Abstract: The MAAP code was used to model AP1000 nuclear power plant and calculate hydrogen concentration in annulus region during severe accident. The leaked hydrogen risk outside containment was evaluated. The results show that hydrogen igniters and hydrogen recombiners employed by AP1000 can effectively control hydrogen concentration and mitigate hydrogen risk in annulus region. Only in the very limited accident scenario that a large amount of hydrogen is produced with the hydrogen igniters and hydrogen recombiners out of operation, the hydrogen concentration in annulus region is high enough to threaten the containment integrity.