Investigation of Hydrogen Risk in the Major Containment Compartments of OPR1000 under SBO and TLOFW Scenarios Using MELCOR Code

Abstract

Considering the lessons from TMI-2 and Fukushima Nuclear Power Plant (NPP) accidents, conservative and reliable analyses for hydrogen risk with thermal-hydraulic phenomena need to be conducted particularly using the safety analysis codes on a regulative perspective. Such analyses are expected to contribute on the improvement of the reliability of hydrogen risk prediction. In this study, postulated hydrogen risks in the current Optimized Power Reactor 1000 MWe (OPR1000) under Station Blackout (SBO) and Total Loss of Feed Water (TLOFW) scenario were investigated by calculating the hydrogen distribution with MELCOR 1.8.6 code, which has been used as the safety analysis code in the nuclear regulatory body in Korea. First, a simplified MELCOR input delineating the containment building of OPR1000 under SBO and TLOFW scenarios was modified to build detailed nodalization. It was developed by focusing on the main compartments for the hydrogen release under both scenarios, i.e. the Reactor Drain Tank (RDT) compartment and the Reactor Cavity. Second, flammability and propensity of hydrogen combustion in each compartment were evaluated based on the criteria for ignition, Flame Acceleration (FA), and Deflagration-to-Detonation Transition (DDT). The Annulus 1st A and B, Regenerative Heat Exchanger (RHX) compartment, and Cavity door were classified as compartments vulnerable for hydrogen risk by examining the ignition criteria in MELCOR BUR package. After applying the σ-criterion and L/7λ-criterion, the vulnerable compartments could be judged to bear not the possibility of DDT but the possibility of FA. However, it should be noted that these criteria for evaluating FA and DDT give necessary but sufficient conditions.