Experimental study on accident transients and flow instabilities in a PWR-type small modular reactor

Experimental study on natural circulation flow instabilities is of great importance for the safety analysis in a PWR-type SMR, especially for accident scenarios such as loss of coolant accident (LOCA) and loss of heat sink accident (LOHS). In this study, an experimental natural circulation facility was built by scaling down from a typical PWR-type SMR. The scaling ratios were derived from non-dimensional field and constitutive equations of the drift flux model. The test facility has a height of 3.44 m with an operating pressure limit of 1.0 MPa. Two kinds of tests, the blowdown test and cold-blowdown test were performed. The blowdown test was designed to simulate the low pressure phase (<1 MPa) of a LOCA event and evaluate the performance of the emergency core cooling system (ECCS). The test results indicate that the ECCS can keep the RPV water level above the core, and the RPV pressure keeps decreasing during the entire accident transients. The cold-blowdown test was designed to simulate the whole transients of the system blowdown at low pressure conditions. The oscillations of the pressure drop, natural circulation rate, void fraction and inlet temperature were observed and analyzed. The cold-blowdown test results were compared with the RELAP5 simulation. Although some discrepancies existed at the initial blowdown phase, the code calculation agreed well with the experiment data.