A nuclear power plant, comprising: a core; a reactor pressure vessel that houses the core; a dry well that houses the reactor pressure vessel; a wet well whose lower portion houses a suppression pool that is connected to the dry well via a LOCA vent pipe, and whose upper portion includes a wet well gas phase; a vacuum breaker that allows gas inside the wet well gas phase to flow back into the dry well; a containment vessel that includes the dry well, the LOCA vent pipe, the wet well, and the vacuum breaker; a passive containment cooling system comprising: a cooling water pool that is placed outside the containment vessel and stores cooling water; a heat exchanger that includes an inlet plenum, an outlet plenum, and a plurality of heat exchanger tubes connecting the inlet plenum and the outlet plenum and being at least partially submerged in the cooling water; a gas supply pipe whose one end is connected to the inlet plenum of the heat exchanger, and whose other end is connected to the dry well so that gas in the dry well is led to the heat exchanger; a condensate return pipe whose one end is connected to a lower portion of the outlet plenum of the heat exchanger, and whose other end is connected to the containment vessel so that condensate inside the heat exchanger is led to the containment vessel; characterised in that the passive containment cooling system of the nuclear power plant further comprises a gas vent pipe whose one end is connected to an upper portion of the outlet plenum of the heat exchanger, and whose other end is connected to an outside of the wet well so that non-condensable gas inside the heat exchanger is released out of the wet well, wherein the gas vent pipe is not connected to the wet well. a core; a reactor pressure vessel that houses the core; a dry well that houses the reactor pressure vessel; a wet well whose lower portion houses a suppression pool that is connected to the dry well via a LOCA vent pipe, and whose upper portion includes a wet well gas phase; a vacuum breaker that allows gas inside the wet well gas phase to flow back into the dry well; a containment vessel that includes the dry well, the LOCA vent pipe, the wet well, and the vacuum breaker; a passive containment cooling system comprising: a cooling water pool that is placed outside the containment vessel and stores cooling water; a heat exchanger that includes an inlet plenum, an outlet plenum, and a plurality of heat exchanger tubes connecting the inlet plenum and the outlet plenum and being at least partially submerged in the cooling water; a gas supply pipe whose one end is connected to the inlet plenum of the heat exchanger, and whose other end is connected to the dry well so that gas in the dry well is led to the heat exchanger; a condensate return pipe whose one end is connected to a lower portion of the outlet plenum of the heat exchanger, and whose other end is connected to the containment vessel so that condensate inside the heat exchanger is led to the containment vessel; characterised in that the passive containment cooling system of the nuclear power plant further comprises a gas vent pipe whose one end is connected to an upper portion of the outlet plenum of the heat exchanger, and whose other end is connected to an outside of the wet well so that non-condensable gas inside the heat exchanger is released out of the wet well, wherein the gas vent pipe is not connected to the wet well. a cooling water pool that is placed outside the containment vessel and stores cooling water; a heat exchanger that includes an inlet plenum, an outlet plenum, and a plurality of heat exchanger tubes connecting the inlet plenum and the outlet plenum and being at least partially submerged in the cooling water; a gas supply pipe whose one end is connected to the inlet plenum of the heat exchanger, and whose other end is connected to the dry well so that gas in the dry well is led to the heat exchanger; a condensate return pipe whose one end is connected to a lower portion of the outlet plenum of the heat exchanger, and whose other end is connected to the containment vessel so that condensate inside the heat exchanger is led to the containment vessel; characterised in that the passive containment cooling system of the nuclear power plant further comprises a gas vent pipe whose one end is connected to an upper portion of the outlet plenum of the heat exchanger, and whose other end is connected to an outside of the wet well so that non-condensable gas inside the heat exchanger is released out of the wet well, wherein the gas vent pipe is not connected to the wet well.