First Results of the Phebus FPT3 Test

The purpose of this paper is to provide a preliminary overview of the phenomena observed during the experimental phase of the PHEBUS Fission Product Test FPT3. This experiment was the last in the series of 5 in-pile integral experiments performed by IRSN in the PHEBUS facility operated by the CEA on the site of Cadarache. 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 mechanisms inside the containment vessel and a chemistry stage devoted to the analysis of the iodine chemistry. During the experiment, both the on-line instrumentation and the periodic samplings worked quite well. The fuel degradation progress could be analysed through both temperatures inside the bundle and gaseous concentration measurements performed in the circuit and inside the containment vessel. Some major events, like fuel clad and absorber rod failures or material relocations, were clearly correlated to both bundle and circuit instrumentation signals. The post test non destructive examinations of the fuel bundle (X-radiography, X- and γ-tomographies and γ-scanning) allowed to compare FPT2 and FPT3 bundle final degradation states. On-line γ-detector 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 and the long term phases. The whole set of measurements appears self-consistent 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. (authors)