Patent Number: 041727614
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2 a cellular grid structure comprises a plurality of tubular ferrules 2 grouped within an encircling band 1 with at least some of the adjacent openings or cells framed by twin ferrules formed from one piece of metal strip and a bridgepiece dividing means described below hereafter dividing the interior of each twin ferrule into two similar openings. In FIGS. 1 and 2 the grid is seen to comprise an outer band 1 of metal strip which encompasses sixty cells for the reception of nuclear fuel rods and sparge pipes. The sixty cells are defined by thirty twin tubular ferrules 2 and five of these twin ferrules are assembled within each of the six sectors into which the area enclosed by the band 1 is divided. The division is effected by six radial webs 3 which are brazed to a central tubular ferrule 4 and the webs extend from the ferrule 4 to the outer band 1 where they are similarly brazed. Near their outer extremities the radial webs are formed with distance pieces shaped as small diameter spacers 5. The central ferrule 4 is adapted to receive a sparge pipe which also acts as a structural member. The twin tubular ferrules 2 are to frame two adjacent openings or cells and are formed from one piece of metal strip. Twin ferrule so formed from metal strip bounds two contiguous cells for the reception of adjacent fuel rods or a fuel rod and a sparge pipe of similar dimensions. The interior is divided by a bridgepiece dividing means in the form of a partition 6. The partition carries a double sided bow spring 7 which is a separate member. In the construction shown in FIGS. 3-5, the twin ferrule 2 is made from a strip metal blank which is in the main uniform width but has a portion near one end which is to constitute the partition 6 about three times the uniform width. The blank also has end tags 9. The blank is first deformed to exhibit four dimples 10 at positions predetermined so that when the strip is pressed to a shape to frame adjacent cells the dimples protrude into the cell interiors to form two rigid fuel rod-engaging stops. In addition two parallel ribs 11 are pressed into the partition-forming portions of the strip. The strip so prepared is pressed into the double cylindrical shape shown in FIG. 3 so as to frame adjacent cells 12, 13 separated by the partition 6 and secured in position by passing tags 9 through slots in the strip. The tags are then folded down flat. The partition 6 is retained in position by a re-entrant fold 14. FIGS. 6-8 illustrate a similar twin cell ferrule in which the bridgepiece is not a part of the strip which frames the openings. Here one length of metal is of substantially uniform width and, having been formed with complementary tag and slot at each end and with dimples 10, is pressed into the shape which frames twin cylindrical cells. The ends overlap and are joined by tag and slot engagement at 15. A separate short length of strip 16 forms the partition between the cells. This shorter length has its longitudinal edges bent at right angles to form short flanges 16a and its other transverse edges cut with recesses 16b. The partition is inserted into the twin ferrule across the neck portion and fixed in position by brazing the mid-length portions into the neck portion of the twin ferrules. In both examples the partition carries a double sided bow spring 7. In the one piece construction FIGS. 3-5 the spring 7 is located between the ribs 11 of the partition whilst in the two piece construction it is located by the two recesses 16b in the end faces of the partitions. The double sided bow spring 7 is shown in FIG. 9 and comprises two lengths of Nimonic (RTM) type material which are oppositely bowed. The lengths are resistance spot welded together at one end only. After the grid has been fully brazed up with the twin ferrules joined together and to the other band 1 and to the web 3 where the parts are in contact with one another, the bow springs 7 are placed over the partition. The free ends of the springs are then joined together by resistance spot welding. The presence of nuclear fuel rods 20 is indicated one sector A of the grid in FIG. 1 with the bow springs 7 pressing the fuel rods against the fixed stops 10. At a central position in each of the six sectors the ferrule opening is occupied by a sparge tube 21 only one of which is shown in FIG. 1. One end of each of the sparge tubes 21 is sealed where it protrudes through the end of the fuel assembly and the other end is available for connection to a source of auxiliary coolant in a substantially conventional manner.