Effect of free falling condition on melt jet breakup length in partially flooded cavity

When a severe accident occurs and proceeds to the failure of the reactor pressure vessel, the fuel-coolant interaction commonly occurs under the partially flooded cavity. The jet acceleration and the air entrainment phenomena, which were caused by the existence of the air space, resulted in different jet breakup lengths compared to the fully flooded cavity condition. Since the jet breakup length is an important parameter regarding the debris bed coolability, its precise prediction has been a significant issue for decades. This study investigated the effect of the free fall on the jet breakup length by observing the Froude number change and the air cavity generation according to the free fall height. Therefore, the Saito correlation was re-expressed as a function of the falling height emphasizing the influence of the jet acceleration in the air. Also, the size prediction model for the air cavity was developed as a function of the volume and the velocity of the jet head (bulge) based on the energy conservation between the bulge and the air cavity. By calculating the actual jet breakup length (the breakup length only from a melt-water interaction) and comparing it with the literature data, the influence of the jet acceleration and the air entrainment was highlighted indicating the necessity of the elaborate observation of the melt jet at the air in the fuel-coolant interaction experiments.