Patent Number: 
Section: description

Referring now to the drawing figures, particularly to FIG. 1, there is illustrated a representative example of a fuel assembly, generally designated 10. Fuel assembly 10 includes a plurality of nuclear fuel rods 12 forming a nuclear fuel bundle 14 disposed with a fuel channel 16. The rods 12 are connected at their upper ends to an upper tie plate 18 and are supported at their lower ends in a lower tie plate grid 20 forming part of a lower tie plate 22. Spacers 18 are arranged at a plurality of vertically spaced locations along the fuel bundle to maintain lateral spacing of the fuel rods 12 relative to one another. The lower tie plate includes an inlet nozzle 24 for receiving coolant water for transmission through the lower tie plate 22, the tie plate grid 20 and upwardly therefrom for flow about the fuel rods for generating steam. Referring now to FIG. 2, there is illustrated a fuel rod support structure 29 comprised of a lower tie plate 30 constructed in accordance with a preferred embodiment of the present invention. The lower tie plate 30 includes a nozzle 31 adjacent its lower end for receiving water for flow upwardly through a transition structure 33 and through a tie plate grid 32 and for flow about the fuel rods 12. The tie plate grid 32 as illustrated in FIG. 2 lies adjacent the upper end of the tie plate 30 and is comprised of an array of cylindrical bosses 34 which extend between upper and lower surfaces of the tie plate grid 32 for receiving the cylindrical end plugs of the nuclear fuel rods for supporting the latter, as described hereinafter. The bosses 34 are arranged in a rectilinear array, a 10xc3x9710 array being illustrated. The centerlines of the bosses 34 are arranged at corners of substantially square matrices thereof. Interconnecting and forming the sides of the square matrices are webs 36 joining the adjacent cylindrical bosses 34. As will be appreciated from a review of FIG. 2, the upper edges 38 of the webs are recessed below the upper cylindrical edges 40 of the bosses. With this configuration, it will be seen that the webs 36 have portions formed along the sides of each square matrix and, together with convex outer portions of the cylindrical bosses 34, define coolant flow openings 42. Thus, coolant flow openings 42 extend between the upper and lower surfaces of the grid for flowing coolant from the inlet nozzle of the lower tie plate through the grid and upwardly about the fuel rods supported on the lower tie plate 30. The debris-catching function is performed by a filter plate 44 carried by the lower tie plate 30. As illustrated in FIG. 2, the filter plate 44 includes a plurality of holes 46 arranged in an identical array relative to the holes 48 of the bosses 34. Consequently, when the filter plate 44 overlies the lower tie plate grid, the holes 46 register with holes 48 through bosses 34, affording a combined opening for receiving the lower end plugs of the fuel rods. Also, registering holes are provided in the filter plate and the grid to receive end plugs of water rods 47. Referring now to FIG. 4, there is illustrated an enlarged view of the filter plate 44. As illustrated the filter plate 44 includes a plurality of apertures 50 between the holes 46. In a preferred embodiment of the present invention, the filter plate comprises a stainless steel plate having a thickness of 0.048 inches with staggered apertures 50 having diameters of 0.0625 inches on 0.094-inch centers. The number of apertures 50 through the filter plate is in excess of ten and preferably in excess of fifteen for each hole 46 through said filter plate 44. The cross-sectional area of each hole 46 is at least fifteen times and preferably twenty times the cross-sectional area of each aperture 50. As illustrated, each aperture 50 has six surrounding apertures in a hexagonal array of apertures. To provide perspective, the filter plate is preferably about 5.070 inches on a side, having holes 46 of 0.0287 inches diameter. The apertures 50 provide about at least 30% open area through the plate, with approximately 132 holes per square inch. The diameters of the holes 46 through plate 44 correspond to the inner diameters of the bosses 34 such that, upon application of the filter plate 44 in overlying relation to the tie plate grid, the plate 44 is wholly supported by the edges 40 of the bosses. Consequently, the flow openings 42 between the bosses and webs lie in direct alignment with the apertures 50. Additionally, because the upper edges of the webs are recessed below the upper edges of the bosses and hence the underside of the filter plate 44, the flow area through the filter plate includes each aperture 50 except those overlying the edges 40 of the bosses. The edges 38 of the webs 36 do not block the vertically registering apertures. It will be appreciated that the fuel rods of the fuel bundle are of different types. For example, certain of the fuel rods comprise tie rods for securing the fuel bundle to the lower tie plate. Those fuel rods comprising tie rods have end plugs at their lower ends which are threaded for threaded engagement with complementary female threads within associated bosses. Thus, as illustrated in FIG. 5, end plugs 58 of tie rods 56 have threaded male projections 57 for threaded engagement with the complementary female threads 59 of the bosses in which the end plugs reside. Additionally, part-length fuel rods, where applicable also have threaded end plugs. Consequently, a certain number of additional selected holes through the bosses are complementary threaded for receiving the threaded end plugs. The remaining holes 48 in the bosses 34 have smooth sides. Thus, the remaining fuel rods 55 have end plugs 60 having end projections 62 with smooth side surfaces for slidable reception within correspondingly smooth-sided bores. It will be appreciated that the end plugs 58, 60 pass through the registering holes 46 of the filter plate for reception in the holes of the bosses. For those fuel rods having smooth-sided end plugs 60, the end plugs are received through the registering holes 46 and 48 of the plate and grid, respectively, with the tapered side surfaces 61 of the end plugs bearing against the margins of the holes through the filter plate 44. This engagement and the weight of the fuel rods holds the filter plate 44 down on and against the upper edges 40 of the bosses 34 of the lower tie plate grid. The tapered surfaces 63 of the end plugs 58 having the threaded male extensions 57, however, are spaced from the margins of the holes 46 through the filter plate 44. That is, there is a discrete gap between the end plugs and the margins defining the holes through the filter plates which receive the threaded end plugs. This affords a tolerance for securing the threaded end plugs in the female threaded bosses. Consequently, there is provided a debris-catching filter plate with substantial reduction in hole size as compared with debris catchers of the prior art for minimizing or eliminating failure of the fuel bundles resulting from debris collection. Moreover, the assembly of the present invention is readily manufacturable and assembled with the tie plate without requiring additional parts. The prior art debris catcher integrally cast with the tie plate is thus eliminated, together with its associated problems. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.