Analysis of hydrogen risk and its mitigation systems in CPR1000 during shutdown conditions

Analysis of hydrogen risk and the effectiveness of hydrogen mitigation systems during shutdown conditions is of great importance to support the development of the low power and shutdown (LPSD) probabilistic safety assessment (PSA) model and improve the capacity of nuclear power plants (NPPs) to reduce the hydrogen risk during shutdown conditions. This study deals with the identification of hydrogen risk and optimization of current hydrogen mitigation systems in the CPR1000 nuclear power plant during shutdown conditions before vessel head open. A detailed MELCOR input is developed, which includes detailed containment nodalization, accurate configuration of the passive autocatalytic recombiner (PAR) systems, and realistic boundary conditions based on design data and technical specifications. Three cases that represent different coolant inventory and specific hydrogen release paths are selected, and the accident progression and hydrogen risk are analyzed subsequently. The analysis indicates that the hydrogen risk is still an important issue during shutdown conditions, although the accident progression is relatively slow. PAR systems with the current configuration are added to the plant model to verify the effectiveness of the hydrogen mitigation system in the CPR1000 during shutdown conditions. The results indicate that the current configuration of PAR systems is still effective in mitigating the hydrogen risk following accidents during cold shutdown POS with PZR manhole close but is insufficient to mitigate the hydrogen risk during an accident with PZR manhole open. Therefore, two strategies to improve the hydrogen mitigation systems are proposed, and their effectiveness are compared. It is concluded that at least 12 additional PAR systems or an ignitor should be added to the PZR room to avoid severe hydrogen risk during shutdown conditions, and the additional PAR systems is the prior consideration.