Overall thermal-hydraulic behavior in an SBO test using HSIT in the ATLAS facility

Highlights: • A passive core makeup during an SBO test using HSIT was conducted in ATLAS facility. • An investigation of the thermal hydraulic behavior of SBO-HSIT test was performed. • Quite a much amount of ECCW inventory remained in closed HSITs in the test. • The DC water levels at LSCs in SBO tests maintained higher compared to SBLOCA tests. - Abstract: A station blackout test called SBO-HSIT (Station Blackout-Hybrid Safety Injection Tank) was performed at the ATLAS (Advanced Thermal-Hydraulic Test Loop for Accident Simulation) facility. An overall review of the SBO-HSIT test was performed, including comparisons with a previous SBO test and small break loss-of-coolant accident (SBLOCA) tests relating to similar thermal-hydraulic behavior. The decay heat release in the SBO-HSIT test was carried out through the main steam safety valves (MSSVs) until the steam generator (SG) dryout, and after that by blowdown through the power operated safety relief valve (POSRV). Most of the major thermal-hydraulic behavior happened during this blowdown period. It is noteworthy that the reactor pressure vessel (RPV) took charge of the pressurizer (PZR) function after it reached a saturated condition and then the PZR became a simple buffer tank along the POSRV flow path. This seemed to hinder the PZR inventory from being drained to the primary circuits. Loop flows showed unstable conditions under a slightly subcooled condition of the upper plenum of RPV (RPV-UP) region, and the natural circulation was broken after the SG U-tube became empty. Loop seal clearings (LSCs) occurred twice during the test and the downcomer (DC) water levels maintained significantly higher values compared to the SBLOCA cases. Pseudo differential pressure (DP) values appeared in some pressure balance lines (PBLs) due to the existence of void in the PBLs, and the injection flowrates from the HSITs tended to increase after the intrusion of voids into the corresponding PBL. Quite a large amount of emergency core cooling water (ECCW) inventory remained in the closed HSITs in the test. Finally, a core heatup occurred at low core water level despite a rather large amount of PZR inventory, whose drainage was hindered by the pressurizer function of the RPV, and of HSITs inventories, whose drainage was interrupted by the closure logic for HSITs, respectively.