Patent Number: 046541853
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an improved nuclear reactor having a calandria in the upper portion thereof, and to a deep beam head adapted for use on a nuclear reactor to seal the top of the reactor pressure vessel. In nuclear reactors, a core is supported in a pressure vessel and a coolant, typically water at critical temperature and pressure is circulated upwardly through the core and outwardly from the pressure vessel to provide power. The coolant flows upwardly through the core and then transversely to the outlets. The reactor also includes control rod assemblies and water displacement assemblies, which are movable into and out of the core to control the same. The use of a calandria structure in the upper region of the pressure vessel has been proposed, the calandria having an upper support plate, lower support plate and a series of hollow members therebetween, with the rod drive mechanisms operating through the hollow members. The coolant passes upwardly into the calandria through apertures in the lower support plate and is directed transversely about the hollow members to the outlet nozzles, while the control rods are protected by their shielding with the walls of the hollow members. Such a system is described in copending application Ser. No. 490,099 filed Apr. 29, 1983 in the names of Luciano Veronesi, et al., entitled "Nuclear Reactor", which application is assigned to the assignee of the present invention, and which application is incorporated by reference herein. In the improved reactor described in said pending application, a yoke member carrying a plurality of control rods is movable in the space between the calandria and the core. In such systems, however, a dome-shaped head or cover for the pressure vessel is used with an open area present between the calandria and the dome-shaped head. This large area of unused space between the calandria and the dome-shaped head causes certain technical difficulties that are overcome by use of the present invention. SUMMARY OF THE INVENTION A deep beam head for use in sealing the top opening of a nuclear reactor pressure vessel and an improved nuclear reactor have a calandria disposed in the upper portion of the reactor vessel, with the upper horizontal support plate of the calandria forming a sealing plate resting on the outer pressure resistant wall of the pressure vessel. A ring member is provided about the upper periphery of the sealing plate and is secured to the top of the pressure resistant wall. A plurality of reinforcing plates extend across the top of the sealing plate and are spaced from each other and the ends thereof secured to the ring, while a plurality of spaced transverse cross-members extend between and are secured to the reinforcing members. The ring member is preferably also secured to the sealing plate such that the ring member, sealing plate and calandria may be removed as a single unit from the pressure vessel to provide access to the upper internals of the reactor.