[en] The main loops of 5 MW nuclear heating reactor and 200 MW nuclear heating reactor are integral natural circulation systems. In case of loss of coolant accident (LOCA), while the water level reaches 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. An experiment described in this paper is carried out on the thermohydraulic test system HRTL-5 of the nuclear heating reactor. The position of the break is selected on the lower part of the exchanger. The system pressure is 1.5 MPa, the heating power under the accident is kept 5% or 20% of steady state power. Results show 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 inquired