Patent Number: 050892215
Section: description

Referring to FIG. 15, an exemplary fuel bundle F is disclosed in which spacers are utilized. Specifically, a lower tie plate N and an upper tie plate U are illustrated. Lower tie plate N supports the fuel rods 9. Upper tie plate U braces the rods in the vertical upstanding side-by-side relation to maintain the fuel rods vertically parallel to one another. A channel 8 surrounds the fuel rods between the tie plates and continues fluid flow from the lower tie plate N to the upper tie plate U. Spacers S1 and S2 are located between the tie plates and around the fuel rods. These spacers function to maintain the sid-by-side alignment of the fuel rods 9 between the tie plates as well as to provide improved fluid flow, especially from the inside of the channel 8 to the outer fuel rods in the fuel bundle. Having set forth the generic construction of an exemplary fuel bundle F, the construction of a spacer grid of the prior art will be set forth. Thereafter, the improvements of the present invention to the spacer grid will be set forth. Finally, various spacer constructions which can be utilized in fuel bundles will be illustrated. Referring to FIG. 1A, an illustration of a discrete cell C' of the prior art is set forth. The cell includes two spring legs, 14, 16. These respective spring legs are spaced apart one from another and are connected at the top by rod encircling arms 18 and at the bottom by rod encircling arms 20. Each of the respective spring legs 14, 16 is deflected inwardly and defines at a central portion thereof respective rod contacting portions 15,17. The respective arms 18,20 when surrounding a fuel rod must form stops onto which the fuel rod is urged. Therefore, the arms in their encirclement of a fuel rod are bent inwardly at respective stop portions. These stop portions include arcuate portions 21, 23 in upward arms 18 and arcuate portions 25, 27 in lower arms 20. Typically, a fuel rod R (shown in broken lines) passes medially through each cell unit. As can be seen, the fuel rod is urged at the rod contacting springs portions 15, 17 onto the respective stops 21, 23 in upper arms 18 and 25, 27 in lower arms 20. Referring the FIG. 1C, the method of fastening together the cells of the prior art can be seen and understood. Specifically, the spring legs 14, 16 of each cell units adjoin the cantilevered arms 18, 20 of each adjacent cell unit. It will hereafter be emphasized that in the construction of the cell units of this invention the spring legs of pair cell units are remote from one another. This remote placement enables the rod encircling bands to extend across the center of the cell pair while the respective spring legs are at remote extremes of each cell pair. (See FIG. 2B and the position of the respective spring legs 24, 26.) Having set forth the prior art cell construction with respect to FIGS. 1A, 1B and 1C, the preferred embodiment of this invention will now be set forth. Referring to FIG. 2A, a front elevation is shown of a blank of spring material precut to form a cell C utilized with this invention. As will be set forth, cell C is similar to cell C' illustrated in FIG. 1A. Specifically the spring metal is preferably an alloy sold under the trademark of Inconel by the International Nickel Company. The metal is 0.008" to 0.012" thick and die cut in the shape of FIG. 2A. Specifically, there is a first spring leg 24, a second spring leg 26 with an upper rod encircling arm 28 and a lower encircling arm 30. Observing the upper portion of the metal to be formed into the cell, it can be seen that an axis 33 is utilized to divide at a medial location the metal blank between the respective spring legs 24, 26. At the upper arm 28, leg 31 is slightly shorter than leg 32. Similarly, the lower arm 30 has legs which differ slightly in length. This difference in the lengths of the legs is a departure from the prior art. The use of these respective long and short legs will be illustrated with respect to FIG. 2B to show the attachment of the two formed cell structures C into a convenient self-bracing structural unit for fabrication of the spacer set forth herein. Additionally, and over the prior art, an upper spring stop 34 and a lower spring stop 35 are formed in the respective arms 28, 30 overlying the spring leg 24. As will be set forth with respect to FIG. 2C, these respective stops prevent bending of the spring leg 24 beyond the elastic limit. Similarly, a spring stop 36 and a spring stop 37 is formed overlying spring leg 26. These respective stops function identically to stops 34, 35 in preventing bending of the spring leg 26 beyond the elastic limit. The reader will understand that during shipment of fuel bundles the spacers frequently become dynamically loaded with the weight of the fuel rods. The respective stops function to prevent overloading of the spring legs during fuel bundle assembly and during transport. Those understanding the construction of the prior art spacers will realize that bending is typically formed by passing the material of FIG. 2A through a series of bending dies. Respective bends are made at each of the dies until the full structure of one of the discrete cells C1 or C2 FIG. 2B is formed. Since the construction of such a series of dies is well within ordinary skill in the art, such construction will not be set forth herein. It will suffice to say that the cells C1 and C2 are individually fabricated as shown in FIG. 2B. Referring to FIG. 2B, two cells C1 and C2 are illustrated in a cell unit P. Each cell includes spring legs 24, 26 with an upper arm portion 28 and a lower arm portion 30. Each of the spring legs 24, 26 has a rod contacting portion 25, 27. Rod contacting portion 25, 27 biases a contained rod into the rod stops 45, 46, 47, 48. It will be understood that bands 45, 46, 47, 48 are given a compound curvature. This curvature is inward toward the contained fuel rod in each cell so that the fuel rod bears against the bands 45, 46, 47 and 48. It will further be seen that bands 45, 46, 47, 48 are convex as exposed to the fuel rods. This convex curvature enables the fuel rods to be inserted within each cell without the end of the fuel rod hanging up on the end of the cell. It can be seen that upper arms 28 and lower arms 30 are bent into encirclement so that the respective arm ends at 50, 51 on cell unit C2 are off-center with respect to the arm ends 50, 51 from cell unit C1. In the construction of the cell unit pair P of cell units C1, C2, the two cell units are preferably confronted one to another. Thereafter and in the confronted position at the respective upper arms 28 and lower arms 30, the units are spot welded. It can be seen that there is formed a structurally rigid unit. The respective spring legs 24, 26 of cell unit C2 and 24, 26 of cell unit C1 form rigid spaced apart vertical legs members to the paired cell units. At the same time, the joined together upper arms 28 and lower arms 30 form horizontal spanning units. As will hereinafter be illustrated, this cell unit pair can be conveniently manipulated to construct varying configurations of a spacer. An aspect of the prevention of spring leg movement beyond the elastic limit may be illustrated with respect to FIG. 2C. Referring to FIG. 2C, it can be seen that it is a vertical side elevation of spring member 26 taken along lines 2C--2C of FIG. 2B. The spring leg 26 is shown in two positions. The solid lines show the spring in its normal position, where it biases the fuel rod against the stops. As can be understood, the rod when so biased will be well away from the respective spring stops 36, adjacent upper arm 28 and stop 36 adjacent lower arm 30. If, however, the fuel rod is biased to fully deflect spring member 26, the rod contacting portion will move to the position shown at 27'. In this position, the rod R will be biased against the respective stops 36. A three point stance will result with no further deflection of the spring member 26. This being the case, it will be understood that by selecting the dimension of the respective spring 26 and of the rod contacting portion 27 and of the upper and lower stops 36, flexure of the spring can be limited so that yielding and permanent deflection of the spring does not occur. Having set forth the cell pair here illustrated, brief reference may be made to FIGS. 3A, 3B, 4A, 4B, 5, and 6 for the disclosure of the varying patterns into which the cell pair may be constructed. Referring to FIG. 3A, there is shown a schematic in which 4 lattice positions are occupied. In this 4 lattice position unit, cell pairs designated P1-P2 are arrayed in side-by-side parallel relation. Referring to FIG. 3B, the side elevation illustrates the minimal structure of the Inconel sections of the disclosed spacer construction. Specifically, arms 28 extend in a plane at the top of the spacer. Further, arms 30 extend in a plane at the bottom of the spacer. The spatial interval there between is spanned by the spring legs 24, 26. Thus, the spring legs 24, 26 act not only for the bias of the contained fuel rods in the cells but additionally form the vertical interconnecting members between the upper grid (formed by arms 28) and the lower grid (formed by arms 30). Thereafter, welding at the abutted interfaces of the cell arms 28, 30 (see FIG. 2B) occurs. Such welding is typically by a fusion weld such as that provided by an inert tungsten gas weld or alternately a laser weld depending upon production preferences. At this juncture, a solid and interlocking grid structure is formed. (See FIG. 3B.) Referring to FIG. 4, a grid having an aperture for a large water rod R shown. The disclosed aperture includes the omission of designated but omitted cell pairs P1, P2, P3, P4, P5 and P6. As can be seen, it is required that the included cell pairs be aligned so that omitted cell pairs P1, P2, P3, and P4 be oriented in one direction while designated but omitted cell pairs P5 and P6 are oriented in an orthogonal direction. Referring to FIG. 5, an embodiment is illustrated having two types of required apertures. A first aperture constructed precisely analogous to that already illustrated with respect to FIG. 4 is for a water rod R. A second set of apertures are for overlying partial length rods. Such partial length rods are disclosed in United States patent application Ser. No. 176,975, filed Apr. 4, 1988 entitled Two-Phase Pressure Drop Reduction BWR Assembly Design. Referring to FIG. 5, the array shown can be best illustrated by observing cell pairs P1, P2, P3 and P4. Specifically, cell pairs P1 and P3 are across the pictorial representation of FIG. 5. Cell pairs P2 and P4 are oriented orthogonally. The four cell pairs each define a place for two rods. A total of eight rods occupies nine lattice positions. In the center of the cell pairs P1, P2, P3 and P4 is an omitted portion of the grid which omitted portion of the grid is for the overlying of a partial length rod. It will be realized that this omission is not trivial. Specifically, and during operation at full steaming rates of such a reactor, the locations overlying partial length rods are known to be volumes of high steam venting. These volumes of high steam venting can experience back pressure even when passing the relatively low profiles of the cell pairs here illustrated. By the expedient of aligning four cell pairs around nine lattice positions with the central position vacant, a preferred lattice structure with improved venting is provided. Those skilled in the art will understand that the remaining pair alignments illustrated at P5-P13 are logical extensions of the pattern illustrated with respect to cell pairs P1-P4. Specifically, it can be seen that in the ten wide lattice array, lattice positions L2,2, L2,4, L2,7, and L2,9 are vacant. These vacancies continue in a similar pattern throughout the ten by ten array here illustrated. It can thus be seen that the cell pairs here illustrated can be uniquely arrayed for the construction of any desired full length fuel rod, partial length fuel rod, or water rod disposition. Referring to FIG. 6, two large water rods R1 and R2 are shown positioned in apertures similar to the aperture previously described with respect to FIGS. 4 and 5. In FIG. 6 the arrangement of cells is slightly different from that of FIGS. 4 and 5. Each corner cell is a single cell, and is not paired with another cell. These cells are oriented so that the springs are oriented away from the corners. The remainder of cells are pairs. Referring to FIG. 6, a construction of the spacer of this invention is illustrated having two large water rods R1 and R2 in parallel, side-by-side relation. It can be seen that each one of the water rods R1, R2 occupies four lattice positions. The remainder of the cell pairs are arrayed to form a complete grid. Referring to FIG. 7, a detail of FIG. 6 at the aperture for water rod R1 is illustrated. Two band members 60 form a lining into which the water rod R1 is braced. The construction of these aligning members are illustrated with respect to FIGS. 8A and 8B. Referring to FIGS. 8A and 8B, it can be seen that the inner band members 60 are formed in two equal segments, which segments have their respective ends at 63, 64. Referring to FIG. 8B, there is an upper member 72 and a lower member 74. These members form the upper and lower portions of the inner band, and encircle a water rod. Referring to FIG. 7, the upper members 72 are welded to the grid cells at locations 66. These respective bands are connected by spring numbers 65, 67 in a manner that is analogous to each of the cell members. Two features of the band members are noteworthy. First, the band members define respective spring legs 65, 66 each having a spring medial spring portion 68 for bearing on the water rod R1. These respective spring members securely brace the water rod in place and maintain such bracing in the absence of appreciable vibration. Secondly, deflecting tabs 70 overlie each of the spring leg members 65, 67 at the upper portion of the band members. These deflecting members serve to deflect water passing upwardly to the adjacent fuel rods braced by the spacer. Referring back to FIG. 7, the attachment of the spacer band 60 interior of the array can be easily understood. Specifically, band ends 63, 64 are identified. It can be seen that by placing two members 60 in the disposition illustrated in FIG. 7 bracing of the respective water rods R1, R2 securely at the spacer can occur. Having set forth the Inconel cellular arrays and their varied constructions, it will be observed that the construction set forth gives a relatively minimum possible amount of spring material in an array for the spacing of fuel rods. In the typical fuel bundle assembly there are on the order of 7 or 8 such spacer arrays. It is necessary that such spacer arrays be surrounded by a continuous band. This continuous band locates the spacer within the channel. It also reduces the coolant flow between the grid and the channel wall, and causes water intermixture into the rising liquid and vapor water coolant flowing on the outside of the fuel rods. Referring to FIG. 9, the grid of FIG. 6 is shown together with the preferred embodiment of the band. The interior cells and water rods are not shown. The band is shown partially exploded away from the sides of the grid. Specifically, band sides B1, B2, are shown spaced apart from the grid G. Bands B3 and B4 are shown in their final position next to the grid. It will be observed that the bands are broken at respective gap 101 (between bands B1 and B2), gap 102 (between bands B2 and B3), and Gap 104 (between band B4, B1). Gap 103 (between bands B3 and B4) has been welded. Each band includes a plurality of inwardly deflecting tabs 110, which tabs have the function of deflecting water flowing near the sidewall of the fuel channel inwardly to and toward the rod array. Referring to FIG. 10, a band segment B3 of the preferred embodiment is illustrated. Band segment B3 terminates along a first side at end 101 and along a second side at an end 102. Band segment B3 defines a plurality of flow deflecting tabs 110. It is the function of these tabs to deflect water flowing along the channel sides to and toward the array of contained fuel rods. The band also includes respective raised portions 120. These respective raised portions enable the band to standoff from the sidewalls of the channels. The respective raised portions 120 additionally can have two alternate functions. First, the respective raised portions can themselves deflect water flowing along the sides of the channel. Such deflection causes a turbulence which ensures mixture of any water layer flowing along the inside of the channel to and towards the fuel rod array. Secondly, the raised portions 120 are oriented to provide stiffness to the band. With a flexible band, flow induced vibration of the band can occur. In the natural mode of vibration the band deflects away from the grid and the maximum deflection occurs halfway between the corners at each side of the spacer. Referring to FIG. 9, the location of maximum deflection on the left side of the spacer is at 103. FIG. 11A shows a side elevation view of the preferred embodiment of the band. The raised portion of the band 120 extends over most of the band width and over slightly more than half of the band height. FIG. 11B shows a section at B--B of FIG. 11A and illustrates the shape of the raised portion 120. This shape is similar to a corrugation, and gives the band side a stiffness several times that of a band with no raised portion. This increased stiffness prevents flow induced vibration of the band. FIG. 11C shows an enlarged top view of the corner region of the spacer, together with the corner region of a channel 130. The distance 132 between the inside of the band and the channel 130 should be small, in order to provide good thermal performance for the outer fuel rods. Unfortunately, as the gap 132 is reduced, the gap between the corner fuel rod and the channel corner is also reduced. For a given gap 132 along the sides, the corner gap becomes smaller as the fuel rod array is changed from an 8.times.8 array to a 9.times.9 and to a 10.times.10 array. As the number of fuel rods in each row and column is increased, the corner rod moves closer to the channel. The channel corner radius cannot be decreased, because the channel corners would then interfere with other reactor components. FIG. 11C has been drawn approximately to scale for a 10.times.10 array. If the corner portion 136 of the Zircaloy band were to lie entirely outside the corner Inconel cell, there would be insufficient clearance between the band and the channel corner, and insertion of the space into the channel would be difficult or impossible. Referring to FIG. 11A, slots 138 have been cut into the corner of the band. The upper and lower arms of the corner cell project into these slots. As can be seen in FIG. 11C, the band corner 136 can then be moved away from the channel corner, toward the corner fuel rod. At the same time, the grid is captured by the band since part of the grid projects into the corner slots. It will thus be understood that the band members at their respective corners accommodate the decreasing diameter of the fuel rod R1. At the same time, these corner sections key firmly the band members B1-B4 around the spacer to the Inconel spacer grid. FIGS. 12A, 12B, 13A, 13B, 13C, 14A and 14B illustrate an alternate embodiment of the Zircaloy spacer band. In this embodiment, the band stiffness is not increased. Instead, Inconel straps are used to tie the band to the Inconel grid. FIG. 12A shows a top view of a segment of this band. As in the prior art, bath tub type indentations 140 are used to space the band away from the channel. FIG. 12B shows a side elevation view of this band. A change from the configuration of FIGS. 11A and 11B is that cutouts 142 are made at the top and bottom of the band. These cutouts are used in conjunction with Inconel straps, which will be described later. Additional cutouts 144 are used at the band corners. These cutouts perform the same function as the corner slots in the preferred embodiment. The upper and lower arms of the corner cell project into the cutouts, allowing the band corner to be spaced away from the channel corner. FIGS. 13A, B, and C show the Inconel strap. FIG. 13A is a side elevation view of an Inconel strap. The length l of the strap is slightly less than the width of the spacer and the width w is equal to the width of the spacer arms. FIG. 13B shows a top view of the strap. The bends 150 project into the cutouts 142 of FIG. 12B. FIG. 13C shows an enlarged top view of the strap. A projection 152 is used to bear against the band, halfway between the bend regions 150. FIG. 14A shows a top view of a portion of the spacer, illustrating how the strap locks the band securely to the spacer grid. The strap acts as a series of springs, bearing against the band at the points 152. The strap is shown prior to welding to the grid. In this position there are gaps 154 between the strap and grid. To attach the strap to the grid, the gaps 154 are closed, bending the strap and applying loads to the band at the contact points 152. The Inconel strap is then welded to the Inconel grid at locations 154. FIG. 14B shows a side elevation view of the strap and part of the band.