Patent Number: 06320925&
Section: summary

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a spacer for a fuel assembly of a nuclear power station, in particular for a fuel assembly of a light water reactor. Spacers are used for fixing fuel rods in a fuel assembly. A spacer forms a matrix of intersecting webs and a base area covered by the spacer corresponds essentially to the cross sectional area of the fuel assembly. Each web has an assembly gap which is disposed at an intersection location with an intersecting web and receives the intersecting web. The webs which are guided in the assembly gaps and subsequently fixed, for example by welding, form cells having an essentially rectangular or square base area. The fuel rods, as well as guide tubes in the case of pressurized water fuel assemblies and possibly water rods in the case of boiling water fuel assemblies, project through the cells formed by the spacer and are held there. In other words, they are fixed in their position relative to the center axis of the fuel assembly. In certain circumstances, under the operating conditions in the core of the nuclear reactor, the spacers of a fuel assembly may undergo longitudinal expansion which may lead to an increase in the external dimensions of the spacers and consequently of the fuel assembly. In an extreme case, the result of the increase in the external dimensions may be that a fuel rod bundle formed by the spacer can no longer be removed from a fuel assembly box, for example in the case of a fuel assembly for a boiling water reactor. In the case of fuel assemblies of pressurized water reactors, the longitudinal expansion of the spacers is unusually high. The increase in the external dimensions may cause complications with the adjacent fuel assemblies during servicing work and during loading and unloading of the reactor core. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a spacer for a fuel assembly of a nuclear power station, which overcomes the heretofore-mentioned disadvantages of the heretofore-known devices of this general type and which undergoes insignificant longitudinal expansion during an operating period. With the foregoing and other objects in view there is provided, in accordance with the invention, a spacer for a fuel assembly of a nuclear power station, comprising webs disposed in a grid defining intersection locations, each of the webs having a wall thickness and having an assembly gap receiving an intersecting web at one of the intersection locations; the assembly gap in each of the webs having regions through which parts of the other of the webs pass in the intersection location of two webs, at least two of the regions having different widths; and the assembly gap in each of the webs having a total length formed by the regions through which the parts of the other of the webs pass in the intersection location of two webs, at most a fraction of the total length having a width substantially corresponding to the wall thickness of the other of the webs. Therefore, the assembly gap has a width, at most over a fraction of its total length, which corresponds essentially to the wall thickness of the intersecting web, but in a remaining region is wider than the wall thickness of the intersecting web. The invention proceeds from the knowledge that, under normal operating conditions, the longitudinal expansion of a spacer may be caused by corrosion of the spacer. A corrosion layer may form on the web wall in the assembly gap, particularly at assembly gaps of a spacer web formed of a zirconium alloy, for example, a Zircalloy plate. In order to achieve a high accuracy of fit of the spacer composed of the webs, the assembly gaps have heretofore been dimensioned in such a way that their width corresponds essentially to the wall thickness of the intersecting web. With that structure of the spacer, there is the risk that the corrosion layers, which grow toward one another from the edges of the assembly gaps and from the surface of the inserted web, will meet one another before the end of the period of use. The corrosion layers then exert a solid pressure on the respective web which may lead to a lengthening of the web. Since each web has a multiplicity of assembly gaps with corrosion-endangered regions, the resulting total longitudinal expansion of the webs may become so great that the change in the external dimension of the spacer exceeds a critical value. Moreover, a varying longitudinal expansion of different webs may lead to the warping of individual webs and/or of parts of the spacer, thus adversely influencing the flow properties of the spacer. The invention proceeds, then, from the notion of reducing the solid pressure exerted on a web by the corrosion layers. According to the invention this is achieved, on one hand, in such a way that the assembly gaps of two webs which are provided at an intersection location with another web, have a width only over a fraction of their total length, through which parts of a web pass in each case, that corresponds to the wall thickness of the intersecting web. By virtue of the reduced bearing surface between the edges of the assembly gaps and the intersecting webs, the solid pressure on the web decreases, thus resulting in a considerable reduction in the undesirable longitudinal expansion of the web. The length of the regions in which the width of the assembly gap corresponds essentially to the wall thickness of the intersecting web is selected in such a way that the spacer matrix has a strength which is sufficient for subsequent machining steps, such as, for example, the welding of the webs at the intersection locations, and for the loads which occur under operating conditions. This affords the advantage of ensuring that, after the webs have been welded together, the assembly gaps do not have to be widened in an additional operation, for example by pickling or corroding, in order to reduce the solid pressure on the web which occurs as a result of corrosion. In accordance with another feature of the invention, each web has a recess on at least one side of each assembly gap, in the region of the narrowest cross section of the latter, wherein the recess is adjacent the assembly gap. This affords the advantage of permitting web deformation caused by corrosion in the region of the narrowest cross section of the assembly gap to be absorbed by the recess. The solid pressure on the remaining part of the web can thereby be further reduced. If the web is to have only minimal longitudinal expansion due to corrosion, without an additional recess, the width of the assembly gap is dimensioned, virtually over its entire length, to be greater than the wall thickness of the intersecting web. In order to acquire the necessary stability of the spacer matrix formed from the webs, the assembly gap has only a few (but at least three) support locations along its axis or principal extent, for example bearing points or bearing regions, at which the assembly gap touches the intersecting web. The support locations are disposed on both sides of each assembly gap, in such a way that the intersecting web is supported in each case on only one side over a sufficiently small area. The advantage of this is that a solid pressure may build up at the few support locations during the operating period and could partially deform the intersecting web transversely to its direction of principal extent, and consequently only some of the solid pressure contributes to the longitudinal expansion of the web. In accordance with a further feature of the invention, a further reduction in the solid pressure leading to longitudinal expansion is achieved through the use of apertures in the middle of the web. Preferably, each web has at least one aperture disposed on an assembly axis of each assembly gap. Advantageously, each assembly gap opens in each case into at least one of the apertures. Thus, corrosion-endangered locations in the web are cut out and, moreover, it becomes possible to have a sufficient cooling water stream for the regions of the spacer which are located in the vicinity of an intersection location, so that corrosion is retarded there. In accordance with an added feature of the invention, there are provided firm connections between the webs, through the use of which a stabilization of the spacer is achieved. Lengthening of the web is prevented by such a firm connection of the intersecting webs at a connection location, preferably through the use of a metallurgical connection, for example through the use of a weld spot in the intersection region. Moreover, metallurgical connections often undergo a lesser degree of corrosion than the untreated material. Such metallurgical connections may, for example, fill those regions in the assembly gap which would be particularly in danger of corrosion. In accordance with an additional feature of the invention, particularly in pressurized water reactors, it has proved particularly appropriate to have spacers with webs which contain at least two web plates bearing against one another at least in the region of an intersection location with an intersecting web. The common or combined thickness of these web plates at the intersection location corresponds to the wall thickness of the web. In the regions outside an intersection location, the web plates may be deformed and/or bent relative to one another so that, for example, flow ducts are obtained between them. In the case of spacers in fuel assemblies for pressurized water reactors, there is therefore a series of further locations, in addition to the assembly gaps, at which the webs or the web plates touch one another. There are, in part, regions in which the webs, in particular the web plates, bear against one another over a large area. For the reasons already explained with regard to the assembly gaps, these regions are likewise preferred regions for corrosion and/or for the formation of corrosion lenses and may cause deformation of the spacers. In accordance with yet another feature of the invention, consequently, in the case of the webs formed of a plurality of web plates, it is advantageous to connect the web plates which are located next to one another metallurgically to one another, in particular in the region of the intersection location of the webs. In accordance with yet a further feature of the invention, the web plates located next to one another are connected, preferably metallurgically connected, to the intersecting web at least at one connection location. The web plates may be connected to one another over a large area at least at one further connection location, preferably in a region in which they bear against one another. The latter is achieved, in particular, by welding at the locations at which the web plates bear against one another over a large area. This may be carried out, for example, when the webs are assembled to form the gridlike spacer. The web plates have expediently already been previously assembled to form webs. In accordance with yet an added feature of the invention, corrosion-endangered locations are avoided if each web is firmly connected, preferably metallurgically connected, to the other web virtually along the entire length of that part of each assembly gap having a width which corresponds essentially to the wall thickness of the other web. This results in long welds which are applied preferably along the narrower region or narrower regions of the assembly gaps by laser beam welding. With regard to the spatial conditions in the spacer, a longer run of the weld seam along the assembly gaps is possible as a result of laser beam welding, as compared with electron beam welding. Moreover, regions in danger of corrosion are those at which the webs and/or the web plates are bent. This relates, in particular, to regions with small bending radii. The cause of this is possibly the reduced cooling water stream in such regions, as compared with regions which have no bending radii. At those endangered locations and/or regions as well, for the reasons mentioned above, corrosion layers may exert a solid pressure on the respective web that may lead to a lengthening of the web. Making recesses, apertures or firm connections at such corrosion-endangered locations effectively prevents the longitudinal expansion of spacers, since a sufficient cooling water stream is ensured once again. In particular, such apertures may also be produced through the use of a bead or a groove. This preferably also relates to further apertures or recesses in the webs and/or web plates along an assembly axis of each assembly gap. In particular, this also relates to further connection locations which connect the web plates to one another, in particular metallurgically to one another, over a large area. In accordance with yet an additional feature of the invention, that part of a web which passes through an assembly gap of another web carries two elevations, between which the other web is held. These elevations fix the intersecting webs before they are firmly connected to one another, for example through the use of weld spots or weld seams. With the objects of the invention in view, there is also provided a spacer for a fuel assembly of a nuclear power station, comprising webs disposed in a grid defining intersection locations, each of the webs having an assembly gap receiving an intersecting web at one of the intersection locations; each assembly gap of each web including one region having a metallurgical connection of two webs in the intersection location of two webs; and each assembly gap of each web including another region in the intersection location of two webs through which part of the other web passes, the other region having a width sufficient to prevent two webs from touching. With the objects of the invention in view, there is additionally provided a spacer for a fuel assembly of a nuclear power station, comprising two mutually parallel first outer strips; two mutually parallel second outer strips perpendicularly to the first outer strips; first webs standing on edge, parallel to the first outer strips and having ends each engaging into and fixed to a respective one of the outer strips; second webs standing on edge, parallel to the second outer strips, intersecting the first webs and having ends each engaging into and fixed to a respective one of the outer strips; the first webs each having an upper edge and a lower edge, the upper edges running from one of the outer strips to another of the outer strips and each having an assembly gap at an intersection location with a respective one of the second webs, the assembly gap directed toward the lower edge and receiving part of an intersecting one of the second webs; the second webs each having an upper edge and a lower edge, the lower edges each having an assembly gap at an intersection location with a respective one of the first webs, the assembly gap directed toward the upper edge and receiving part of an intersecting one of the first webs; each of the assembly gaps closed at one of the edges of each of the webs by a metallurgical connection of one web with an intersecting web; and each of the assembly gaps having a region through which part of the intersecting web passes, the region wide enough to prevent two of the webs from touching. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a spacer for a fuel assembly of a nuclear power station, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.