Dispersion and deposition parameters for calculating exposure to radiation in hypothetical reactor accidents

Fallout constants are required for calculating both ingestion doses and gamma doses caused by ground contamination, as well as cloud depletion by deposition. For the calculation of ingestion dose, deposition on the ground and vegetation is important insofar as it enters the food chain, whereas for cloud depletion deposition over wide areas, including, for example, woodlands, with their large surface areas, is of prime importance. With regard to deposition, distinction must be made between iodine and aerosols. The velocity of deposition depends mainly, for aerosols, on particle diameter. For iodine, the order of magnitude depends on its chemical composition (free iodine or organic compounds). For free iodine, which is deposited in the largest quantities, biological and meteorological influences of varying importance are known. In JUELICH deposition velocities for both iodine and aerosols have been determined for calculating the ingestion dose; exhaustive testing is needed to ascertain whether average values can be applied to accident situations or to what extent account must be taken of the particular circumstances in each case. For deposition over a wide area as characterized by the cloud depletion, few data are available. Since washout cannot readily be investigated by experimental means, no sufficiently reliable washout coefficients are available