Patent Number: 039376532
Section: summary

This invention relates to a diagrid for supporting the core of a nuclear reactor of any type but especially a liquid-sodium cooled fast reactor, the heat generated by the reactor core being recovered in separate heat exchangers which are preferably mounted within the vessel which contains the reactor core and the circulating sodium, the reactor core and heat exchangers being thus incorporated in a so-called integrated structure. The aim of the invention is to endow the coresupport diagrid within the reactor vessel with high mechanical strength, especially with respect to the loads to be carried, while making it possible in particular and especially in the event of accident or of damage to any of its elements to disassemble said diagrid as a single unit, to withdraw the diagrid from the reactor vessel and to replace it by another diagrid of identical design. More precisely, the invention relates to a reactor diagrid of the above-mentioned type constituted by a box structure having end-walls formed by two metallic plates braced by hollow cylindrical support columns which are suitably disposed at intervals within the box structure on a well-defined and uniform pitch. The open ends of said hollow support columns extend through apertures formed in the upper end-wall of the box structure for positioning and supplying liquid sodium to the fuel assemblies which are supported by the diagrid and the arrangement of which in combination forms the reactor core. Finally, the invention is even more specifically concerned with a diagrid of this type comprising a horizontally extending flat upper plate and the axis of the reactor core being vertical, and means whereby the box structure formed by the two braced plates is applied against a support forming part of the vessel which contains the reactor core and the heat exchangers. In accordance with the invention, the diagrid under consideration is characterized in that the lower end plate of the box structure has the shape of a spherical segment or dome which terminates at its periphery in a flat rim and that the upper flat end plate has a circular flange which is parallel to the peripheral rim of the lower plate, said rim and said flange being braced with respect to each other, the flange of the upper end plate being capable of resting on a ring-girder which provides the box structure with a peripheral side restraint and the flat rim of the lower end plate being supported by a flat circular flange formed on a shell element which is rigidly fixed to the reactor vessel. Apart from this main characteristic feature, a reactor diagrid as constructed in accordance with the invention has further accessory features which are preferably to be considered in combination but could be considered separately if necessary and relate in particular to the following points: The flat rim of the lower end plate and the parallel flange of the upper end plate are braced by small vertical stiffening columns. The ring-girder is provided with a top horizontal bearing surface of circular shape having substantially the same radius as the flange of the upper end plate, said flange being applied against said bearing surface by means of coaxial circular grooves, the interengagement of which forms a labyrinth seal against the coolant sodium. Positioning of the flange of the upper end plate with respect to the circular bearing surface of the ring-girder is carried out by means of studs having the shape of sectors which are carried by the flange and engaged in recesses formed in the bearing surface or conversely. The shell element which is rigidly fixed to the reactor vessel has a conical shape and is placed in the line of extension of the lower domical end plate of the box structure. A flexible seal is interposed between a shouldered portion of the flat circular flange of the shell element and the flat rim of the lower end plate of the box structure. The ring-girder is constituted by a hollow metallic torus which is open laterally towards the box structure, said torus being such as to constitute a manifold for supplying the diagrid with the coolant sodium.