Thermal-hydraulic mechanisms of core liquid level depression and recovery during small break LOCA experiment

A 5% cold leg break loss-of-coolant accident experiment was conducted by using the Large Scale Test Facility (LSTF) of the Rig of Safety Assessment (ROSA)-IV program. In the early stage of experiment, the core collapsed liquid level was depressed to the bottom of the simulated core and the dryout of the core was observed. The maximum superheat of heater rod surface temperature was about 100 K at the middle of core and the entire core was quenched just after the loop seal clearing. The manometric effect due to the liquid seal formation in the loop seal and the liquid holdup in the steam generator (SG) U-tubes upflow-side caused a depression of the core collapsed liquid level in spite of the coolant injection from the emergency core cooling system (ECCS). The liquid holdup in the U-tubes upflow-side was observed after the termination of the two-phase circulation due to the phase separation at the U-tubes top