Nuclear Aerosol Filtering in the PHEBUS FP FPT4 Debris Bed Experiment

S. GAILLOT, S. GUENTAY, R. ZEYEN EUROPEAN AEROSOL CONFERENCE 1999 PRAGUE, September 6 - 10, 1999

The PHEBUS Fission Product Programme is a wide international research effort to investigate LWR severe accident phenomena, in particular in core melt progression up to melting and pool formation and the subsequent source term release. The Programme comprises 6 integral in-pile experiments, carried out in the Phébus facility operated by the French Institut de Protection et de Sureté Nucléaire (IPSN) in collaboration with the European Commission (CEC), the French Utilities (EdF), and with a number of countries using nuclear power : USA (NRC), Canada (COG), Japan (NUPEC and JAERI), South Korea (KAERI) and Switzerland (HSK and PSI). The ultimate objective is mainly to reduce the uncertainty on the evaluation of the amount and nature of radioactive products which could be released into the environment (source term) in case of a LWR core melt-down accident [1]. Most of the experiments involve part of a PWR fuel bundle containing about 10 kg of pre-irradiated fuel rods; one test, however, FPT4 contains a debris bed made up of crushed PWR fuel after 33 GWd/t pre-irradiation in a commercial reactor. The objective of this particular test is the study of the release during the transition from solid debris to a molten pool and from a molten pool itself. The mass of fuel amounts to 3.2 kg, mixed to 0.8 kg of oxidised zircaloy cladding shards. The debris bed is surrounded by a thermal shield made of thoria and zirconia tubes. The released material will be trapped in tubular filters operated between 1170K and 520K located in the upper part of the in-pile test assembly. Five filters will be used sequentially during the different phases of the experimental temperature transient up to melting at 2700K. The proposed filter material is Inconel or Hastelloy-X sintered metal tubes from both PORAL and PALL companies. The differential pressure is the only parameter, as measurable during the test, which can give on-line information on loading and assist the test conduct for switching from one sequential filter to another. Shortly after the experiment the test section will be de coupled from the circuit and stored in a vertical examination and control station (PEC). This PEC houses a radial and axial gamma scan and radiography facility connected to computer tomography. In a second phase, several months later, the test section will be transported in a specific shielded flask to a hot lab where horizontal and vertical cuts will be performed. These small sections will then be dispatched to outside laboratories for further detailed elemental analysis and FP inventory measurements on fuel remnants and filters. Computer codes support the modelling and pre calculation of the experiment and will be validated through its results. Large uncertainties exist in the modelling of releases of low volatile FPs and fuel species from a debris bed.