Hydrogen risk for advanced PWR under typical severe accidents induced by DVI line break

Abstract

Advanced passive pressurized water reactor (APWR) relies on In-Vessel Retention (IVR) of molten core debris under severe accidents, which makes hydrogen generation and distribution different with other PWRs. Hydrogen generation and hydrogen risk for APWR is studied with the integrated severe accident plant model, including Reactor Coolant System (RCS), engineered safety features (ESFs), simplified secondary side, containment and passive containment cooling system (PCCS). Initial events of DVI line break with passive core cooling system (PXS) compartment not flooded (named Case 1), DVI line break with PXS compartment flooded through broken DVI line (named Case 2), and DVI line break with IRWST direct injection available based on Case 1 (named Case 3) are selected to study hydrogen generation in pressure vessel, distributions in containment, and hydrogen risk controlled with igniters. Results show that hydrogen generated in pressure vessel is 257kg, 461kg and 499kg for Case 1 to Case 3 due to different thermal hydraulic characteristics. During maintaining the water level in the pressure vessel with the safety measures, zirconium-steam reaction is violent and the duration of hydrogen generation is longer than the other two cases due to the availability of plenty of steam, resulting in an amount of hydrogen generation, and hydrogen distribution in upper containment is nearly 10%. Case 3 is screened to analyze the effectiveness of hydrogen control system consisting of igniters, showing that the hydrogen control system can mitigate hydrogen risk when igniters are available at the SAMG inlet condition.