In-containment hydrogen, produced during severe accident, can largely challenge the integrity of containment, which has been studied for many years. However, Fukushima Dachii accident has shown the possibility of explosion outside containment due to leaked hydrogen, which was never considered before. In the paper, ex-containment hydrogen analysis is conducted with and without mitigation measures based on the newly-built CAP1400 MAAP4 containment model. Middle annulus is screened as the only location for the possible hydrogen accumulation. Due to steam condensation, hydrogen concentration can be unexpectedly high enough to exceed that in the containment, but either igniters or PARs can mitigate its risk even under conservative conditions. The possibility of containment failure due to hydrogen leakage is very low as long as mitigation measures are functional.

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