Radiation doses due to long-range transport of airborne radionuclides

Radioactive material released into the atmosphere may be transported over long distances before it is deposited on the ground. The material is scavenged by dry deposition through absorption and gravitation, or by wet deposition through precipitation. This deposition may expose people to radiation doses over a long period. The report described TRADOS model for calculating dispersion of the released radioactive material and the resulting individual and collective doses. Dispersion paths, the transport trajectories, are calculated using numerically analysed wind fields at constant pressure levels. Dispersion parameters, i.e. atmospheric stability, mixing height, occurrence and type of precipitation etc., are estimated along each trajectory. The model then calculates the concentrations of radioactive material and amounts of material deposited along the trajectories as well as expected radiation doses due to different exposure pathways. In one application of the model the expected doses and conditional cumulative probability distributions of collective doses were calculated for an assumed reactor accident in northern Central Europe. The dispersion of radioactive material from the release on the first day of the Chernobyl accident has also been estimated. The corresponding population doses in Finland have been estimated and compared with the estimates made on the basis of radiation measurements. There was relatively close agreement between these estimates