Patent Number: 050948059
Section: summary

The invention relates to a fuel assembly for a pressurized water nuclear reactor, having a bundle of parallel fuel rods each containing nuclear fuel in a cladding tube. Fuel assemblies for pressurized water reactors usually are square in cross section. They contain a number (for instance 17.times.17) of closely adjacent fuel rods, which are disposed in a grid pattern in rows, are joined into a bundle and are surrounded by moderating water during operation. The pressurized water reactor is provided with a core that is made up of a number of such individually replaceable fuel assemblies disposed within one pressure boiler. Control rods are longitudinally movably guided in guide tubes of the fuel assemblies. Guide tubes can also be provided, which contain an additional moderator rod. A relatively large number of fuel rods, guide tubes and as a rule one instrumentation tube form one bundle, which is combined into the fuel assembly by means of a support structure. The support structure has a fuel assembly head at the top, a fuel assembly foot at the bottom, and spacers with a grid-like structure each being disposed in one of a plurality of intermediate planes, with one fuel rod, guide rod or instrumentation tube being guided through each of the meshes in the spacer, so as to keep these rods and tubes in a desired position by means of suitable support devices (such as button and spring combinations). In U.S. Pat. No. 4,059,484, a spacer is made up of square meshes that either all have a relatively small surface area or are combined in the interior of the bundle into islands with larger meshes, resulting in a non-uniform grid structure. The guide tubes and thicker fuel rods which have differently enriched nuclear fuel and are placed in suitably thick cladding tubes, are located in the larger meshes. In the pressurized water reaction, certain moderator inconsistencies in fact occur because of structural reasons, specifically because of the heterogeneous radial distribution of the guide tubes and possibly an instrumentation tube and because of a water gap between adjacent fuel assemblies. The inconsistent local moderator distribution over the cross section of one fuel assembly leads to a correspondingly uneven local output generation. In the vicinity of the guide tubes, for instance, which are filled with the moderator (such as water), a higher local output generation occurs in comparison with the mean fuel assembly value. This results in a heterogeneous output generation from one fuel rod to the other and thus not only leads to uneven utilization of the nuclear fuel but also causes pronounced differences in fuel rod consumption, and in local fuel rod corrosion. Published European Application No. 196,655 shows a pressurized water fuel assembly and a boiling water fuel assembly. The boiling water fuel assembly includes relatively few fuel rods, and is surrounded by a box or chest for hydrodynamic and mechanical reasons. Thin fuel rods are disposed in one corner of the box, and thicker fuel rods are relatively widely spaced apart in the interior of the box, while fuel rods of an average cross-sectional area are disposed at the remaining edge of the box with close spacing between one another. Such variably thick and unevenly distributed fuel rods do not fit in a grid-like spacer having square meshes of equal mesh area. As is generally the case in boiling water fuel assemblies, no guide tubes are provided. The pressurized water fuel assembly of Published European Application No. 196,655 contains thicker and thinner fuel rods which fit into square meshes of a spacer having equal surface areas. Each of the guide tubes, which is distributed over the cross section of the fuel assembly, is surrounded by four thinner rods and forms a cross-shaped structure therewith. The fuel rods on the edge of the bundle also have a smaller cross section with equal spacing from center to center, while all of the other fuel rods are thicker. Published European Application No. A 0 199 197 discloses a special fuel assembly for a boiling water reactor, in which some fuel rods are provided with a jacket of gadolinium, which is a neutron poison, and thus have a larger outside diameter than other fuel rods. However, in that configuration this is not done to even out the output density distribution but instead is dictated by the fact that the pellet clad with gadolinium must necessarily have a larger diameter for the same fuel content than conventional pellets. This is also due to the fact that although two water rods are shown in FIG. 2 of that reference, no provisions are shown or described for the output generation in the vicinity of the water rods. The fuel assembly known from French Patent No. 1,549,344 likewise contains only a few fuel rods and no guide tubes, because it is intended for a boiling water reactor. The fuel rods are of equal thickness and are disposed in straight or curved rows in such a way that the spacing among the fuel rods increases toward the central axis of the fuel assembly. Published French Application No. 2,258,691 shows a sodium nuclear reactor with a hexagonal fuel assembly having edge rods which are shifted toward the outside in order to increase the spacing from fuel rods located farther inward. It is accordingly an object of the invention to provide a fuel assembly for a pressurized water reactor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which is improved in terms of the uneven local output generation. It is intended to provide an economically improved utilization of the fuel present in the core of the reactor, combined with a reduction in the maximal local thermal fuel rod load that arises, if this is at all possible without impairing the mechanical stability and functional ability (guide tubes, spacers, etc.) of the pressurized water fuel assembly. With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a pressurized water reactor, comprising fuel assembly edges, and a fuel assembly interior, a bundle of mutually parallel fuel rods with cladding tubes each containing nuclear fuel, a given number of guide tubes for control rods, and optionally at least one instrumentation tube; a support structure including an upper end with a fuel assembly head, a lower end with a fuel assembly foot, and a plurality of spacers disposed perpendicular to the fuel rods, the spacers having a uniform grid structure with edge meshes formed therein at the fuel assembly edges and inner meshes formed therein in the fuel assembly interior, each of the inner meshes being surrounded by for respective immediately adjacent meshes each having essentially the same area and each containing a respective one of the guide or cladding or instrumentation tubes; and whereby, for each inner mesh containing a guide tube, at least one of said respective immediately adjacent meshes contains a fuel rod in which the spacing of the cladding tube periphery from said inner mesh is smaller than the corresponding spacing of the cladding tube periphery of a fuel rod contained in an edge mesh from an inner mesh. The cladding tube of the fuel rod in the at least one mesh immediately adjacent the given inner mesh with a guide tube, may have a larger outside diameter than a fuel rod in an edge mesh and/or it may have a central axis being closer to the guide tube than the center of the at least one mesh. Therefore, the assembly contains spacers having square meshes of equal area, each receiving one fuel assembly or one guide tube (for instance an instrumentation tube, in the event that instrumentation tubes are provided in the interior of the bundle of fuel rods). The positions of the fuel rods in the various meshes and the diameters thereof are selected as a function of the location of the various meshes relative to the edge of the bundle, the guide tubes, and the density of thermal neutrons prevailing there, in such a way that the most uniform possible output distribution over the cross section of the fuel assembly is established. Due to the specified square structure of the equal-area meshes, one of the four broad sides of each mesh in the interior ("inner mesh") abuts the broad side of another mesh, which therefore directly adjoins the mesh. In contrast, a mesh at the edge of the bundle (or "edge mesh") has two edge meshes and at best only one further inner mesh directly adjoining it. In terms of the spacing of the centers of the fuel rod or the outer diameter of the cladding tubes in the edge meshes, a reduction in the spacing between a guide tube and the fuel rods in the immediately adjacent inner meshes is virtually equivalent to an enlargement of the outer diameter of the cladding tube, since both leave the periphery of the fuel rod cladding tube closer to the guide tube than the fuel rod cladding tubes in the other meshes. In both cases, a disruption resulting from the disposition of a guide tube instead of a fuel rod in the corresponding mesh is largely neutralized. Reducing the fuel rod to guide tube spacing is attained, despite the equal-area meshes, by providing that the central axis of the fuel rod is offset out of the middle or center point of the corresponding mesh in the direction toward the guide tube. As a result, the spacing of the thus offset fuel rod with respect to fuel rods that are located in the middle of other immediately adjacent meshes also increases. With fuel rods of equal thickness, this increased fuel rod to fuel rod spacing therefore works like a reduction of the thickness of the other fuel rods, so that as a result, by means of a reduced thickness of the edge rods or an increased spacing from fuel rod to fuel rod between a given edge rod and a rod located farther inward, the same effect on the fuel rods located farther inward can be attained. A larger cladding tube diameter further offers the opportunity of accommodating more fuel. However, for the sake of simple manufacture and storage, it is advantageous to provide the same diameter for all of the fuel rods, in the embodiment having an unequal fuel rod distribution or in other words having fuel rods which are shifted out of the center points of the equal-sized meshes, or to provide the uniform fuel rod distribution with the constant standard spacing in an embodiment for different diameters. The standard spacing is understood in this case to mean the basic spacing of the rectangular basic pattern. In particular, this can be the conventional basic spacing (for instance, 12.6 or 12.7 or 14.3 mm). Through the selection of different outside diameters of the fuel rods, the local output can be reduced to approximately the standardized mean value in the vicinity of the guide tube filled with the moderator and optionally in the vicinity of the instrumentation tube. This makes the output distribution in the fuel assembly more uniform. In other parts of the fuel assembly cross section, the option of unequal distribution of the fuel rods also enables finer local matching to the standardized mean value of the output generation in the fuel assembly. Through the use of the above-described embodiment with different spacings of the fuel rods at and/or near the edge of the fuel assembly, the local moderation increased there, thus improving fuel utilization. Moreover, in a pressurized water reactor, this construction enables reduction of the pressure loss that occurs in the moderator (that is, coolant) as it flows through the fuel assemblies. With this construction that reduces the pressure loss, problems in terms of hydraulic compatibility, for instance of fuel assemblies having Zirkaloy spacers and Inconel spacers, are also solved. The improved cooling of the fuel rods at the edge of the fuel assembly resulting from the increase in the flow cross section at that location also reduces cladding tube corrosion. These provisions homogenize the moderator distribution in the fuel assembly and thus achieve particularly good fuel utilization. On the other hand, hot spots and cladding tube corrosion of the fuel rods are lessened. There are fuel assemblies in which even when the measures according to the invention are taken, the fuel weight is to be kept constant once it is great enough, and there are fuel assemblies in which when making taking the measures according to the invention, the fuel weight is to be increased because it is too low. In both types of fuel assembly, a reduction in hot spots and a lessening of cladding tube corrosion are attained. Negative economic effects, resulting from a reduction in the number of cycles and an increase in the specific fuel assembly manufacturing costs (from the use of different pellet diameters and/or fuel rod diameters) can be largely compensated for by the high fuel weight in the fuel assembly. In accordance with another feature of the invention, some of the meshes including all of the four meshes immediately adjacent each given inner mesh with a guide tube, contain fuel rods with a larger outside diameter than a fuel rod in an edge mesh, and/or of the meshes including all of the four meshes immediately adjacent each given inner mesh with a guide tube, contain fuel rods with a central axis being closer to the guide tube than the center of the at least one mesh. In accordance with a further feature of the invention, all of the fuel rods in the edge meshes have cladding tubes with a smaller outside diameter than a fuel rod in the at least one mesh immediately adjacent a given inner mesh containing a guide tube. In accordance with an added feature of the invention, the cladding tube of the fuel rods in inner meshes adjoining edge meshes but not adjacent a given inner mesh containing a guide tube, have a smaller diameter than the cladding tubes of the fuel rods in the at least one mesh immediately adjacent a given inner mesh containing a guide tube. In accordance with an additional feature of the invention, all of the fuel rods having a smaller diameter have the same small diameter, and all of the other fuel rods have the same larger diameter. In accordance with yet another feature of the invention, some of the fuel rods have central axes which intersect center points of the meshes in which they are disposed and which are spaced apart by a given standard spacing, and at least one of two fuel rods disposed in an edge mesh and in an inner mesh immediately adjacent thereto have central axes which are spaced from the center points of the meshes in which they are disposed and which are spaced apart by a spacing which is greater than the given standard spacing. In accordance with yet a further feature of the invention, adjacent fuel rods have central axes being mutually spaced apart by equal spacings in a uniform distribution. In accordance with yet an added feature of the invention, the cladding tubes of the fuel rods have equal outside diameters, and the fuel rods are non-uniformly distributed. In accordance with yet an additional feature of the invention, at least one of the fuel rods has a different ratio of cladding tube wall thickness to cladding tube outside diameter than another of the fuel rods. With this further feature, the weight of the fuel (such as uranium) in the fuel assembly can likewise be increased, with the positive economic effects referred to above. In addition, with the reduction in cladding tube material in the fuel assembly that this makes possible, and the associated improvement in neutron economy, an improvement in overall economy is likewise attained. The invention is particularly advantageous with the use of fuel assemblies having plutonium uranium mixed oxide fuel. This is because mixed oxide fuel assemblies of this kind normally have a particularly inhomogenous output distribution over the cross section thereof. With the objects of the invention in view, there is also provided a fuel assembly comprising a grid-like spacer having a rectangular and preferable square spacer cross section and having square spacer meshes formed therein with corners, sides and equal areas, the meshes having center points defined by intersecting diagonal lines each passing through two of the corners, a bundle of mutually parallel fuel rods with central axes and cladding tubes containing nuclear fuel, a bundle interior, a fuel bundle edge, and guide tubes disposed in the bundle interior, the fuel rods and the guide tubes each having a circular cross section and each being guided through a respective mesh, a first fuel rod group having a number Z1 of the fuel rods, the central axes of the fuel rods of the first fuel rod group passing through the center points of the meshes in which the fuel rods are disposed, a second fuel rod group having a number Z2 of the fuel rods, the central axes of the fuel rods of the second fuel rod group passing outside of the center points of the meshes in which the fuel rods are disposed, Z1+Z2&gt;0, and if Z2=0, the first fuel rod group includes a first fuel rod guided through a spacer mesh with a side adjoining a mesh through which one of the guide tubes is guided, and the first fuel rod group includes a second fuel rod with a cladding tube having a smaller diameter than the cladding tube of the first fuel rod, and the second fuel rod is disposed at the bundle edge. It is therefore seen that the fuel rods disposed in the center of the meshes can be combined into a first fuel rod group, while the fuel rods that are offset can be combined into a second fuel rod group. Accordingly, in order to attain the object of the invention, a suitably selected unequal distribution of the fuel rods then already suffices. In other words, the desired effect is achieved by the fuel rods of the second group. As mentioned above, not even a single fuel rod of the first group need be present in the fuel assembly. In other words, the number Z1 of fuel rods of the first group can equal 0. As a rule it will be necessary, at least in a mesh directly adjoining a mesh that contains a guide tube, to shift the fuel rod toward the guide tube with respect to the center point of the mesh. In some cases, however, the object can be attained satisfactorily by simply providing fuel rods of the first group. In that case, the number Z2 of fuel rods of the second group can be selected to be equal to 0. In that situation, in at least one mesh directly adjacent an inner mesh containing a guide tube, the outside diameter of the cladding tube is selected to be larger than the outside diameter of a fuel rod at the edge of the cladding tube. In accordance with again another feature of the invention, Z1 and Z2 are both at least equal to 1, and all of the fuel rods of the first fuel rod group have equal outside diameters. In accordance with again a further feature of the invention, all of the fuel rods of the second fuel rod group have equal outside diameters. In accordance with again an added feature of the invention, all of the fuel rods have equal outside diameters. In accordance with again an additional feature of the invention, Z1 and Z2 are both at least equal to 1, and the fuel rods of the second fuel rod group have different outside diameters than the fuel rods of the first fuel rod group. In accordance with still another feature of the invention, the fuel rods of the second fuel rod group have different diameters. In accordance with still a further feature of the invention, the fuel rods of the second fuel rod group are guided in the meshes at the bundle edge. In accordance with still an added feature of the invention, the fuel rods of the second fuel rod group are guided in the meshes at the bundle edge and have smaller outside diameters than the fuel rods of the first fuel rod group. In accordance with still an additional feature of the invention, the fuel rods guided in the meshes at the bundle edge have cladding tubes with lesser wall thickness than the cladding tubes of others of the fuel rods. In accordance with a concomitant feature of the invention, the fuel rods of the second fuel rod group are guided in the meshes adjacent the meshes in which the guide tube are disposed. 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 for a pressurized 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.