An experimental study for transient behaviors under flow break of natural circulation by nuclear heating reactor LOCA

The main loops of 5 MW nuclear heating reactor and 200 MW nuclear heating reactor are integral natural circulation systems. The natural circulation of the primary coolant is built up by the buoyancy from coolant density difference between the riser and the downcomer. The rupture of coolant penetrating tube is a typical accident to cause coolant loss. When the water level drops down to the upper edge of the inlet of the heat exchanger, flow break occurs. It has influence on cooling core and stability of the main loop. To analyze the design safety of the nuclear heating reactors (NHRs) and to investigate the characteristics of LOCAs during discharging and flow break of natural circulation, a series of tests is completed using the thermohydraulic test loop HRTL-5 in INET, Tsinghua University. Results show that there is a mainly stable drop of the pressure, and the heated element temperature is not too high and could not be burnout. But the flow oscillation in the primary coolant circuit occurs when the flow breaks completely at the end, which is unfavorable. The whole process of flow oscillation is described and its mechanism is discussed