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The explosion on 26 April 1986 at the Chernobyl nuclear power plant, which is located 100 km from Kiev in Ukraine (at that time part of the USSR), and the consequent reactor fire, which lasted for 10 days, resulted in an unprecedented release of radioactive material from a nuclear reactor and adverse consequences for the public and the environment. The resulting contamination of the environment with radioactive material caused the evacuation of more than 100 000 people from the affected region during 1986 and the relocation, after 1986, of another 200 000 people from Belarus, the Russian Federation and Ukraine. Some five million people continue to live in areas contaminated by the accident. The national governments of the three affected countries, supported by international organizations, have undertaken costly efforts to remediate the areas affected by the contamination, provide medical services and restore the region’s social and economic well-being. The accident’s consequences were not limited to the territories of Belarus, the Russian Federation and Ukraine, since other European countries were also affected as a result of the atmospheric transfer of radioactive material. These countries also encountered problems in the radiation protection of their populations, but to a lesser extent than the three most affected countries. Although the accident occurred nearly two decades ago, controversy still surrounds the real impact of the disaster. Therefore the IAEA, in cooperation with the Food and Agriculture Organization of the United Nations (FAO), the United Nations Development Programme (UNDP), the United Nations Environment Programme (UNEP), the United Nations Office for the Coordination of Humanitarian Affairs (OCHA), the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the World Health Organization (WHO) and the World Bank, as well as the competent authorities of Belarus, the Russian Federation and Ukraine, established the Chernoby

The ability to predict the consequences of an accidental release of radioactive nuclides relies mainly on the level of understanding of the mechanisms involved in radioactive nuclides interactions with different components of agricultural and natural ecosystems and their formalisation into predictive models. Numerous studies and databases about contaminated agricultural and natural areas have been obtained but their use to enhance our prediction ability has been largely limited by their unresolved variability. Such variability seems to stem from an incomplete knowledge about radioactive nuclide interactions with the soil matrix, soil moisture, biological elements in the soil and additional pollutants, which may be found in such soils. In this project, we investigate mainly the role of the biological elements (plants, mycorrhiza, microbes) in: radioactive nuclide sorption/desorption in soils and radioactive nuclide uptake/re lease by plants. Because of the importance of the chemical nature of the involved radioactive nuclides, we will follow the bioavailability of three radioactive nuclides: caesium, strontium, and technetium. The role of additional non-radioactive pollutants will also be scrutinised as they may interfere with the mechanisms governing radioactive nuclide transfer to plants. Knowledge acquired from the experiments will be incorporated into two mechanistic models BIOPHAST and BIORUR specifically modelling radioactive nuclide sorption/desorption from soil matrices and radioactive nuclide uptake by/release from plants. These mechanistic models will be incorporated into assessment models to enhance their prediction ability. One expects to extract from these experiments scientific bases for the development of bioremediauon methods of radioactive nuclidecontaminated soils.

After the release of radionuclides into the environment it is important to be able to readily identify major routes of radiation exposure, the most highly exposed individuals or populations and the geographical areas of most concern. There have been significant recent improvements in our capability to estimate spatial variation in the environmental behaviour of radionuclides through better understanding of the underlying processes and an ability to spatially apply this knowledge by integrating relevant spatial information in the form of digital data sets using geographical information systems. Thus, it is now possible to refine the estimation of spatial variation in radiation exposure, both for routine releases and in accident situations. Within a recent EC-funded concerted action (the Radioecological Sensitivity Forum), there has been a renewed consideration of the concept of radioecological sensitivity with a particular focus on identifying sensitive areas as well as processes and communities. The concept should be relevant for both terrestrial and aquatic ecosystems, and might even be applied for consideration of doses to biota, although the current focus has been restricted to a consideration of human exposure. The conclusions of the action are outlined, considering the potential usefulness of the concept, and methods by which it can be applied. A framework for the estimation of radioecological sensitivity is proposed and the various measures by which it can be considered, including (i) aggregated transfer coefficients, (ii) action loads, (iii) fluxes and (iv) individual exposure of humans are discussed. The importance of spatial and temporal consideration of each of these outputs is emphasized.