Abstract: A closure hold-down system for a prestressed concrete reactor vessel is disclosed which employs a concrete plug having an annular liner thereon and adapted to be received within the upper open end of a cavity in the reactor vessel. A prestressed concrete retaining ring is mounted on the reactor vessel concentric to the plug through a plurality of circumferentially spaced prestressed tendons anchored directly to the reactor vessel, and engages the concrete plug so that pressure forces acting on the plug from within the cavity are transmitted directly from the plug to the prestressed retaining ring through a contact surface therebetween. An omega seal is disposed between the closure plug and a cavity liner to prevent escape of fluid pressure from the cavity.