Patent Number: 058898328
Section: description

DETAILED DESCRIPTION PREFERRED EMBODIMENT The control cluster shown in FIG. 1 comprises a spider 10 having twenty-four rods 12 suspended therefrom. The spider comprises a hub 14 from which sixteen fins 16 are mounted to radial carrying fingers 18 in which the top end plugs of absorbent rods 12 are fixed. The fingers 18 project well away from the fins in a downward direction. A counterbore 20 is formed in the hub 14 and has grooves for receiving a device (not shown) for connection to a drive shaft for displacing the cluster vertically. In the bottom portion of the hub there is provided a bore 22 for receiving a shock absorber. The shock absorber comprises a socket 24 and a spring 25 bearing against an internal partition separating the bore from the counterbore. A screw 26 whose position is adjusted by being screwed into tapping formed through the partition serves to fix the rest position of the socket 24, in which position it projects downwards from the hub. After adjustment, the screw is locked into place by being welded to the bottom of the counterbore 20. The fins are conventionally fixed to the hub by brazing. In FIG. 2, the fingers 18 are fixed to the fins by a tenon-and-mortise joint and by brazing. The terminal portion (or intermediate portion for the inner fingers) constitutes a tenon which is engaged in a slot in the finger, which slot constitutes a mortise. In the prior art, each finger is formed with by a tapped blind hole for fixing the plug 26 of a rod 12. This blind hole cannot extend beyond the bottom edge of the fins because of the tenon-and-mortise assembly. FIG. 3 shows one possible way of fixing a rod in the tapped blind hole of a finger 18. An extension of the plug is terminated by a threaded length 29 which is screwed into a tapped portion of the hole. Between the threaded length and the plug proper 46, there is provided a portion of smaller diameter situated between two bulges. A pin or peg 27 locks the connection. FIG. 4 shows the structure of a spider constituting a first embodiment of the invention. The spider is made as a single piece that is either molded directly or obtained by electro-erosion. To make it easier to mold, the edges of the fins are constituted by flat portions that are mutually parallel, constituting steps that are interconnected by sloping flat portions. The end fingers 18a are of a height that corresponds exactly to that of the portions of the fins 16 remote from the axis of the hub. The intermediate fingers 18b have the same height as the portions of the fins that are nearer to the axis of the hub. FIG. 4 does not show the bores formed along the axes of the fingers, but it does show some of the bullet-shaped nuts 28 for fixing rod plugs. The spider shown in FIG. 4 can receive a shock absorber of the same kind as that shown in FIG. 1. Nevertheless, in order to facilitate manufacture and assembly, it is advantageous to adopt the disposition shown in FIG. 5, where members corresponding to those of FIG. 1 are given the same reference numerals. The rest position taken up by the socket 24 under drive from the spring 25 is fixed by bearing against a washer 30 mounted in an end counterbore 33 of the bore 22 and welded into place. The socket 24 has a shoulder enabling it to bear against the inside portion of the top face of the washer 30. In the embodiment shown in FIG. 6, the spider is in two parts. It comprises firstly a tubular part or sleeve 14a constituting the top portion of the hub, and secondly a one-piece bottom part comprising the bottom portion of the hub together with the fins 16 and the fingers 18. The sleeve 14a is machined. The bottom portion which includes the bottom portion of the hub, the fins, and the fingers is obtained by molding a solid part or by electro-erosion. The connection between the top portion 14a and the bottom portion can be achieved in various ways. As shown in FIG. 6, a cylindrical step is machined in the top end of the bottom portion. The bottom end of the top portion 14a has a complementary step that engages in the bottom portion. A circular weld 42 serves to hold together the two portions of the spider. The bores 22 formed in each of the two portions have the same diameter so as to avoid any discontinuity in the bore. In the modified embodiment shown in FIG. 7, the connection between the bottom end of the top portion and the bottom portion is a screw connection. For this purpose, the upper portion of the fraction of the bore situated in the bottom portion of the spider is tapped while the sleeve is threaded. Thus, the two portions of the spider can be fixed together by screw engagement until the shoulder 44 of the upper portion presses against the top end of the fins. The spider structures shown in FIGS. 6 and 7 enable the shock absorber to be assembled without any additional element for retaining the socket 24. It suffices to provide a lip 34 -at the bottom end of the bore in the bottom portion against which a shoulder formed on the socket 24 can bear. The socket 24 and the spring 25 are put into place prior to the sleeve being assembled on the bottom portion of the spider (FIG. 7). This shock absorber structure makes it possible to provide a cooling fluid flow duct through the spider. No screw is required. The cooling fluid can flow along an internal channel 36 in the socket and through a central hole 38 in the collar against which the spring 25 bears. There is no longer any need to adjust and weld a screw. The arrangement illustrated in FIG. 7A mainly differs from that of FIG. 7 in that there is a thimble 45 for locking against rotation, so as to prevent the threaded connection between the bottom part and the lower end of the top part from unthreading. The thimble 45 is in abutment against the shoulder of the top portion and has at least one finger which engages into a slot of the bottom portion. After the top portion has been screwed down, the thimble is secured by some welding points at 47. The modification shown in FIG. 7B differs from that of FIG. 7A in that the thimble 45 is replaced with a key 72 located in a recess 74 of the top portion and projecting into a slot 76 of the bottom portion. The parts are again screwed into each other and the recess 74 and slot 76 are located in front of each other. The key, which preferably has half-cylindrical ends, is slid into place as indicated by an arrow. It is then secured to the top portion by welding along its upper edge. In FIG. 7C, the smooth bore of the bottom portion has a lower counterbore for receiving a nut 78 apt to be screwed on the threaded end portion of hub 14A. After the nut has been screwed down, it may be secured by welding at points 80, through the metal, between two fins. The rods can be fixed to the spiders shown in FIGS. 4, 6, and 7-7C by means of the type described in the above-mentioned French Patent No. 2,599,884. Nevertheless, it is preferable to adopt one of the dispositions described below which give flexibility to the cluster of rods facilitating insertion into the guide tubes of an assembly or of guide sheaths in the upper internal of the reactor. In the case shown in FIG. 8, each rod 12 has a plug 46 provided with an extension having one or more portions 48 of small section, and rounded connections with adjacent portions of nominal diameter to avoid breakage starters. There may be up to three generally-cylindrical portions of small section. The end portion 50 of the extension may also be fixed to the respective finger by means of the kind described in the above-mentioned French patent document. In the variant shown in FIG. 9, the fixing means still comprise an extension of the plug 46. The extension has a portion 54 of small diameter located between two swellings. It is fixed in a connector 52. The connector 52 has two cylindrical bearing surfaces 56 on either side of the length of small diameter. Only the bearing surfaces bear against the wall of an open bore formed in the finger 18. A flat 57 in the bore receives a flat on the connector 52 so as to prevent the connector from rotating. The top bearing surface 56 extends beyond the upper surface of the finger and guides a cap shaped nut 58 screwed onto the threaded terminal portion of the extender. When the cap nut is tightened, it presses a shoulder of the extender 52 against the bottom edge of the finger. Once the nut has been fully tightened, it can be prevented from rotating by deforming a thin ferrule of the kind described in French Patent No. 2,599,884. In the variant shown in FIGS. 10 and 11, the plug 46 of the rod is fixed to the finger 18 by an extension of a structure such that the rod is slightly loose. The extension has a threaded terminal portion for receiving a cap similar to that of FIG. 9. A pin or peg 61 prevents the plug from rotating. A groove 68 may be formed in the extension to enable it to be engaged when the pin is in place. The extension also presents a portion of small section situated in the hole of the finger and lying between two projections, swellings or bulges 62 and 64. The top swelling 62 has a sliding fit in the hole and has a centering function. The diameter of the bottom swelling 64 is slightly smaller than that of the hole so as to allow the rod a limited amount of lateral movement. An internal shoulder 66 may be provided in the hole for receiving the extension to constitute a bearing surface for the top face of the swelling 62. The embodiment shown in FIG. 12 differs from that of FIG. 10 essentially in that the projection 62 is connected to a portion of smaller diameter via a frustoconical zone 70 that co-operates with a corresponding portion of the hole to hold the rod in translation. The rod is prevented from rotating during assembly or disassembly by tooling (not shown). A portion of reduced section and/or a set of bottom swellings also ensure that the rod can move laterally. The modified embodiment illustrated in FIG. 12A is designed for providing an accurate degree of centering of the extension of the fuel rod into the finger 18 and at the same time to prevent jamming due to tilting when the fuel rod is mounted or dismounted. For that, the projection 62 has a very short cylindrical portion (lesser than 1 mm) extended upwardly by a frusto-conical abutment zone 70, having an apex angle of about 110.degree., and extending downwardly as a frusto-conical zone 82 merging with the portion of reduced diameter. FIG. 13 and 14 show a possible distribution for locking pegs or pins 61 used in the embodiment of FIG. 10. This distribution gives easy access to all of the pegs 61 for installation or removal purposes.