tropospheric exchange processes. Radon and thoron, and their associated decay products, are a second group of natural isotopes which may be utilized, for example, in studies of the atmospheric circulation processes of the lower atmosphere.The movement of radionuclides in the environment has been studied for many years, with the principal objective of tracing the routes by which they accumulate in the food chain and become available for human consumption. The explosions in Reactor No. 4 of the nuclear power station of Chernobyl in the Ukraine provided a point source for distribution of radionuclides and a unique opportunity to trace the mechanisms by which they are distributed. The quantity and composition of the emissions are capable only of being estimated from environmental measurements, not by direct calculations from the composition of the reactor core and its mode of disintegration. Material deposited around Chernobyl and in neighbouring countries was traced through direct observation of radioactivity and radiochemical analyses through all stages from atmospheric dispersal to deposition and subsequent transfer. A major significance of Chernobyl was the relatively shortlived `pulse' of emission which it caused. This has proved immensely valuable to the research community as a form of time marker from which the rates of subsequent processes may be estimated.
The basic properties of isotopes of interest and their sources are described in the opening chapter; the coverage is limited mainly to those whose release is due to human activities. A series of case-studies details the incidents which have been important for adding to knowledge of radionuclide behaviour within atmospheric, terrestrial, aquatic and urban compartments of the environment and the pathways through which transfer occurs. Major chapters then give detailed reviews of current knowledge of environmental pathways of radioactivity and recommendations for future research. This is followed by assessments of dosimetry and environmental effects.
Radionuclides which occur naturally in the environment have a pattern of distribution which is in dynamic equilibrium, with disturbances due to cultivation, earth movement and weathering. This equilibrium has had superimposed upon it the isotopes, some with comparatively short half-lives, which have arisen from mining, processing and accidents. The local disturbances allow the tracing of pathways through which the radionuclides are dispersed. Those which are most abundant and/or display the greatest activity provide most of the information for this study. These are principally 134, 137Cs, 90Sr, 103, 106 Ru, 141, 144 Ce, 129,131 I and 239, 239, 240 Pu. Isotopes of other elements such as 110m Ag, 127, 129, 132 Te, 95 Zr, 95 Nb, 138, 140 La, 99 Tc and 241Am are included but not treated in great detail.
The artificial creation of radionuclides may result from physical processes involving nuclear fission, nuclear fusion and neutron activation. The most important source of artificially created radionuclides is neutron-induced nuclear fission. The chemical and physical forms of the active species determine deposition, migration and uptake of radioactivity by living organisms.
A variety of systems and processes may introduce radioactivity into the environment. Human activities involving nuclear weapons and the nuclear fuel cycle (including mining, milling, fuel enrichment, fabrication, reactors, spent fuel stores, reprocessing facilities and waste storage) are important, leading to significant creation and release of radioactivity. Human technology also releases pre-existing natural radionuclides which would otherwise remain trapped in the Earth's crust. The burning of fossil fuels (oil and coal) dominates direct atmospheric release of pre-existing natural radioactivity.
Since radionuclides are in some respects easier to study than stable nuclides there is an application in their use as environmental tracers. The most important tracers which are of artificial origin are the fission and activation products from nuclear tests. In addition, radioisotopes produced as a result of the bombardment of the Earth's atmosphere by cosmic rays provide useful tracers for studies of
stratospherictropospheric exchange processes. Radon and thoron, and their associated decay products, are a second group of natural isotopes which may be utilized, for example, in studies of the atmospheric circulation processes of the lower atmosphere.
The physical and chemical form of radionuclides may vary depending on the release and transport condition in addition to the elements' properties. A general distinction can be made between gases, aerosols and particulate material. Particles with high activity concentration, known as `hot particles', may result from atmospheric nuclear weapons tests or nuclear reactor accidents. This activity is diluted as material is transferred to soil and water directly or via vegetation and movement through other biota.
The presence of radioactivity in the environment may be due to a variety of sources. One such source arises from routine releases which are made, for example, as a result of reprocessing activities undertaken at Sellafield, UK, and Cap de la Hague, France, or due to plutonium production operations at Hanford, USA. Besides planned discharges arising from operations associated with the nuclear fuel cycle, radionuclides may enter the environment as a consequence of accidental releases. Some important non-routine releases which have occurred include that due to an explosion in a high-level waste storage tank at Kyshtym, former USSR (1957), and those involving nuclear reactors at Windscale, UK (1957); Three Mile Island, USA (1979) and Chernobyl, USSR (1986). Additional releases have occurred upon re-entry of satellites powered by nuclear sources, such as SNAP 9-A (1964) and Cosmos 954 (1978). Whilst the Chernobyl accident is the most serious to have occurred in the history of nuclear reactor operation, other smaller releases are also of importance for the elucidation of the potential pathways of radionuclides in the environment.
The fate of radionuclides in the atmosphere is determined by various physical processes. The degree of dispersion and point of deposition depends on factors such as release parameters, the radionuclide form and meteorological conditions. Deposition of radioactive particles and gases to the ground may occur via dry deposition which includes direct gravitational settling, as well as turbulent transfer, or due to intervention of some form of precipitation (wet deposition). The involvement of precipitation processes may act as a concentration mechanism leading to areas of enhanced deposition. Entrainment of previously deposited material in the atmosphere (resuspension), due to the action of wind, mechanical agitation or falling hydrometeors, is another mechanism resulting in the presence of radionuclides with the potential for causing spread of contamination to adjacent clean areas and constituting an inhalation hazard. In some coastal areas a proportion of radionuclides discharged into the sea is blown back onto land as seaspray.
Measurements of the concentration of radionuclides in the air and on the ground are complemented by modelling studies, which require appropriate radiological and meteorological input data. A variety of air sampling methods may be used for measuring air concentrations of radionuclides, and a network of air monitoring stations is maintained in many nations. Many sites throughout the world have also been established to monitor the deposition of radionuclides on the Earth's surface. A major problem in most monitoring programmes lies in obtaining samples which are representative of the average deposition over a large area, as this is often highly variable on a small spatial scale. Techniques employed to acquire meteorological data as input to models depend on the release size. Data may be collected via on-site instrumentation or from a standard meteorological observing station, whilst at sea observations may be made either on board ships or gas and oil platforms, or using drifting buoys. Measurement of upper-air conditions are made using balloon-borne radiosondes. Aircraft and weather radar are other sources of valuable information regarding meteorological conditions and, in the case of the latter, to models tracking pollution.
A wide range of atmospheric dispersion and deposition models is available. Detailed modelling studies are an aid in understanding the role and interaction of specific microphysical and meteorological factors, whilst more general models are directly applicable to estimating the environmental consequences of radionuclides released to the atmosphere, which are of particular importance in guiding monitoring teams following accidental releases. The specific purpose for which a model is utilized governs the complexity of the model selected. It is, nevertheless, important that model simplifications, and limitations induced by the indeterminate nature of the atmosphere, should be recognized. Unique opportunities for model evaluation have been provided by the accidents which occurred at Windscale, Three Mile Island, and Chernobyl, where simulations of atmospheric transport have been compared with radiological observations.
The type of land and vegetation, and deposition mechanism, determine radionuclide behaviour in the terrestrial environment. The amount of a nuclide intercepted by surface vegetation depends on the surface area of leaves, roughness and botanical composition. Some small transfer of activity also occurs by translocation from the roots. Movement from roots and direct deposits on soil depend on vegetation type, soil geology and water movement. Grazing animals and cultivation cause redistribution in the topmost layer. Soluble materials may be removed from the soil by leaching due to the passage of water. Transfer back to vegetation, and thence to animals, occurs via root systems. A further transfer cycle is associated with mechanisms relating to metabolism, removal in food products, excretion and resuspension due to wind. The chemical similarity of potassium and caesium, and of strontium and calcium, has been a focus of studies concerning physiology of radionuclide uptake. As a result of bomb tests in Nevada and the South Pacific there is much information available for arid soils and movements in tropical islands. However this has little relevance with regard to Chernobyl fallout where effects in Arctic tundra ecosystems, particularly uptake by lichens, and in wetlands are of more importance. Knowledge is particularly needed on transfer of radionuclides in ecosystems such as forests and tropical agriculture.
The pathways of radionuclides in the aquatic environment (which is composed of both aqueous and solid phases and includes rivers, lakes, estuaries, shelf seas, deep oceans, ice sheets, glaciers, groundwater and ground-ice) are of importance since a major fraction of the Earth's surface is occupied by this compartment. The transfer of radionuclides present within the aquatic environment is affected by both nonliving and living components. Radionuclide behaviour in the aquatic environment is determined by transport of the solution and solid phases, the chemical interactions between phases and their biological cycling. Of importance in determining radionuclide behaviour and its solid/solution distribution are the chemical characteristics of the water. An important contrast in the behaviour of radionuclides is observed between aerobic and anaerobic waters, and major differences in ionic composition which exist between saline water and freshwater (and in the latter case between alkaline and acid waters) are of significance in determining radionuclide speciation and behaviour. Furthermore, the solid/solution distribution of radionuclides in the aquatic environment is found to vary with grain size and composition of sediment grains, and bedrock grain surfaces. The nature of the grain surface, particularly the extent to which it is modified by inorganic (oxyhydroxide), or organic deposits, may also be of importance. Moreover, the potential mobility of radionuclide particulate fractions is determined by grain size. Transport of radionuclides within the aquatic environment not only leads to their redistribution, but results in dilution, fractionation and mixing. Accumulation of the particulate phase by sedimentation also affects the radionuclide distribution, with the radionuclide inventory in sediment deposits found to vary widely depending on radionuclide sources and deposit characteristics.
Since almost three-quarters of the population of the current developed world is located in urban areas, the study of radionuclides in towns and cities is of importance. The distribution pattern of fallout depends on the weather conditions (i.e. wet or dry) and variation in the degree of interception of fallout is found depending on the nature of the surface and the
physico-chemical form of radionuclides. Large variability in surface contamination within cities may occur due to
washoff from impervious urban surfaces. The nature and rate of removal processes determines the retention behaviour of pollutants. In the outdoor environment wash-off probably governs weathering, whereas within houses cleaning constitutes the major removal process. Predicted dose reductions achievable in the urban environment through the application of various decontamination techniques reveal that digging gardens and defoliating trees should be of highest priority, based upon consideration of cost and effectiveness. Artificial radionuclides have been successfully employed in the study of individual urban environment pathways. Further applications lie in tracer studies of wet deposition, resuspension and wash-off, with applications to pollutant retention behaviour of greatest importance.
A consequence of the presence of radionuclides in the environment is the potential for increased radiation exposure of living organisms. The impact of radionuclide releases on organisms may be assessed by consideration of the likelihood and extent of radiation exposure (i.e. radiation dosimetry) and potential consequential effects (i.e. the radiobiological response). With regard to characteristics influencing the maintenance of the population, for the majority of species (other than humans and rare/endangered wild species or valuable domestic animals) it is the population level rather than the individual response to radiation exposure which is considered to be of greatest importance. Nevertheless effects due to increased radiation exposure at, or above, the population level cannot appear without the occurrence of a clear response in individuals which comprise the population. Studies have revealed that no significant effects would be apparent in wild populations of terrestrial animals provided that the dose rate to the most exposed individuals did not exceed 0.04 mGy h-1, or in terrestrial plants and aquatic plants/animals provided that the dose rates to the most exposed individuals did not exceed 0.4 mGy h-1. With very few exceptions controlled waste disposal operations have not, and would not, cause these values to be exceeded. An exception is the deep ocean disposal of packaged low-level radioactive waste, where high dose rates could potentially be delivered to benthic organisms at the disposal site since the ultimate human exposure, which comprises the present basis for control, occurs far from the dumpsite and more than 100 years into the future. In the case of a major nuclear accident such as Chernobyl, although responses have been detected in pine trees, and to a lesser extent in animals, any serious risk to long-term survival of local populations is debatable. However, since nuclear accidents cause global concern, there is need of careful periodic assessment of safety systems and their upgrading to minimize probability of accident occurence.
In summary, the findings contained in this volume constitute an authoritative body of basic scientific information utilizing practical experiences relating to the pathways of radioactivity, which may assist environmental managers and policy makers in achieving a swift and effective response to the assessment of any future (planned or accidental) releases. Wider applications of relevance lie, for example, in advancing knowledge generally of environmental processes and biogeochemical cycling of chemical elements or their compounds. The Chernobyl accident has provided a great impetus to work on this subject by highlighting the need for rapid, reliable predictive capabilities in the event of a nuclear accident. Such predictions of radionuclide behaviour will only be reliable if based upon a solid and comprehensive foundation of research. The tracking of contamination arising from Chernobyl has clearly illustrated areas where present knowledge is partially or even wholly inadequate.
Much fundamental and applied research remains to be done. In the atmospheric science field, much is still to be learnt about the physico-chemical nature of radionuclides originating from within a hot emission source, and its evolution during atmospheric transport of the various gaseous and particulate forms of airborne radionuclides. The Chernobyl accident demonstrated that large (> 10 µm) particles could be transported over long distances (> 100 km) within the atmosphere and these processes need to be better understood if the development of future accident scenarios is to be reliably predicted. Chernobyl also highlighted the importance of wet processes and orographic effects in deposition of radioactivity. Research is needed into the interaction of radioactive aerosol particles with cloud droplets and into wet deposition generally in convective and frontal systems. The high spatially variable nature of such deposition indicates a need for better understanding of radionuclide atmospheric dispersion in complex terrain and frontal systems, and of `occult' deposition in cloud and mist. Present understanding of both terrestrial and marine resuspension processes is rudimentary and improved quantitative knowledge is sorely needed.
In the terrestrial environment, knowledge of radionuclide physico-chemical speciation is presently poor, particularly in relation to bioavailability to plants and animals. Chernobyl highlighted inadequacies in the knowledge of radionuclide behaviour in organic soils and further research is needed in this area, particularly in relation to long-term processes. Additionally, since much of the deposition in the near-field is in the form of particles, a better understanding is required of their behaviour, both in terms of physical migration of the particles themselves and leaching of the various radionuclides associated. The role of micro-organisms in the cycling of radionuclides and of agricultural practices upon radionuclide migration are inadequately understood. Radionuclide behaviour in forests and other natural and semi-natural ecosystems is poorly quantified as is the role of secondary sources of radionuclide contamination such as resuspension. Information is generally relatively abundant in relation to major fission products, but is lacking in respect of some `exotic' species such as 110m Ag. Numerical models of food-chain transfer are steadily improving, but require further validation, especially in regard of transfer parameters. Models for young animals require development and testing.
Research recommendations relating to the aquatic environment highlight the need for studies of freshwater pathways, especially within lakes, due to the sensitivity of the systems to pollution. Collaborative international efforts to study `hot' zones require improved co-ordination and intensification. International radioecological studies of polar seas are also recommended. As with the terrestrial environment, the role of micro-organisms in biogeochemical cycling is not well understood. The role of the surface microlayer in aquatic systems in influencing radionuclide behaviour is comprehended inadequately and requires further study; improved techniques of sampling the microlayer will be necessary. In freshwater and estuarine systems the importance of interactions of radionuclides with colloids is well documented. Colloidal processes in marine systems however require further study, particularly in relation to size association and speciation of radionuclides.
In the urban environment, the major concern is to understand processes leading to contamination and radionuclide retention and to optimize decontamination. Use of artificial radionuclides will continue to be of value in the study of deposition, retention and clean-up processes, leading to the development of improved models.
A consequence of the rapid development of digital computers is that detailed and complex numerical models of multi-phase environmental transport, dispersion, deposition and food chain transfer can be constructed. These will of necessity become ever more sophisticated and embrace an ever wider range of biogeochemical pathways. One crucial need is for the results of field and laboratory research investigations of individual processes to be translated into compact but effective parameterizations for incorporation within the numerical models.
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