Abstract: A nozzle dam for providing a fluid-tight seal across a primary fluid nozzle in the plenum of a nuclear steam generating vessel includes two fluid-impermeable seals, each comprising a circular three-section foldable seal plate covered with a flexible diaphragm and circumferentially encompassed by an inflatable seal member disposable in frictional sealing engagement with the inner surface of the nozzle. The two seal apparatuses are interconnected by a central tubular coupling coaxial with the nozzle and non-rotatably fixed to each seal, the coupling having a quick-disconnect feature operable from outside the nozzle dam. The coupling provides a conduit for inflation of one of the seals. The two seals are also interconnected by four post assemblies to maintain the seal plates parallel with each other, each post assembly including a cylindrical member pivotally movable between a release position and a supporting position, and provided with a slidable encompassing sleeve to lock it in the supporting position.