According to an embodiment, a nuclear power plant has a core; a reactor pressure vessel; a dry well; a wet well; a vacuum breaker; a containment vessel including the dry well, the LOCA vent pipe, the wet well, and the vacuum breaker; a cooling water pool placed outside the containment vessel; a heat exchanger at least partially submerged in cooling water; a gas supply pipe connected to the inlet plenum of the heat exchanger and the dry well; a condensate return pipe connected to the outlet plenum of the heat exchanger and the containment vessel; and a gas vent pipe connected to the outlet plenum of the heat exchanger and an outside of the wet well so that non-condensable gas inside the heat exchanger is released out of the wet well. The gas vent pipe is not connected to the wet well.