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Under Hypothetical Core Disruptive Accidents (HCDA) in Sodium-cooled Fast Reactor (SFR), Molten Fuel-Coolant Interaction (MFCI) will cause intensive fragmentation of molten fuel materials and violent sodium boiling, possibly resulting in recriticality due to sodium void effect and reactor core compaction. And the formation and sufficient cooling of core debris bed are significantly affected by the fragmentation of molten fuel materials in sodium pool. Due to approximate ambient Weber Number (Wea) with molten metallic fuel, kilogram-level molten copper jet is dropped into sodium pool to study the fragmentation characteristics under varied experimental parameters in the present experiments. The nucleate boiling of liquid sodium is estimated to occur at the instantaneous contact interface between molten copper and liquid sodium. The molten copper jet suffers from intensive fragmentation and solidification to produce many fragments under thermodynamic and hydrodynamic effects. Besides, experiment with molten aluminum jet injected into sodium pool is conducted without occurrence of sodium boiling to independently study the effect of hydrodynamic interaction on interphase penetration and fragmentation. During the experiments, temperature variations and dynamic pressure change in sodium pool are monitored to evaluate energy release. The resultant fragments are recorded and measured for morphology analysis and size distribution to examine the fragmentation characteristics. Based on experimental results, the fragmentation mechanism is discussed against hydraulic and thermal factors. The present findings contribute to further understanding the fragmentation characteristics of molten corium in sodium pool and optimization of mitigation systems against MFCI for SFR.

In this study, the effect of the stratified interface instability on the thermal focusing effect in two-layer corium pool were investigated by numerical simulations performed with CFD code Fluent. The Rayleigh numbers ( Ra ′ ) obtained in this study range from 109 to 1015. By setting different decay heat power and turbulence intensity, crust of different melting degree at stratified interface can be obtained. Through the comparison of the corium pools with the crust of different melting degree, the differences of temperature distribution and boundary heat flux distribution are obtained. The coupling mechanism of two layers of corium pools and a new criterion for the occurrence of stratified interface instability are also presented. The results show that when the crust is slightly damaged, the thermal focusing effect is intensified by the reduced thermal resistance due to the crust failure at the interface and the unevenness of the thickness of the crust on the side wall of the metal layer, and if the crust is highly damaged, the thermal focusing effect is weaken by the melting of the crust at the wall of the lower head. The results of this study can provide reference for reactor IVR (In-Vessel Retention) safety analysis and optimization design.

Hypothetical Core Disruptive Accidents (HCDA) are dominantly concerned during safety assessment and evaluation in Sodium-cooled Fast Reactor (SFR). With molten core materials discharged into liquid sodium, positive reactivity is potentially introduced due to sodium boiling and molten core compaction, which can cause terrible recriticality. The possibility of recriticality and efficient cooling on the relocated debris bed are significantly affected by the fragmentation behavior of molten core in liquid sodium. With few available mechanism models and benchmarks, many investigations have been conducted on the fragmentation characteristics during molten fuel-coolant interaction (MFCI). In the present study, molten copper is used for molten simulant to be discharged into the liquid sodium pool through guiding tube based on a multifunctional experimental facility (COSA). The simulants are heated by electromagnetic induction system in customized ceramic crucible and the molten materials are controlled by magnetic lifting system to be drained through the guiding tube into the bottom liquid sodium pool. Temperature variation and pressure change in the liquid sodium pool are acquired against the energy release during MFCI. Furthermore, the fragments cleaned by water medium are measured and recorded for distribution and morphology analysis. Significant pressure pulses and temperature gradient almost not occur during MFCI and the molten copper is finely fragmented possibly due to hydrodynamic and thermodynamic effects. And the experimental results are helpful to confirm the prediction of fragmentation mechanism and to validate physical model, which can be applied to the development and validation of analysis code.

Molten Fuel-Coolant Interaction (MFCI) is especially focused in Sodium-cooled Fast Reactor (SFR) with the consideration of possible recriticality and predominant effect on resultant consequences. As the nuclear fuel cladding material of SFR, kilogram quantities of molten stainless steel jet are used in the present experiments to study the fragmentation behavior when discharged into sodium pool. During the interaction between molten stainless steel jet and liquid sodium, the great differences in temperature, velocity and thermophysical properties result in violent sodium boiling, hydrodynamic instability and viscous shearing at the contact interface. The molten stainless steel jet suffers from significant fragmentation under multiple interfacial forces, which increases contact interface area to enhance heat transfer. However, the accelerated solidification of molten stainless steel jet at the contact interface greatly raises the criteria of fragmentation, conversely decreasing the heat transfer. In the present study, the sodium pool depth is varied to study the effect of the dropping height of molten stainless steel and inventory of liquid sodium on the fragmentation characteristics. The temperature variations in sodium pool are measured to estimate energy release during molten stainless steel-liquid sodium interaction. The resultant stainless steel debris is recorded and measured against debris mass with respect to size intervals for morphology analysis and size distribution. Furthermore, experiments with kilogram quantities of molten copper jet are conducted in the present study. The results with molten stainless steel jet are compared to those with molten copper jet and those with molten stainless steel droplets to distinguish fragmentation characteristics. According to the present findings, adequate fragmentation of molten material can be caused by increased relative velocity, high superheat and considerable energy release capability, enhancing heat transfer and accelerating solidification of molten material. With few researches on MFCI in SFR, a fragmentation mechanism is proposed based on the growth and collapse of sodium bubbles to further understand the fragmentation behavior of molten material in sodium pool.

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.