Investigations of hydrogen hazard mitigation by deliberate ignition in small modular reactor during severe accident using GASFLOW-MPI

Abstract

Hydrogen explosion, which represents a complex safety issue in the water-cooled Small Modular Reactors (SMRs), could lead to over-pressurization and threaten the integrity of the reactor containment in case of severe accidents. In order to mitigate this hydrogen hazard, ignitors could be used as an option in SMRs by deliberately burning the hydrogen. Computational Fluid Dynamics (CFD) tool GASFLOW-MPI has been developed and validated to perform hydrogen safety analysis of the SMR with steel containment during severe accidents, including the hydrogen distribution, hydrogen explosion risk assessment, hydrogen removal by deliberate ignition and hydrogen combustion. The combustion models implemented in GASFLOW-MPI have been validated against the THAI-HD hydrogen deflagration experiments. The relevant analysis methodology is improved according to the GASFLOW-MPI features. The calculation results show that the ignitor system can effectively reduce the hydrogen and mitigate the hydrogen explosion risk. However, H2-air-steam mixtures could be inerted by the extremely high steam concentration near the ignitors. A large amount of hydrogen could be accumulated in containment, which will become flammable due to the condensation of steam. To study the consequences of the failure of the deliberate ignition, accidental ignitions at various locations are triggered in the SMR containment. Depending on the turbulence intensity and the geometrical complexity of the internal structures, a pressure peak higher than the design pressure occurs. It indicates that the capability of deliberate ignition system is restricted by the burnable limit of the mixtures. It seems that a combination of deliberate ignition system and passive hydrogen removal system is more preferable to eliminate the hydrogen risks in the SMR containment.