Patent Number: 058898328
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to control clusters for a nuclear reactor, each cluster comprising a spider made of a hub fitted with drive shaft fastening means and with fins radiating from a bottom portion of the hub and provided with parallel vertical fingers distributed in a regular array, together with rods provided with plugs fixed removably to the fingers. At present, control clusters of this type include sixteen fins. Each fin carries a finger at its end. Every other fin also has an intermediate finger. Thus, the cluster includes twenty-four absorbent rods. Spiders are presently constituted as welded assemblies. The fins are put into place around the hub and brazed. The brazing operation is lengthy and difficult when seeking to avoid any positioning error of the rods and to avoid any deformation due to thermal effects. In most cases, the fingers are fixed to the fins by respective tenon-and-mortise connections: the end portion of each fin constitutes a tenon which is engaged in a slot of the associated finger. That connection is finished off by brazing. When this solution is adopted, it is impossible to provide a vertical hole passing through the finger, and consequently the plugs of the rods must be fixed in blind holes in the portions of the fingers located beneath the level of the fins. That solution has turned out to be not very satisfactory. French Patent No. 2,599,884 describes a solution that makes it possible to make the rods removable. It is complex and gives the rod assembly a secured connection with the spider. Such a connection is liable to impede sliding of the rods in the guide tubes of an assembly that is to receive the cluster. SUMMARY OF THE INVENTION An object of the invention is to provide an improved control cluster. For this purpose, in an important aspect of the invention, the invention provides a control cluster in which the fins, their fingers, and the bottom portion of the hub constitute a single unitary piece obtained by molding or by electro-machining. The top portion of the hub may be integral with the bottom portion; this generally requires manufacture by electro-erosion. In an advantageous embodiment, the top portion of the hub, constituted by a single cylindrical sleeve, is made separately from the single piece and is engaged in the bottom portion and is connected thereto, e.g., by means of a threaded connection or by welding. The above design makes it possible to considerably simplify the structure and assembly of the damper generally provided in the spider to reduce the shock to which the top endpiece of the fuel assembly receiving the cluster is subjected when the cluster is dropped for a scram. Conventionally, the shock absorber comprises a socket that slides in the hub and that is urged downward by a spring toward a position in which it projects from the hub. The projecting position of the socket is defined by an abutment constituted by the head of a screw placed axially in the hub. That structure requires initial adjustment of the screw. Also, the presence of the screw impedes the flow of cooling liquid along the hub and, by a dash-pot effect, lengthens the time required for the cluster to drop. When the sleeve is made up of two parts, it is possible to provide an internal collar in the bottom portion situated at a suitable location and constituting an abutment that defines the projecting position of the socket. In addition, in an internal flare of the sleeve on which the spring bears, it is advantageous to provide a hole allowing cooling fluid to flow through the socket and the sleeve, thereby reducing the dash-pot effect. Another object of the invention, which can be achieved regardless of whether some of the preceding dispositions are used, consists in allowing the rods a degree of freedom or "looseness" enabling them to adapt to the exact location of the guide tubes designed to receive them in the fuel assemblies and/or guide sheaths in which the clusters are housed when they are raised into the upper internals of a reactor, in the event of misalignment due to manufacturing tolerances. To this end, the invention proposes a cluster whose plugs include, between the rods and the means for fixing to the fingers, an extension having at least one portion of reduced diameter for increasing its flexibility. Because of the one-piece structure of the fingers and of the fins, through holes can be formed in the fingers to enable easily-released fixing for replacement of some of the rods. Finally, it should be mentioned that the "loose" nature of the way in which the rods are mounted can be advantageous even with a spider that does not have the above-defined structure, e.g., a spider of the kind constituting the subject matter of above-mentioned French Patent No. 2,599,884. The above characteristics and others will appear more clearly on reading the following description of particular embodiments of the invention given by way of example, and by the comparison made thereof with prior art dispositions. The description refers to the accompanying drawings.