Patent Number: 058964363
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a prior art boiling water reactor fuel assembly 1 which comprises a long tubular container, of rectangular cross section, referred to as fuel channel 2. The fuel channel 2 is open at both ends to form a continuous flow passage through which the coolant of the reactor flows. The fuel assembly 1 comprises a large number of equally long tubular fuel rods 3, arranged in parallel in a bundle, in which pellets 4 of a nuclear fuel are arranged. The fuel rods 3 are retained at the top by a top tie plate 5a and at the bottom by a bottom tie plate 6. The fuel rods 3 are kept spaced from each other by means of spacers 7 and are prevented from bending or vibrating when the reactor is in operation. Through two of the openings in the top tie plate 5a, there are adapted to run extra long fuel rods 3a, which partly extend above the top tie plate 5a. The two extra long fuel rods 3a are fixed to the bottom tie plate 6 and are each provided with a nut 8 at the upper side of the top tie plate 5a. The other rods 3 rest on the bottom tie plate 6 and make contact with the lower side of the top tie plate 5a by means of a helical spring 9 arranged around the respective end plugs 10 of the rods 3. The helical springs 9 press the top tie plate 5a against these nuts 8, whereby the nuts 8 limit the maximum distance between the top and bottom tie plates 5a, 6. The two extra long fuel rods 3a are arranged in a tensile-force-transmitting connection between the top and bottom tie plates 5a, 6 while at the same time limiting the maximum distance between the top and bottom tie plates 5a, 6. FIG. 2 shows in a section A--A in FIG. 1, in the upper righthand corner, a prior art top tie plate 5a for a sub-assembly and, in the lower lefthand corner, a top tie plate 5b for a sub-assembly according to the invention. The prior art top tie plate 5a is described under the background art. The top tie plate 5b for a sub-assembly according to the invention is shown more clearly in FIGS. 4a and 4b. FIG. 4a shows the top tie plate 5b intended for a sub-assembly in a view from above. Only one of the fuel rods, designated 3b, is provided with a nut 8 at the upper side of the top tie plate 5b. The top tie plate 5b shown in FIG. 4a has a more open structure than that shown in FIG. 3 in that a plurality or bars 5c are removed whereby the thickness of the material has been reduced. In principle, the top tie plate 5b may be designed as a conventional spacer 7. From FIG. 4b, in a view from the side of the top tie plate 5b, it is clear that only one of the fuel rods 3, that is, the fuel rod designated 3b, is arranged to fix the top tie plate 5b between a nut 8 and a collar 12 on the end plug 10. Two of the fuel rods 3a are extra long and adapted to extend above the top tie plate 5b. These two extra long fuel rods 3a are arranged fixed to the bottom tie plate 6 to obtain a tensile-force-transmitting connection therewith. The fuel rods which are fixed to the top tie plate 5b and/or the bottom tie plate 6 are detachably arranged therein. The other rods 3 in the bundle rest on the bottom tie plate 6 and are adapted to freely run through the top tie plate 5b. FIG. 4c shows an alternative embodiment of a top tie plate 5d intended for a sub-assembly. The top tie plate 5d comprises a plurality of tubular cells 5e. The cells 5e have a diameter which substantially corresponds to the distance between the center lines of the fuel rods 3 retained by the top tie plate 5d. The cells 5e are joined to each other in an orthogonal lattice. The end plugs 10 of the fuel rods 3 are arranged in the spaces 5f formed between the cells 5e in the top tie plate 5d. The cells 5e surrounding the extra long fuel rods 3a are designed such that a substantially square cross-section is imparted to the space 5g, suitable for receiving and supporting these cells. FIG. 5 shows, in a view from above, an alternative embodiment of a top tie plate 5h for a fuel assembly comprising a bundle of fuel rods 3. One of the fuel rods, designated 3b, is provided with a nut 8 at the upper side of the top tie plate 5h. Eight of the fuel rods are extra long fuel rods 3a and are arranged fixed to the bottom tie plate 6 to obtain a tensile-force-transmitting connection therewith.