Patent Number: 052079783
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT The reactor partially shown schematically in FIG. 1 has a general construction which is well-known at the present time and is, for instance, as disclosed in U.S. Pat. No. 4,092,216. It will therefore be described only. The core is formed of mutually juxtaposed fuel assemblies and contained in a pressure vessel 10 closed by a lid 12. A shroud 14, supported by the vessel, defines an annular space through which the coolant admitted by nozzles (not shown), flows down to the space formed under a lower plate carrying the core. The coolant then rises through the core and leaves it through passages formed in an upper core plate 16 belonging to the upper internals of the reactor. The internals also comprise a support plate 18, supported by the vessel, connected to the core plate by structural columns (not shown), and by guide devices 20, each for receiving a control cluster. The clusters are formed of a bundle of elongated elements containing neutron absorbent material, e.g., twenty-four in number suspended from a "spider" fixed to a drive shaft 22. Each device 20 comprises, between plates 16 and 18, a tubular casing 24, of approximately square section as shown and, above the support plate 18, an extension having a closure plate 25, formed with a hole for passage of the drive shaft 22 therethrough. Horizontal guide plates 26 are evenly spaced apart along casing 24. They are fixed to the casing by external projections of the plates engaged in slots of the casing 24, and generally by welding. Those plates 26 which are situated at the upper part of casing 24, six in number in the embodiment shown in FIG. 1, have an internal cut-out such that the plates guide the elements and leave the arms of the spider free to move. The elements are thus guided discontinuously, at intervals corresponding to the spacing between plates 26. The lower plates, four in number in the embodiment shown in FIG. 1, are connected together and to the foot 28 of casing 24 by continuous guide means, and are securely connected to the casing, for instance by welding. As shown in FIG. 3, the guide means are formed as split tubes 30, each intended to receive an element, such as the element shown at 32, and as sleeves 34 and 36 each guiding a pair of elements carried by a same arm of the spider. Openings 38, elongated in the vertical direction, are formed in casing 24 in the continuous guide zone. These openings 38 constitute a path for the coolant from the core into the manifold defined by plates 16 and 18 and by shroud 14; from there, the coolant flows out of the reactor through nozzles 40. Finally, the guide device comprises a frusto-conical guide 42 for centering the drive shaft 22, when lid 12 is being positioned on the vessel. In general, the pressures are not balanced across the closure plate 25, which causes a turbulent flow in the annular clearance, between the wall of the hole in the plate and shaft 22. This flow may be upward or downward. When it is upward, it forms a jet which, before being diffused, is subjected to a double reversal, as shown by arrows f on FIG. 6. The turbulence of such a flow causes considerable excitation of the shaft which is communicated to the elements by the spider. The guide device according to the invention, a specific construction of which is shown in FIG. 2, considerably reduces the wear phenomenon due to vibrations of the elements. The device of FIG. 2, where the elements corresponding to those of FIG. 1 have the same reference numbers to which the index A has been added, may often replace that of FIG. 1 in an existing reactor as a retrofit. Again, it comprises a casing 28a to which horizontal guide plates 26a are fixed. The four lower plates 26a and the foot 28a of the device are again connected together by continuous guide means, formed of tubes 30a, and sleeves 34a and 36a (top part of FIG. 3) which pass through the plates, and connected to casing 24a, for instance by welding. As shown in FIG. 2a, the openings 38a have an approximately rectangular shape and are placed just below those plates 26a which are in the continuous guide zone. Each opening 38a extends as far as the plate 26a placed above it. To better distribute the flow which leaves the core among the superposed openings 38a, the three uppermost plates of the continuous guide zone are preferably formed so that they offer a coolant cross-sectional flow area smaller than the cross-sectional area of the lowest plate. To this end, the internal periphery of the three upper plates may preferably have the cut-out shape shown in the upper half of FIG. 3, while the lowest plate keeps the usual cut-out shape shown in the lower half of FIG. 3. It can be seen that the flow cross-sectional area is reduced in the upper plates by extending the plate inwardly along four sleeves 34a, placed at 90.degree. from each other. As a result, flow occurs in a passage consisting of a central zone and four radially directed zones having a width which only slightly increases radially outwardly. In a modified embodiment, the plate may extend inwardly along all sleeves, for giving a substantially constant width to the radial zones. To reduce the coolant speed between the sleeves and the pressure fluctuations, the internal end of the sleeves, in the radial direction, is advantageously rounded as shown in FIG. 3. To generate a pressure differential which applies the elements against the tube wall, each split tube may have a wall without any orifice other than the slit. Sleeves 34a are without apertures in their upper portion, above the second plate, only three elongate apertures 44 (FIG. 5) being left. Sleeves 36a likewise have apertures 46 at their lower part only. These apertures may be completed by two sets of three aligned holes 48, in the low part of the sleeves and in the vicinity of the slit. All these arrangements significantly reduce the risks of vibrations caused by the flow along the elongated elements. To reduce the vibrations induced by the flow along drive shaft 22, apertures 50 each in the form of a slit are formed in conical guide 42 to upward flow coolant. The apertures 50 may be in the form of slits spaced evenly angularly apart, elongated in the longitudinal direction and formed at the top part of the guide. Thus, the upward flow takes place in the direction of arrows fa in FIG. 6. Sixteen apertures 50 may typically be used, only three of these being shown schematically in FIG. 6.