Patent Number: 
Section: description

In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown in a sectional view of an exemplary embodiment of a fuel element 1 shown transversely with respect to a fuel-element axis with fuel rods B and a spacer. In this case, the fuel rods B are disposed substantially parallel to the fuel-element axis. The axis is perpendicular to a polygonal internal cross section 7 of the fuel element 1, which in this embodiment is bounded by an inner edge of a fuel-element box 9. The fuel rods B are thus also disposed substantially perpendicularly to the internal cross section 7, which is square in this embodiment, and pass through the internal cross section 7 over an area which is in each case determined by a cross section Q of the fuel rod B. A sum of all the cross sections Q1 in a first region 3 defines an area through which the fuel rods B1 pass. A remaining area defines a free area of the first region 3. A sum of all the cross sections Q2 defines an area in a second region 5 through which the fuel rods B2 pass. The first region 3 and the second region 5 together form the internal cross section 7. In this case, the first region 3 forms a first corner 11 of the internal cross section 7. Other corners 12, 13 of the internal cross section 7, in particular the corner 12 opposite the first corner 11, are formed by the second region 5. Since, in the exemplary embodiment shown, a distance D between outer surfaces A1, A2 of in each case two adjacent fuel rods B1, B2 increases monotonically in particular along a diagonal W of the internal cross section 7 of the fuel element 1 starting from the first corner 11 of the fuel element 1 toward the opposite corner 12. A ratio of the free area of the internal cross section 7 to the area through which the fuel rods B1, B2 pass is smaller in the first region 3 than in the second region 5. In particular, a distance D1 between the outer surfaces A1 of in each case two adjacent fuel rods B1 in the first region 3 is on average smaller than a distance D2 between the outer surfaces A2 of in each case two adjacent fuel rods B2 in the second region 5. In this case, the cross section Q of the fuel rod B is constant over the entire internal cross section 7 of the fuel element 1. However, as the graph below the fuel element 1 in FIG. 1 shows, the distance Di between in each case two immediately adjacent fuel rods B1, B2xe2x80x94that is the xe2x80x9cimmediate-neighborxe2x80x9d distancexe2x80x94increases along the diagonal W of the internal cross section 7. The distance Di as a function of the path even increases linearly in this embodiment. In the exemplary embodiment of a fuel element 1A shown in FIG. 2, according to the graph in FIG. 2, the distance D between the outer surfaces Ai of in each case two adjacent fuel rods Bi in one direction, starting from the first corner 11 of the fuel element 1 toward the opposite corner 12, does not increase strictly monotonically manner but only monotonically. In the first region 3A, the distance Di has a constant value D1 and then increases abruptly at the boundary to a second region 5A, in which the distance Di has a likewise constant, but larger value D2. This is illustrated by the plot of Di along the diagonal W in the graph in FIG. 2. In this embodiment, the distance D1 corresponds to the average distance between the outer surfaces A1 of in each case two adjacent fuel rods B1 in the first region 3A. Likewise, the distance D2 corresponds to the average distance between the outer surfaces A2 of two adjacent fuel rods B2 in the second region 5A. In both of the exemplary embodiments in FIGS. 1 and 2, all of the fuel rods B of the fuel element 1 have the same cross section Qi and thus the same diameter. FIG. 2 shows an embodiment of the fuel element 1A essentially with a fuel-rod bundle 15 with 10xc3x9710 positions for the fuel rods Bxe2x80x94again as a section view transversely to the fuel-element axis. However, the representation in FIG. 2 applies in just the same way to a pressurized-water-reactor fuel element. In a pressurized-water-reactor fuel element, a corresponding fuel-rod bundle would contain, for example, 17xc3x9717 or 18xc3x9718 of the fuel rods B, with control rods disposed between the fuel rods B. In this case, for example, the fuel element according to U.S. Pat. No. 4,849,161 may be taken as a basis, the configuration of the fuel rods being modified in the manner according to the inventionxe2x80x94in a segment, for example, as in FIG. 2. In the exemplary embodiment of a fuel element 1B shown in FIG. 3, the distance Di between the outer surfaces of in each case two adjacent fuel rods Bi, starting from the first corner 11 of the fuel element 1 toward the corner 12, increases along the diagonal W of the internal cross section 7 in accordance with a convex function with respect to the path along the diagonal W (see the graph in FIG. 3). In the embodiment 1B, although a distance M between centers of two adjacent fuel rods B1 in the first region 3B is the same as the distance M between centers of two adjacent fuel rods B2 in the second region 5B, the cross section Q1 of the fuel rod B1 in the first region 3B has a greater value than the cross section Q2 of the fuel rod B2 in the second region 5B. This leads to the convex increase (shown in the graph in FIG. 3) in the distance Di along the diagonal W of the internal cross section 7 in the embodiment 1B of the fuel element. In the embodiments of the fuel element 1, 1A and 1B that are shown in FIGS. 1 to 3, in each case the fuel rods Bi are disposed over the internal cross section 7 virtually in mirror symmetry relative to the diagonal W, going from the first corner 11 to an opposite corner 12, of the internal cross section 7 of the respective embodiment 1, 1A, 1B, the internal cross section 7 in each case being substantially square. In addition, the embodiment of the fuel element 1B shown in FIG. 3 has a water tube R, which is substantially parallel to the fuel rods B of this embodiment. According to the explained configuration of the fuel rods B1 in the first region 3B and of the fuel rods B2 in the second region 5B, the ratio of the free area of the internal cross section 7 to the area through which fuel the fuel rods B1 pass in the first region 3B is smaller than the corresponding ratio of the free area of the internal cross section 7 to the area through which the fuel rods B2 pass in the second region 5B. Since the first region 3B forms the first corner 11 of the internal cross section and in particular borders on the water tube R with an inner corner 17 which is opposite the first corner 11, the open cross section of flow in the first region 3B is substantially smaller than in the second region 5B and in particular in a section 19 of the second region 5B. The section 19 certainly has the same base area as the first region 3B. However, since the ratio of the free area of the internal cross section 7 to the area through which the fuel rods B2 pass is greater in the section 19 than in the first region 3B, the cross section of flow in the section 19 is greater than in the first region 3B. These differences result in a pressure gradient from the first region 3B to the second region 5B, in particular to the section 19. This leads to a redistribution U of the flow of a coolant from the first region 3B to the section 19xe2x80x94that is into the region 5B. In particular, the steam portion present in a two-phase flow escapes especially quickly into the second region 5B, since it has a substantially smaller mass moment of inertia than the liquid portion of the coolant. Depending on the selection of an increase in the distance Di in one direction, in particular along the diagonal W, the redistribution U can be accurately metered. Examples of the different increase in a distance D is shown in each case in a graph in FIGS. 1, 2, 3 and 4. FIG. 4 shows a further exemplary embodiment of a fuel element 1C. In this case, the fuel element 1C has the water tube R sitting eccentrically relative to the fuel-element axis. In addition, the fuel-rod bundle 15 as an entity is moved along the diagonal W of the internal cross section 7 of the fuel element 1C in the direction of a first region 3C or the first corner 11, that is away from the opposite corner 12. The result of this measure in this embodiment is that the ratio of the free area of the internal cross section 7 to the area through which the fuel rods B1 pass is smaller in the first region 3C than in a second region 5C. This is because, in this case, a gap K1 between an outer margin of the fuel-rod bundle 15 and a side S1 starting from the first corner 11 is smaller than a distance G1 of an outer margin of the fuel-rod bundle 15 from a second side S2 which starts from another corner 12 lying opposite the first corner 11. In this case, in particular the fuel rod B adjacent to the first side S1 is at a smaller distance K from the first side S1 than the fuel rod B which is adjacent to a second side S2 and is at a distance G from this second side S2. Since a redistribution U of the coolant flow from the first region 3C into the second region 5C is partly already achieved in this exemplary embodiment 1C by a displaced fuel-rod bundle 15 as an entity along the diagonal W towards the first region 3C, a change in the distance D between the outer surfaces A of two adjacent fuel rods B can be less pronounced than would be necessary, for example, in the embodiment in FIG. 1 in order to achieve approximately the same redistribution. This can be seen for the exemplary embodiment 1C from the graph in FIG. 4, in which the distance D increases linearly as a function of the path along the diagonal W. However, the increase in D as a function of the path along W is less than in the exemplary embodiment of the fuel element 1 shown in FIG. 1.