Vibration phenomena in large scale pressure suppression tests

Structure and fluid vibration phenomena were observed during blow-down experiments simulating a LOCA in the GKSS full scale multivent pressure suppression test facility. The source related excitations during the two regimes of condensation oscillation and of chugging are described together with the response vibrations of the facility's wetwell. Modal analyses of the wetwell were run using excitation by hammer and by shaker in order to separate phenomena that are particular to the GKSS facility from more general ones, i.e. phenomena specific to the fluid related parameters of blow-down and to the geometry of the vent pipes only. The lowest periodicities at about 12 and 16 Hz stem from the vent acoustics. A frequency of about 36 to 38 Hz prominent during chugging seems to result from the lowest local modes of two of the wetwell's walls when coupled by the wetwell pool. Further peaks found during blow-down in the spectra of signals at higher frequencies correspond to global vibration modes of the wetwell. (author)