Main lessons learnt from 40 years of R&D on iodine source term prediction: Identification of the main parameters governing iodine volatility in PHEBUS FP tests

Iodine chemistry and phenomenology in the containment has been studied for several decades. The main phenomena leading to the formation of volatile iodine have been identified step by step and their kinetics has been modeled and capitalized over the years in ASTEC-SOPHAEROS IRSN Severe Accident (S.A) code. Recently, the uncertainties for each phenomenon have been quantified and uncertainty propagation calculations have been performed on PHEBUS FPT-0/1/2/3 tests within the objective to identify which phenomena govern iodine volatility. The main highlights from PHEBUS studies are that (1) the sump reactions do not contribute to iodine volatility and (2) the gaseous phase chemical reactions are the main contributor to iodine volatility and (3) only a few gaseous reactions govern iodine volatility in PHEBUS containment. Another objective was to narrow the estimated range of %I 2_RCS (gaseous iodine fraction coming from the RCS). The results show that, considering 43 uncertain parameters, the iodine volatility plume is compatible with the experimental data whatever 2% < %I 2_RCS < 50% that mostly govern iodine volatility in the first days. It also indicates that, as soon as the FP release from the core is stopped and whatever 2% < %I 2_RCS < 50%, the influence of %I 2_RCS decreases over time so that the main processes leading to iodine volatility are slowly switched from %I 2_RCS (short term) to other gaseous phenomena (long term). The influence of %I 2_RCS on iodine volatility is thus important in the short term but becomes less and less significant in the long term (after several days). A more complete analysis is necessary for reactor applications to identify if the same conclusions can be drawn.