Patent Number: 053274725
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of International Application PCT/DE91/00133, filed Feb. 20, 1991. The invention relates to a boiling water reactor, having a pressure vessel, a steam turbine connected to the pressure vessel, at least one nuclear reactor fuel assembly and controllable absorber elements disposed in a reactor core in the pressure vessel, the fuel assembly including an elongated fuel assembly case with mutually parallel side walls laterally closing off the fuel assembly, an inlet end for liquid coolant and an outlet end for a liquid/steam mixture of the coolant, and fuel rods containing nuclear fuel and being disposed side by side and parallel to the case walls, the absorber elements being disposed outside the fuel assembly, the fuel rods being disposed in lengthwise rows and crosswise rows intersecting the lengthwise rows in such a manner that they reach through meshes of a grid extending practically at right angles to the side walls, and each four fuel rods disposed in two adjacent lengthwise rows and two adjacent crosswise rows forming one flow subchannel for the coolant, being parallel to the side walls. The invention also relates to a fuel assembly for the boiling water reactor. The structures of fuel assemblies for boiling water reactors described above are conventional and are used in the invention as well. They are therefore part of the fuel assembly and the reactor of the invention. In the boiling water reactor core, the liquid coolant enters the fuel assembly case at a lower or inlet end and leaves it as a liquid/steam mixture at an upper or outlet end of the case. Since an increasing proportion of the coolant, which simultaneously acts as a moderator, is in the form of steam in the upper part of the fuel assembly, the vertical flow in the reactor core must be channelled in such a way that sufficient moderator is present there as well. To this end, tubes for the liquid coolant (water tubes) can be used, which replace one or more fuel rods. Such a water tube may also have a larger cross section than an individual fuel rod, so that it extends over the cross section of a plurality of meshes in the grid, or it may have some other cross section (such as cross-shaped). The case walls also serve to channel the vertical flow and in particular must prevent coolant vapor from accumulating on the absorber elements because of an uncontrolled horizontal flow, which would interfere with the proper course and control of the reactions in the reactor That kind of horizontal flow is even considered desirable for pressurized water reactors, because it brings about a temperature equalization between hotter and cooler regions of the fuel assembly and improves cooling. Such a horizontal flow, which in a boiling water fuel assembly is largely suppressed by the case walls and in any event is kept away from the absorber elements between the fuel assemblies, can therefore be generated in a pressurized water fuel assembly by suitable baffles or vanes in the flow subchannels. Such vanes may be disposed at the ribs of spacers, which are necessary in any event to fix the mutual spacing of the fuel rods, or on the ribs of their own mixing grids, as is shown, for instance, in FIGS. 1 and 2 of German Published, Non-Prosecuted Application DE-OS 15 64 697. That produces a circular flow around the individual fuel rods by means of which all of the flow subchannels bordering a fuel rod communicate with one another. The superimposition of the individual circular flows then leads to horizontal flows, which pass transversely through the entire fuel assembly. The same circular flows also develop in the grid structures of pressurized water reactors as is shown in FIGS. 1, 8 and 9 of U.S. Pat. No. 4,224,107. In German Published, Non-Prosecuted Application DE--OS 2 157 742, it is proposed that four vanes be disposed in propeller-like fashion in each flow subchannel of a pressurized away from the coolant flow, tapering and protruding obliquely from the wall of the water tube into the interstices between the adjacent fuel rods. Such baffles deflect the liquid film on the water tube wall and mix it turbulently with the hot liquid/steam mixture of the coolant flowing between the fuel rods. However, that kind of turbulence increases the pressure loss in the vertical flow and can therefore largely cancel out or even overcome the advantages of improved cooling that are sought. In contrast, in pressurized water reactors, the creation of a mixture of liquid and steam is prevented by a high pressure in the reactor core, so that even at the hot outlet end of the fuel assembly, there is sufficient liquid moderator available, and dryout of the fuel rods need not be feared. A fuel assembly case, which would merely represent unnecessary consumption of material and addition neutron absorption, is not present, and the absorber elements are distributed as uniformly as possible over the cross section of the fuel assembly. As a result, completely different flow conditions prevail, In particular, a horizontal flow between adjacent flow subchannels of a fuel assembly and between adjacent fuel assemblies themselves can form. The absorber elements are disposed outside the fuel assemblies, in interstices between the individual fuel assemblies. Normally, a film of water is located on the surface of the fuel rods, which carries heat produced in the nuclear-heated fuel rods, leads to evaporation of the liquid coolant in the upper part of the fuel assembly, and transfers heat into the interior of the interstices between adjacent fuel rods. The interstices form flow subchannels for the vertical flow in the reactor core. If the output of the fuel assembly is excessive, this water film can tear away or dry out. A boiling transition or dryout of the rods is then said to have occurred. Such an occurrence worsens the heat transfer from the rods to the coolant, and undesirable local overheating of the fuel rods occurs. On the other hand, in the upper part of the fuel assembly as well, there is still a considerable proportion of the coolant in the form of liquid droplets and a liquid film creeping along the case walls and along a water tube if applicable, which could be utilized to improve cooling in the event of high outputs. U.S. Pat. No. 4,749,543 has therefore proposed that grooves ("flow trippers") be machined on the insides of the case walls, at which grooves a film of liquid is rendered turbulent. According to German Petty Patent DE-G 88 02 565.9, in a boiling water reactor with a central water tube, baffles are attached to the side of a spacer facing water reactor, in order to produce turbulence in the flow subchannels and to produce a greater heat transfer at the fuel rods. In U.S. Pat. No. 4,725,403, for the same purpose, one additional sheath is mounted on the spacer ribs, which has two vanes on its side facing away from the coolant flow that are inclined toward one another in such a way that a swirl is imposed upon the pressurized water reactor coolant flowing through the additional sheath. According to German Published, Non-Prosecuted Application DE 35 19 421 A1, the additional sheath itself is also wound spirally within itself, and it has four such vanes, which are bent outward in the direction of the desired swirl. However, such vanes of pressurized water reactors represent a considerable flow resistance and therefore cause a pressure drop, worsening the utilization of the reactor, so that the desired improvement practically does not ensue. Therefore, if such vanes are used at all in pressurized water reactors to produce a horizontal flow, particular care must be taken to ensure that the vertical flow turbulence, which is unavoidable at such vanes, and the attendant pressure loss, are kept as small as possible. According to Published European Application No. 0 291 748, corresponding to U.S. Pat. No. 4,844,860, an especially low pressure loss arises at the vanes of a gridlike spacer if the vanes at the intersections are bent in pairs toward one another and are welded together in such a way that one pair of diagonally disposed triangles, aimed at the fuel rods, is created in each flow channel. That produces swirling flows which cosine to make horizontal flows extending diagonally through the pressurized water fuel assembly. In boiling water reactors, if a horizontal flow should develop at all, it is interrupted at the case walls in any event, so that it need not be expected that any substantial improvement would be attainable by using vanes to generate a horizontal flow. Instead, it is precisely in boiling water reactors, that the pressure loss in the flow subchannels is especially critical. It is accordingly an object of the invention to provide a boiling water nuclear reactor and a nuclear reactor fuel assembly for the boiling water reactor, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which utilize the liquid portion of the coolant contained in the liquid/steam mixture to increase the output of the fuel assembly and to prevent dryout. It has surprisingly been found that vanes or similar baffles, which are used in pressurized water reactors to produce and reinforce horizontal flows and are intended to generate as little turbulence as possible in the vertical flow, are so advantageous in boiling water reactors, precisely because of this turbulence, that the disadvantage of the increased pressure loss is acceptable. Horizontal flows only play a subordinate role, because they are deflected by the case walls and have to be kept away from the absorber elements. With the foregoing and other objects in view there is provided, in accordance with the invention, a boiling water reactor, comprising a pressure vessel, a steam turbine connected to the pressure vessel, a reactor core disposed in the pressure vessel, at least one nuclear reactor fuel assembly disposed in the reactor core, and controllable absorber elements disposed in the reactor core outside the at least one fuel assembly; the at least one fuel assembly including an elongated fuel assembly case with mutually parallel side walls laterally closing off the fuel assembly, an inlet end for liquid coolant, an outlet end for a liquid/steam mixture of the coolant, fuel rods containing nuclear fuel being disposed side by side and parallel to the side walls of the case, and a grid extending practically or substantially perpendicularly to the side walls of the case, the grid having grid meshes formed therein through which the fuel rods extend; the fuel rods being disposed in lengthwise rows and in crosswise rows intersecting the lengthwise rows, and each four of the fuel rods disposed in two adjacent lengthwise rows and in two adjacent crosswise rows forming one flow subchannel being parallel to the side walls of the case for coolant flowing in a given direction, the flow subchannels defining a center axis and side walls of the flow subchannels; and at least two vanes disposed in each of at least a plurality of the flow subchannels, the vanes being tapered in the given direction, being three-dimensionally curved or at least bent to form an angle with respect to the center axis, for creating a swirl in the coolant around the center axis. In accordance with another feature of the invention, the at least two vanes are four vanes disposed in each of the plurality of flow subchannels, the four vanes being substantially or practically rotationally symmetrical about the center axis. In accordance with a further feature of the invention, the grid is formed of ribs having edges facing away from the coolant flow, and the vanes are disposed on the edges of the ribs. In accordance with an added feature of the invention, the ribs have retaining elements fixing a spacing between the fuel rods and the side walls of the case. In accordance with an additional feature of the invention, all of the vanes are disposed and formed substantially rotationally symmetrically in one direction about the center axis in one of the flow subchannels, and all of the vanes are disposed and formed substantially rotationally symmetrically in an opposite direction in the flow subchannels adjacent the one flow subchannel. With the objects of the invention in view, there is also provided a nuclear reactor fuel assembly of a boiling water reactor, comprising an elongated case having an interior, mutually parallel side walls laterally closing off the interior, an inlet end for a liquid coolant flow, and an outlet end for a liquid/steam mixture of the coolant; a grid extending substantially perpendicularly to the side walls of the case, the grid having mutually parallel first lengthwise ribs and mutually parallel second crosswise ribs meeting the first ribs at intersections, the ribs having lateral surfaces being parallel to each other and to the side walls of the case, the ribs having edges facing toward the coolant flow, and the ribs defining grid meshes therebetween; fuel rods containing nuclear fuel being disposed side by side and parallel to the side walls of the case, each of the fuel rods passing through a respective one of the grid meshes, and the fuel rods being disposed in lengthwise rows and in crosswise rows intersecting the lengthwise rows; each four of the fuel rods disposed in two adjacent lengthwise rows and in two adjacent crosswise rows forming one flow subchannel being parallel to the side walls of the case for conducting the coolant flow in a given direction, the flow subchannels defining a center axis and side walls of the flow subchannels; and two vanes being disposed at one of the intersections in each of at least a plurality of the flow subchannels, the two vanes being disposed on the edges of the first ribs facing away from the coolant flow at two sides of the second ribs, and the vanes being tapered in the given direction and being curved, for instance three-dimensionally curved or at least bent in order to form an angle with respect to the center axis in different directions for creating a swirl in the coolant around the center axis. In accordance with another feature of the invention, there are no controllable absorber elements disposed in the interior of the case. In accordance with a further feature of the invention, there are provided two three-dimensionally curved further vanes tapering in the flow direction and being disposed on the edge of the second rib facing away from the coolant flow; one of the further vanes being disposed on each respective side of the first rib, and the further vanes being curved toward different meshes than the vanes on the first rib. In accordance with an added feature of the invention, there is no intersection of one of the second ribs and one of the first ribs between an intersection of the first rib and the second rib and one of the further vanes. With the objects of the invention in view, there is additionally provided a nuclear reactor fuel assembly, comprising an elongated case having an interior, mutually parallel side walls laterally closing off the interior, an inlet end for a liquid coolant flow, an outlet end for a liquid/steam mixture of the coolant; fuel rods containing nuclear fuel being disposed side by side and parallel to each other and to the side walls of the case, the fuel rods being disposed in lengthwise rows and in crosswise rows intersecting the lengthwise rows; a grid extending substantially perpendicularly to the side walls of the case, the grid having mutually parallel sheaths seated in spaces and forming meshes of the grid being penetrated by the fuel rods, the sheaths having ends facing away from the coolant flow; each four of the sheaths containing the fuel rods in two adjacent lengthwise rows and in two adjacent crosswise rows forming one flow subchannel being parallel to the side walls of the case for coolant flowing in a given direction, the flow subchannels defining a center axis and side walls of the flow subchannels; and one vane being disposed on each respective end facing away from the coolant flow of at least two of each four of the sheaths forming a flow subchannel, the vanes being diagonally opposite in the flow subchannel, being tapered in the given direction, and being inwardly curved (for instance bent or curved) three-dimensionally into the flow subchannel and rotationally symmetrical with respect to the center axis, for creating a swirl in the coolant around the center axis. In accordance with a concomitant feature of the invention, the sheaths include main sheaths and additional sheaths parallel to the main sheaths in the flow subchannels, the additional sheaths have the ends facing away from the coolant flow, and the vanes are formed onto the ends of the additional sheaths facing away from the coolant flow. The swirl generated by the vanes attached to the coolant outflow end of the grid in the cross section of the fuel assembly case spins the water droplets in the two-phase coolant mixture out of the flow subchannel in the direction of the outer surface of the fuel rods. There, even at an elevated output of the nuclear fuel assembly, they form a water film that prevents a dryout of the rods and an attendant poor heat transfer. It is also seen that the film of water is formed on the outer surface of the fuel rods to an increased extent, in order to increase the allowable output of the nuclear fuel assembly. 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 boiling water nuclear reactor and a nuclear reactor fuel assembly for the boiling water reactor, 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.