Patent Number: 052271269
Section: summary

The invention relates to an improved internal structure of a fast neutron nuclear reactor. Fast neutron nuclear reactors of the integrated type comprise a main vessel enclosing the core of the reactor and an internal structure which are immersed in a liquid metal, usually constituted by sodium, for cooling the core. Intermediate heat exchangers and pumps for circulating the liquid metal cooling the core are also disposed inside the main vessel which is closed in its upper part by a very thick slab comprising throughway passages through which the exchangers and the pumps are introduced. The internal structure of the reactor disposed in the main vessel comprises in particular an inner vessel defining a zone of the main vessel receiving the hot liquid metal leaving the core and a zone receiving the cooled liquid metal issuing from the intermediate heat exchangers. The inner vessel comprises generally an assembly of shells coaxial with the main vessel in its upper part and a wall having a generally toroidal shape termed a step shaped area, in its lower part. The heat exchangers and the pumps extend through the step shaped area so that the lower inlet part of the pumps is immersed in the cooled sodium and the intermediate exchangers comprise openings on each side of the step shaped area for ensuring respectively the inlet and the outlet of the liquid metal cooling the core, termed primary fluid, circulating inside the heat exchanger. The primary fluid circulating inside the heat exchanger is cooled in thermal contact with a secondary fluid generally constituted by a liquid metal such as sodium. The secondary fulid heated by the primary fluid is circulated inside the intermediate heat exchangers and in a secondary circuit outside the main vessel of the reactor on which steam generators are disposed. Internal vessels of fast neutron neuclear reactors are known which have a step shaped area constituted by a single wall of toroidal shape whose meridian may have for example the shape of an ogive. In a reactor of known type at present in use, the plating resting on the bottom of the vessel supports the bed on which are fixed the fuel assemblies of the core and a false bed disposed around the bed on which rest the elements providing the lateral neutronic protection of the reactor. The lower part of the step shaped area of the inner vessel is fixed to the plating and the bed is supplied with hot liquid metal coming from the pumps, through piping extending through the plating in radial directions. Sealing devices are placed between the supply piping and the plating and between the bed and the support of the bed. The fact that the supply piping of the bed must extend through the plating, or possibly through the lower part of the inner vessel, complicates the design of the reactor and requires use of sealing devices such as bellows in the region of the passages of the piping to permit maintaining the pressure difference between the part of the vessel receiving the hot liquid sodium, or hot collector, and the part of the vessel receiving the cooled liquid, or cold collector. The lateral size of the sealing devices and of an intermediate support structure such as a bed support renders the design of the primary circuit of the reactor more difficult and results in radial dimensions of the elements of the reactor, and in particular of the main vessel, which may be large. The sealing devices which must be provided between the bed and the support element of the inner vessel such as the plating, for conducting the leakage flow of the primary fluid necessary for the cooling of the main vessel, may have a complex structure. Further, zones are formed around the bed in which the primary sodium is practically stagnant or flows at a low rate; the existence of these zones results in complex thermal stresses on the structures of the reactor during the transitional operating periods of the reactor. A first solution has been envisaged and disclosed in particular in the document FR-A-2,558,635. This solution consist in using a complex structure performing the functions of both the bed and the plating. However, it is still necessary in the case of such a structure to provide a lateral connection and a sealed passage for the primary sodium supply piping of the core. Further, the design of such a integrated structure which is heavy and hyperstatic may be complex; the hyperstatic nature of the structure renders it incapable of following in a satisfactory manner the thermal transitional periods in the course of the operation of the reactor. Lastly, the design of an integrated structure presents drawbacks when assembling or adjusting the reactor unit. An object of the invention is therefore to provide an internal structure of a fast neutron neuclear reactor comprising a main vessel enclosing the core of the reactor and the internal structure which are immersed in a liquid metal for cooling the core, the internal structure comprising an internal vessel defining a zone of the main vessel receiving the hot liquid metal issuing from the core and a zone receiving the cooled liquid metal and comprising a single wall or step shaped area of substantially toroidal shape in its lower part, a support element for the assemblies of the core and for feeding and distributing the liquid cooling metal in the core, termed bed, and a support element for the bed, or plating, resting on the bottom of the main vessel, said internal structure of being of simple design and avoiding the drawbacks of the aforementioned devices of the prior art. For this purpose, this step shaped area of the internal vessel is directly fixed by welding in its lower part to the upper part of the bed and the bed is connected to means for supplying cooling liquid metal in a zone located at its periphery and outside the internal vessel and the plating and rests on the plating through the medium of sliding supports one of which is located on the periphery of the bed and provides a cooling metal seal between the bed and the plating. In order to explain the invention, there will now be described, by way of a non-limitative example with reference to the accompanying drawings, an embodiment of an internal structure of a fast neutron nuclear reactor cooled by liquid sodium according the the invention.