A containment system 10 for a nuclear reactor comprising: a pressure vessel 12 defining a containment region 14; a working floor/platform 16 extending from the containment region to outside the pressure vessel; an access opening 18 in the pressure vessel providing access from the working floor outside the pressure vessel to the working floor inside the pressure vessel; and an access door 20 provided in the pressure vessel configured to slide downwards relative to the pressure vessel from a closed position to an open position, respectively closing & opening the access opening, wherein the access door comprises an integral floor plug 28 that plugs a space 22 in the working floor proximal to the access door when the access door is in the open position. At the space, the floor may comprise shoulders for seating the integral floor plug. A second integral floor plug 34 may be included to plug the space when the access door is in the closed position.
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