Discussion on possibility of early film collapse for corium particles during FCI based on experimental results from unexpected collision between solid sphere and thermocouple

During a severe accident progression, fuel-coolant interaction (FCI) occurs in a lower head of reactor vessel or a pre-flooded cavity. Since the corium temperature is very high (~3000 K), film boiling regime is the main heat transfer mechanism during a FCI. This film boiling regime sustains until the corium reaches the minimum film boiling temperature. However, the heat transfer of some corium debris with the film boiling in a pre-flooded cavity would be influenced by the steam explosion. At the instant moment of the steam explosion, analysis codes apply the high heat transfer coefficient even though the corium particles are in the film boiling region in terms of the temperature: 5∙104 W/m2K in MC3D, ~106 W/m2K for TEXAS-V [1]. After the steam explosion, the calculation of analysis codes returns to the normal boiling equation implemented in each code. This seems acceptable since some experiment showed that the specimen recovers the film boiling heat transfer right after the peak heat transfer resulted from the instant film collapse with the pressure shock [2]. However, the early collapse and the complete escape from the film boiling heat transfer was observed for the solid sphere with the water in experiments which we present here.