Systematic hydrogen risk analysis of OPR1000 containment before RPV failure under station blackout scenario

This study postulates a hydrogen risk analysis for the Korean Optimized Power Reactor 1000 MWe (OPR1000) until the reactor pressure vessel (RPV) fails under a station blackout (SBO) scenario, using the MELCOR 1.8.6 code. A detailed MELCOR input for OPR1000 was developed through the modified nodalization of the containment building considering hydrogen release paths. The reactor drain tank compartment is the major compartment of interest, because it is a proximate hydrogen release path before the failure of the RPV under the SBO scenario. The flammability and the flame propagation regime for hydrogen combustion in each compartment were evaluated by applying the criteria for ignition, flame acceleration (FA), and deflagration-to-detonation transition (DDT). As a result, the Annulus 1st A and B regions were identified as the flammable compartments exceeding the ignition criteria embedded in the MELCOR BUR package. In terms of the flame propagation regime, the major compartments were determined as being exposed to the possibility of FA meeting the σ-criterion. However, the possibility of DDT was not predicted when the L/7λ-criterion was applied. Nevertheless, it should be noted that these criteria for evaluating FA and DDT give required, but not necessarily sufficient conditions.