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Enhancing Nuclear Safety



FPT3 PHEBUS test : first results on iodine behaviour

Congress title :CSARP 2006 Cooperative Severe Accident Research Program
Congress town :Albuquerque
Congress date :25/09/2006


The international Phebus FP programme was initiated in 1988 by the French “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN), in cooperation with the European Commission (EC), to investigate key phenomena involved in light water reactor (LWR) severe accidents. This experimental programme consists in 5 in-pile integral experiments performed by IRSN in the PHEBUS facility operated by the CEA on the site of Cadarache. Its aim is to study the fuel degradation, the release of fission products and their transport through the reactor coolant system to the containment building. For safety concern, attention is focused on the iodine radiochemistry as it can directly impact on the consequences of any radioactivity release to the environment. Ultimately, the Phebus FP programme results will be used to validate the different codes used in water reactor safety analysis. The purpose of the presentation is to provide a preliminary overview of the iodine behaviour during the last PHEBUS FP test, FPT3, performed on 18th November 2004, emphasizing iodine data collected in the containment vessel. Unlike the previous tests, FPT3 used boron carbide as absorber material instead of silver-indium-cadmium, so varying an important parameter impacting physico-chemical phenomena. FPT3 test course was in agreement with the pre-defined test protocol including a 8.5-day irradiation phase, a fuel bundle degradation phase which lasted less than 5 hours and a 4-day long term phase that consisted of an aerosol stage dedicated to the analysis of aerosol deposition mechanism inside the containment vessel and a chemistry phase devoted to the analysis of the iodine chemistry. During the experiment, both the on-line instrumentation and the periodic samplings work quite well. On-line gammadetector measurements coupled with numerous post test gamma-counted sequential samplings help for the characterization of the iodine behaviour inside the containment vessel during the degradation phase and the long term phases. The whole set of measurements appears selfconsistent and provides new data for the iodine solubility inside the sump, the iodine gaseous fraction and the organic versus molecular iodine distribution inside the containment atmosphere. These results are of major interest, as they revealed a very important gaseous iodine instantaneous containment inventory, especially during the degradation phase and a high water solubility of iodine inside the sump during the long term phase. Whether these phenomena are due to the presence of boron carbide or the absence of Silver, Indium or Cadmium will be further investigated. Nevertheless, after further interpretation and transposition to the reactor case, these data should be relevant for a more accurate estimation of the iodine impact on potential source term to the environment in case of a hypothetical severe accident.
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