Fuel rod support grids for fuel assembly

Abstract: A grid structure is presented in which grid cells (2) are formed by orthogonally intersecting straps (1) prefitted in the longitudinal direction with a plurality of slited spring frame members (10). Therefore, when the straps (1) are assembled, the spring frame members (10) are also orthogonally intersected. The spring frame members (10) are made of Inconel while the straps (1) are made of zircalloy, thus providing relatively long-lasting grids without the loss of fuel rod retaining power, which would result from radiation and thermal damages on holding devices made of zircalloy.

Description: Background of the Invention Field of the Invention The present invention relates to a fuel rod support grid in which straps and springs are made of different materials. Technological Background Fuel assembly of the following construction is generally known. In a typical fuel assembly, a plurality of support grids are disposed at a predetermined spacing between the upper nozzle and the lower nozzle. A grid comprises a plurality of straps intersecting at right angles to each other thereby defining a plurality of grid cells. In the grid cells are disposed a control rod guide tube and instrument tubes which are retained by the grid and whose ends are attached to the upper and lower nozzles. A plurality of fuel rods pass through the grid cells, in which each rod is elastically retained in the grid cell by the springs joined to the straps of the grids. In the conventional technology presented above, the material of construction of the grid is Inconel (nickel alloys for use in nuclear reactors) which provide excellent elastic properties. However, it is desirable to make the grid with zircalloy (zirconium alloys), in view of improving the neutron economy and lowering the exposure to neutron irradiation. However, the use of zircalloy alloys for the springs presented a problems that this material, which is inherently not highly elastic, is prone to degradation after neutron irradiation. Therefore, zircalloy springs are inferior in performance to Inconel springs. A solution to the above problem which has been under consideration is to use zircalloy for the straps, and Inconel for the springs. A problem in this combination of different materials for making a grid is that when the straps and springs are made of different materials, welding cannot be used for joining the straps to springs. Therefore, there has been an outstanding need to develop a grid configuration in which the springs are firmly joined to the straps, the joint structure is relatively simple and grid manufacturing is facilitated. Summary of the Invention The present invention relates to a fuel rod support grid (hereinafter referred to as grid) and an objective is to present a grid configuration of a relatively simple structure which can be manufactured readily, so that the straps are engaged firmly with the springs, the neutron economy is improved, the exposure to neutron irradiation is reduced, and the elastic characteristics of the springs are maintained during the service life. Another objective of the present invention is to present a grid configuration which enables welding to be applied to join the same metals even though the straps and the springs are made from different metals. Yet another objective of the present invention is to present a grid having a structure which prevents thermal effects of welding of the spring frame members to adversely affect the performance of the straps. A fuel rod support grid is described in FR 2132705 which comprises a plurality of straps made from zircalloy interconnected to constitute grid cells for holding fuel rods, the support grid further comprises separate spring frame members made from Inconel connected to the strap and which include spring sections that protrude into each grid cell to co-operate with bosses formed on the straps to hold fuel rods. A similar fuel rod support grid is described in FR 2352373. According to an aspect of the present invention, there is provided a fuel rod support grid for nuclear fuel assemblies, said support grid having a plurality of straps intersecting each other thereby constituting a grid for defining grid cells for holding fuel rods, said straps having a plurality of bosses, separate spring frame members connected to said straps which include spring sections that protrude into each grid cell to co-operate with said bosses for holding fuel rods, said support grid being characterized in that: each of said straps is provided with a plurality of window sections disposed in a longitudinal direction thereon at a predetermined spacing and consisting of an interlock section and a recess section said plurality of spring frame members, each of which is previously engaged with each of said straps, intersecting each other thereby constituting another grid, each of said spring frame members having said plurality of spring sections disposed in a longitudinal direction thereof, being provided with a plurality of protrusion sections disposed in a longitudinal direction thereof at a predetermined spacing for respectively engaging with said window interlock sections of the straps, and each of said protrusion sections respectively having a frame member welding tab which is housed in said window recess section so as to leave spacing to prevent adverse thermal effects on said straps during welding of said tabs and wherein said frame member welding tabs are orthogonal to each other and firmly welded; wherein each of said fuel rods is resiliently held in each of said cell by said bosses and said spring sections. In the grid of such a configuration, fuel rods are retained in the grid cells with mutually crossing spring frame members engaged with the straps in the horizontal direction. The spring frame members themselves support each other firmly and the mutually orthogonal spring frame members are attached firmly to the straps. The grid cell configuration of the present invention thus allows engaging of the spring frame members smoothly but firmly with the straps. Furthermore, since the straps and the spring frame members are made of different materials, it provides advantages in neutron economy and lowering of neutron exposure, thereby enabling to maintain the spring properties for extended period of time. The construction is relatively simple, thus facilitates manufacturing of the grids. Brief Description of the Drawings Figure 1 is a perspective view of the essential parts of a partially assembled grid of an embodiment of the present invention. Figure 2 is an exploded perspective view of the unassembled straps and a spring frame member of the grids shown in Figure 1. Preferred Embodiment of the Invention An embodiment of the present invention will be explained with reference to Figures 1 and 2. In these figures, the reference numeral 1 refers to straps of a thin strip form made of zircalloy. A large number of grid cells 2 of the grids are formed by cross assembling the straps 1 with each other. A plurality of bosses 3, arranged in rows and projecting out of one wall of the grid cells 2, are formed on both ends (the top and bottom ends in Figure 1) of the straps 1. In the spaces between the two columns of bosses 3 are disposed rectangular-shaped central windows 4. As shown in Figure 2, punch-out windows 5 (window sections) of a cross shape, consisting of an interlock section 5a and a recess section 5b at right angles to each other, are formed above and below and between the central windows 4. Near the center region of the strap 1, a series of through holes 6 of roughly rectangular shape are formed on the extension line joining the vertical pairs of window sections 5. At the top and bottom edges of the straps (1) on the extension line joining the vertical pairs of window sections 5 are disposed a series of vertical pairs of strap welding tabs 7. With regard to the first type of strap 1 as shown in Figure 2, on one edge (i.e., the bottom strap welding tabs 7 and window sections 5 in Figure 2) of the strap 1 is formed a series of slits 8 which start at the bottom strap welding tab 7 and pass through the lower window section 5 to reach the center region of the strap 1. The second type of the strap 1 has the same configuration as the first type except that a series of slits are formed from the top strap welding tab 7. When the first and second types of straps 1 are assembled at right angles by inserting each other at the slits 8, the intersections 9 are formed as shown in Figure 1. A ladder-like spring frame members 10 made of Inconel is engaged with strap 1 by engaging at window sections 5 formed on the straps 1. The spring frame member 10 is disposed on the strap 1 at a height of central windows 4, and comprise: spring sections 11 jutting out oppositely to the bulging direction of the bosses 3; and a pair of connecting plate sections 12 which connect to the top and bottom sections of the spring section 11. Protrusion sections 13 which protrude in the opposite direction to the spring section 11 are formed on the connecting plate section 12 to be engaged with the interlock section 5a of the window section 5. Further, vertically opposing pairs of frame member welding tabs 14 are formed at the top and the bottom edges of the spring frame member 10. With regard to the first type of the spring frame member shown in Figure 2, one lower horizontal frame member welding tabs 14 are provided with slits 15. The second type of the spring frame member (not shown) is provided with slits at upper horizontal frame member welding tabs 14. When the first and second spring frame members are assembled, they are inserted to each other through the slits 15. The frame member welding tabs 14 are assembled with each other at right angles by inserting through the slits 15, a welding section 16 is formed at the intersection of the frame member welding tabs 14, as illustrated in Figure 1. The welding section 16 is housed in the over-sized recess section of the window section 5 so as to prevent adverse thermal effects on the strap 1 during the welding operation of spring frame members 10. The method of constructing a grid using the strap configuration presented above will be explained with reference to Figure 2. First, the spring frame members 10 are attached to the strap 1 by engaging the protrusion sections 13 of the spring frame members 10 in each interlock section 5a of the two rows of window sections 5. The straps 1 with the spring frame members 10 attached thereto are then cross-inserted through the slits 8 to construct a temporary structure shown in Figure 1. Next, the strap welding tabs 7 crossing at the intersections 9 of the straps 1 are laser welded (exemplified by reference numeral 20 in Figure 1), and the frame member welding tabs 14 of the spring frame member crossing at the welding section 16 are similarly laser welded. In this way, the grid assembly structure itself is constructed with the mutually crossing straps 1 made of zircalloy while only the spring sections 11 formed with mutually crossing spring frame members 10 made of Inconel are disposed inside the grid cell 2. Before loading fuel rods into the grids of the above construction, keys (not illustrated) are inserted through the through hole 6 horizontally into the grid cells 2 to press open the spring section 11, then fuel rods can be slid in the grid cells 2 without interference from the springs. Thenafter, the keys are removed to return the spring section 11 to the original position and retain the fuel rods with the spring sections 11 and the bosses 3.

Claims: A fuel rod support grid for nuclear fuel assemblies, said support grid having a plurality of straps (1) intersecting each other thereby constituting a grid for defining grid cells (2) for holding fuel rods, said straps (1) having a plurality of bosses (3), separate spring frame members (10) connected to said straps which include spring sections (11) that protrude into each grid cell (2) to co-operate with said bosses (3) for holding fuel rods, said support grid being characterized in that: each of said straps (1) is provided with a plurality of window sections (5) disposed in a longitudinal direction thereof at a predetermined spacing and consisting of an interlock section (5a) and a recess section (5b); said plurality of spring frame members (10), each of which is previously engaged with each of said straps (1), intersecting each other thereby constituting another grid, each of said spring frame members (10) having said plurality of spring sections (11) disposed in a longitudinal direction thereof, being provided with a plurality of protrusion sections (13) disposed in a longitudinal direction thereon at a predetermined spacing for respectively engaging with said window interlock sections (5a) of the straps (1), and each of said protrusion sections (13) respectively having a frame member welding tab (14) which is housed in said window recess section (5b) so as to leave spacing to prevent adverse thermal effects on said straps (1) during welding of said tabs (14) and wherein said frame member welding tabs (14) are orthogonal to each other and firmly welded; wherein each of said fuel rods is resiliently held in each of said cell (2) by said bosses (3) and said spring sections (11). A support grid as claimed in claim 1, wherein each of said plurality of straps (1) is provided with a plurality of strap welding tabs (7) for welding orthogonally intersecting straps at both ends of intersection. A support grid as claimed in claim 1 or 2, wherein each of said spring frame members (10) has a ladder-like structure formed by a pair of connecting plate sections (12) provided parallel to each other, and said protrusion section (13) and said frame member welding tabs (14) are formed on said connecting plate section (12) between the adjacent pair of spring sections (11). A support grid as claimed in claim 1, 2 or 3, wherein each strap (1) is provided with central windows (4) at a location to correspond with each spring section (11) of said spring frame member (10). A support grid as claimed in claim 2, wherein said plurality of said welding tabs (7) of each strap (1) face each other across a transverse direction, and said window sections (5) of each strap (1) face each other across said transverse direction, wherein each of said strap welding tabs (7) and said window sections (5) disposed on one side of said transverse direction is provided with a slit (8). A support grid as claimed in any preceding claim, wherein said frame member welding tabs (14) of each spring frame member (10) face each other across a transverse direction, and said protrusion sections (13) of each spring frame member (10) face each other across said transverse direction, wherein each of said frame member welding tabs (14) and said protrusion sections (13) disposed on one side of said transverse direction is provided with a slit (15). A support grid as claimed in any preceding claim, wherein each of said straps (1) is made of zircalloy, and each of said spring frame members (10) is made of Inconel.

Inventors: BABA ATSUSHI, IMAIZUMI MASAHIKO, KADONO HIROAKI, KATO AKIHIRO, KAWAGOE TOSHIYUKI, MORI MASAJI, SUGAI KINICHI, YAMAGUCHI YASUNAO, BABA,ATSUSHI, IMAIZUMI,MASAHIKO, KADONO,HIROAKI, KATO, AKIHIRO, KAWAGOE,TOSHIYUKI, MORI, MASAJI, SUGAI, KINICHI, YAMAGUCHI,YASUNAO, BABA,ATSUSHI,C/O MITSUBISHI GENSHIRYOKU KOGYO K.K., IMAIZUMI,MASAHIKO,C/O NUCLEAR DEVELOPMENT CORP., KADONO,HIROAKI,C/O TOHKAI SEISAKUSHO MITSUBISHI, KATO, AKIHIRO, C/O TOHKAI SEISAKUSHO, KAWAGOE,TOSHIYUKI,C/O MITSUBISHI GENSHIRYOKU KOGYO, Mori, Masaji, c/o Kobe Shipyard &amp; Machinery Works, Sugai, Kinichi, c/o Tohkai Seisakusho, YAMAGUCHI,YASUNAO,C/O MITSUBISHI JUKOGYO K.K.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI NUCLEAR FUEL, MITSUBISHI JUKOGYO KABUSHIKI KAISHA, MITSUBISHI NUCLEAR FUEL CO.
Application year: 1993
Main classification: G21C3
IPC codes: G21C3