Experimental study and modeling Investigation on counter-current flow limitation phenomenon in reactor downcomer with DVI injection

During the refilling phase of a large-break loss-of-coolant accident (LBLOCA), the large upward flow of steam generated in the core region enters the reactor downcomer, causing the cooling water to fail to descend into the reactor core in time. This results in counter-current flow limitation (CCFL). The passive safety injection system of the HPR1000 reactor incorporates direct vessel injection (DVI) technology in the reactor pressure vessel (RPV). DVI has different injection characteristics compared to traditional loop injection, leading to differences in the phenomena and processes during a LBLOCA. To investigate the gas–liquid two-phase characteristics of the CCFL behavior in the RPV downcomer section with DVI injection under LBLOCA conditions, an RPV integral test facility was designed and constructed based on scaling criteria and the design features of the prototype reactor, and experimental and modeling studies were then performed. Emergency core coolant (ECC) experimental data were obtained under different inlet ECC flow rates, core steam flow rates, etc. Based on these experimental results, a conservative analysis was performed using the LBLOCA-specific analysis code — CATHARE GB. The results indicate that lowering the axial height of the DVI nozzle and installing a DVI flow diversion structure can effectively suppress ECC bypass. The CATHARE GB model used in the analysis of the LBLOCA analysis with DVI injection is conservative, and the current reactor incorporates DVI technology within the RPV. The ECC design can maintain an adequate safety margin.