Circuit and containment aspects of Phébus experiment FPT-2

The international reactor safety project Phebus FP is devoted to the study of core meltdown in the case of a severe accident scenario. Its main objective is to study the fuel degradation, the release and transport of fission products and structure material in the reactor coolant system (RCS) and containment building, with special attention to iodine radiochemistry. The test sequence of those experiments included an irradiation phase, a fuel bundle degradation phase and a long-term phase dedicated to the analysis of aerosol and iodine radiochemistry in the containment vessel. The FPT-2 test was performed using a bundle configuration in a steam poor environment and with injection of boric acid. On-line measurements and post-test analyses indicate that releases of noble gases (Xe, Kr) from the fuel bundle are very high (up to 80% of initial bundle inventory); other elements can be classified as high-volatile with releases higher than 50% of bundle initial inventory (FPs: Cs, I, Te, Mo), as semi-volatile with releases comprised between 10 and 50% (FPs: Rb and control rod material: Cd) and low-volatile with releases lower than 10% (FPs: Ru, Ba and control rod material: Ag, In). Important material deposition was observed in the circuit, so that material inventory reaching the containment is significantly lower than the fractions released from the degraded bundle. The release and transport kinetics of material could be determined and seems strongly correlated with bundle degradation events (first fuel Zircaloy cladding oxidation phase, late oxidation phase and fuel liquefaction and displacement). In the circuit, specific instrumentation devoted to iodine speciation revealed that iodine exists mainly in a vapour form in the hot leg (700°C), most probably as vapour of metal iodide. Between the circuit hot leg and the cold leg (150°C), condensation of those species both on the circuit surface and on transported aerosols is quite extensive. Therefore, iodine transiting through the circuit cold leg before arriving in the containment vessel was found mainly to be in an aerosol form. It is to be noted that a minor gaseous iodine fraction was evidenced in the cold leg of the circuit. The Circuit and Containment Aerosol Interpretation Circle CACIC is devoted to give a final interpretation of the main outcomes of the experiment related just to circuit and containment, reviewing all the calculation carried out and providing a code validation/quality assurance. The interpretation is based on test data, computational simulations by state-of-the-art severe accident analysing codes and on the discussions at the meetings of the Phebus CACIC organized by IRSN, Cadarache and EC JRC, Petten.