Environmental dispersion of long-lived radionuclides in the geosphere and biosphere

Knowledge of the migration of radionuclides in the terrestrial environment is one of the key elements in assessment of the risk resulting from potential or actual releases from nuclear facilities. Among the various radionuclides arising from the nuclear fuel cycle, long-lived fission products and actinides are of particular concern for the different disposal options considered for radioactive wastes. Current knowledge of the processes that are responsible for their migration through different compartments of the environment (geosphere and biosphere) is reviewed. Migration in the geosphere is governed by a number of different mechanisms that have a wide range of reaction rates. Certain processes may be identified that control short-term behaviour and others long term. In assessing these processes, which relate to the interaction between solid and liquid phases, special attention is paid to the time-scale as well as the dynamic nature of the controlling processes. The current state of knowledge and the major research efforts under way concerning the predominant processes identified in different geological disposal options are reviewed. Various mechanisms control the behaviour of long-lived radionuclides and actinides in the soil and their migration in the biosphere. Regarding the biological uptake, particular attention is drawn to the 'ageing' phenomena and their consequences on radioactivity transfer in the different food chains. Similarities in the biogeochemical properties of natural and man-made elements that could further our understanding of the long-term behaviour of radionuclides are discussed. Consequently, all these considerations justify study of the biogeochemical aspects of the transfer of radioactivity in the environment. In the actual state of knowledge the uncertainties in the parameters used in the models may lead to difficulty in long-term dose estimation. Assessment of the dose received by man appears to be more strongly limited by the quality of available data than it is on the ability of the modellers to simulate mathematically the processes involved in radionuclide transport. (author)