Numerical model for bubbels break-up during blowdown

A two-phase model was developed for evaluating coolant discharge rates from loss-of-coolant accidents (LOCA). The equal velocity unequal temperature (EVUT) two-phase model was incorporated into a numerical code whose solution is based on the method of characteristics. Specifically, the code contains a bubble break-up model that depends on two different instability criteria: Rayleigh-Taylor and the Kelvin-Helmholtz instabilities. The code was successfully tested against sets of experimental data for various test conditions. The two instabilities had a similar influence on the bubble breakup rate. Either mechanism reproduced quite accurately the wavy transients of various tests, both in amplitudes and pressure oscillation frequencies. The inclusion of the bubble breakup mechanism was essential to predict accurately the oscillatory pressure responses and prevent under-predictions of evaporation rates. This induced a sharp increase of the effective interface area and thereby intensified both the vaporization rate and the pressure buildup.