Patent Number: 040381366
Section: summary

This invention relates to a structure for supporting the lateral neutron shield system of a fast reactor core. It is known that the core of a fast reactor which is cooled with liquid sodium is surrounded by a neutron shield system. The top and bottom shield are formed by elements which are introduced directly above and beneath the fuel assemblies which constitute the reactor core. The lateral shield of the reactor core is constituted by elements which have the same shape as the fuel assemblies and surround the reactor core. A better understanding of the problem to be solved will be obtained by referring to FIG. 1 of the accompanying drawings, in which is shown diagrammatically the lower portion of a sodium-cooled fast reactor. There can be seen in this figure the lower portion 2 of the metallic vessel which contains the complete reactor assembly, that is to say essentially the reactor core 4, the primary sodium and the primary pumps 5 and the primary heat exchangers 3. The reactor core 4 rests on a diagrid 6 in which the bottom end-fittings of the fuel elements constituting the core 4 are inserted. The diagrid also permits the flow of cold primary sodium within said fuel assemblies, said primary sodium being injected into the diagrid 6 through ducts such as the duct 8 which are connected to the outlets of the primary pumps 5. The diagrid rests on a support grid 10 which is rigidly fixed to the wall 2 of the reactor vessel by means of a frusto-conical shell-plate 12. The lateral neutron shield is generally designated by the reference numeral 14 and surrounds the reactor core. The neutron shield elements are, for example, round stainless steel members of hollow construction, the top portion of which is level with the top portion of the fuel elements constituting the reactor core 4. The bottom end-connectors of the lateral shield elements are inserted in a so-called "false grid" 16 or ring-shaped support structure which rests on the periphery of the diagrid 6. The false grid is higher than the diagrid and surrounds the reactor core 4, with the result that a peripheral side restraint is applied to the base of the core. The design function of the false grid 16 is therefore to ensure that the neutron-shielding elements 14 are rigidly maintained in position and that said elements can also be cooled by the liquid sodium. In addition, the false grid must afford peripheral side restraint for the reactor core. One known false grid arrangement consists in designing this latter in six sectors which are not joined together but connected to the diagrid by means of a system of keys. Each sector is formed by a bottom plate and a top plate which are pierced by bores at the bottom of the system of shield assemblies and separated by hollow spacer members placed at the level of said bores. The bottom end-connectors of the shield assemblies are intended to be fitted in the bores of said spacer members. Tie-rods which serve to connect the top and bottom plates ensure powerful clamping of these latter against the spacer members. Sodium coolant is supplied to the lateral shield assemblies through lateral holes which are formed in said spacer members and which must therefore occupy precise positions both angularly and in height. For this reason, after the spacer members have been machined to the required tolerances, they must accordingly be oriented on the false grid at the time of assembly. This result is relatively difficult to obtain and calls for the presence of positioning studs carried on the top face of the bottom plate. Moreover, by reason of the fact that the sectors are independent, the false grid does not permit of effective application of peripheral side restraint to the fuel assemblies of the reactor core. The precise aim of the present invention is to provide a support structure for the lateral shield system of a fast reactor core which overcomes the disadvantages mentioned in the foregoing by making it possible in particular to provide a peripheral side restraint for the reactor core, simple forms of construction and higher structural rigidity of the core as a whole and of its neutron shield systems. The structure for supporting the lateral neutron shield system of a fast reactor core is distinguished by the fact that the structure has the shape of a horizontal hexagonal ring whose internal contour coincides exactly with the external contour of the reactor core, the bottom face of said ring structure being intended to rest on the periphery of the core support structure, said ring structure being constituted by a plurality of layers of metallic plates, said layers being maintained in relative positional relation by means of clamping members, the metallic plates of one layer being angularly displaced with respect to the plates of adjacent layers, said ring structure being penetrated by vertical through-holes in which are fitted the bottom end-connectors of the elements constituting the lateral shield system. In a first embodiment, the layers of metallic plates are contiguous. In a second embodiment, two successive layers of metallic plates are separated by washers of small thickness. As a preferable feature, the members for clamping the layers together consist of tie-bolts.