A simple probabilistic approach to evaluate radioiodine behavior at severe accidents: Application to PHEBUS test FPT1

A simple probabilistic approach is proposed to quantify the relative importance of selected processes and mechanisms which govern the behavior of radioiodine under conditions pertaining to a postulated severe accident. In principle, these are sensitivity studies of a dedicated, small event tree which describes the iodine behavior at an accident. The emphasis is put on the processes taking place in containment at a late phase of the accident. Here, volatile species of iodine (volatile iodine: I2I2 and various organic iodides) could form from the species which were transported from RCS to containment originally as non-volatile. The relative importance of the processes leading to volatile iodine formation in containment is, for a long time, a subject of some dispute among the international research communities. The proposed probabilistic insight could be one of the many ways to address these complicated questions. In this work, the probabilistic approach was tested against the results of the well-documented PHEBUS FPT1 experiment. The small containment iodine event tree was processed by the EVNTRE code with a direct call from within the tree to the detailed iodine chemistry program calculating the speciation of iodine in containment. The French program IODE, as a part of the ASTEC suite of codes, has been used for this purpose. The results of this sensitivity study of PHEBUS FPT1 are promising as for the possibility to use such (or similar) probabilistic method for thorough analyses of uncertainties linked with the iodine behavior in containment at an accident. Similar approach could be used even for the iodine behavior modeling in whole-plant PRA analyses.