Patent Number: 054715148
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1-3 show a fuel channel 1 of substantially square cross section. The fuel channel 1 surrounds, with no significant free space, an upper square portion 2a of a bottom part 2 which otherwise comprises a conical portion 2b and a cylindrical portion 2c. The bottom part 2 has a downwardly-facing inlet opening 3 for cooling water. Besides supporting the fuel channel 1, the bottom part 2 supports a supporting plate 4. At its lowermost part, the fuel channel 1 has a relatively thick wall portion which is fixed to the bottom part 2 and the supporting plate 4 by means of a plurality of horizontal bolts, indicated by dash-dotted lines 5. By means of a hollow supporting member 6 with cruciform cross section, the fuel channel 1 is divided into four vertical tubular parts 7 with at least substantially square cross section. The supporting member 6 is welded to the four walls 1a, 1b, 1c and 1d of the fuel channel 1 and has four hollow wings 8. The central channel formed by the supporting member 6 is designated 9 and is connected at the bottom to an inlet tube 10 for moderator water. Each tubular part 7 comprises a bundle 11 of twenty-five fuel rods 12. The rods 12 are arranged in a symmetrical lattice in five rows each containing five rods 12. Each rod 12 is included in two rows perpendicular to each other. Each bundle 11 is arranged with a bottom tie plate 13, a top tie plate 14 and a plurality of spacers 15. A fuel rod bundle 11 with bottom tie plate 13, top tie plate 14, spacer 15 and fuel channel 1 forms a unit which, in this application, is referred to as a sub-assembly, whereas the device illustrated in FIGS. 1-3 and comprising four such sub-assemblies is referred to as a fuel assembly. The four bottom tie plates 13 are supported in the fuel assembly by the supporting plate 4 and are each partially inserted into a corresponding square hole 16 therein. The holes for the passage of the water through the bottom tie plate 13 are designated 17a. According to the present invention, debris catchers 18 are arranged below the bottom tie plates 13 in the bottom part 2, that is, in the flow path for the water which flows to each one of the bottom tie plates 13. An example of debris catchers 18 according to the invention is shown in FIG. 1 and FIGS. 3-5. The debris-catching element consists of a plurality of helical springs 19 arranged in slots in a holder. In this embodiment the holder consists of three--a first 20, a second 21, and a third 22--substantially parallel plates where the two flat sides of the second plate 21 are provided with concentric, annular slots 23 whereas the first 20 and third 22 plates only have one flat side provided with such slots 23. Since the plates 20-22 are arranged with the flat sides adjoining each other, a parallelepiped is formed with annular channels 24 of circular cross section. The springs 19 are fixed in two levels between the plates 20-22 in these channels 24. The plates 20-22 are provided in the axial direction with through-holes 17 for the flow of cooling water through the fuel assembly. The plate 20 is provided with a flange 20a for fixing the debris catcher 18 in the bottom part 2. FIGS. 5 and 6 show an alternative embodiment in which the holder consists of concentric rings 25 fixed to each other in the axial and radial directions via a cruciform support member 26, to which the rings are fixed, for example, by welding. In an alternative embodiment (not shown), the rings 25 can be fixed by means of pins which extend through holes in several rings 25, to which pins the rings 25 are welded. The helical springs 19 are fixed between two layers of rings 25 in the spaces, slots 27, which are formed between the concentric rings 25. FIGS. 7-9 show a debris-catching element which consists of one single helical spring 19 adapted so as to form, per se, a plane spiral. FIG. 7 shows a holder seen in a section corresponding to 4--4 in FIG. 1. The holder comprises a plate 28 which is formed such that the slot 30 therein consists of a plane spiral corresponding to the plane spiral helical spring 19. In the axial direction the plate 28 is provided with through-holes 17 for the flow of cooling water through the fuel assembly. FIG. 8 shows a holder for the plane spiral helical spring 19 seen from above. The holder is formed in the same way as the holder shown in FIG. 5. The plane spiral helical spring is fixed, like the annularly arranged helical springs in FIG. 5, in the space, the slot 27 between the concentric rings 25. At the transition of the spring 19 between two slots 27, the spring 19 is stretched out such that a part of a winding turn is arranged around the ring 25 which lies between the slots 27 in question. FIG. 9 shows a holder which, like the plane spiral helical spring 19, is formed as a plane spiral 29. The spring 19 is arranged between the spiral turn of the plane spiral 29, that is, in the slot 30. The spiral is fixed radially and axially by means of a cruciform support member 26 corresponding to the support member shown in FIGS. 5, 6 and 8. The material in the holders consists, for example, of stainless steel or another material resistant to corrosion by the reactor water. The material in the springs 19 preferably consists of Inconel. The pitch of the helical springs 19 is determined by the demand for debris catching capacity. The helical springs 19 are made with tolerances which prevent the occurrence of a play between the springs 19 and the holders, thus eliminating the risk of abrasion caused by vibrations. To compensate for the pressure drop which arises across the debris catcher 18, the diameter of the through-holes 17a in the bottom plate 13 can be somewhat increased. According to a particularly preferred embodiment of the invention, the debris catcher 18 consists of a device which is parallel to the bottom tie plate 13 such that the symmetry axis of the helical springs 19 is horizontal. If the debris catcher 18 is arranged below the bottom tie plate 13, it has, in addition to the above-mentioned advantages, the advantage of being able to be inspected and cleaned. It is obvious that a debris catcher 18 of the kind described can be used in a fuel assembly which is not, as in the exemplified case, divided into sub-assemblies with separate bottom tie plates 13 but which consists of one single assembly with one single bottom tie plate 13 and is thus in analogous manner placed in the flow path of the water to the single bottom tie plate 13. The number of levels with helical springs 19 can, of course, be greater or smaller than what has been shown in the embodiments. It is also obvious that a debris catcher 18 of the kind described can be arranged below the bottom tie plate 13 of a pressurized-water reactor. In a pressurized-water reactor, because of the constructive design of the flow path of the water to the bottom tie plate, it is normally most suitable to allow the debris catcher 18 to make contact with the underside of the bottom tie plate.