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 a fuel assembly containing a multiplicity of fuel rods 4 to 6 which, in the operating state, extend vertically between a lower rod-holding plate 1 and an upper rod-holding plate 3. The rod-holding plates 1 and 3 are provided with non-illustrated coolant passages. The fuel rods 4-6 are disposed parallel to one another and are clamped in spacers 11 to 18. While the fuel rods of normal length do not rest or only rest loosely on the lower rod-holding plate 1, the part-length rods are securely anchored in the rod-holding plate 1 by their lower ends. A fuel assembly channel 2 (only partially illustrated in FIGS. 1 and 3) which is open at the top and bottom encloses the bundle of fuel rods 4 to 6 and forms a closed shroud for a liquid coolant which enters through the lower rod-holding plate 1. On its way through the fuel element channel 2, the coolantxe2x80x94preferably waterxe2x80x94is heated by the fuel rods 4 to 6 and begins to evaporate, so that a mixture of liquid coolant and coolant in vapor form takes up the heating capacity of the fuel rods 4-6 in an upper region of the fuel assembly. The mixture of liquid and vapor has a larger volume than the pure liquid. In order nevertheless to avoid an undesirably high flow velocity with a low mass throughput, it is known per se to shorten some of the fuel rods, so that the clear passage cross section in the upper region of the fuel assembly channel 2 is greater than in the lower region. In configuration terms, the spacers 11 to 18 are divided into a lower group A (12 to 15) and an upper group B (15 to 18), a distances between the spacers 12 to 15 in group A being identical to one another. It may be that just two rod lengths (full length and a single part length) will be sufficient, but two part lengths are more advantageous (and more complex). Accordingly, distances between the spacers 15 to 18 in the upper group B are shorter than in the lower group A, in particular becoming shorter the further up they are. To more precisely achieve values that are required in order to optimize a maximum transition power, the fuel rods 5 and 6 are shortened by different extents and in some cases end above the spacer 14 and in other cases end directly above the spacer 15 which forms the boundary between the spacer groups A and B. As a result, the configuration is further away from the maximum power for transition to boiling, so that the configuration according to the invention, in configuration terms, provides an additional optimization parameter. FIG. 2 likewise shows the fuel assembly with eight spacers 11 to 18 which, in configuration terms, are divided into a group A and a group B. For the sake of clarity, the fuel assembly channel 2 is not shown in FIG. 2. To provide a configuration situation which differs from that used for the configuration shown in FIG. 1, in this case fuel rods 7 are also provided. The fuel rods 7 are shortened to a lesser extent than the fuel rods 5 and 6. Further variations on fuel assemblies configured in accordance with the invention are shown in FIGS. 3 and 4. In these two solutions, in each case nine spacers 11 to 19 are provided, three different fuel rod lengths being used in FIG. 3, as in FIG. 1, and four different fuel rod lengths being used in FIG. 4, as in FIG. 2. FIGS. 1 to 4 in each case show only one of, for example, 9 to 11 rows of fuel rods positioned one behind the other, all of which can be differently equipped with fuel rods of different lengths. All the above measures together or on their own allow the maximum power for transition to boiling to be optimized over a wide range. The configurations shown in FIGS. 1 to 4 are advantageous in particular because they allow unrestricted measures for segregating the liquid phase and the vapor phase in the upper region of the fuel assembly. Devices on the spacers 11 to 19, which are illustrated in FIGS. 5 and 6, are used for this purpose. FIG. 5 shows, on a greatly enlarged scale, one of the spacers 11 to 19 which contains metal strips 20 which cross one another at right angles and penetrate through one another. The metal strips 20 form approximately square mesh openings for accommodating the fuel rods 4 to 7 which are clamped securely in mesh openings by lugs 21 and springs 22. As well as crossing points of the metal strips 20, in each case upwardly directed, laterally bent-off sheet-metal vanes 23 are provided, of which in each case those which are disposed next to the same crossing location 25 act in the same direction on a partial flow of the coolant which is flowing through the spacers 11 to 19 parallel to the fuel rods 4 to 7, so that a turbulent impulse D is imparted to a partial flow 26. The resultant rotary movement generates centrifugal acceleration in the partial flow 26, which forces the liquid phase of the coolant onto the fuel rods 4 to 7 and increases the cooling of these rods. In principle, the spacer as shown in FIG. 6, in which the mesh openings which are provided to receive the fuel rods 4 to 7 are formed by hollow cylindrical sleeves 24, which likewise bear the sheet-metal vanes 23 and impose the turbulent impulse D on the partial flow 26 of coolant flowing past them, acts in the same way. The vanes 23 shown are provided beneath the very top spacer on some (preferably all) spacers 23 belonging to group B, but are not present in group A or are only much smaller in this group. In this way, the hydraulic stability is increased, since compared to the upper part the pressure drop in the lower part of the fuel assembly should not be too low. It is also possible, with a view to achieving a low pressure loss in the upper part B, to dispense with springs, lugs or similar holding elements for the fuel rods on one or more spacers (e.g. at position 17 in FIGS. 1 to 4). FIGS. 5 and 6 show a spacer region, the mesh openings of which all have in each case one fuel rod passing through them (except for the positions which are taken up by a water tube). However, above the part-length fuel rods there are mesh openings through which no fuel rod passes. In this case, the configuration of the vanes 23 is advantageously unchanged, and at any rate the vanes still do not project into the region of the area which lies in a rectilinear continuation of the part-length fuel rods illustrated, i.e. into the area which is formed above the part-length fuel rods. However, springs, lugs or similar supports for the fuel rods may be absent in these mesh openings. In boiling water fuel assemblies, a configuration with at least one coolant tube is advantageous, in order to ensure that sufficient liquid moderator (cooling water) is present in the center of the fuel assembly even in the vapor/liquid zone of the fuel assembly. This is precisely the effect achieved by fuel rods whose length is shortened to from half to ⅔ of the normal fuel rod length. Hitherto, it has been assumed that, in the regular pattern in which the fuel rods are distributed across the cross section of the fuel assembly, all positions that are adjacent to the coolant tube configuration or a part-length fuel rod must be occupied by fuel rods of full length. The part-length fuel rods PL are disposed according to this rule in FIGS. 7 and 8. However, it is advantageous if at least a plurality of fuel rods which are directly adjacent to the coolant tube configuration are occupied by shortened fuel rods, as illustrated using fuel rods PLxe2x80x2 in FIGS. 7 and 8. FIG. 7 shows an example with two D-shaped tubes 30, 31 as the coolant tube configuration 30, 31, while FIG. 8 shows a single coolant tube 32 which is square in cross section as the coolant tube configuration. In both cases, the coolant tube configuration 30-32 covers a plurality of fuel rod positions and there is no fuel rod in the center of the fuel assembly. In a reactor core, a corner of a fuel rod channel 33 serves as a guide for a cross-shaped control rod 34, while the diametrically opposite corner is adjacent to an instrumentation tube 35 for measuring probes. This configuration of the measuring probes 35 and the control rod 34 in the gaps between outer surfaces of adjacent fuel assembly channels causes a relatively great width of the water-filled gaps, the gaps that carry the control rods often being wider than the other gaps. This leads to an uneven distribution of absorption material, moderator and fuel and therefore to inhomogeneity in the neutron flux and the power and burnup of the fuel rods. To achieve good utilization of the fuel at a sufficient distance from the power for transition to boiling, i.e. cooling which is adapted to the inevitable inhomogeneity, it is advantageous if the coolant tube configuration is not central, but rather is offset diagonally away from the control rod toward the opposite corner. This is achieved simply if the coolant tube configuration 30-32 is positioned correspondingly eccentrically in the pattern of the fuel rods. FIGS. 7 and 8 show some of the fuel rods 36 of full length and the webs of spacers 37, which are held at a predetermined distance from the inner surfaces of the fuel assembly channel 33 by suitable distancing elements 38, 39 which are disposed on the outer web of the spacers. FIGS. 7 and 8 show that advantageously the entire pattern of fuel rods disposed around the coolant tube configuration is also held in the same eccentric configuration in the channel. Accordingly, the distancing elements 38 produce a wider gap between the outer web of the spacer and the channel inner surface than the distancing elements 39. Furthermore, FIG. 8 shows that the shortened fuel rods are also advantageously distributed in a similarly eccentric manner across the fuel assembly cross section. A diagonal DG indicates the direction in which the coolant tube configuration and the entire pattern of fuel rods are offset with respect to the center axis of the fuel assembly channel. In the diagonal half of the channel cross section which is disposed symmetrically about the diagonal DG and is adjacent to the instrumentation tube 35 (i.e. is delimited by the second diagonal DGxe2x80x2 and includes at least the greater part of the coolant tube configuration), there are more shortened fuel rods PL and PLxe2x80x2 than in the other half which is disposed symmetrically about the diagonal DG and is adjacent to the control rod 34. Moreover, FIG. 8 shows that in the half which is delimited by the diagonal DGxe2x80x2, the walls of the water passage configuration 32 are advantageously closer to the adjacent fuel rods (e.g. PGxe2x80x2) compared to the fuel rods in the other half. In these figures, PL denotes fuel rods which are shortened to a lesser extent than the fuel rods PLxe2x80x2, i.e. the fuel rods PLxe2x80x2 have the shortest length. In accordance with FIG. 8, a plurality of fuel rods which are directly adjacent to the coolant tube configuration 32 are advantageously shortened to the shortest length, and at least most of the fuel rods with the shortest length are situated in the corresponding diagonal half of the channel cross section which is adjacent to the instrumentation tube 35. These rules relating to the configuration of the coolant tube configuration, of the fuel rods and in particular of the part-length fuel rods may advantageously be used even with boiling water fuel assemblies whose spacers are disposed at constant axial distances from one another. However, a particularly advantageous configuration results if the spacers in a lower region are at a constant distance from one another, but the spacers in the upper region are at a mean distance from one another which is shorter than the constant distance in the lower region.