Measurement and characterization of released fission products from the deposits on the RCS wall surface in severe accident conditions

The assessment of fission products and structural materials release due to reactions on the surfaces of the primary circuit is essential for nuclear safety research, objective being that their effects could be incorporated in accident analyses. Total fuel inventory of iodine, its short half-life and its radiotoxicity make iodine one of the most important fission products in the context of reactor safety. Consequently, understanding its behavior under severe accident transient conditions is a major point in the optimization of the accident management and mitigation. In order to improve the modelling of iodine chemistry in the primary circuit, an experimental facility has been specially designed at VTT for studying iodine transport experiments. It allows the study of released fission products from the deposits on the RCS wall surface in severe accident conditions. The measuring technology is sophisticated and unique in the field of aerosol research. On-line measurement devices provide information as for the size distribution of the transported aerosol particles using a Scanning Mobility Particle Sizer (SMPS) as well as with Electric Low Pressure Impactor (ELPI). Aerosol concentration is monitored by Tapered Element Oscillating Microbalance (TEOM). Moreover, efforts are underway to measure and identify chemical species of iodine. Although fraction of release gaseous iodine is the main goal, speciation of gaseous iodine is a key point for validation of the models in computer codes, notably because it can solve the question of possible kinetic limitations. In that purpose, one takes advantage of properties of iodine by trapping it in different solutions, characterized by a combination of analytical techniques (ICP-MS, UV-visible, ITP-CE).