Patent Number: 053655575
Section: summary

The invention relates to a fuel assembly with a bundle or cluster of fuel rods, and webs between the fuel rods that extend approximately parallel to one another in a plane approximately perpendicular to the fuel rods and that have tabs on their upper edge. The invention also relates to fuel assemblies in which lengthwise webs extending approximately parallel to one another and crosswise webs connecting the lengthwise webs with one another, extend between the fuel rods. Webs with tabs are usually used in fuel assemblies for pressurized water reactors, in order to deflect a flow of coolant and liquid moderator, which is passed through interstices ("flow channels") between the fuel rods and along the rods into adjacent flow channels. As a result, mixing of the coolant flows is brought about in the various flow channels, and therefore the temperature distribution and the cooling of the rods are made uniform. In boiling water reactors, tabs have been proposed that are curved in such a way that they each cause a spin in the coolant flow in the various flow channels. The intent of such a spin is to attain separation of the liquid/steam mixture, particularly in the upper portion of the fuel assembly, where a considerable portion of the coolant is in the form of steam, with the liquid being spun outward, or in other words toward the hot fuel rods, while the steam in the middle of the flow channel is carried away at a high speed. Such spin-generating tabs may be provided on the webs of spacers, on which the fuel rods of the bundle or cluster are supported through protrusions and/or springs, or on the webs of grids, which serve merely as carriers of such spin generators, and therefore at most have protrusions of the webs that prevent contact between the tabs and the fuel rods, without effecting a fast contact of the fuel rods with the webs. It is accordingly an object of the invention to provide a fuel assembly of a nuclear reactor with a grid structure for producing spin, which achieves the maximum possible spin with the maximum possible effective separation of vapor bubbles and liquid droplets, without overly severely hindering the mixture of liquid and steam flowing along the fuel rods and which does so in a structurally simple way. With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly, comprising a bundle of fuel rods; approximately mutually parallel upright webs extending between the fuel rods in a plane approximately perpendicular to the fuel rods; the webs having upper edges and tabs on the upper edges, the tabs having ends, and the tabs being twisted continuously or increasingly more severely toward the ends relative or as compared to the webs. With the objects of the invention in view, there is also provided a fuel assembly, comprising a bundle of fuel rods defining flow channels and an axis approximately parallel to the fuel rods; lengthwise webs extending approximately parallel to one another between the fuel rods in a first plane extending approximately perpendicular to the fuel rods; crosswise webs being disposed in a second plane approximately parallel to the first plane and joining the lengthwise webs to one another between the fuel rods; and tabs being twisted about the axis and disposed in the flow channels, the tabs joining the lengthwise webs to the crosswise webs and merging steadily with the webs at a twisting of 90.degree. relative to the webs. The invention assumes that the least possible spin is to be generated in the flow of liquid, in order to minimize any pressure loss caused thereby. Nevertheless, in order to promote the separation of liquid and steam, high spin must be imposed on the flowing mixture of liquid and steam. Only a slight spin is therefore generated in the regions near the fuel rods, where the water collects, or in the regions of the webs, where the liquid in the coolant flow is still poorly separated from the steam, but this slight spin is increased more and more in the course of the tab, in order to separate the mixture of liquid and steam there. The invention therefore provides for the tabs to be initially only slightly twisted, but for them to be twisted continuously more and more markedly relative to the web toward their end. In accordance with another feature of the invention, in order to avoid "dead zones" downstream in terms of the flow in the twisted tabs, the variation in the flow direction at the transition from the webs to the tabs is made continuously and gently and not in the form of a sharp bend. Advantageously, the tabs are twisted so severely that they have tab parts which are at right angles to the webs. The ends of the tabs are accordingly twisted through 90 .degree., 270 .degree. and 350.degree. (in general, n.times.180.degree.-90.degree.) , for instance, relative to the web. These tab parts merge with web parts, which are welded on as separate parts or in particular are formed integrally onto the tabs, or in other words can be stamped out of a metal sheet together with the tabs and the web. The tab parts of the tabs o f adjacent webs may be welded together, in order to form crosswise webs for connecting the adjacent webs parallel to one another. This creates a grid structure, which intrinsically is formed only of stamped-out lengthwise webs that are disposed in one web plane, but these lengthwise webs are joined, in a second plane located above that plane, by crosswise webs that are formed of the afore-mentioned, welded-together tab parts. Thus, the lengthwise webs, on their side facing away from the flow direction, merge with the twisted tabs to become cross-wise webs. As a result, the number of sheet-metal parts to be stamped out is reduced, while the number of weld points remains practically the same as compared with a lattice-like grid that is manufactured in the prior art from lengthwise and crosswise webs that are inserted into one another and welded together. In accordance with a further feature of the invention, the twisting is made easier if the tabs taper continuously toward their end. However, this tapering is especially advantageous because as a result the broad parts of the tabs, which extend up to the vicinity of the fuel rods, protrude with only moderate twisting into the liquid collected there. In other words, there is little disturbance (little pressure loss) in the liquid flow. However, the more severely twisted ends of the tabs impose a more severe spin upon the steam flowing in the center of the flow channels, thereby providing the rest of the separation of the liquid. In accordance with an added feature of the invention, if this kind of grid structure is used as a spacer, then support elements are necessary to support the fuel rods. To that end, the part of a web extending between two fuel rods has two protrusions formed from the web part, on which protrusions one of the two fuel rods is supported, and a spring, on which the other fuel rod is supported. In accordance with a concomitant feature of the invention, the fuel rods, particularly in fuel rod assemblies for boiling water reactors, are seated in square holes in an imaginary grid, or in a spacer formed of crosswise and lengthwise webs in the holes of the spacer grid. The spin generators are then provided in flow channels formed by four adjacent fuel rods, or in other words at the intersections of the grid. With adjacent intersection points carrying tabs, the two tabs are advantageously twisted in mirror fashion relative to one another. This creates a checkerboard-like pattern of flows in the square grid, with the flows alternatingly rotating clockwise and counterclockwise. At the constrictions between two adjacent fuel rods, or in other words at the transitions between two flow channels, the tangential direction of the two spinning flows is therefore made to turn in the same direction, so that the two spinning flows do not slow one another down. 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 of a nuclear reactor with a grid structure for producing spin, 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.