Characterisation of radioactive particles in the environment†

Abstract

When radioactive particles are released into the environment, information on the size distribution pattern, radionuclide and matrix composition, morphology and structure is essential for assessing weathering and the subsequent mobilisation and biological uptake of associated radionuclides. The particle characteristics will depend on the source in question in addition to the release scenario, dispersion processes and deposition conditions. Following high temperature accident scenarios associated with nuclear installations (e.g., Chernobyl accident) a range of different uranium fuel particles and condensed particles were observed, varying in composition, morphology and structure (e.g., crystalline and amorphous phases). Following low temperature releases (e.g., prefire Winscale releases), flake-like uranium fuel particles significantly different from those collected at Chernobyl were identified. Furthermore, a major fraction of radionuclides in effluents from reactors and reprocessing plant during normal operations are associated with particles 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 weathering rate will depend on the particle composition (e.g., UO₂ fuel), structural 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, assessment of mobilisation, transfer and long-term consequences of radionuclide releases may be underestimated. In order to improve the predicting power of transport models, ecosystem transfer models and dose assessment models, experimental information is required with respect to source term radionuclide speciation and in particular the association of radionuclides with particles and colloids, influencing mobility and biological uptake. This work was therefore focused on analytical techniques applicable to the fractionation, identification and characterisation of radioactive particles and colloids (e.g., hollow fibre fractionation, electron microscopic techniques and reactivity studies) released from a source and deposited in the environment. Results from research carried out during several years at the authors’ laboratory are presented to illustrate the usefulness of the techniques.