Patent Number: 052079783
Section: summary

FIELD OF THE INVENTION The present invention relates to a nuclear reactor control cluster guide device, and is particularly suitable for use in pressurized water reactors and, among the latter, in thermal neutron reactors or spectral shift reactors. BACKGROUND OF THE INVENTION In present day pressurized water reactors (PWRs), the reactivity is controlled mainly with control clusters each comprising a bundle of elongated elements suspended from a spider with radial arms carried by a vertical cluster drive shaft, for moving the cluster between a lower position in which it is inserted almost completely in the core of the reactor and an upper position in which it is situated in the upper internal equipment of the reactor. The elements are guided in the core by guide tubes provided in the fuel assemblies. Above the core, these elements, which are very long and thin, must be guided in the upper internals by a device which may comprise: a tubular casing, having, at its upper part, a closure plate formed with a hole for passing the drive shaft therethrough; horizontal plates spaced apart along the casing and cut out to let the spider pass and to guide the elements; means for continuously guiding the elements in the low part of the casing, formed by guide tubes for individual elements and/or by guide sleeves for pairs of elements each having a slit of sufficient width to let an arm of the spider pass, in the intervals between the lowest plates, intended to protect the elements from the action of the turbulent flow of coolant which leaves the core and is deflected towards coolant outlet openings formed in the casing and situated in said intervals and to avoid buckling of the elements when the cluster is released and falls while being braked by the dash-pot effect of the guide tubes. Up to now, attempts have been made to reduce as much as possible the pressure gradients which are exerted on the elements in a horizontal direction, in the above-mentioned intervals. Such attempts have been based on the assumption that such pressure gradients apply the elements against the lips of the slit through which the spider passes and cause frictional wear of the elements. To achieve pressure balancing, pressure equalizing holes have been formed in the wall of the guide sleeves; the sleeves have been replaced by castings leaving a pressure balancing clearance (U.S. Pat. No. 5,006,305); and indentations have been formed in the lips of the slit which constitute pressure balancing openings (U.S. Pat. No. 4,562,038). The subjects matter of such patents are included in the present specification by reference. Other examples of prior art guiding devices may be found in French Patent No. 2 397 043 (WESTINGHOUSE) and European No. 329,536 (FRAMATOME). SUMMARY OF THE INVENTION The present invention is based on the finding that, at least in certain types of reactors cause of wear of the elements and of the plates is vibration of the elements stimulated by pressure oscillations within the flow, and it is an object of the invention to reduce such stimulation. For that purpose, there is provided a guide device of the above-defined type in which the openings are formed immediately below the plates at the top of each gap; the tubes for guiding a single element are without openings, so as to generate a differential pressure which applies the element against the tube and prevents it from being driven by the coolant which flows towards the openings; and the lowest plate has such a cut-out that it offers to the coolant a flow cross-sectional area greater than that of the plates placed above and defining said intervals, so as to better distribute the flow leaving the casing through said openings. The openings advantageously have a rectangular shape and terminate upwardly at the horizontal level of the transverse plates, this prevents local reversal of the fluid streams and reduces the pressure oscillations. The internal edges of the sleeves guiding two elements carried by a same spider arm are typically shaped so as to reduce the flow speed between the sleeves and thus the intensity of pressure fluctuations. The sleeves are advantageously devoid of coolant passage apertures in their upper part, so as to generate pressure differences biasing the elements placed closest to the axis of the casing and avoiding their being driven by the flow of coolant. Conventional guide devices generally comprise a frusto-conical guide, flaring out downwardly, placed above the closing plate of the casing and intended to center the control rod of the cluster during closure of the lid of the vessel of the reactor. The pressure differential which appears during operation between the two faces of the closure plate of a casing may cause a turbulent flow in the annular clearance which exists between the wall of the hole in the plate and the shaft. The turbulent flow, upward or downward, depending on the case, causes excitation of the rod, particularly in the case of a rising flow which undergoes a double change of direction caused by the conical guide. Such excitation is transmitted to the elements by the spider. The vibrations which result therefrom cause additional wear of the top part of the elements, which strike the horizontal guide plates when they oscillate. It is an ancillary object of the invention to reduce the vibrations having the above identified cause and the attendant corrosion; for that purpose there are provided, in the frusto-conical guide, slit-shaped apertures spaced evenly angularly apart, advantageously elongated in the longitudinal direction and facilitating upward flow of the coolant, particularly by avoiding a double change of direction and acceleration.