![]() These estimates of the source term were based solely on the estimated deposition of radionuclides on the territory of the Soviet Union, and could not take into account deposition in Europe and elsewhere, as the data were not then available. This did not include the noble gases, and had an estimated error of ±50%. At that time, it was estimated that 1 to 2 exabecquerels (EBq) were released. The IAEA International Nuclear Safety Advisory Group (INSAG) issued in 1986 its summary report (IA86a) based on the information presented by the Soviet scientists to the Post-Accident Review Meeting. This corresponds to the emission of 6 t of fragmented fuel. ![]() This estimate was later revised to 3.5 ± 0.5% (Be91). The early estimate for fuel material released to the environment was 3 ± 1.5% (IA86). In the initial assessment of releases made by the Soviet scientists and presented at the IAEA Post-Accident Assessment Meeting in Vienna (IA86), it was estimated that 100% of the core inventory of the noble gases (xenon and krypton) was released, and between 10 and 20% of the more volatile elements of iodine, tellurium and caesium. The initial estimates were presented as a fraction of the core inventory for the important radionuclides and also as total activity released. Subsequent assessments support this view, certainly for the caesium radionuclides (Wa87, Ca87, Gu89). ![]() This was clear at the IAEA Post-Accident Review Meeting in August 1986 (IA86), when the Soviet scientists made their presentation, but during the discussions it was suggested that the total release estimate would be significantly higher if the deposition outside the Soviet Union territory were included. The initial estimation of the Source Term was based on air sampling and the integration of the assessed ground deposition within the then Soviet Union. ![]() Not only are the levels of radioactivity released important, but also their distribution in time as well as their chemical and physical forms. The "source term" is a technical expression used to describe the accidental release of radioactive material from a nuclear facility to the environment. ![]()
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