The group at Louvain-la-Neuve has developed novel methods for generating uranium based aerosols at high temperatures similar to those experienced within the reactor during the Chernobyl accident. These have been characterised using a variety of physico-chemical methods and have then been used to determine the leachability of individual radionuclides contained within the UO(2) matrix and the flux rates of the aerosol to isolated branches of trees. The Imperial College group has developed a novel low temperature generation technique for uranium-based aerosols which has been used to determine dry deposition velocities within complete model canopies within a wind tunnel. Resuspension, losses due to weathering under ambient conditions and the effects on deposition rates of forest edges have also been investigated and quantified by this group. The University of Barcelona have used field techniques to determine the rate of vertical transport of thermogenerated aerosols within forest soils in Catalonia. Detailed information on the physio-chemical forms of radiocaesium and radiosilver within these aerosols and their subsequent physico-chemical behaviour is available as a result of this study.

Taken together, the results provided by the three groups over the last three years have significantly advanced our knowledge of the rates of physical transport processes of radionuclides introduced to forest ecosystems in aerosol form. This information is of considerable importance to other groups working within the framework of the European Commission's Radiation Protection Research and Nuclear Fission Safety Programmes, in which dynamic modelling of radionuclide behaviour and fate within forest ecosystems is currently an urgent priority.
During 1990-92 the applicants addressed the capture of radioactive aerosols by forest canopies and their subsequent fate. Due to lack of information on these topics following the Chernobyl accident this study is of major importance, a view confirmed by data recently available from the USSR. 1990-92 has seen the forging of successful links between the four groups with research focussing on the ability of individual trees and 'model' forest canopies to intercept aerosols depositing from the atmosphere under dry conditions and to subsequently retain these deposits over time. In addition, the Louvain-la-Neuve & Barcelona groups extended their interests to an examination of the physical migration of caesium in multilayered forest soils typical of north and south Europe. For the funding period 1992-94 several lines of investigation are proposed, as a result of both experimental findings and methodological development during this time. Dry deposition to forest canopies is of relevance to both near- and far field situations. The complexity of dry deposition results of 1990-92 is greater than initially expected and in the current proposal we stress the need to elucidate further the effects of canopy morphology and related micro-meteorological conditions and to strengthen our understanding of small scale diffusion and impaction of aerosols within canopies. Further wind tunnel work is proposed at LLN and IC-CARE which will make use of thermogeneration and delayed neutron counting techniques to synthesise and detect UO2 aerosols. Together with new gusting facilities in the IC-CARE wind tunnel this represents a unique combination of techniques which promise dry deposition studies of the utmost originality. Furthermore, CEA will extend wind tunnel findings to real canopies and the data will be used in the validation of an atmospheric deposition model currently being developed at Fontenay-aux-Roses. The question of radiocontamination of forest soils in the near-field situation will also be addressed. The thermo-generation technique developed at LLN allows synthesis of vitrified hot particles which will be used in studies of fuel fragment behaviour in forest soils. This highly original technique will extend considerably our understanding of the environmental behaviour of hot particles within European forest ecosystems.