Patent Number: 052271269
Section: description

FIG. 1 shows the main vessel 1 of a fast neutron nuclear reactor lined externally by a safety vessel 2 and enclosing the core 3 of the reactor constituted by fuel assemblies of prismatic shape disposed vertically and side by side. The fuel assemblies 4 are engaged by their lower part in a bed 5 which maintains the fuel assemblies in vertical position. The bed 5 rests on a plating 6 which rests on the bottom of the main vessel 1 of the reactor. The main vessel 1 of the reactor comprises a substantially cylindrical part and a curved convex bottom formed by part-spherical case portions welded together. The internal vessel 8 of the reactor is constituted in its upper part by an assembly of coaxial shells which have a common axis corresponding to the axis of the main vessel and define therebetween different annular containers for the primary liquid sodium cooling the main vessel. In the central part of the vessel defined by the internal vessel 8, the primary liquid sodium has an upper level 10 which may vary between certain limits as a function of the operating stages of the reactor. The lower part 9 of the internal vessel constituting the step shaped area is made in the form of a single wall of toroidal shape whose meridian shown in FIG. 1 has the shape of an ogive. The internal vessel 8 defines, inside the main vessel 1, a first zone 12 located inside the internal vessel 8 and enclosing the core 3 of the reactor, and a second zone 13 located outside the internal vessel 8 below the step shaped area 9. The first zone 12 constituting the hot collector of the reactor contains the primary liquid sodium, heated in contact with the core 3, up to the upper level 10. The step shaped area 9 of the internal vessel comprises openings for the passage of the primary pumps 14 and the intermediate exchangers 15 in the region of which the wall of the step shaped area is connected to shells providing sealed passages for the pumps and the exchangers and isolation of the liquid sodium contained in the hot collector 12 from the liquid sodium contained in the cold collector 13. The vessel 1 of the reactor is closed in its upper part by a very thick slab 11 provided with throughway openings for the passage of the pumps 14 and the heat exchangers 15. In the case of a 1500 MW fast neutron nuclear reactor, the primary circuit of the reactor permitting the circulation of the primary cooling liquid sodium of the core comprises three primary pumps 14 spaced 120.degree. apart around the axis of the vessel, and six intermediate heat exchangers interposed between the primary pumps. Each of the primary pumps 14 is laterally connected by a discharge piping 18 directly to the bed 15 on which the assemblies of the core 3 rest. The primary liquid sodium, taken by the pumps 14 from the cold collector 13, is discharged inside the bed 5 which ensures the distribution of the sodium cooling the core in each of the assemblies 4. The sodium injected at the base of the core passes through the assemblies upwardly in the vertical direction and issues from the core in the hot collector 12. The intermediate exchangers 15 comprise an upper opening 19 through which the hot sodium enters the heat exchanger and a lower opening 20 through which the sodium cooled inside the heat exchanger enters the cold collector 13. There will now be described with reference to FIG. 2 the internal structure of the nuclear reactor comprising the internal vessel 8, the bed 5 and the plating 6. The plating 6 and the bed 5 are constituted by mecanowelded structures whose strength is so calculated as to withstand the forces applied thereto and in particular the weight of the core 3. The plating 6 resting on the bottom of the main vessel 1 has a structure which is substantially identical to the structure of platings of the prior art. The bed 5, constructed in the form of a cylindrical box structure of small height, rests on the upper surface of the plating 6 through the medium of a circular bearing flange 22 downwardly extending the outer lateral wall of the cylindrical bed 5. Spherical bearing means 21 are also interposed between the lower surface of the bed 5 and the upper surface of the plating 6. As will be explained hereinafter, the circular flange 22 and the spherical bearing means 21 constitute sliding supports allowing a certain displacement of the structure of the reactor in the event of earthquakes or thermal transitional periods accompanied by expansions or retractions of certain parts of the structure. In its central part, the upper wall of the plating 6 comprises an opening in the region of which is fixed a flange 45 and an adjusting packing piece co-operating with a pivot 25 connected to the lower part of the bed 5. When the bed 5 is in position on the plating 6, the pivot 25 engaged in the flange 45 prevents lateral displacements between the bed 5 and the plating 6. A key 27 preventing rotation is placed at the periphery of the bed so as to avoid any rotation of the bed with respect to the plating about an axis coincident with the axis of the vessel. The lower annular flange 22 of the bed 5 ensures a certain sealing with respect to the cooling liquid sodium of the reactor. It rests on an adjusting packing piece 35 for achieving the correct attitude of the bed when mounting. The major part of the flow of liquid sodium discharged into the bed 5 through the piping 18 (arrow 28) enters through the base of the core for cooling the fuel assemblies (arrow 29). A small part of this flow constituting a leakage flow is directed downwardly toward the plating 6 (arrow 30). Owing to the presence of the annual flange 22 providing a seal in the contact between the bed and the plating at the periphery of the bed, the flow 30 passes through the plating and reaches the vicinity of the bottom of the vessel where this flow is recovered and flows in contact with the inner wall of the main vessel 1 and cools this wall. The step shaped area 9 constituting the lower portion of the internal vessel 8 is directly welded, in the region of its lower end, to the peripheral portion of the bed 5. The assemblies constituting the lateral neutronic protection 31 of the reactor disposed around the core 3 formed by fuel assemblies rest directly on the upper part of the bed 5. In fact, the absorbent assemblies of the lateral neutronic protection 31, as well as the fuel assemblies of the core 3, each comprise a foot which is engaged in a pillar of corresponding shape fixed in the box structure of the bed 5. FIG. 3 shows a part of the bed 5 made in the form of a cylindrical box structure comprising a lower sole 32 and an upper sole of circular shape, which are parallel to each other and fixed at their periphery to a cylindrical shell constituting the outer wall of the bed. The lower portion of the outer shell 34 constitutes the annular flange 22 supporting the bed resting on a packing piece 35 which adjusts the position of the bed and bears on the upper sole 36 of the plating 6. There is also shown a pillar 37 for fixing an assembly constituted by a tubular element fixed in its lower part to the lower sole 32 of the bed and, in its upper part, to the upper sole 33 in the region of an opening extending through the upper sole of a cavity machined in the lower sole. The pillars 37 of the bed 5 receiving the fuel assemblies comprise openings 38 in their lateral wall for the passage of the sodium cooling the fuel assemblies. The pillars receiving the assemblies providing the lateral neutronic protection of the reactor do not include said lateral openings 38. The lower end of the step shaped area 9 of the internal vessel 8 is fixed by welding 39 to an attendant part of the outer shell 34 of the bed projecting above the upper sole 33. The spherical bearing means 21 through which the bed bears against the plating constitute localized bearing devices arranged over the area of the lower sole 32 and comprise a spherical male part connected to the lower surface of the sole 32 of the bed and a spherical female part connected to the upper sole 36 of the plating 6. The annular flange 22 and the packing piece 35 on one hand and the spherical bearing devices 21 on the other constitute slidable bearing means whereby it is possible to absorb forces exerted in the transverse direction between the bed and the plating in the event of an earthquake and the expansions occurring in the course of transitional operation periods of the reactor. FIG. 4 is a partial view of the outer portion of the bed 5. The outer shell 34 of the bed 5 comprises an upper portion constituting an attendant welding portion on which is fixed, by a weld joint 39, the lower end of the step shaped area 9. The shell 34 also includes two portions of annular shape which project inwardly and are machined so as to constitute with corresponding machinings provided on the peripheral surface of the circular soles 32 and 33, welding chamfers 39 and 40 which are filled with welding metal upon the assembly of the box structure constituting the bed 5. A pillar 37 is also shown which supports an assembly of the lateral neutronic protection and is fixed inside an opening extending through the upper sole 33 of the bed, by a weld 41 achieved by laser welding and engaged in a blind opening machined in the lower sole 32 of the bed. The annular flange 22 constituting the lower portion of the outer shell 34 of the bed and including a hard layer on its lower surface rests on the upper sole 36 of the plating through the medium of an adjusting packing piece 35 covered with a hard layer achieved by aluminization or formed by chromium nitride. This packing piece lies in an annular cavity machined in the upper sole of the plating. In this way it is possible for the bed to slide to a limited extent relative to the plating with a reduced wear of the parts in contact with one another. Further, the contacting surfaces of the annular flange 22 and the upper sole 36 of the plating are perfectly planar thereby providing a fluidtight closure of the space between the bed and the upper surface of the plating which is consequently isolated from the cold collector 13. This space constitutes a part of the leakage collector receiving the liquid sodium cooling the inner wall of the main vessel. Nozzles, such as the nozzle 43, provided for connecting piping 18 supplying cooling liquid sodium are fixed by welding with a welding metal in the region of openings extending through the outer shell 34 of the bed 5. As is shown in FIG. 2, the pipings 18 are directly connected to the outer shell of the bed 5 in zones located outside the inner vessel 8 and the plating 6. In this way, the use of deformable sealed passage devices such as bellows is avoided. FIG. 5 shows the central portion of the bed comprising the pivot 25 connected to the lower sole 32. The upper sole 36 of the plating 6 has an opening in its central portion along the edge of which is fixed a flange 45 whose inner bore receives the pivot 25 with interposition of an adjusting packing piece 46. The adjusting packing piece 46 is machined when mounting the internal structure of the reactor. The pivot 25 has a diameter which is defined as a function of earthquake forces which are liable to be exerted on the structure of the reactor and capable of bringing about a lateral displacement between the plating and the bed, and as a function of the spacing between the lower surface of the sole 32 of the bed and the upper surface of the sole 36 of the plating. This spacing may be fixed at a small value and is usually around 80 mm. The central pivot resists not only the forces occurring during an earthquake but also the forces due to thermal dissymmetries inside the vessel of the reactor. The bed may be made from an outer forged shell on which the nozzles for the piping are fixed by welding, and soles formed by circular planar plates fixed by welding to the outer shell. The outer shell may be formed by a weld-less single forged element or by a plurality of forged shell sections welded together. The outer shell could also be made from a thick sheet or plate. Note that in the case of the structure according to the invention, the step shaped area of the internal vessel is directly fixed to the bed through which the core is supplied with cooling fluid. In the case of devices of the prior art, the step shaped area of the internal vessel was usually fixed to a structure supporting the bed, such as plating. Further, the seal between the bed and the plating is achieved by a sliding annular peripheral bearing element and not by flexible sealing elements. Greater strength and greater reliability of the device according to the invention result. The assemblies of the lateral neutronic protection of the reactor are supported by the bed itself in a manner identical to the fuel assemblies constituting the core and not by an additional structure such as a false bed. The device according to the invention has in particular the advantage of being less complex and having fewer component parts and a simpler shape than the devices of the prior art. For a higher reactor power, the radial size of the structure according to the invention is markedly reduced so that there is a substantial reduction in the diameter of the internal vessel and of the main vessel of the reactor. The use of deformable or flexible sealing means such as bellows or strips is also avoided so that the reliability of the device is improved and the time required for its assembly distinctly reduced. The formation of zones in which the cooling liquid metal is stagnant or flows at a low rate is also avoided. There is an improved thermal behavior of the structure during the transitional periods of operation of the reactor. The various parts of the structure according to the invention are more readily accessible for inspection or repairs. The amount of steel required for the construction according to the invention is distinctly less than the corresponding amount of the devices of the prior art. The saving in the amount of steel required for the construction of the internal structure of the reactor is of great importance in particular owing to the fact that the structure must be made from stainless steel. The scope of the invention is not intended to be limited to the embodiment described. Thus there may be envisaged a structure comprising a bed having a shape different from that described and bearing and retaining means between the bed and the plating different from those described. The step shaped area of the internal vessel may have a shape different from that described. However, this step shaped area must be formed by a single wall which may be connected by welding to the upper part of the bed. The structure according to the invention may be employed in the case of any fast neutron nuclear reactor of integrated type cooled by a liquid metal.