Conception for the measurement of the critical environmental exposure by radioiodine in the case of reactor accidents

On the occasion of a reactor accident, radioactive nuclides may be released into the atmosphere. By means of the diffusion calculation with realistic assumptions for the relative emission source strength of fission gases and radioiodine it is shown that, in the case of typical dispersion conditions, the inhalation of the iodine nuclides constitutes the critical exposure pathway, provided that the ingestion of contaminated food is considered avoidable. For the purpose of initiating accident prevention measures it is therefore necessary to have a precise knowledge of the inhalation dose in the locations affected. Various concepts for determining the inhalation dose are presented and discussed. It is explained why the concepts based on the diffusion calculation do not furnish sufficient information for emergency measures due to insufficient knowledge of the emission source strength and dispersion function in the case of individual emissions. The only safe approach to determining the inhalation dose is the direct measurement of the time-integrated iodine concentration in the locations affected in conjunction with remote transmission of measured values to the accident control centres. The requirements to be met by measuring devices are explained and justified, and the possible design of an iodine monitor is described. (author)