Patent Number: 
Section: description

The invention relates to a fuel assembly for a boiling water reactor. A fuel assembly of this type comprises a bundle of fuel rods filled with nuclear fuel, which are held axially by a plurality of spacers. A water channel runs approximately centrally in the fuel-rod bundle, the lower end of the water channel carrying a stopper, through which a bore passes. The water channel is attached to the fuel assembly foot with the aid of said stopper. The fuel assembly foot itself, or at least part thereof, is formed by a frame part surrounding a sieve plate. In a fuel assembly known from EP 1 280 163 A1, the sieve plate includes a bore, into which a threaded sleeve is inserted from the underside of the sieve plate and soldered there. A soldering of this type requires an increased manufacturing outlay and is also problematic with respect to fatigue strength during use in a nuclear reactor. It is an object of the invention to provide a fuel assembly for a boiling water reactor, in which the fuel assembly foot is connected to the water channel in an alternative manner. This object is achieved by a fuel assembly with a skirt that is integrally formed on the underside of the stopper, which is arranged at the lower end of the water channel, and extends, in a rotationally-fixed manner, into an opening passing through the sieve plate. Furthermore, a bush is provided, having a first and a second longitudinal section, wherein the second longitudinal section projects from the underside of the sieve plate into the opening of the latter in a rotationally-fixed manner and wherein the outer side of a radial shoulder located between the two longitudinal sections bears against the underside of the sieve plate. Finally, the threaded bolt of a screw passes through the bush, which engages in a thread of the bore of the stopper. It is an advantage of the described connection that it completely dispenses with the complicated production of welded or soldered seams and therefore has a high fatigue strength, on the one hand, and low assembly outlay, on the other hand. The rotationally-fixed engagement of the skirt in the opening of the sieve plate rules out a change in the rotational position between water channel and fuel assembly foot. The rotational connection is preferably achieved in that the skirt has a polygonal contour and a region (which surrounds the skirt) of the screen-plate opening has a cross-sectional area complementary thereto. In a preferred embodiment, the mutually facing end faces of the second longitudinal section and of the skirt bear against one another, wherein the lengths of bush and skirt are such that there is a gap between the underside of the stopper and the upper side of the sieve plate. Thereby the prestress of the threaded bolt which is required for a fixed screw-connection is achieved by producing stress between the bush and the stopper. The sieve plate remains free from any introduction of force in the axial direction here. This is advantageous if the sieve plate is formed by elements flexible in said direction, for example by webs, which are arranged parallel to one another and have a curvature in the axial direction. When applying force in the axial direction, there is a risk that the webs will flex and the screw-connection will lose strength as a result. The proposed refinement, however, prevents this effectively. In order to prevent the screw from loosening by itself, the bush is arranged in the opening of the sieve plate in a rotationally-fixed manner and the screw is connected to the bush in a rotationally-fixed manner. In a preferred refinement, this is achieved in that the second longitudinal section of the bush has a polygonal contour and a region (which surrounds the bush) of the screen-plate opening has a cross-sectional area complementary thereto. The rotational lock between bush and screw is such that a circumferential region of the first longitudinal section of the bush interacts with the head of the bush in a torque-locking manner on account of a radially inwardly facing plastic deformation. The water channel is attached to the fuel assembly foot transversely with respect to the axial direction without play and in particular secure against tilting on account of the threaded bolt having a preferably thread-free longitudinal section, which extends away from the screw head and bears against the inner face of the bush by its outer circumferential face. The invention is now explained further by means of the exemplary embodiments illustrated in the attached drawings, wherein: FIG. 1 shows a fuel assembly of a boiling water reactor comprising a bundle of a multiplicity of fuel rods 1 filled with nuclear fuel. The fuel rods are attached laterally by a plurality of spacers 2 arranged in different axial positions. A water channel 4 extends in the axial direction 3 approximately centrally in the fuel-rod bundle. A fuel assembly top fitting 5 is arranged at the upper side of the fuel assembly and a fuel assembly foot 6 is arranged at the lower side. The fuel assembly foot 6 substantially comprises, as can be seen in FIG. 2, for example, a frame part 7 and a sieve plate 8. The sieve plate 8 is formed by a multiplicity of webs 9, which extend between two opposite walls 10, 12 of the frame part 7. The webs 9 are arranged, at a distance from one another, transversely with respect to their longitudinal direction. They are connected to one another in the same direction by struts 13 integrally formed thereon. An opening 14 passes through the sieve plate 8 at an eccentric position. The inner cross-sectional area of the opening 14 is in the shape of an octagon. To be more precise, the rims 15 (which define the opening 14) of the webs 9 bear against an imaginary cylinder of octagonal outline. For the purpose of attaching to the fuel assembly foot 6, a stopper 17, through which a threaded bore 16 passes, is arranged at the lower end of the water channel 4. A skirt 19 surrounding the threaded bore 16 projects in the axial direction from the underside 18 of the stopper 17, which underside faces the sieve plate 8 in the installed state. The outer circumferential face of the skirt 19 forms an octagonal cylinder casing and extends, in a rotationally-fixed manner, into the complementary opening 14 of the sieve plate 8. The stopper 17 itself has an approximately square contour. The edges 20 delimiting the underside 18 of the stopper 17 are beveled. A bush 22, which is subdivided into a first and a second longitudinal section 23, 24, is furthermore provided for the purpose of attaching the stopper 17 and the water channel 4. A radial shoulder 25 is arranged between the two longitudinal sections, as can best be seen in FIG. 6. The first longitudinal section 23 is annular and has a larger diameter than the second longitudinal section 24, which has an octagonal outline complementary to the opening 14. In the installed state (FIG. 3, 5, 6), said second longitudinal section 24 extends into the opening 14 in a rotationally-fixed manner. The bush 22 bears here against the underside 27 of the sieve plate 8, and the webs 9 forming it, by the outer side 26 of the radial shoulder 25. The length of the skirt 19 and of the second longitudinal section 24 of the bush 22 is in each case such that their sum is smaller than the thickness 28 (FIG. 5) of the sieve plate 8 or the height of the webs 9 forming it. Therefore there is an axial distance 32 between the mutually facing end faces 29, 30 of skirt 19 and bush 22. Furthermore, a screw 34 having a threaded bolt 35 and a head 36 is provided for attaching the water channel 4, wherein the threaded bolt 35 extends through the bush 22 and into the threaded bore 16 of the stopper 17. The threaded bolt 35 has a thread-free collar 33 extending axially away from the head 36. A recess 37 is located between the collar 33 and that section of the threaded bolt 35 which has a thread. The head 36 is supported on the inner side 38 of the radial shoulder 25. The bush 22 is thereby pressed against the underside 27 of the sieve plate 8 by the outer side 26 of the radial shoulder in the stressed state. The underside 18 of the stopper 17 here presses against the upper side 40 of the sieve plate. The fuel assembly foot 6, the bush 22 and the screw 34 are made of austenite. The water tank and the stopper 17 bounding it on the underside, on the other hand, are made of zircaloy. The first longitudinal section 23 of the bush 22 has a wall thickness which permits a radially inwardly directed deformation 41. As can be seen in FIGS. 3 and 4, two such deformations 41 have been realized at diametrically opposed locations of the first longitudinal section 23. The deformations 41 extend into two diametrically opposed notches 39 in the head 36. This fixes the screw 34 with respect to the bush 22 in terms of rotation. The latter, in turn, is fixed in terms of rotation in the opening 14. The screw 34 is thereby prevented from loosening by itself. The exemplary embodiment illustrated in FIG. 7-9 differs from that mentioned above merely in that the longitudinal section 24a of the bush 22 is longer, wherein the length of said section and the length of the skirt 19 are such that their sum is greater than the thickness 28 of the sieve plate 8. This forms a gap 42 between the upper side 40 of the sieve plate 8 and the underside 18 of the stopper 17. During stressing, the stopper 17 is therefore not supported on the upper side 40 of the sieve plate 8, but on the bush 22. This eliminates the risk that the webs 9 corrugated in the axial direction 3 will flex resiliently in this direction, which would lead to an undesired deformation of the webs in the axial direction under certain circumstances to produce the required prestress.     1 fuel rod    2 spacer    3 axial direction    4 water channel    5 fuel assembly    6 fuel assembly foot    7 frame part    8 sieve plate    9 web    10 wall    12 wall    13 strut    14 opening    15 edge    16 threaded bore    17 stopper    18 underside    19 skirt    20 edge    22 bush    23 first longitudinal section    24 second longitudinal section    25 radial shoulder    26 outer side    27 underside    28 thickness    29 end face    30 end face    32 axial distance    33 color    34 screw    35 threaded bolt    36 head    37 recess    38 inner side    39 notch    40 upper side    41 deformation    42 gap