Experimental Study of the Cooling Performance of the PAFS (Passive Auxiliary Feedwater System) During Transient Operation

Abstract

PAFS (Passive Auxiliary Feedwater System) is one of the advanced passive safety systems adopted in the APR+ (Advanced Power Reactor plus) being developed in Korea. The PAFS aims at completely replacing the conventional active auxiliary feedwater system by cooling down the steam generator’s secondary side with a natural driving force mechanism; i.e., it can remove the core decay heat by condensing steam in a passive condensation heat exchanger (PCHX) submerged inside the passive condensation cooling tank (PCCT). With an aim of validating the cooling and operational performance of the PAFS, the separate effect test facility named as PASCAL (PAFS Condensing Heat Removal Assessment Loop) has been constructed. In this study, postulated transient scenarios occurring in the PAFS were simulated to evaluate the performance of the condensation heat transfer and investigate the thermal hydraulic phenomena of the two-phase natural convection flow. The transient test matrix is composed of inadvertent MSSV (Main Steam Safety Valve) opening test (MO), PAFS start-up actuation test (SU), and non-condensable gas effect test (NC). In the MSSV opening test, MSSV was intentionally opened and closed several times and the characteristics of the natural convection flow were investigated. The experimental results showed that the cooling performance of the PAFS could be recovered after inadvertent opening and closing the MSSV. Start-up actuation test simulated the initial transient when the PAFS actuation signal was generated and the natural convection flow was initiated in the loop, and any significant two-phase flow instability was not observed in the test. The purpose of the non-condensable gas effect test is to study the characteristics of the condensation heat transfer in the heat exchanger when the nitrogen gas was injected. The test results proved that the existence of the non-condensable gas up to 1% of the steam did not produce a meaningful decrease of the cooling capability in the PAFS. From the experimental results described above, the cooling and operating performance of the PAFS was validated with respect to occurrence of the various transient scenarios and it was proved that the function of the PAFS can be effectively performed during the transient situation. The result will be also utilized in validation of the thermal hydraulic system code in the future.