingest beach material and will also be externally irradiated. Radionuclides may also be transferred to the land in seaspray. As well as direct inhalation of the seaspray people may also receive doses from the ingestion of terrestrial foods contaminated by radionuclides in the spray. EXAMPLES The river model has been applied to calculate doses to individuals arising from the past and potential future discharges of radionuclides to a small European river from a nuclear site\ The methodology dealing with radionuclide discharges to the marine environment has been used a number of times, most recently in an assessment of collective doses from all liquid discharges to coastal waters from EC nuclear sites*.

8 276 Water Pollution FUTURE DEVELOPMENTS Future developments of the river model include the incorporation of a plume model to represent dispersion in the first compartment, instead of assuming complete dispersion. It is also planned to include separate compartments to represent radionuclides transferred to the river banks by either natural processes or by dredging. The marine dispersion model will be divided into a number of modules which will simplify its use in the majority of cases. The implementation of some or all of the aquatic models on microcomputers will be investigated. REFERENCES 1. Commission of the European Communities. NRPB/CEA, Methodology for evaluating the radiological consequences of radioactive effluents released in normal operations. Doc. No. V/3865/79-EN (1979). 2. NRPB, Methodology for Assessing the Radiological Consequences of Routine Releases of Radionuclides to the Environment. Commission of the European Communities (to be published). 3. Schaeffer R, 'Consequences du deplacement des sediments sur la dispersion des radionucleides' in Proc. Conf. on Impacts of Nuclear Releases into the Aquatic Environment, Otaniemi 1975, p 263. Vienna, IAEA (1976). 4. McColl N P, A model to predict the level of artificial radionuclides in environmental materials in the Severn Estuary and the Bristol Channel. J. Fish BioL, 33 (Supplement A), (1988). 5. International Atomic Energy Agency, Sediment K^s and Concentration Factors for Radionuclides in the Marine Environment. IAEA Technical Reports Series No. 247 (1985). 6. ICES, Bulletin Statisque des Peches Maritimes. ICES, Copenhagen. Annual reports. 7. Lawson G et al., Assessment of the Radiological Impact of Radioactive Liquid Waste Discharges to the Molse Nete River. NRPB-M330. NRPB, Chilton (1991). 8. Charles D et al., Radiological Impact on EC Member States of Routine Discharges into North European Waters. NRPB-M172. NRPB, Chilton (1990). National Radiological Protection Board