Effect of par deactivation by carbon monoxide in the late phase of a severe accident

Abstract

Passive auto-catalytic recombiners (PARs) are installed inside the containments of water-cooled reactors worldwide in order to mitigate the risk of hydrogen explosions in the course of an accident. After failure of the reactor pressure vessel (ex-vessel phase of a severe accident), the gas mixture released during the molten core-concrete interaction (MCCI) includes the flammable components hydrogen and carbon monoxide. Carbon monoxide is well-known as potential catalyst poison due to its strong adsorption properties which ultimately may prevent hydrogen and oxygen reaching active reaction sites. While high operational temperatures support the additional conversion of carbon monoxide to carbon dioxide inside the recombiner, experiments have revealed that catalyst deactivation by carbon monoxide is possible at lower oxygen concentrations. Based on experimental data, a correlation was derived indicating the catalyst poisoning temperature as a function of the gas composition. After enhancing the in-house REKO-DIREKT code according to the experimental data base, a COCOSYS accident simulation based on a generic accident scenario has been performed. The simulation results predict that the PARs in relevant regions will stop operation approx. 3.4 h after MCCI starts. From that time on, hydrogen and carbon monoxide are released from MCCI without available mitigation measure. As a consequence, the hydrogen concentration reaches significantly higher values at the end of the calculated scenario when compared with calculations without taking into account PAR poisoning. The present study demonstrates that carbon monoxide could significantly affect PAR operation in the course of a severe accident involving MCCI. As a consequence, substantial enhancement on all disciplines – experimental data, PAR models, and accident scenario assessment – is required in order to further detail the present findings.