PARs interaction with in-containment safety systems during severe accidents in Western PWR containments

Abstract

The combustion of flammable gases, such as hydrogen (H2) and carbon monoxide (CO), during severe accidents in Light Water Reactors might threaten the containment integrity. In this context, the AMHYCO project, funded by the European Commission, aimed to enhance the understanding of combustion risks in Pressurized Water Reactors and assess accident management strategies. The presented study, conducted within AMHYCO, uses the integral code MELCOR v2.2 to systematically analyse the combined role of Passive Autocatalytic Recombiners (PAR) and safety systems in mitigating combustion risks. The H2 and CO evolution is simulated in two reference scenarios for a generic 1000 MWe Pressurized Water Reactor: a Large Break Loss of Coolant Accident (LBLOCA) and a Station Black-Out (SBO). Results show that PARs effectively reduce flammable gas concentrations, though their performance depends on accident conditions. In low-steam scenarios and early vessel failure, such as a LBLOCA, combustible gas accumulation poses a significant challenge, especially during the ex-vessel phase. In these scenarios, reducing the containment cooling capacity has advantages for the combustion risk without affecting PAR's recombination capacity. Nonetheless, if pressure approaches the design pressure, reducing it might be a priority. In contrast, high steam concentration in SBO dilutes flammable gases, lowering combustion risk. The study quantifies the impact of spray-system recovery time, highlighting the importance of delaying activation timing so that PARs deplete enough combustible gases and oxygen to prevent deflagrations. These findings emphasize the need for specific accident management strategies to optimize PAR operation and cooling systems.