Integral effect test on operational performance of PAFS (passive auxiliary feedwater system) for CIV opening stroke

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. 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). PAFS-CIV-01 test was performed for validating cooling rate of the reactor according to the CIV opening stroke at the FLB accident, which was analyzed as the most severe case in the APR+ SSAR (Standard Safety Analysis Report). With an aim of simulating a FLB+CIV accident of the APR+ as realistically as possible, the three-level scaling methodology was taken into account to determine the test conditions of the steady-state and the transient. From the experiment, major thermal-hydraulic phenomena such as the system pressures, the collapsed water levels, the break flow rate, and the condensate flow rate in PAFS were investigated and discussed. With decreasing the flow area of the CIV, the gradient of system pressure and temperature were reduced due to the decrease of the heat removal performance. As the CIV was re-opened, the two-phase natural convection flow in the loop of the PAFS was recovered. From the present experimental result, it could be concluded that the cooling rate of the core was controlled by the adjustment of the CIV opening stroke when the APR+ PAFS was operating. (author)