Integral Effect Test on Operational Performance of the PAFS (Passive Auxiliary Feedwater System) for a SGTR (Steam Generator Tube Rupture) Accident

Abstract

The PAFS is one of the advanced safety features adopted in the APR+ (Advanced Power Reactor Plus) which is intended to completely replace a conventional active auxiliary feedwater system. The PAFS cools down the steam generator secondary side and eventually removes the decay heat from the reactor core by adopting a natural convection mechanism; i.e., condensing steam in nearly-horizontal U-tubes submerged inside the PCCT (Passive Condensation Cooling Tank). With an aim of verifying the operational performance of the PAFS, the experimental program of an integral effect test is in progress at KAERI (Korea Atomic Energy Research Institute). The test facility, ATLAS-PAFS was constructed to experimentally investigate the thermal hydraulic behavior in the primary and secondary systems of the APR+ during a transient when the PAFS is actuated. Since the ATLAS-PAFS facility simulates a single train of the PAFS, the anticipated accident scenarios in the experiment include FLB (Feedwater Line Break), MSLB (Main Steam Line Break), and SGTR (Steam Generator Tube Rupture). Among them, SGTR was considered as one of the design basis accidents having a significant impact on safety in a viewpoint of radiological release. Therefore, the SGTR test was determined to be the integral effect test item in the frame of the ATLAS-PAFS experimental program. In this study, the PAFS-SGTR-HL-02 test was performed to simulate a double-ended rupture of a single U-tube in the hot side of the steam generator of the APR+. The three-level scaling methodology was taken into account to determine the test conditions of the steady-state and the transient. The pressures and temperatures of the system and the data related to the PAFS operation were collected with the measurement of the break flow. The initial steady-state conditions and the sequence of event of SGTR scenario for the APR+ were successfully simulated with the ATLAS-PAFS facility. And it was shown that the pressure and the temperature of the primary system were continuously decreased during the heat removal by the PAFS operation. The water pool in the PCCT was heated up to the saturation condition and the evaporation of the water made a decrease of the PCCT water level. It could be concluded from the present experimental result that the APR+ has the capability of coping with the hypothetical SGTR scenario with adopting the PAFS and the proper set-points of its operation.