Study of strategies to avoid hydrogen deflagration in venting pipelines during severe accident scenarios

Highlights: • A topic related to BWR Mark II containment safety analysis is considered in this study. • A three-dimensional vent pipeline model was developed for the CFD type code GASFLOW. • Inertization with nitrogen at specific locations is investigated to avoid hydrogen deflagration. • Volume enlargement at vent pipe's end section is also investigated to avoid hydrogen deflagration. - Abstract: Venting timing and duration are key issues for the development and assessment of severe accident guidelines and mitigation alternatives. In BWRs, venting from wetwell has the advantage of gaining fission product scrubbing. In this study, two strategies are investigated to avoid hydrogen deflagration in venting pipelines. The starting point of the vent pipe is a penetration on the wall of wetwell's suppression chamber of a BWR Mark II containment. A three-dimensional pipeline model was developed for the CFD type code GASFLOW, to better determine conditions leading to risk of flame acceleration and hydrogen deflagration. The analysis starts with a base case, in which venting occurs when pressure reaches 4.5 kg_f/cm² and the vent pipe is full of air. Then, the first strategy to reduce hydrogen deflagration risk consists of inertization with nitrogen at specific locations along the vent pipe through rupture disks with three opening pressure setpoints (2.0, 3.0. 4.0 kg_f/cm²). Three different locations are considered in this study. The second strategy is the volume enlargement of the last section of the vent pipeline. Two different expansions additional to the base case were considered for analysis. The results show that the inertization with nitrogen at the lower pressure setpoints (2.0 and 3.0 kg_f/cm²) does effectively, for practical applications of safety analysis, highly reduces the risk of flame acceleration anywhere in the vent pipeline. However, lowering the opening pressure value implies earlier venting. If it is preferable to keep the disk opening pressure at the higher pressures (4.0 and 4.5 kg_f/cm²), the results show that it is necessary to choose an appropriate location to set the rupture disk, to effectively diminish flame acceleration risk. Regarding the second venting strategy, the results show that increasing the volume of the last section of the vent pipe is also an effective way to reduce hydrogen deflagration risk. Thus, although flame acceleration still could occur, those conditions for that to happen will be restricted to a shorter period. For actual practical applications, this second strategy seems more plausible to be carried out, because all relevant changes to the vent pipeline would be focused on the parts already outside reactor building.