Patent Number: 053717694
Section: description

PREFERRED EMBODIMENT OF THE INVENTION An embodiment of the present invention will be explained with reference to FIGS. 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 FIG. 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 FIG. 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 FIG. 2, on one edge (i.e., the bottom strap welding tabs 7 and window sections 5 in FIG. 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 FIG. 1. A ladder-like spring frame member 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 FIG. 2, 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 grind 2. Before loading fuel rods into the grids of the above construction, keys (not illustrated) are inserted thorough 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.