Patent Number: 050948059
Section: description

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a 17.times.17 fuel assembly 1, which includes a fuel assembly head 2 with holding-down springs 3, a fuel assembly foot 4, a number of guide tubes 5 (in this case the number n equals 24), an instrumentation tube 5a, square grid-like spacers 6, and round fuel rods 7, 8, 9 and 10, which are filled with fuel in the form of cylindrical pellets. Edge fuel rods located at the edge or edge leg 11 shown in FIG. 2, are identified by reference numeral 10. The fuel rods 7, 8, 9 and 10 are conventionally disposed in a grid configuration with a strictly square cross section, or in other words in a specified equal distribution. As will become clear below, in at least one embodiment of the invention, a departure from this strictly square grid configuration is made. The possible structural difference between the fuel rods 7-10 will be explained later. The fuel rods 7 may be conventional standard fuel rods, for example. As a rule, in a reactor core, a given number of fuel assemblies 1 which are identical to one another, are provided. The fuel assemblies are disposed in a reactor pressure vessel and a coolant K flows through them from bottom to top, as indicated by an arrow. The fuel rods 7, 8, 9, 10 of the fuel assembly 1 are retained in a support structure, which is formed of the fuel assembly head 2, provided with the four holding-down springs 3, the fuel assembly foot 4, the guide tubes 5 inbetween four non-illustrated control rods and the central instrumentation tube 5a, depending on the fuel assembly type. The grid-like spacers 6 which have a square spacer cross section and meshes or openings of equal area, are secured to the guide tubes 5. Each of the fuel rods 7, 8, 9, 10 is guided through one respective mesh, and the grid-like spacers 6 combine the fuel rods into a bundle and hold them in such a way that they can freely expand axially. The laterally open construction of the fuel assembly 1 enables a crosswise mixing of the coolant K and makes for uniform heating of the coolant. FIG. 2 shows a complete cross section of a grid-like spacer 6 of a fuel assembly 1 of FIG. 1, as an example of a pressurized water reactor having 17.times.17=289 grid meshes. The fuel assembly 1 has 264 fuel rods with reference numerals 7, 8A and 10, twenty-four symmetrically distributed guide tubes 5, and one central instrumentation tube 5a. The drawing only shows fuel rods 7, 8A, 10 with the same mutual spacing P1 among the cross-sectional points, both in the longitudinal and the transverse direction of the grid (making a uniform distribution in a square basic configuration). For instance, the spacing P1 can be 12.6 mm. In other words, the spacing is basic to the grid, throughout the grid, and can be considered a standard spacing. For the sake of particularly good economical utilization of the nuclear fuel (for instance, uranium oxide, or in particular uranium plutonium mixed oxide), at least one fuel rod 8A (shown with shading inclined from the lower left to the upper right) in the immediate vicinity of a guide tube 5 and/or of the instrumentation tube 5A, should have a cladding tube with an outside diameter of 9.5 mm or 9.7 mm, for instance, which is larger than that of a fuel rod located farther away from the guide and instrumentation tubes 5, 5a. The thinner fuel rods which are located farther away from the guide and instrumentation tubes 5, 5a (with the shading inclined from the lower right to the upper left), are predominantly identified by reference numeral 7. In FIG. 5, the preferred case is shown, where all of the immediately adjacent fuel rods have the larger diameter, while all of the edge or peripheral meshes containing the edge fuel rods 10 and all of the other meshes adjoining the edge meshes contain thinner fuel rods. The group identified as "fuel rods located farther away" includes predominantly the four rows of edge fuel rods 10. The outside diameter of the cladding tubes of the fuel rods 7 located farther away is 9.14 or 9.30 mm, for example. In this exemplary embodiment, all of the fuel rods 7 located farther away, or in other words all those with shading oriented from the lower right to the upper left have a smaller diameter than the fuel rods 8A immediately adjacent the tubes 5, 5a. The group of "fuel rods 7 located farther away" can also predominantly (fully or partly) include the four adjacent fuel rods 7 located diagonally away. This depends on the desired uniformity. Accordingly, this exemplary embodiment also shows that the fuel rods 10 at the edge of the fuel assembly 1 have an outside diameter of the cladding tube of 9.14 or 9.30 mm, for instance, which is also smaller than all of the fuel rods 8A in the center of the fuel assembly 1. This center can be defined by the (almost circular) circumferential line described by the outer guide tubes 5. In FIG. 3, a portion of a fuel assembly cross section as seen from above is shown, in accordance with the first exemplary embodiment of FIG. 2. The fragmentary or detail view again shows the region in the fuel assembly 1 around a guide tube (or instrumentation tube 5a), but on a larger scale. It can be seen clearly in this case that the two fuel rods 8A shown immediately adjacent the guide tube 5 (or instrumentation tube 5a), have a larger outside diameter of the cladding tube than the fuel rods 7 located farther away from the tube 5 (or 5a). In a departure from the drawing, the fuel rod 7 located diagonally to the right from the tube 5 could also have a larger outside diameter of its cladding tube. Correspondingly, the two non-illustrated fuel rods 8A located to the left of and below the guide tube 5 could have a larger diameter than the fuel rods 7. As a result of this dimensioning of the four fuel rods 8A, more fuel can be accommodated in the fuel rods 8A near the tube 5, given an increased pellet diameter and the same wall thickness, for example, than in the fuel rod 7. As a result, the previously excessive local moderation in the region around the tube 5 (or 5a) is reduced. The dimensioning of the fuel rods 8A assures the uniform utilization of fuel over the cross section. The adjacent fuel rods 7 and/or 8A, located in a crosswise or lengthwise row, have spacings P1 of the cross-sectional centers or central axes thereof which are equal to one another. In other words, the center points or central axes of the fuel rods 7, 8A of each vertical or horizontal row are located along a straight line. Button and spring combinations for retaining the fuel rods 7, 8A in the various meshes are shown in this case at reference numeral 13. FIG. 4 shows a portion of a second embodiment of a fuel assembly cross section as seen from above. The second embodiment of the fuel assembly 1 is only insignificantly more difficult to manufacture than the embodiment of FIG. 3. Once again, particularly good utilization of the nuclear fuel is attained. The fragmentary or detail view shows an enlargement in the region of the fuel assembly 1 around a guide tube 5 (or instrumentation tube 5a). It can be seen that all of the fuel rods 7, 8B in this case are selected to have the same diameter. They preferably contain the same quantity of nuclear fuel, or in other words the same number of pellets of the same size. However, different weights are once again possible. It can also be seen that in this case the spacing P2 between a cross-sectional center point or point along the central axis M8 of the two fuel rods 8B that are immediately adjacent to and in the same row direction as the guide tube 5 (or instrumentation tube 5b) on one hand, and the cross-sectional center point M5 of the guide tube 5 (or instrumentation tube 5a) on the other hand is less than the standard spacing P1 of the cross-sectional center points M7 of (adjacent) fuel rods 7 located farther away. In other words P1 is larger than P2. The fuel rods 8B in this case are accordingly shifted closer to the tube 5, 5a. Accordingly, in the vicinity of the tubes 5, 5a a "non-uniform" distribution is obtained. In other words, the centers of the fuel rods and the guide rods describe the intersection points of a grid that is not uniform like the grid of the spacer but rather is deformed in the vicinity of the tubes 5, 5a, in order to assure that the moderation is made uniform in this case. The spacings P1, P2 are once again measured in the row direction. It should also be noted that the fuel rod 7 located in the diagonal direction can also be shifted closer to the tubes 5, 5a. This is also true for the three other non-illustrated adjacent fuel rods 7 in the diagonal direction. FIG. 5 shows a portion of a third embodiment of the corner of a fuel assembly cross section as seen from above. In this case a spacing P3 between the cross-sectional centers M9 of some fuel rods 9C in the second row and the cross-sectional center M10 of the adjacent fuel rod 10 at the edge of the fuel assembly (that is, on the edge or edge leg 11 of the spacer) is greater than the spacing P1, P2 of the inner fuel rods 7, 9C. That is, P3&gt;P1&gt;P2, and P3=2P1-P2. The fuel rods 7, 9C, 10 all have the same outside diameter. In other words, in FIG. 5 the fuel rods 9C of the second row in each case are shifted away from the adjacent edge fuel rod 10. Although it is not illustrated in FIG. 5, the fuel rod 7 in the second row can be shifted away from one or both adjacent fuel rods 10 of the edge row. In other words, the cross-sectional center M7 of the fuel rod 7 (shifted diagonally inward) can be located at the point P7. The diameters of all of the fuel rods 7, 9C, 10 are equal and the fuel rods can be standard fuel rods. Due to this construction, uniformity of the hot spots and a local increase in fuel utilization are also assured in the vicinity of the edges 11. In FIGS. 4 and 5, the "unshifted" fuel rods 7 or the fuel rods 7 and 10 form a first group, and the "shifted" fuel rods 8B or 9C form a second group. FIG. 6 shows how this result can be assured in a different way as well. In this case, each of the fuel rods 10 of the edge row has a smaller outside diameter than a fuel rod 7 located farther inward, which for instance is a standard fuel rod. Thus with the same cladding tube wall thickness, they contain less fuel. Moreover, the fuel rods 9D of the second row are again thinner than the fuel rods 7, which as noted above can, for instance, be standard fuel rods. Differing from the drawing, in this case, the fuel rods 9D and 10 can preferentially have the same outside diameter in order to reduce the number of types of fuel rods and thus keep manufacturing costs low. The spacings P1 between all of the adjacent fuel rods 7, 9D and 10 are equal. The effect of the construction chosen in FIG. 6 is equivalent to that of the construction chosen for FIG. 5, if the spacings P2, P3 are suitably selected in FIG. 5. FIG. 7 shows two adjacent fuel rods 12a and 12b having cladding tubes with the same outside diameter D1, in which one fuel rod 12a has a different ratio s1/D1 of its cladding tube wall thickness s1 to its cladding tube outside diameter D1 than the adjacent fuel rod 12b. The ratio s2/D2 is larger than s1/D1. The fuel rod 12a can therefore hold more fuel than the fuel rod 12b. FIG. 8 shows two adjacent fuel rods 12c, 12d with cladding tubes having different outside diameters D2 and D3 (D3 is larger than D2). It is assumed in this case that the wall thickness s3 is the same for the cladding tubes of both fuel rods 12c, 12d. Accordingly the following applies to the ratios: s3/D2&gt;s3/D3. In this case, the fuel rod 12d can hold more fuel than the fuel rod 12c. FIG. 9 shows two adjacent fuel rods 12e and 12f, where the relationship between the outside diameters D4 and D5 of their cladding tubes are such that D4 is smaller than D5, while the relationship of their cladding tube wall thicknesses s4 and s5 are such that s4&lt;s5. The dimensioning is selected in such a way that the following applies: s4/D4=s5/D5. This assures mechanical stability. In this case, the fuel rod 12e can contain more fuel than the fuel rod 12f. As shown in FIG. 10, a pressurized water reactor has a pressure vessel 22, in which a reactor core with diagrammatically illustrated nuclear reactor fuel assemblies 1 is located. The fuel assemblies have fuel rods in accordance with an exemplary embodiment as described above. An outlet 24 for water (both coolant and moderator) from the pressure vessel 22 and thus from the reactor core having the nuclear reactor fuel assemblies 1, is connected to one end of a primary tube 26 of a steam generator 27. An inlet 25 for leading the water into the pressure vessel 22 and thus into the reactor core is connected to the other end of the primary tube 26. The primary loop formed by the pressure vessel 22 and the primary tube 26 is a closed loop, so that no steam can form in this primary loop and thus in the reactor core. Steam does form on the secondary side of the steam generator 27, which has a delivery connection 28 for feedwater 29 and an outlet connection 30 for steam. The steam may, for instance, be carried from the outlet connection 30 to a non-illustrated steam turbine.