Radionuclides released from a source may be present in different physicochemical forms, varying from low molecular mass (LMM) species, colloids and pseudocolloids, to particles and fragments. Following releases from severe nuclear events such as nuclear weapon tests, weapon grade materials such as U and Pu are predominantly transported and deposited as radioactive particles or fragments. These entities can carry substantial amounts of refractory fission products, activation products and transuranics. Similarly, radioactive particles are also released following conventional explosions of nuclear weapons or depleted uranium ammunitions, or following nuclear reactor accidents involving explosions or fires. Finally, radioactive particles and colloids are present in effluents from reprocessing facilities and civil reactors entering the environment, and radioactive particles are observed in sediments in the close vicinity of radioactive waste dumped at sea. Thus, releases of radioactive particles containing refractory radionuclides should also be expected following severe nuclear events in the future.
To perform long-term impact assessments for organisms in radioactive contaminated areas by contamination, information on the source term, i.e. activity concentrations, isotopic ratios as well as the radionuclide speciation is essential. If areas are contaminated with radioactive particles, particle characteristics such as the particle size distribution, crystallographic structures and oxidation states are important for assessing particle weathering rates and the subsequent mobilisation and biological uptake of associated radionuclides. Thus, advanced solid-state speciation techniques such as electron microscopy combined with synchrotron radiation X-ray microscopic techniques are needed in radioecology. Many years of research on radioactive particles from different sources has demonstrated that the activity concentrations and the isotopic ratios are source dependant, while particle characteristics such as particle size distribution, crystallographic structures and oxidation states for matrix elements also reflect the release scenario, dispersion processes and deposition conditions.