Patent Number: 047524383
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS The invention will be described, by way of example, as applied to a pressurized water reactor of the kind shown in FIG. 1. Referring to FIG. 1, a reactor comprises a pressure vessel 10 for withstanding the pressure of the water acting as coolant and moderator. The vessel is closed by a lid 12 which carries mechanisms 14 for moving the control clusters, a single one of which is shown at 16. Vessel 10 contains the core 18 formed by adjacent fuel assemblies each having a skeleton and a bundle of fuel rods distributed in a regular polygonal array (square or triangular). The skeleton of each assembly comprises guide tubes (not shown) slidably receiving absorbing rods 20 belonging to a control cluster (or possibly to a cluster for varying the neutron energy spectrum). A mechanical welded assembly, which is generally designated as the upper internals, is located above the core. This assembly fulfills several functions. It collects the high temperature water flow leaving the core and diverts it towards the output nozzle 22 of vessel 10. It retains and indexes the fuel assemblies 18. Finally, it guides the control clusters 16 insertable into some at least of the assemblies during their vertical movements. The upper internals shown in FIG. 1 comprise a lower plate 24 which receives the upwardly directed thrust of the assemblies and an upper plate 26 which transfers the forces applied to the assembly (upper internals) to vessel 10. Plates 24 and 26 are connected together by spacer columns 28 of tubular shape, formed with wide openings for coolant flow. Finally, guides 30 for clusters 16 extend from the lower plate 24 up to the upper plate 26 and beyond. Each guide generally comprises a sleeve 34 whose top part is rigidly fixed to the upper plate 26, and slidably receive the part (frequently called "spider") to which the rods 20 are fixed. Transverse plates 32, often called "cards", formed with openings allowing the spider and the rods to pass therethrough, provide discontinuous guiding of the latter. At the lower part of the guide, the elements are guided continuously by split tubes 36 or equivalent members. For allowing relative expansion of the guides and of the upper internals, the guides are generally each guided in an opening 38 in the lower plate 24, but are not secured thereto. The arrangement which has just been described is generally known. A description thereof can be found in prior art documents already mentioned and for example in French Pat. Nos. 8409275 or 2501892. In the embodiment of the invention which is shown in FIGS. 2 to 4, a sleeve 34 of the guide has a tubular body 40 whose top end is provided with a flange 41 for connection to the upper plate 26 and a base 42 fitted onto the lower part of the body and fixed thereto by pins 44 (FIGS. 2 and 3). The lower part of the base is machined to form a set of rigid blades or lugs 46 and a set of flexible blades 48. Referring to FIG. 4, the blades of the two sets are alternated and distributed evenly angularly. Opening 38 is cylindrical in shape and it has a shoulder 50 for retaining base 42 if required. The rigid blades have radially outer part-cylindrical surfaces having a diameter less than that of opening 38 so as to leave a radial clearance in the 1 mm range. That order of magnitude was selected as a compromise between acceptable manufacturing tolerances and accuracy of mutual alignment of the guides and fuel assemblies. The flexible blades have a thinner part, for giving them the required flexibility, and an end bulge which the elasticity of the blade tends to apply against the edge or cylindrical surface which defines opening 38. The number of blades in each set may be greater than four and in general the blades will be even in number. So that, upon breakage of a flexible blade, this blade is not carried away by the coolant, individual retaining means are advantageously provided for the blades. As shown in FIGS. 2 and 4, the retaining means comprise bridges 52 fixed to the wall of the opening 48, for example by welding, and each surrounding one of the flexible blades. As shown in FIG. 4, indentations may be formed in some of the bridges and/or in some of the thick rigid blades 46 for avoiding interference with control cluster rods, such as those rods 54 whose distribution is shown schematically in FIG. 4. When, as in the embodiment shown in FIGS. 2 to 4, the contacting surfaces of base 42 and body 40 are circular in cross-section, the base may be set angularly on the body in the appropriate orientation for which the blades are in alignment with bridges 52, after the lowest card 32, which carries the tubes 36 for continuous guiding, has been mounted. The modified construction shown in FIG. 5 differs from that of FIG. 4 solely by the absence of bridges. That of FIG. 6 comprises an opening 38 having a cross section which is not circular but polygonal, each of the thick blades 46 being located in a respective angle of the polygon. This arrangement prevents the lower part of the cluster guide from rotating about the axis of the guide.