Patent Number: 054886341
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to the representative example of a fuel assembly, generally designated 10 in FIG. 1, there is illustrated a plurality of nuclear fuel rods 12 forming a nuclear fuel bundle. The rods 12 are connected at their upper ends to an upper tie plate 14 and are supported at their lower ends in a lower tie plate grid, generally designated 16, forming part of a tie plate assembly, generally designated 23. Spacers 18 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 20 whereby coolant water inlet through the bottom nozzle or inlet opening 22 of the tie plate assembly 23 flows upwardly therefrom through a transition structure 25 defining an enlarged flow volume 27 for flow through the lower tie plate grid 16 thereof and about the fuel rods whereby steam is generated. As indicated previously, it is important that debris in the flow of the coolant water be prevented from flowing into the area between the fuel rods. Accordingly, a lower tie plate assembly 23 including a unitary one-piece lower tie plate grid 16 serving the dual purpose of catching debris and supporting the fuel bundle according to the present invention is described and illustrated with respect to subsequent drawing figures. Referring now to FIGS. 2 and 3, there is illustrated a unitary one-piece lower tie plate grid 16 according to the present invention forming a part of the lower tie plate assembly 23. Lower tie plate grid 16 may be integrally cast with the lower tie plate assembly 23 but is preferably formed separately and secured thereto as by welding its depending side walls 29 to assembly 23. Lower tie plate grid 16 has a lower grid portion 26 for separating debris from the flow of water through the tie plate with minimum pressure drop and an upper portion 28 which, together with the lower portion, support the fuel rods above the lower tie plate grid 16. The upper portion 28 affords flow spaces which assist to minimize the pressure drop across the lower tie plate grid and enable the fluid to expand within the flow spaces uniformly and smoothly for subsequent flow about the fuel rods. Turning first to FIG. 2, there is illustrated a plurality of generally cylindrical bosses 36 which extend between the upper and lower surfaces 30 and 32 (FIG. 3), respectively, of tie plate grid 16 for receiving the cylindrical end plugs of the nuclear fuel rods and supporting the latter, the bosses 36 having portions projecting upwardly from the upper surface 35 of the lower grid portion 36. As best seen in FIG. 2, the cylindrical bosses have centerlines arranged at corners of substantially square matrices of such bosses 36. Interconnecting and forming the sides of the square matrices are webs 38 adjoining the adjacent cylindrical bosses 36 along radial lines of bosses 36 and extending between the upper and lower surfaces 30, 32 of the lower tie plate grid 16. Consequently, it will be seen that above the lower grid portion 26, the webs 38 have portions formed along the sides of each square matrix and, together with convex outer portions of the cylindrical bosses 36, define side walls of upper flow spaces 40. As described below, the lower portion 26 has a plurality of openings for flowing coolant through the lower portion and into each of the flow spaces where the flow expands smoothly at reduced velocity for flow upwardly about the fuel rods supported by the lower tie plate assembly 23. The debris catching function of the tie plate is performed by the lower grid portion 26. To accomplish this debris catching function, lower portion 26 includes a plurality of openings 42 which open through the lower surface 32 of grid 16 and through the upper surface 35 of the lower portion 26 into the flow spaces 40. From a review of FIGS. 2 and 3, it will be seen that the openings through the cylindrical bosses 36 extend between the upper and lower surfaces 30, 32, respectively, of grid 16. Openings 42, however, extend through only the lower portion 26 from lower surface 32 to surface 35 and open through surface 35 thereof into the flow spaces 40. The openings 42 are configured and dimensioned to maximize the debris catching function, while simultaneously minimizing the pressure drop across the lower grid portion resultant from the need to filter the debris from the coolant water. As illustrated in FIG. 2, the central portion of the tie plate has openings 45 for water rods, not shown, which alter the arrangement of the openings 42 in the adjacent flow spaces defined by adjacent bosses 36 and webs 38. As stated hereafter, the openings 42 through lower portion 26 are of at least two types and both types comprise the openings through these central flow spaces as well as all other flow spaces. More specifically, and referring to FIGS. 4-8, the openings 42 are arranged in a pattern relative to each of the flow spaces 40. To achieve maximum flow into each flow space 40 with minimum pressure drop, the openings 42 are arranged in a generally square pattern rotated 45.degree. relative to the sides of the square matrices formed by the bosses. A review of FIG. 2 reveals that, while a different number of openings 42 open into the various configured flow spaces 40, i.e., the marginal flow spaces, the intermediate flow spaces shown particularly in FIG. 4 and the inner flow spaces adjacent the water rod openings 45 of FIG. 2, each such flow space 40 communicates with openings 42 arranged in this offset square pattern and openings of two different types, as will now be explained. In a preferred form, each opening 42a (FIG. 4) of a first array of openings opening into each flow space 40 has a generally square configuration with linear sides and radiussed corners between adjacent sides (see FIG. 8). Each opening 42b (FIG. 4) of a second array of openings has a plurality of sides in excess of four sides, preferably five sides, with adjacent sides of each opening of the second array having a radius therebetween (see FIG. 7). In order to maximize the debris catching function and minimize the pressure loss, the first and second arrays of openings 42 are particularly arranged relative to one another and to the corresponding or associated flow spaces 40. Thus, each opening 42a of the first array of openings 42 is located such that a vertical centerline through the opening intersects a diagonal of the square matrices and which diagonal passes through the convex portions and the vertical centerlines of the cylindrical bosses 36. Thus, the dashed lines D in FIG. 4 represent diagonals between the centerlines of diagonally related bosses 36 and which diagonals are intersected by the centerlines of the openings 42a of the first array of openings 42. Note also that the openings 42b of the second array have a linear side generally parallel to the webs 38 interconnecting the bosses and that two of their remaining sides are generally parallel to the sides of the openings 42a of the first array. Referring to FIGS. 5 and 6, the edges 45 of the openings 42 opening through the lower surface 32 of the tie plate are radiussed to provide a smooth, non-turbulent transitional flow from the inlet plenum of the tie plate assembly into the opening 42. Referring to the cross-sectional views of FIGS. 5 and 6, the openings are tapered at 47 from a minimum throat area 49 at the end of the inlet radius 45 to a maximum dimension at the top surface 35 of lower portion 26. This gradual enlargement of the openings 42 provides a diffuser or venturi action so that the flow can expand from the minimum flow area with minimal pressure loss. There is a change in flow area from the exits of the openings 42 into the volume, i.e., flow spaces 40 above the openings which causes a pressure loss. While this pressure loss can be reduced by increasing the aperture exit area, increasing the flare angle of the apertures, or increasing the thickness of the lower portion while maintaining the flare angle constant, practical problems in the manufacturing process as well as possible flow separation may occur. Thus, the described and illustrated arrangement and shape of the openings 42 are optimal. As will be appreciated, the coolant flows through the openings 42 and into the flow spaces 40. In view of the larger volume of the flow spaces 40, the flow pattern through openings 42 flares and transitions smoothly into the associated flow space 40 with minimal pressure drop. In a particular preferred form of the present invention, there is provided a generally square lower tie plate 5.41 inches on a side, having a total cross-sectional area of 29.27 square inches. The flow area through the openings 42 of the lower grid portion is 4.3 square inches and hence the ratio of the flow area through the openings 42 to the area of the lower grid portion 26 of the tie plate is about 0.15. Additionally, the thickness of the lower portion 26 is preferably less than about 25% of the overall thickness of the tie plate. In a preferred embodiment, the thickness of tie plate grid 16 between upper and lower surfaces 30 and 32 is about 0.590 inches and the thickness of the lower grid portion 26 is 0.150 inch. Accordingly, in a preferred embodiment hereof, the ratio of the overall thickness of the tie plate grid to the thickness of the lower grid portion thereof is within a range of 3-4 to 1 and preferably about 3.9 and should also be preferably less than about two times the size of the opening 42 along a line perpendicular to its edges. Additionally, the radii 45 of the openings 42 adjacent the lower surface of the tie plate is about 0.029 inch and the width of the metal between adjacent openings 45a is about 0.058 inch as indicated at x in FIG. 6. Referring again to FIGS. 4-8, and in a preferred form of the present invention, the generally square openings constituting the first array of openings, are preferably 0.088 inches on a side at the throat 49, with a radius r of 0.030 inches between adjacent sides. Each opening of the second array of openings, as illustrated in FIG. 7, has a dimension a at throat 49 equal to 0.100 inches, a dimension b equal to 0.090 inches and a dimension c equal to 0.010 inch, with radii r between adjacent linear sides of 0.030 inch. The distance between adjacent linear sides of the openings at throats 49 is 0.058 inch. The centerline-to-centerline distance between the bosses along the sides of the square matrices is preferably 0.566 inches, with the thickness of the webs and convex portions of the bosses being 0.070 inches. FIGS. 9 and 10 show the arrangements of the openings in the edge and corner regions, respectively, of the tie plate grid. The aperture shapes are basically the same as in the central region of the grid, with slight modifications where the holes are adjacent to bosses or webs. FIGS. 11, 12 and 13 show an alternate embodiment of the invention wherein like numerals indicate like parts as in the prior embodiment. In this embodiment, the walls of the openings 42 do not flare outwardly and are generally parallel to one another. Also, the lower grid portion is thinner than in the prior embodiment. This embodiment facilitates casting the grid because the ratio of lower grid thickness to the dimension of the opening 42a is smaller. For example, the thickness of the lower grid portion is 0.100 inch and therefore a preferred ratio of overall thickness of the tie plate grid to the lower grid portion is within a range of 5-7 to 1 and preferably about 5.9. However, the pressure loss through the grid, in this embodiment, is greater because of the diffuser action through the holes is not present. FIGS. 14, 15 and 16 illustrate a still further embodiment of the invention similar to the first embodiment hereof and wherein like numerals are used to denote like parts followed by the suffix "c." Thus, the lower grid portion 26c has a thickness identical to the thickness of the preferred embodiment illustrated in FIGS. 3-8, and the openings have an outward flare 47c from a minimum or throat area 49c to the top exits of the openings. However, there is step change in the opening size at the end of the entrance radius and which step is indicated at 52. More particularly, the openings 42c in the lower portion 26c into the flow spaces 40c defined by the bosses 36c and webs 38c are defined in part by a radius 45c at the juncture of the opening and the lower surface 32c of the tie plate. Above the radius, however, the side walls of the openings 42c taper at 47c divergently away from one another in a vertically upward direction. Additionally, the step 52 lies at the transition between the radiussed lower edge 45c at the minimum inlet dimension and the tapered walls 47c of the opening. Step 52 faces in an upward vertical direction. It will be appreciated that with this overall configuration, each such opening forms a diffuser with the flow first converging along radii 45c and then gradually diverging along tapered walls 47c as the flow enters the flow space 40c above the lower grid portion. This enables the flow to expand smoothly in the upward direction, reducing the pressure loss. The step 52 is of a minimum dimension consistent with manufacturing requirements and tolerances to minimize any increase in pressure loss by virtue of the step. The step ensures as smooth a transition between the radiussed inlet 45c of the opening 42c and the divergent walls 47c consistent with the casting process used to manufacture the one-piece integral unitary tie plate. This step 52 can be used to alleviate some of the problems in casting the grid 26. For example, the tie plate grids are investment castings produced using the lost wax process. First a "wax" model of the final part is made. This wax model is coated with several layers of ceramic which later form a mold for the final metal part. Then the wax is evaporated and metal is poured into the remaining ceramic mold. The mold for the "wax" is in two parts which mate when the inlet radius 45c ends. If the two halves of the mold do not match exactly, the upper part of each hole will be offset relative to the lower and wax will project into the holes at the interface between the upper and lower molds. The final metal part will have exactly the same shape as the wax. Any material projecting into the openings must be removed, and any sharp edges due to offset must be rounded. The step in the openings minimize these problems while causing increased pressure loss due to the sudden expansion at the step. The pressure loss is intermediate between that of the preferred embodiment with smooth tapers and the embodiment with no taper. Referring now to FIGS. 17a-17e, it will be seen that there are five different configurations or arrays of openings 42, although the openings shown in FIGS. 17a and 17c are considered generally square openings with radiussed corners between adjacent sides, notwithstanding that the opening illustrated in FIG. 17c has one side slightly curved to accommodate the convex portion of the web. Thus, the openings of FIGS. 17a and 17c are arranged similarly as in the previous embodiment along diagonals parallel to or interconnecting the centerlines of diagonally related bosses of the square matrices. The remaining openings illustrated in FIGS. 17b, 17d and 17e each have five linear extending sides with corner radiusses therebetween. In a preferred embodiment of the present invention, the dimensions of the openings are as follows: FIG. 17a: e=0.118; f=0.098; g=0.088; FIG. 17b: h=0.130; i=0.110;j=0.100; k=0.105; l=0.095; m=0.090; FIG. 17c: n=0.103; o=0.093; p=0.088; q=0.118; r=0.098; s=0.088; FIG. 17d: t=0.105; u=0.095; v=0.090; w=0.130; x=0.110; y=0.100; and FIG. 17 e: aa=0.130; bb=0.110; cc=0.110; dd=0.121; ee=0.101; ff=0.091. Radii R1, R2 and R3 in FIG. 14a are 0.030, 0.035 and 0.045 inches, respectively. The total cross-sectional area and flow area through the openings of the tie plate of this embodiment are similar to that of the previously described embodiment. The thickness of the lower grid portion is 0.150 inch, the overall thickness of the tie plate grid is 0.590 inch and the ratio of the overall thickness of the tie plate to the thickness of the grid portion thereof in this second embodiment is about 3.93 and preferably within a range 3-4 to 1. Additionally, the radii of the openings adjacent the lower surface of the tie plate is about 0.029 inch. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.