There is disclosed a containment system for a nuclear plant, the containment system comprising a pressure vessel defining a containment space for containing nuclear plant structures, a working floor extending from within the containment space to outside the pressure vessel, an access opening in the pressure vessel for providing access from the working floor outside the pressure vessel to the working floor in the containment space, and an access door provided within the pressure vessel. The access door is configured to slidably move downwardly with respect to the pressure vessel from a closed position in which it closes the access opening to seal the pressure vessel, and an open position in which it reveals the access opening and is received in a door space in the working floor. The access door comprises a first integral floor plug which is configured to plug the door space in the working floor when the access door is in the open position, to provide a substantially continuous working floor surface from outside the pressure vessel to within the containment space.