Debris-resistant lower tie plate assembly

Abstract: A relatively low profile lower tie plate assembly (14) for interconnecting the lowermost end of an array of reactor fuel rods and guide tubes (11) and for capturing debris carried by flowing coolant comprises substantially planar, spaced apart, upper and lower members (20,22) and a skirt member (28) extending along the periphery of, enclosing and interconnecting the upper and lower planar members (20,22), the lower planar member (22) including a relatively small number of large openings (25) adapted for flow of coolant into the assembly in a vertical direction, the upper planar member comprising an array of a large number of relatively small, debris-resistant, coolant flow holes (24) dimensioned to trap at least a portion of the debris particles carried by the coolant. The assembly further comprises a plurality of guide tube screws (31) extending through and fastened to the lower planar member (22) and further extending through the upper planar member (20) to a height sufficient to support and position lowermost extremes of associated guide tubes (11) and surrounding guide tube sleeves (35) which project upwardly from the upper member (20), the portions (39) of the guide tube screws (31) which extend through the lower member (22) being adapted for engaging alignment (40) pins associated with a support plate (26) for the tie plate assembly.

Description: FIELD OF THE INVENTION This invention is directed to a debris-resistant nuclear fuel assembly and, more particularly to such an assembly employing a lower tie plate unit constructed to collect potentially harmful debris carried in the flow of coolant material (e.g. water). The invention is particularly useful in combination with a debris-resistant grid spacer arrangement of the type described in U.S. Patent No. 4,849,161, granted to Charles A. Brown et al. on July 18, 1989, which patent is assigned to the same assignee as the present invention. BACKGROUND OF THE INVENTION As is stated in the Brown et al patent, in the operation of nuclear reactors such as pressurized water reactors (PWR's), it has been found that debris such as nuts, bolts, metal cuttings, wire, splices and grommets, drill bits, and pieces of wire sometimes accumulate in the reactor during construction, repair or the like. Certain mid-range sizes (1/2″ to 4″) of this type of debris are particularly troublesome, since that debris is likely to be carried by cooling water to the area near the bottom (lower ends) of the fuel rods. The debris vibrates in the moving coolant and impacts principally upon lower ends of the rods, ultimately abrading and wearing away the fuel rod cladding at that point. As is noted in the Brown et al patent, one prior approach to this problem was to use extra long lower end caps on the fuel rods, which end caps did not contain fuel and therefore there would be no escape of radiation if a crack or other opening occurred in the end caps. However, that approach of using elongated end caps may result in a reduction of power output for a given overall size of the reactor. In the Brown et al. patent, a lowermost grid spacer is described which is positioned on or only slightly above the lower tie plate. The geometry of the Brown grid spacer is arranged to divide the coolant flow openings in the lower tie plate into smaller openings and thereby trap at least part of the debris in the zone near the lower tie plate before the debris comes in contact with the fuel rods. As an indication of the significance of the debris problem, reference may be made, for example, to recently issued U.S. patents such as No. 4,652,425 - Ferrari et al., granted March 24, 1987; No. 4,684,495 - Wilson et al., granted August 4, 1987; No. 4,684,496 - Wilson et al., granted August 4, 1987; No. 4,781,884 - Anthony, granted November 1, 1988; No. 4,828,791 - De Mario, granted May 9, 1989 and No. 4,832,905 - Bryan et al., granted May 23, 1989. Particularly in US-A-4,832,905 and 4,919,883 a debris collecting lower tie plate assembly for collecting debris is disclosed which is assembled in the form of a welded "egg crate" grid. In a lower plane the cross area is devided into several rectangular elongated openings while in an upper plane directly above the lower plane the cross area of the assembly is divided in rectangular elongated openings the direction of elongation in the upper plane being perpendicular to the direction in the lower plane. Guide tubes are fastened to the tie plate assemblies by screws which are introduced from below through sleeves forming hollow legs for positioning the tie plate assembly on a core plate and extend through the tie plate assembly into the lower ends of the guide tubes to which they are screwed. While a number of the foregoing proposals to reduce the debris problem have focussed remedial attention on the region in the vicinity of the lower tie plate, certain of the proposals have required that additional space in the vicinity of the tie plate be taken from the length of the active fuel rods in order to insert means to accomplish the desired collection of debris. Other proposed approaches also may result in an unwanted drop in coolant pressure, thereby adversely affecting the desired heat transfer to the coolant. The present invention, on the other hand, is directed towards a lower tie plate assembly of reduced vertical dimension (i.e. a low profile tie plate) which is capable of trapping significant additional debris and, furthermore, is well suited for use in combination with the debris-resistant grid spacer described by Brown et al. referred to above. STATEMENT OF THE INVENTION In accordance with one aspect of the present invention, a relatively low profile lower tie plate assembly for interconnecting the lowermost end of an array of reactor fuel rods and guide tubes and for capturing debris carried by flowing coolant comprises substantially planar, spaced apart, upper and lower members and a skirt member extending along the periphery of, enclosing and interconnecting the upper and lower planar members, the lower planar member including a relatively small number of large openings adapted for flow of coolant into the assembly in a vertical direction, the upper planar member comprising an array of a large number of relatively small, debris-resistant, coolant flow holes dimensioned to trap at least a portion of the debris particles carried by the coolant. Upper planar member, lower planar member and skirt member form a cavity. The assembly further comprises a plurality of guide tube screws extending through and fastened to the lower planar member and further extending through the upper planar member to a height sufficient to support and position lowermost extremes of associated guide tubes and surrounding guide tube sleeves which project upwardly from the upper member, the portions of the guide tube screws which extend through the lower member being adapted for engaging alignment pins associated with a support plate for the tie plate assembly. In accordance with a further aspect of the present invention, a lower tie plate assembly for interconnecting the lowermost end of an array of reactor fuel rods and guide tubes and for capturing debris carried by flowing coolant comprises substantially planar, spaced apart, upper and lower members and a skirt member extending along the periphery of, enclosing and interconnecting the upper and lower planar members, the lower planar member including a relatively small number of large openings adapted for flow of coolant into the assembly in a vertical direction, the upper planar member comprising an array of a large number of relatively small, debris-resistant, coolant flow holes dimensioned to tray at least a portion of the debris particles carried by the coolant. The relatively large openings in the lower planar member are flared inwardly towards a cavity formed by the upper planar member, the lower planar member and the skirt member, the large openings being dimensioned to produce turbulence in the flow of coolant into the cavity to facilitate trapping debris in the cavity. BRIEF DESCRIPTION OF THE DRAWING In the drawing: FIGURE 1 is an elevation view, partially in section, of a fuel assembly incorporating one version of the present invention, the assembly being foreshortened in height and partially broken away for convenience and clarity; FIGURE 2 is a view, drawn to a different scale than FIGURE 1, along the line A-A′ shown in FIGURE 1 (i.e. a top view of the assembly); FIGURE 3 is a sectional view, drawn to the same scale as FIGURE 2, along the line B-B′ shown in FIGURE 1; FIGURE 4 is a view, drawn to the same scale as FIGURE 3, along the line C-C′ shown in FIGURE 1 (i.e a bottom view of the assembly); FIGURE 5 is a plan view of the lower tie plate and associated guide tubes shown in FIGURE 1; FIGURE 6 is a sectional view taken along the line D-D′ in FIGURE 5, including a portion of a lowermost grid spacer which is shown in FIGURE 1 but was eliminated from FIGURE 5 for clarity; and FIGURE 7 is a sectional view similar to FIGURE 6 of an alternative embodiment of the invention having different dimensional relationships, as well as a different guide tube end fitting and cap screw arrangement. DETAILED DESCRIPTION Referring to Figure 1, a 14 x 14 fuel bundle assembly is indicated generally by the reference numeral 10. The fuel assembly 10 includes an upper tie plate 12 and a lower tie plate 14 capturing at opposite ends a plurality of (e.g. 176) fuel rods 13. A plurality of (e.g. five) guide tubes 11 are secured to upper tie plate 12 (see Figure 2) and, as will appear below, to lower tie plate 14. A plurality of grid spacers 15 (e.g. eight) are disposed along the length of the fuel rods 13 at locations between the tie plates 12, 14 and form cells, as is well known, through which the fuel rods 13 and guide tubes 11 extend. A lowermost one 15′ of the grid spacers is illustrated as a debris-resistant spacer of the type shown and described in U.S. Patent No. 4,849,161 of Brown et al. The description of that debris-resistant spacer 15′ contained in the Brown et al. patent is herein incorporated by reference. Each of the fuel rods 13 includes a stack of fuel pellets 16. The pellets 16 in each stack are maintained in close proximity to each other by means of a spring 17 disposed between an upper end of the rod 13 and the uppermost one of the pellets 16. The lower end cap 18 of each fuel rod is in close proximity to but spaced away from the upper portion of lower tie plate 14 to take into account the expected linear growth of the rods 13 in the normal operation of the reactor. The total height from the bottom of lower tie plate 14 to the top of the uppermost pellet 16 (i.e. the top of the active fuel) may, for example, be several centimeters less than 3,66 m (i.e. a few inches less than twelve feet). Referring now to Figures 4-6, details of the lower tie plate assembly 14 constructed in accordance with the present invention are shown. As can be seen in the cross-sectional view of Figure 6, lower tie plate 14 comprises a multi-apertured, upper planar member 20 (the plan view of which is shown in Figure 5) and a multi-apertured, lower planar member 22 (the bottom view of which is shown in Figure 4). The upper planar member 20 contains a relatively large number of relatively small diameter (e.g. 0.238 inches diameter ≈ 6 mm) holes 24 while the lower planar member 22 includes a relatively small number (e.g. four) of large openings 25 (e.g. 2.25 inches ≈ 57 mm in diameter), the large openings 25 being adapted for flow of coolant into the assembly in a vertical direction as shown by the arrow in Figure 6. The lower planar member 22 is designed to rest upon a lower core support plate 26 (shown partially broken away) such that the large openings 25 are aligned with coolant channels 27 in lower core support plate 26. As is shown in Figure 6, the inlet (lower) diameter of the large openings 25 is greater than the outlet (upper) diameter of the immediately adjacent coolant channels 27 in core support plate 26. Furthermore, the outlet (upper) diameter of the large openings 25 is greater than the inlet diameter of openings 25. To that end, a flared (quarter round) outlet 21 is provided for opening 25 to effect a desired pressure drop and particular turbulent flow of the coolant as it enters a cavity 23 formed between upper planar member 20, lower planar member 22 and a surrounding skirt member 28. As can be seen in Figures 1 and 6, skirt member 28 may be provided, if desired, with coolant bypass holes 29, according to the flow requirements of a particular reactor installation. Referring to Figure 6, it should be noted that the thickness of lower planar member 22 is approximately twice the thickness of upper planar member 20. The greater thickness of lower planar member 22 facilitates threading of a plurality (four) of holes 30 which extend through lower support member 22 and receive guide tube screws 31. The guide tube screws 31 are of special shape and material (preferably Inconel) and include first, second and third shoulders 32, 33 and 34 of progressively decreasing diameter which are dimensioned to abut, respectively, against complementary shoulders in upper planar member 20, at the lowermost end of a guide tube sleeve 35 and at the lowermost shoulder of an internally threaded guide tube nut 36. The guide tube nut 36 is fastened (e.g welded) into the lower end of guide tube 11 and projects beyond the end of guide tube 11 so as to fit within guide tube sleeve 35 and engage a threaded uppermost end of guide tube screw 31. In the normal assembly of the fuel assembly 10, the guide tubes 11 and complementary guide tube sleeves 35 are assembled in a fixture along with the fuel rods 13. Thereafter, when the guide tubes 11 and sleeves 35 are brought into contact with the upper surface of the upper planar member 20, the guide tube screws 31 are threaded into lower planar member 22 and ultimately into guide tube nuts 36 to fasten the lower tie plate 14 to the fuel assembly 10. The guide tube screws 31 are formed with a hollow conical central portion 39 at their lower portion, the hollow portion 39 being complementary to alignment pins 40 extending upwardly from lower core support plate 26. A guide tube bleed hole 41 is also provided through the center of each of guide tube screws 31. The central guide tube llc passes through upper planar member 20 and is provided with a lateral bleed hole 41c extending into the cavity 23. The central guide tube 11c as illustrated is supported in a conventional manner without the use of a guide tube sleeve or guide tube screw. The small diameter (6,05 mm ≃ 0.238″) holes 24 in upper planar member 20 are illustrated as vertically oriented apertures. However, in certain installations, other geometric shapes may be appropriate (see for example, U.S. Application Serial No. 89 E 7499 of Yates, Patterson and Ewing which is assigned to the same assignee as the present invention). The apertures in upper planar member 20, in conjunction with the apertures in lower planar member 22 and the skirt 28 operate to trap a large proportion of debris which has been found to cause undesired wear of the lower ends 18 of rods 13. Specifically, it has been observed that the flow of coolant through the large openings 25 closely follows the rounded corner 21 of the outlet of openings 25 into cavity 23. As a result, typical debris such as metal shavings, wires, nuts and the like carried by the coolant are diverted away from a straight flow path upwardly through cavity 23. The resulting turbulent coolant flow into cavity 23 causes the debris to remain in cavity 23 to a significant degree. The small apertures 24 in upper planar member 20, of course, prevent passage of large pieces of debris as well. The overall effect of the relative sizes and shapes of the large apertures 25, small apertures 24 and cavity 23 have been found to produce an advantageous effect, the degree of which would not be likely to be anticipated prior to actual use of the apparatus. It should also be noted that the advantageous filtering effect of lower tie plate 14 has been obtained at the same time that the height of the lower tie plate 14 has been reduced by more than 25% (i.e. from 68 mm ≈ 2.677 inches to 50,8 mm ≈ 2.00 inches). This reduction in height of lower tie plate 14 allows for a corresponding addition to the end caps 18 of fuel rods 13 without reducing the power capabilities of the reactor. The longer end caps 18 provide a greater margin of safety for debris resistance. The reduced height of lower tie plate 14, however, can be obtained without reducing the flow of coolant to an undesirable extent. Furthermore, although the thickness of the upper planar member 20 is reduced as compared to earlier designs, the redesigned guide tube attachment screws 31 which are fastened (threaded) to lower planar member 22 provide the dual function of guide tube attachment and mating with the reactor alignment pins 40 carried on lower core support plate 26. The use of Inconel material for the guide tube screws 31 is particularly advantageous for reducing the potential for damage to screws 31 in handling or by impact forces. The foregoing arrangement provides an enhanced degree of resistance to debris. At least a portion of the enhancement is believed to be attributable to the fact that back pressure created in cavity 23 by virtue of the presence of the multi-apertured upper member 20, in combination with the differences in diameters of the juxtaposed coolant flow channels 27, 25 at the inlet to the lower tie plate 14, causes the coolant flow to follow the radius at outlet 21 and thereby create turbulence in cavity 23. Movement of debris particles through lower tie plate 14 is thereby interrupted and debris particles are advantageously collected and retained in cavity 23. Referring now to Figure 7, details of an alternative lower tie plate assembly 14′ constructed in accordance with a further aspect of the present invention are shown. Reference numerals used in Figure 7 are similar to those used in Figure 6, but are followed by a prime symbol (′) to indicate a difference compared to a corresponding part in Figure 6. In Figure 7, lower tie plate 14′ comprises a multi-apertured, upper planar member 20 and a multi-apertured, lower planar member 22′. The upper planar member 20 contains a relatively large number of relatively small diameter (e.g. 6 mm ≈ 0.238 inches diameter) holes 24 while the lower planar member 22′ includes a relatively small number (e.g. four) of large openings 25′ (e.g. 57 mm ≈ 2.25 inches in diameter), the large openings 25′ being adapted for flow of coolant into the assembly in a vertical direction as shown by the arrow in Figure 7. The lower planar member 22′ is designed to rest upon a lower core support plate 26′ (shown partially broken away) such that the large openings 25′ are aligned with coolant channels 27′ in lower core support plate 26′. As is shown in Figure 7, the inlet (lower) diameter of the large openings 25′ is equal to or slightly greater than the outlet (upper) diameter of the immediately adjacent coolant channels 27′ in core support plate 26′. Furthermore, the outlet (upper) diameter of the large openings 25′ is greater than the inlet diameter of openings 25′. To that end, a flared (quarter round) outlet 21′ is provided for opening 25′ to effect a desired pressure drop and particular turbulent flow of the coolant as it enters a cavity 23 formed therebetween upper planar member 20, lower planar member 22′ and a surrounding skirt member 28. As can be seen in Figure 6, skirt member 28 may be provided, if desired, with coolant bypass holes according to the flow requirements of a particular reactor installation. In some applications, the lower tie plate assembly 14′ may include bosses 34′ which abut against the lowermost end of a guide tube sleeve 35 and the lowermost shoulder of an internally threaded guide tube nut 36. The guide tube nut 36 is fastened (e.g. welded) into the lower end of guide tube 11 and projects beyond the end of guide tube 11 so as to fit within guide tube sleeve 35 and engage a threaded uppermost end of guide tube screw 31′. In the normal assembly of the fuel assembly 10, the guide tubes 11 and complementary guide tube sleeves 35 are assembled in a fixture along with the fuel rods 13. Thereafter, when guide tubes 11 and sleeves 35 are brought into contact with the upper surface of guide bosses 34′, the guide tube screws 31′ are threaded into guide tube nuts 36 to fasten the lower tie plate 14′ to the fuel assembly 10. The guide bosses 34′ may be formed with a hollow conical central portion 39′ at their lower portion, the hollow portion 39′ being generally complementary to alignment pins 40 extending upwardly from the lower core support plate 26′. A guide tube bleed hole may also be provided through the center of each of guide bosses 34′. Although the thickness of the upper planar member 20 is reduced as compared to earlier designs, the redesigned guide tube bosses 34′ allow attachment screws 31 to be fastened (threaded) to the guide tube nut (end fitting) 36 which provides sufficient clearance for alignment pins 40 carried on lower core support plate 26′. While the invention has been described in terms of particular embodiments, obvious modifications may occur to persons skilled in this art, and such modifications are intended to be covered by the following claims.

Claims: A relatively low profile lower tie plate assembly (14) for interconnecting the lowermost end (18) of an array of reactor fuel rods (13) and guide tubes (11) and for capturing debris carried by flowing coolant, the tie plate assembly comprising; substantially planar, spaced-apart, upper and lower members (20; 22) and a skirt member (28) extending along the periphery of, enclosing and interconnecting said upper and lower planar members (20; 22), said lower planar member (22) including a relatively small number of large openings (25) adapted for flow of coolant into said assembly in a vertical direction, said upper planar member (20) comprising an array of a large number of relatively small, debris-resistant, coolant flow openings (24) dimensioned to trap at least a portion of debris particles carried by said coolant; a cavity (23) formed by said upper planar member (20), said lower planar member (22) and said skirt member (28); the assembly further comprising a plurality of guide tube screws (31) extending through and fastened to said lower planar member (22) and further extending through said upper planar member (20) to a height sufficient to support and position lowermost extremes of associated guide tubes (11) and surrounding guide tube sleeves (35) which project upwardly from said upper member, the portions of said guide tube screws (31) which extend through said lower member (22) being adapted for engaging alignment pins (40) associated with a support plate (26) for said tie plate assembly. A relatively low profile lower tie plate assembly according to Claim 1 wherein: said coolant flow openings (24) in said upper planar member (20) comprise generally vertically oriented holes extending through said upper member, said holes being of substantially equal size and in a regular array. A relatively low profile lower tie plate assembly according to Claim 1 wherein: said coolant flow openings (25) in said lower planar member (22) comprise generally vertically oriented holes extending through said lower member, said holes being of substantially equal size and in a regular array. A relatively low profile lower tie plate assembly according to Claim 1 wherein: said relatively large openings (25) in said lower planar member comprise a substantially cylindrical lower portion and a flared upper portion (21) opening into said cavity (23). A relatively low profile lower tie plate assembly according to Claim 4 wherein: said flared upper portion (21) comprises a rounded lip at an entry into said cavity (23) proportioned to produce turbulent flow of said coolant into said cavity. A relatively low profile lower tie plate assembly according to Claim 1 wherein: said lower planar member includes a plurality of threaded holes (30) for fastening said guide tube screws (31) to said assembly. A relatively low profile lower tie plate assembly according to Claim 1 wherein: said guide tube screws (31) include a hollow central portion (39) at the lowermost portion thereof adapted to engage alignment pins (40) on an associated support plate. A relatively low profile lower tie plate assembly according to Claim 1 wherein: said guide tube screws include a threaded end portion extending through said upper planar member for engagement with an associated guide tube nut (36). A relatively low profile lower tie plate assembly according to Claim 8 wherein: said guide tube screws further include at least one shoulder (32; 33; 34) for engaging said upper planar member (20). In a fuel assembly (10) comprising an upper tie plate (12) and a lower tie plate (14), a plurality of fuel rods (13) extending in a region between said tie plates, a plurality of guide tubes (11) extending between and secured to said upper and lower tie plates (12; 14), a plurality of grid spacers (15; 15′) disposed along said fuel rods at locations between said tie plates and forming cells through which said fuel rods and guide tubes extend, at least said lower tie plate including a plurality of relatively large openings (25) at a lowermost extreme through which coolant flows into said assembly, said lowermost extreme of said lower tie plate (14) being adapted for support by a lower core support plate (26), said support plate having alignment pins (40) projecting therefrom, and said guide tubes (11) being adapted for support within guide tube sleeves (35) extending upwardly from said lower tie plate, the improvement comprising: said lower tie plate (14) comprising substantially parallel, planar spaced-apart upper and lower members (20; 22) and a skirt member (28) extending along the periphery of and interconnecting said upper and lower planar members, said lower planar member (22) including said relatively large openings (25) adapted for flow of coolant, said upper planar member (20) comprising an array of relatively small, debris-resistant, coolant flow openings (24) dimensioned to trap debris particles carried by said coolant; a cavity (23) formed by said upper planar member (20), said lower planar member (22) and said skirt member (28); the assembly further comprising a plurality of guide tube screws (31) extending through and fastened to said lower planar member (22) and further extending through said upper planar member (20) to a height sufficient to support and position the lowermost extremes of associated ones of said guide tube sleeves (35) and said guide tubes (11), the lowermost portions of each of said guide tube screws (31) having a hollowed central portion (39) adapted for engaging said alignment pins (40). In a fuel assembly, apparatus according to Claim 10 wherein: said coolant flow openings in said upper planar member comprise generally vertically oriented holes (24) extending through said upper member (20), said holes being of substantially equal size and in a regular array. In a fuel assembly, apparatus according to Claim 10 wherein: said coolant flow openings (25) in said lower planar member (22) comprise generally vertically oriented holes extending through said lower member, said holes being of substantially equal size and in a regular array. In a fuel assembly, apparatus according to Claim 10 wherein: said relatively large openings (25) in said lower planar member comprise a substantially cylindrical lower portion and a flared upper portion (21) opening into said cavity (23). In a fuel assembly, apparatus according to Claim 13 wherein: said flared upper portion (21) comprises a rounded lip at an entry into said cavity (23) proportioned to produce turbulent flow of said coolant into said cavity. In a fuel assembly, apparatus according to Claim 10 wherein: said lower planar member includes a plurality of threaded holes (30) for fastening said guide tube screws (31) to said assembly. In a fuel assembly, apparatus according to Claim 10 wherein: said guide tube screws include a threaded end portion extending through said upper planar member for engagement with an associated guide tube nut (36). In a fuel assembly, apparatus according to Claim 16 wherein: said guide tube screws further include at least one shoulder (32; 33; 34) for engaging said upper planar member (20). In a fuel assembly (10) comprising an upper tie plate (12) and a lower tie plate (14), a plurality of fuel rods (13) extending in a region between said tie plates, a plurality of guide tubes (11) extending between and secured to said upper and lower tie plates (12; 14), a plurality of grid spacers (15, 15′) disposed along said fuel rods at locations between said tie plates and forming cells through which said fuel rods and guide tubes extend, at least said lower tie plate including a plurality of relatively large openings (25) at a lowermost extreme through which coolant flows into said assembly, said lowermost extreme of said lower tie plate (14) being adapted for support by a lower core support plate (26), said support plate having coolant flow channels (27) therethrough and having alignment pins (40) projecting therefrom, and said guide tubes (11) being adapted for support within guide tube sleeves extending (35) upwardly from said lower tie plate (14) the improvement comprising: said lower tie plate comprising substantially parallel, planar spaced-apart upper and lower members (20; 22) and a skirt member (28) extending along the periphery of and interconnecting said upper and lower planar members (20; 22), said lower planar member (22) including said relatively large openings (25) adapted for flow of coolant, said upper planar member comprising an array of relatively small, debris-resistant, coolant flow openings (24) dimensioned to trap debris particles carried by said coolant; said relatively large openings (25) in said lower planar member (22) being flared inwardly towards a cavity (23) formed by said upper and lower planar members (20; 22) and said skirt member (28) and dimensioned to produce turbulence in the flow of coolant into said cavity to facilitate trapping debris in said cavity. Apparatus according to Claim 18 wherein: said relatively large openings in said lower planar member comprise a substantially cylindrical lower portion and a flared upper portion (21) opening into said cavity (23). Apparatus according to Claim 19 wherein said flared upper portion comprises a rounded lip at an entry into said cavity (23). Apparatus according to Claim 19 wherein the diameter of said upper portion is greater than the diameter of said lower portion of said large openings (25). Apparatus according to Claim 19 wherein said coolant flow openings in said planar upper member (20) comprise generally vertically oriented holes (24) extending through said upper member. Apparatus according to Claim 22 wherein said holes through said upper member (20) comprise an opening (24) in a lower surface of said upper member with a maximum dimension between opposite sides of said last-named opening of 6,35 mm (one-quarter inch) or less. A lower tie plate assembly for interconnecting the lowermost end (18) of an array of reactor fuel rods (13) and guide tubes (11) and for capturing debris carried by flowing coolant, the tie plate assembly comprising: substantially planar, spaced-apart, upper and lower members (20; 22) and a skirt member (28) extending along the periphery of, enclosing and interconnecting said upper and lower planar members (20; 22), said lower planar member (22) including a relatively small number of large openings (25) adapted for flow of coolant into said assembly in a vertical direction, said upper planar member (20) comprising an array of a large number of relatively small, debris-resistant, coolant flow openings (24) dimensioned to trap at least a portion of debris particles carried by said coolant; said relatively large openings (25) in said lower planar member (22) comprising a substantially cylindrical lower portion and a flared upper portion (21) opening into a cavity (23) formed by said upper planar member (20), said lower planar member (22) and said skirt member (28). A lower tie plate assembly according to Claim 24 wherein: said flared upper portion (21) comprises a rounded lip at an entry into said cavity (23) proportioned to produce turbulent flow of said coolant into said cavity. A lower tie plate assembly according to Claim 24 wherein: said lower planar member (22) includes a plurality of bosses for fastening guide tubes to said assembly. A lower tie plate assembly according to Claim 24 wherein: said relatively large openings (25) are dimensioned substantially equal to or greater than coolant flow openings (27) in an associated support plate (26). A lower tie plate assembly according to Claim 24 wherein: said coolant flow openings (24) in said upper planar member (20) comprise generally vertically oriented holes extending through said upper member, said holes being of substantially equal size and in a regular array. A lower tie plate assembly according to Claim 28 wherein: said coolant flow openings (25) in said lower planar member (22) comprise generally vertically oriented holes extending through said lower member, said holes being of substantially equal size and in a regular array.

Inventors: BROWN CHARLES A, FORD KENNETH L, BROWN, CHARLES A., FORD, KENNETH L.
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1991
Main classification: G21C19
IPC codes: G21C3, G21C19