Patent Number: 054024579
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International application Ser. No. PCT/DE92/00433, filed May 27, 1992. BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a grid structure for an elongate fuel assembly with rods disposed parallel and next to one another, between which a stream of coolant is conducted through the fuel assembly. The invention also relates to such a fuel assembly with a spacer and/or a mixing grid with such a grid structure. Published European Application No. 0 364 623 which discloses such a fuel assembly for a pressurized water reactor and German Published, Non-Prosecuted Application DE 15 64 697 A1 which uses spacers with mixing vanes, are described below in the description of the drawings. It has also already been proposed to place such grid structures with mixing vanes even in planes of the pressurized water reactor which in fact do not require any spacer for reasons of stability, in order to reinforce the lateral mixing-through of the coolant and to permit a radial temperature compensation. In such a device, the webs, in the case of spacers have holding assemblies (for example in the form of dimples and springs) for fixing the lateral position of the fuel rods, if anything also include safety stops, in order to prevent the rods from striking against the mixing vanes and damaging them in the event of vibration. Such mixing vanes have also already been proposed in the case of boiling water reactors, although mixing-through over a number of fuel assemblies is not possible therein due to the fuel channels and their walls. In that case, however, a swirl produced in flow subchannels leads to vortices in the axial flow, which especially in the upper part of the fuel assembly, where the coolant is in the form of a liquid/vapor mixture, leads to a wetting of the fuel rods by liquid droplets contained in the mixture. As a result, the occurrence of hot spots is reduced, which particularly at high power outputs could lead to a drying out of the liquid film flowing along the fuel rods and to a worsening of the heat transfer ("dry out"). In that case, it is often necessary for the same flow resistance for the through-flowing coolant to occur in each case at all of the fuel assemblies of a reactor core, since differences in the flow resistance also cause corresponding differences in the flow distribution and cooling effect at the individual fuel assemblies. Thus, for example, if after an operating cycle some of the old fuel assemblies are replaced by fuel assemblies with corresponding fittings to improve the temperature distribution, the flow resistance with respect to the neighboring fuel assemblies is worsened specifically because of the grid structures being employed, and consequently the coolant throughput is reduced in the "improved" fuel assemblies. That often has the effect of compensating or overcompensating for the success which is to be achieved. Such a troublesome pressure loss on one hand occurs at a constriction of the flow cross section and an adjoining widening to the original size, which are caused by the webs and their crossing points as well as the spacing assemblies and mixing vanes disposed thereon (compression and expansion). On the other hand, the vortices and turbulences produced by the mixing vanes also result in further pressure losses. Therefore, it has already been proposed to compensate for such pressure loss by reducing the fuel rod diameter, for example from 9.5 mm to 9.14 mm. In the case of fuel rod production that means that machines, tools and storage spaces have to be additionally set up for the new type of fuel rod. However, due to the reduction of the fuel rod diameter, the fuel content and the possible burn-up in the fuel assemblies is also reduced. It may, however, also be an object from the outset to distribute the envisaged fuel content over fuel rods which are as thin as possible, with a correspondingly greater number of them having to be chosen. In that case, however, the flow resistance increases with the number of fuel rods and requires a fluidically favorable construction of the fuel assembly internals. In U.S. Pat. No. 3,928,131 a spacer with longitudinal webs and with transverse webs which cross the longitudinal webs approximately perpendicularly is described. The crossing points all lie in one plane and the longitudinal and transverse webs are all of the same width at the crossing points. The webs in that case have dimples for supporting the fuel rods approximately in the center between two crossing points and on a level which lies approximately in the center between the upper edge and lower edge of the crossing points. In order to increase the flexibility of that grid structure, the transverse webs and longitudinal webs are significantly narrowed in the vicinity of the dimples, producing zigzag-shaped edges, on which the period of the zigzag-shaped course or progression corresponds to the distance between the center axes of neighboring fuel rods, that is the width of a flow subchannel. Published French Application No. 2 578 348 shows spacers on which upper edges of approximately perpendicularly crossing webs run partially in zigzag form with a period corresponding to the half-period of the zigzag always above a level which corresponds approximately to the center plane of the spacer and on which the crossing points also lie, and the corresponding lower edges always run below that level. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a fuel assembly with a grid structure between the rods, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which keeps flow resistance for the coolant flowing through a fuel assembly as small as possible. In order to achieve this object, the invention begins with the grid structure of the corresponding mixing grids and/or spacers. The flow resistance is so great there in particular because the webs extend as straight strips in a single plane perpendicular to the fuel rods, which is to say that the compression and decompression in all of the flow subchannels in each case take place in the same plane. The mixing vanes of all of the flow subchannels also lie in one plane, so that the pressure loss caused by the swirl also occurs in a single plane. Therefore, in order to achieve the object, the invention provides for the crossing points at which the longitudinal webs cross the transverse webs, or the mixing vanes, which are disposed in the flow subchannels, to be distributed over at least two crossing planes. With the objects of the invention in view, this produces the grid structure described above for an elongate fuel assembly of the type described initially above, in which the fuel rods are guided by the meshes of a grid and the grid is formed by webs which extend in zigzag form between an upper crossing plane (preferably an upper crossing plane perpendicular to the rods) and a lower crossing plane (preferably a lower crossing plane perpendicular to the rods), wherein the lateral surfaces thereof run between the rods and pass through one another at crossing points lying between the fuel rods, with a first group of crossing points lying in the upper crossing plane and a second group of crossing points lying in the lower crossing plane. This grid structure is already advantageous whenever dedicated mixing vanes are not considered necessary, for example in the case of each spacer which includes spacing means (for example springs and dimples) in order to fix the position of the rods laterally. For example, at the lower margin of the reactive zone, that is in the vicinity of the fuel assembly bottom end pieces, no mixing-through is necessary but instead only a fixing of the fuel rods. Thus, there a grid structure with spacing means but without mixing vanes is advantageous. With the objects of the invention in view, there is additionally provided a grid structure for the mixing vanes which, in the case of a mixing grid of a fuel assembly, lie at the corresponding crossing points of the grid. In the case of this grid structure, the fuel rods are thus likewise guided by meshes of a grid, with mixing vanes being disposed at the crossing points of the grid that lie between the fuel rods. In this case, in accordance with another feature of the invention, a crossing point of a first group of crossing points in each case has at least one mixing vane disposed in an upper crossing plane perpendicular to the rods, while each crossing point of a second group of crossing points has at least one mixing vane in a lower crossing plane perpendicular to the rods. In the case of these embodiments, the grid is preferably formed by webs having lateral surfaces which are aligned parallel to the rods and run in zigzag form only between two planes being perpendicular to the rods and lying one behind the other in the direction of flow. However, webs which are inclined slightly with respect to the rods in the individual flow subchannels may also be used. The crossing planes, that are disposed one behind the other, may also be inclined with respect to the rods or be made up of individual, differently inclined partial planes. It is not necessary with regard to the fundamental principle of a grid structure with low flow resistance for the webs of the grid to cross in the flow channels. Thus, for example, grid structures are known which include interconnected sleeves by which the rods of the fuel assembly are guided and are held by means of springs and dimples. If all of the sleeves lie with their welded joints and spacing means in a plane on these elements a flow resistance is also produced which can be reduced considerably if the elements are distributed over a plurality of planes lying one behind the other. Therefore, with the foregoing and other objects in view there is provided, in accordance with the invention, a grid structure for an elongate fuel assembly with rods which are disposed next to one another and parallel to one another and form interspaces therebetween which form flow subchannels for a stream of coolant approximately parallel to the rods, the rods being guided by the meshes of a grid and the grid being formed by webs which have an end edge facing away from the stream of coolant, an end edge facing the stream of coolant and lateral surfaces approximately parallel to the stream of coolant. Each of the two end edges extends approximately in zigzag form between upstream extreme points and downstream extreme points, with at least some of the downstream extreme points of the end edge facing the stream of coolant lying approximately equally far or further downstream than at least some of the upstream extreme points of the end edge facing away from the stream of coolant. The half-period of the zigzag in this case corresponds to the distance between neighboring fuel rod axes, which is to say the width of a flow subchannel. 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 fuel assembly with a grid structure between the rods, 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.