Nordic Owners Group Study On Radiolysis Gas Management Post-Fukushima

摘要

During a severe accident (SA) in a BWR, such as core meltdown, it is assumed that a significant amount of non-condensable gases is produced. The condensable gases are condensed either in the suppression pool or by the containment spraying system. The main part of the produced non-condensable gases is hydrogen which is mainly originating from the steam/metal-reaction. The containments of the Nordic BWRs are inerted with nitrogen gas during normal operations in order to mitigate the risk for generating combustible gas mixtures within the containment. Also, in order to maintain containment integrity during a SA, all Nordic BWRs are equipped with a filtered containment venting system. Furthermore, the Nordic BWRs are equipped with recombiners in order to further reduce the risk related to combustible gas mixture. However, as a response to the Fukushima accident, Nordic Owner's Group (NOG) decided to investigate in more detail if there is any scenario or process that could lead to the accumulation of a mixture of combustible gases within the Nordic BWR containments during SA conditions. The main focus in this study was on the radiolysis of water due to the potential production of oxygen, but also other potential processes and in-leakage of air was considered. The study was performed by analyzing SA scenarios, simulating the selected scenarios with MAAP code; the simulation output was then used as input for the LwrChem code in order to simulate the impact of radiolysis gas production on the containment atmosphere composition. The study also investigated the gas transport paths and possible accumulation points for combustible gases within the containment and the release to the filtered venting system for five representative units. Furthermore, the existing instrumentation and equipment related to gas management was studied with respect to the selected BWRs' SA strategy in order to determine their possible use during such conditions.