Recent studies on fission products behaviour in a severe accident

Brief presentation of the research lead at VTT on fission products transport and chemistry was made, and focus was made on the behaviour of FPs in the primary circuit piping. Starting from the observation of a significant fraction of gaseous iodine in the containment during some Phébus-FP tests, hypotheses concerning reactions from caesium iodide, which can modify the transport of iodine, were presented. Focus was on {Cs-I-O-H} system and the influence that may have molybdenum and boron. Experimental works, performed at IRSN, on the study of the chemical reactions on the gas stream and performed at VTT, on chemical reactions on the surfaces of the pipe walls were shown. The influence of the two elements, acting as caesium consumers in both gaseous and condensed phased, was demonstrated. Moreover, comparative study between experimental results and calculations obtained with SOPHAEROS, module of ASTEC code, was made for the experiments done in gaseous phase. Role of possible {Cs, I, O, H} kinetic reactions was pointed out.