A nuclear power plant comprising: a core; a reactor pressure vessel that houses the core; a containment vessel that houses the reactor pressure vessel; a containment vessel head of the containment vessel; a reactor building that surrounds at least part of the containment vessel; a sidewall that surrounds a periphery of the containment vessel head and extends upward; a bottom portion that is connected to a lower end of the sidewall and is connected to the containment vessel; a reactor well that is formed by the containment vessel head, the sidewall, and the bottom portion; a reactor well upper lid that is provided in an upper portion of the reactor well; an operation floor that is joined to an upper end of the sidewall and is provided on a periphery of the sidewall; an operation floor area wall that surrounds a periphery and upper portion of the operation floor to form an operation floor area and is part of the reactor building; a standby gas treatment system including: a suction pipe to take in gas inside the reactor building; an exhaust fan to drive the gas that the suction pipe takes in; a standby gas treatment system exhaust pipe to release the gas that the suction pipe takes in to an environment outside the reactor building via the exhaust fan; a heater that is disposed between the suction pipe and the standby gas treatment system exhaust pipe and uses electric power to heat the gas that the suction pipe takes in; and a filter to filter the gas heated by the heater and to send the gas to the standby gas treatment system exhaust pipe; and a reactor well exhaust section to release the gas inside the reactor well to the environment without releasing the gas into the operation floor area in an event of a severe accident. a core; a reactor pressure vessel that houses the core; a containment vessel that houses the reactor pressure vessel; a containment vessel head of the containment vessel; a reactor building that surrounds at least part of the containment vessel; a sidewall that surrounds a periphery of the containment vessel head and extends upward; a bottom portion that is connected to a lower end of the sidewall and is connected to the containment vessel; a reactor well that is formed by the containment vessel head, the sidewall, and the bottom portion; a reactor well upper lid that is provided in an upper portion of the reactor well; an operation floor that is joined to an upper end of the sidewall and is provided on a periphery of the sidewall; an operation floor area wall that surrounds a periphery and upper portion of the operation floor to form an operation floor area and is part of the reactor building; a standby gas treatment system including: a suction pipe to take in gas inside the reactor building; an exhaust fan to drive the gas that the suction pipe takes in; a standby gas treatment system exhaust pipe to release the gas that the suction pipe takes in to an environment outside the reactor building via the exhaust fan; a heater that is disposed between the suction pipe and the standby gas treatment system exhaust pipe and uses electric power to heat the gas that the suction pipe takes in; and a filter to filter the gas heated by the heater and to send the gas to the standby gas treatment system exhaust pipe; and a suction pipe to take in gas inside the reactor building; an exhaust fan to drive the gas that the suction pipe takes in; a standby gas treatment system exhaust pipe to release the gas that the suction pipe takes in to an environment outside the reactor building via the exhaust fan; a heater that is disposed between the suction pipe and the standby gas treatment system exhaust pipe and uses electric power to heat the gas that the suction pipe takes in; and a filter to filter the gas heated by the heater and to send the gas to the standby gas treatment system exhaust pipe; and a reactor well exhaust section to release the gas inside the reactor well to the environment without releasing the gas into the operation floor area in an event of a severe accident.