Abstract: A lumped parameter code was used to study the AP1000 hydrogen igniter system and demonstrate its performance. In the worst case defined in this paper, the maximum hydrogen release rate was up to 300 kg/min. The results show that, if no ignition is initiated, the hydrogen concentration in some compartment is very high, the gas is burnable and it may induce the deflagration to detonation transition (DDT). While igniters on, and the concentration is reduced below the lower flammable limits. These demonstrate that the igniter system can effectively reduce the risk of hydrogen in severe accident.