Radioprotection, Suppl. 1, vol. 40 (2005) S27-S32 © EDP Sciences, 2005 DOI: 10.1051/radiopro:2005s1-005 Radioactive particles released from various nuclear sources B. Salbu and O.C. Lind Isotope Laboratory, Department of Plant and Environmental Sciences, Agricultural University of Norway, 1432 Aas, Norway Abstract. Radionuclides released to the environment may be present in different physico-chemical forms, ranging from ionic species to colloids, particles and fragments. Following releases during nuclear events such as nuclear weapon tests or use of depleted uranium munitions, and from nuclear accidents associated with explosions or fires, radionuclides such as uranium and plutonium are predominantly present as particles, mainly fuel particles. Similarly, radioactive particles are present in effluents from reprocessing facilities, and radioactive particles are observed in sediments in the close vicinity of radioactive waste dumped at sea. Thus, releases of radioactive particles occur far more often than earlier anticipated. Soils and sediments can act as a sink for colloids, particles and fragments, while contaminated soils and sediments may also act as a potential diffuse source, depending on particle characteristics and processes influencing particle weathering and remobilisation of associated radionuclides. To assess long-term impact from radioactive particle contamination, information on the source term is essential, i.e. activity concentrations and isotopic ratios as well as the particle size distribution, crystallographic structures and oxidation states influencing particle weathering rates and the subsequent mobilisation and biological uptake of associated radionuclides. The activity concentrations and the isotopic ratios will be source dependant, while particle characteristics will also reflect the release scenario, dispersion processes and deposition conditions. The present paper will summarise information on various nuclear sources summarise having released radioactive particles in the past. 1. INTRODUCTION Radionuclides released from a source may be present in different physico chemical forms such as low molecular mass (LMM) species, colloids and pseudocolloids, aerosols, particles as well as fragments. According to [1], radioactive particles in the environment are defined as localised aggregates of radioactive atoms that give rise to an inhomogeneous distribution of radionuclides significantly different from that of the matrix background. Particles in the aquatic environment are defined as entities having diameters larger than 0.45 µm, while in air these entities are referred as aerosols. Colloids or pseudo- colloids are defined as localised heterogeneities ranging in size from 1 nm to 0.45 µm, while LMM refer to species less than 1nm or molecular mass less than 1 kDa. As radioactive particles ranging from colloids to fragments are inhomogeneously distributed in a matrix such as soils, sediments and waters, identification of localised heterogeneities indicates the presence of particle [2]. A significant fraction of refractory radionuclides released by high temperature nuclear events, such as nuclear weapons tests and accidents in nuclear reactors is associated with fuel particles. In addition, radioactive particles have been released under low temperature conditions such as atmospheric emission during normal operations, and in authorised effluents from nuclear installations. Particles containing refractory radionuclides are formed due to critical or subcritical destruction of fuel matrices (e.g., explosions, fires, corrosion processes), cluster formation, condensation processes or interactions with available surfaces during release and dispersion. Releases of particles from Article published by EDP Sciences and available at http://www.edpsciences.org/radiopro or http://dx.doi.org/10.1051/radiopro:2005s1-005