Patent Number: 054815789
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, a representative example of a fuel assembly is shown generally at 10. The assembly includes a plurality of fuel rods 12 forming a bundle. The rods 12 are connected at their upper ends to an upper tie plate 14 and are supported at their lower ends by a lower tie plate grid, generally designated 16, which forms part of a lower tie plate assembly, generally designated 18. Spacers 20 are arranged at a plurality of vertically spaced locations to maintain lateral spacing of the fuel rods 12 relative to one another. The fuel bundle is disposed within a fuel bundle channel 22 whereby coolant water introduced through the bottom nozzle or inlet opening 24 of the tie plate assembly 18 flows upwardly through a flow volume 26 defined by a peripheral wall 28 of the lower tie plate assembly 18, through the lower tie plate grid 16, and then along and about the fuel rods 12. As indicated previously, it is important that debris in the coolant be prevented from flowing through the lower tie plate assembly and into the area between the channeled fuel rods 12. Referring now to FIGS. 2 and 3, there is illustrated a lower tie plate grid 16 according to the present invention, forming a part of the lower tie plate assembly 18. Lower tie plate grid 16 is preferably formed separately from the lower portion (including the peripheral wall 28 and the bottom nozzle 24) of the assembly, and secured thereto by, for example, welding. The lower tie plate grid 16 supports the fuel rods 12 above the grid and to this end, the grid 16 includes a plurality of generally cylindrical, vertically extending bosses 30 having centerlines arranged at corners of substantially square matrices of such bosses. Interconnecting (and forming the sides of) the square matrices are webs 32 adjoining the adjacent cylindrical bosses 30 along radial lines of the bosses 30 and extending between the upper and lower surfaces of the grid 16. Consequently, it will be seen that the webs 32 have portions formed along the sides of each square matrix and, together with convex outer portions of the cylindrical bosses 30, define side walls of openings or flow areas 34 which permit coolant to flow through the grid 16 and into the channeled fuel bundle assembly. The debris catching function is performed by a plurality of perforated tubes 36 (see FIG. 5), each having a cylindrical shape including a peripheral wall 38 formed with a plurality of substantially uniformly distributed perforations or flow openings 40. Each tube 36 is open at its lower end and, depending on how the fuel rods 12 are secured to the grid 16, may have open or closed upper ends. More specifically, for those lower tie plate arrangements where the fuel rod end plugs are simply supported by the bosses 30 of the grid 16, the upper ends of the tubes 36 will be closed or capped, and the tubes may be secured by any suitable means to the lowermost ends of the bosses 30. For those lower tie plate configurations, however, where the fuel rod end plugs are threaded into the grid bosses 30, the tubes may be formed with open upper ends (as shown in FIG. 5) and, again, the tubes 36 may be secured to the bosses 30 by any suitable means. A plate 42 (see FIG. 4) is secured to the upstream or lowermost ends of the array of tubes 36, such that flow openings 44 formed in the plate are aligned with the tube ends. The plate 42 is also sized and shaped so that it can be secured continuously about its periphery to the inner surface of the wall 28 of the lower tie plate assembly. In this way, substantially all coolant is constrained to flow into the tubes 36, through perforations or flow openings 40 and then through the flow areas 34 in the grid 16 and finally, upwardly into the fuel rod bundle. In alternative arrangements, the tubes 36 may be secured initially to the plate 42 and then plate 42 can be secured to the inner surface of wall 28. In this arrangement, the downstream ends of tubes 36 need only abut the underside of the grid, such that flow induced vibrations are held within acceptable limits. It will be appreciated that some leakage either in the area of plate 42 or at the tube/grid interface can be tolerated, so long as flow rate and pressure drop are not significantly altered. It is significant to the debris catching function of the tubes 36 that the coolant is forced to change direction in order to exit the tubes. In other words, as the flow direction changes substantially 90.degree., momentum of the debris entering the tubes 36 will generally carry the debris past the perforations 40 and impinge on the closed upper end of the tube or on the lower surface of the fuel rod end plug. Some debris, of course, may be too large to pass through the openings 40 in any event, while other debris of the long narrow type may have a cross-sectional area which might otherwise pass through the openings, but is nevertheless unable to negotiate the tortuous path through the tubes 36. Thus, the debris catcher effectively prevents debris from entering the fuel bundle area. With regard to pressure drop across the debris catcher, the flow area through the tubes 36 (i.e., through openings 40) is directly proportional to the axial length of the tubes. Thus, the flow area can be made large enough to produce lower fluid velocities through the openings 40. The objective is to have the total area of openings 40 at least equal the flow area through the grid to minimize velocity changes. To achieve this goal, the tubes 36 should have an axial length of between about 0.5 and about 1.0 inch and a diameter approximately equal to the bosses 30. By increasing the tube length, even better flow characteristics can be achieved. It will be recognized, of course, that the configuration of the lower tie plate assembly places practical limitations on the lengths of the tubes 36, and that only so many holes or openings 40 can be provided in the tubes 36 before the structural integrity of the tubes is negatively impacted. With these caveats, the objective is nevertheless to maximize flow area of the tube openings relative to the area of the grid opening. The overall result is minimal or no additional flow resistance and thus little or no additional pressure drop attributable to the debris catcher. 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.