Fragmentation characteristics of molten materials jet dropped into liquid sodium pool

Abstract

The nuclear reactor is locally melted to form molten corium under instantaneous total blockage accidents in fuel subassembly of Sodium-cooled Fast Reactor (SFR). Molten Fuel-Coolant Interaction (MFCI) will result in violent interphase heat transfer and intensive fragmentation of molten core materials, which significantly affects the formation and cooling of core debris bed. Due to approximate ambient Weber Number (Wea) with molten metallic uranium, kilogram quantities of molten copper are used as fuel simulant to drop into liquid sodium pool to study the fragmentation characteristics during MFCI. Considering the influence of instantaneous solidification at the contact interface on fragmentation, initial temperature of molten copper is decreased to obtain the instantaneous contact interface temperature (Ti) less than the melting point (Tmp) of copper. Furthermore, hydrodynamic fragmentation is independently examined through experiments with molten aluminum jet under different initial temperatures of molten aluminum jet. The Ti between molten aluminum jet and liquid sodium is varied within the range of Tmp of aluminum and boiling point (Tbp) of liquid sodium. According to the present results, nucleate boiling of liquid sodium is proved to occur during molten copper-liquid sodium interaction and thermodynamic effects are much more predominant than hydrodynamic effects to cause finer fragmentation. The fragmentation of same molten material generally increases consistently with Ti. Due to considerable energy release capability of molten copper, molten state can be potentially maintained at the contact interface to cause adequate fragmentation under the same thermal conditions. Instantaneous solidification originating from contact interface greatly increases the criteria of fragmentation. Due to poorer thermal attributes, molten UO2 is provided with smaller variation rate of Ti under the approximate Ti, which suggests longer duration for violent sodium boiling to stimulate fragmentation. As the temperature of liquid sodium increases, interfacial instability is enhanced to fracture the solid crust with reduced thickness, contributing to fragmentation. The fragmentation caused by transition boiling seems to be more significant than that of nucleate boiling. Furthermore, the fragmentation mechanism of molten materials jet discharged into sodium pool is further discussed in the present study.