An operating floor confinement has an operating floor, a sidewall that surrounds the operating floor, a ceiling that is provided on an upper portion of the sidewall, a reactor well, a fuel pool, a dryer and separator pit, an equipment hatch that is provided on the sidewall, an air lock that is provided on the sidewall, and an isolation valve that is provided in a penetration line. The operating floor confinement forms a pressure boundary having pressure resistance and a leakage protection function. The operating floor confinement is separated from an equipment area of the reactor building and has no blowout panel.