A non-equilibrium multiphase model for pressure escalation in fuel coolant interaction during explosion

A non-equilibrium model is used to simulate the multiphase interactions of molten fuel and coolant at explosion stage. In the current model, fluids are liquid coolant, vapor, molten drop and molten fragment. According to the non-equilibrium concept, an energy balance is built at vapor film surrounding the melt. Heat transferred from the melt to the vapor film interface directly participates to evaporate the liquid coolant, without heating the bulk liquid. Mass transfer between molten drop and molten fragment are dominated by thermal and hydrodynamic fragmentation mechanisms. The conservation equations are solved by the modified MCBA-SIMPLE method, with a semi-implicit iteration. Shock tube tests in two-phase medium and two simulations of the KROTOS experiments are used to validate the model. Simulations of KROTOS42 are performed for sensitivity study on fragmentation model, melt diameter, void fraction and melt temperature. Results reveal that the non-equilibrium multiphase model can give reasonable predictions on pressure generation and escalation in fuel coolant interaction. Fragmentation model analysis reveals that thermal fragmentation at trigger stage has unobvious effect on hydrodynamic fragmentation and hydrodynamic fragmentation is the major factor on pressure escalation. With the escalation of pressure, hydrodynamic fragmentation based on stripping mechanism results in a weaker pressurization against Rayleigh-Taylor mechanism. It is suggested that Rayleigh Taylor instability will be more preponderant at higher relative velocity in vapor explosion. Besides, the sensitivity study indicates that the steam generation and quenching process during the premixing have significant effect on explosion. Further, the model is preliminarily applied to a reactor simulation and integrity of the cavity wall is evaluated.