CHARACTERIZATION OF RADIOACTIVE PARTICLES IN THE ENVIRONMENT

When radioactive particles are released into the environment, informat ion on the size distribution pattern, radionuclide and matrix composit ion, morphology and structure is essential for assessing weathering an d the subsequent mobilisation and biological uptake of associated radi onuclides. The particle characteristics will depend on the source in q uestion in addition to the release scenario, dispersion processes and deposition conditions. Following high temperature accident scenarios a ssociated with nuclear installations (e.g., Chernobyl accident) a rang e of different uranium fuel particles and condensed particles were obs erved, varying in composition, morphology and structure (e.g., crystal line and amorphous phases). Following low temperature releases (e.g., prefire Winscale releases), flake-like uranium fuel particles signific antly different from those collected at Chernobyl were identified. Fur thermore, a major fraction of radionuclides in effluents from reactors and reprocessing plant during normal operations are associated with p articles and colloids. Hence the presence of radioactive particles or colloids in releases from nuclear sources occurs more frequently than usually expected. After deposition, weathering of particles occurs and associated radionuclides are mobilised with time. Hence the transfer of mobilised radionuclides within the ecosystem will be delayed until weathering takes place, and the assessment of short term consequences of releases may be overestimated if particles are present. The weather ing rate will depend on the particle composition (e.g., UO2 fuel), str uctural changes occurring during the event (e.g,, transformation from UO2 to U3O8) and chemical conditions after deposition (e.g., pH, redox ), Unless the impact of particle weathering is taken into account, ass essment of mobilisation, transfer and long-term consequences of radion uclide releases may be underestimated. In order to improve the predict ing power of transport models, ecosystem transfer models and dose asse ssment models, experimental information is required with respect to so urce term radionuclide speciation and in particular the association of radionuclides with particles and colloids, influencing mobility and b iological uptake. This work was therefore focused on analytical techni ques applicable to the fractionation, identification and characterisat ion of radioactive particles and colloids (e.g., hollow fibre fraction ation, electron microscopic techniques and reactivity studies) release d from a source and deposited in the environment. Results from researc h carried out during several years at the authors' laboratory are pres ented to illustrate the usefulness of the techniques.