Numerical study of hydraulic fragmentation of melt jet in coolant: Breakup length and melt-coolant contact area

Hydraulic fragmentation of melt jet is an important phenomenon during fuel-coolant interaction (FCI) of nuclear reactor severe accident. If the melt jet has not broken up completely before contacting the lower head, it may ablate the vessel wall and a compact debris bed may form. The jet breakup length and melt-coolant contact area are important parameters describing melt fragmentation. In this study, therefore, a numerical model for analyzing melt jet hydraulic fragmentation was developed and validated against the experiment. Through the analysis of UO₂ melt jet fragmentation in coolant under various conditions, the relationship between the morphological structures and axial velocities of melt jet was clarified, and an accurate criterion for judging jet breakup length was proposed. The effects of initial melt jet velocity and diameter on the dimensionless breakup length were investigated, and an empirical equation for calculating breakup length was established with a maximum deviation about 10%. The dimensionless contact area of melt and coolant increased as the melt was injected into the coolant, but it finally reached a stable value for each case. It increased as the increase of initial injecting velocity, but was not affected by the initial melt jet diameter. The empirical equation for calculating the dimensionless contact area was established as well, and the prediction deviation was less than 10%.