Patent Number: 049816403
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS The dismantling cell 12 which will be described by way of example is intended to receive a fuel assembly of the type shown in FIG. 1, generally used at the present time in pressurized water reactors of 900 MWe. Such an assembly comprises a skeleton and a bundle of fuel rods 2 distributed at the nodal points of a square lattice. The skeleton is formed of two end pieces 3 and 4, joined together by guide tubes 6 which replace the rods at some of the nodes, and grids 8 spaced apart along the guide tubes. These grids, eight in number in the embodiment described, are fixed to the guide tubes and define passages for guiding and holding the rods in position. Cell 12 is particularly suitable for use in a dismantling installation having the general construction shown in FIG. 2, described in U.S. Pat. No. 4,731,219, to which reference may be made. This installation, placed in a swimming pool for receiving assemblies extracted from the core of a reactor, comprises a support cage 10. Cell 12 for receiving and dismantling an assembly 14 to be dismantled rests on the bottom of the structure. The cell comprises a rigid structure 12 carrying a comb mechanism 18 for holding the fuel rods in position at the nodes of the nominal spacing lattice and means for holding in position their lower end piece 4 and the grids. The installation also comprises tools which are movable horizontally and vertically so that they can work on the assembly 14 placed in the dismantling cell. A cutting tool 24 is provided for cutting the guide tubes 6 joining end pieces 3 and 4 together below the level of the upper grid 8a (FIG. 3) and for withdrawing the assembly formed of end piece 3, the cut-out sections of the guide tubes 6 and the upper grid 8a. A gripping tool 28, suspended from a carriage 30, makes it possible to extract the rods and to handle them. Tool 28 as shown comprises a frame work 32 for vertical guidance of the rods, having combs 34, which may be of the type described in FR-A-2 460 027. Head 36 is movable vertically within the frame work 32 by a jack 38. The installation shown in FIG. 2 further comprises a quiver 39 for the defective rods and a regrouping or consolidation cell 40 having a pyramidal quiver 42 for regrouping the rods in a bunch. In the dismantling cell of the invention shown schematically in FIG. 4, structure 16 has a square cross-section and is formed of angle sections assembled together by frames. Its height is such that the upper end piece 3 is above the cell when the assembly is in position. Structure 16 carries, from bottom to top: the mechanism 20 for fixing and centering the lower end piece of the assembly 14 contained in the cell; PA1 mechanisms 22 for only holding two grids of the assembly in position; PA1 a mechanism 44 for holding and centering the grid 8b of the assembly which is highest, after removal of the assembly comprising the upper end piece; and PA1 the mechanism 18 with two sets of combs, for maintaining the upper ends of the rods at their nominal spacing, and which is carried by the upper end of the structure. The different mechanisms mentioned above will now be described. Comb mechanism (FIGS. 5 to 8) FIGS. 5 and 6 show the entire comb mechanism 18. For the sake of clarity, only one of the combs is shown in inserted position, whereas actually opposite combs move simultaneously. Mechanism 18 comprises a frame 46 with two stages each allotted to a set of two opposite combs. The upper set is formed of two combs 48a moving horizontally in the plane of the figure between a position in which they are spaced apart and a maximum insertion position in which the teeth 50 of the combs are practically in contact. In the embodiment shown in FIG. 6, the teeth 50 of each comb have a length corresponding to the half width of the rod bundle. Each comb 48a slides over two guide ramps 52 formed on the frame. Means for simultaneously moving the two combs 48a comprise two fluid pressure jacks 54, whose cylinder is carried by the frame 46 and whose piston is connected to one of the two combs by a cross-piece 56. To provide smooth and even movement of combs 48a and to avoid jamming thereof, each comb has two lateral racks 58 which mesh with corresponding pinions 60 on a common shaft 62 rotating in bearings fixed to frame 46. Combs 48b of the lower set are mounted and actuated in the same way as combs 48a, but the movements of the combs of one set take place orthogonally to those of the combs of the other. For the sake of simplicity, the same reference numbers have been used in FIG. 5 for designating the corresponding elements associated with combs 48a and 48b. Referring to FIG. 7, where only a few rods 2 have been shown, frame 46 is fixed to a top flange of structure 16 at a level such that the combs 48a and 48b are inserted between the rods just above the level N where the guide tubes 6 are cut. With this arrangement, a checker pattern of passages is obtained, shown in FIG. 8, immediately below the position where the rods will be gripped by the means for removing them. Lower end piece fixing and centering mechanism Mechanism 20 shown in FIGS. 9 and 10 comprises two centering studs 64 carried by the bottom of structure 16 for engagement in recesses of corresponding shape in the lower end piece 4. The mechanism again comprises a single set of jaws 66 acting on two opposite faces of end piece 4 and disposed symmetrically with respect to this end piece. Each jaw 66 is movable between a position for clamping the end piece 44 (shown in continuous lines in FIG. 9) and a position in which it releases this end piece, (shown in dash-dot lines). Each jaw has a bearing face allowing it both to hold end piece 4 firmly in position and to center it. In the embodiment shown in FIGS. 9 and 10, the bearing face of each jaw 66 is provided with studs 68 which pass between the adjacent fuel rods 2, bear against the upper face of end piece 4 and prevent it from moving upwardly. Jaws 66 are guided during their movement by pins 70 fixed to the jaws and imprisoned in elongate holes 72 in structure 16. The movements of the jaws are controlled by respective jacks 74 (FIG. 9) whose cylinder is pivotally mounted to a lug 76 belonging to structure 16 and whose piston is connected to a shaft 78 passing through the jaws 66. Grid holding mechanism (FIGS. 11 and 12) The cell shown in FIG. 4 comprises two mechanisms 22 for simply holding in position two current grids 8 of the assembly, to which is added a mechanism for holding and centering the uppermost grid 8b which will be described further on. The two simple holding mechanisms 28 each grip a grid on two faces only. The lower mechanism 22, for example, grips the third grid from the bottom on two faces parallel to the plane of FIG. 4, and the other mechanism 22 grips the fifth grid from the bottom, on the two faces perpendicular to the plane of FIG. 4. The two mechanisms 22 are identical. The one shown in FIGS. 11 and 12 has a construction very similar to that of mechanism 20: it also comprises two jaws 80 each movable between a clamping position and a release position by a jack 82. Each jaw 80 has a flat face for bearing on grid 8 and studs 84 of triangular cross-section engaged in the free spaces between the fuel rods 2 so as retain the grid axially. The bearing points for studs 84 may have the arrangement shown schematically in FIG. 13 for one of the jaws 80, an arrangement offset by the distance between two successive rods for the other jaw. Thus, each grid 8 is retained at fourteen points. In the cell 12 shown in FIG. 4, the lower mechanism 22 grips the grid 8 on two faces orthogonal to the faces gripped by mechanism 20 for fixing and centering the lower end piece and orthogonal to the faces gripped by the other mechanism 22. Thus a distribution of the engagement studs is obtained which is as shown schematically in FIG. 14 and guarantees a firm holding. Mechanism for holding and centering the uppermost grid The mechanism 44 for holding and centering the uppermost grid 8b (seventh grid from the bottom in the case of the assembly shown in FIG. 1) is provided for gripping the grid on its four faces. It may have the construction shown in FIGS. 15 and 16, with two jacks 88 only, located opposite each other and arranged for moving two sets of jaws 86 and 87 mutually opposed in sets of two. The means for guiding the jaws will not be described, for they may have exactly the same construction as those already illustrated in FIGS. 9 and 11. Simultaneous control of the four jaws is obtained by fixing the pistons of the two jacks 88 to the same mobile horizontal plate 92 which surrounds the structure and which carries vertical links 90 provided with pins 94 for pivotal movement of the jaws. In the embodiment illustrated, the jaws 86 of one only of the two sets are provided with retention studs 96. The action of jaws 86 and 87 on the four faces of grid 8b straightens up the fuel assembly should deformation of the assembly have caused flexure of the rods or twisting. A cell according to the invention may be provided for fuel assemblies different from that shown in FIG. 1, for example for assemblies for a 1300 MWe reactor comprising an increased number of grids. The different mechanisms described may again be used. All mechanisms are of simple construction and yet guarantee transverse and longitudinal holding of the lower end piece and of the grids and holding of the rods in position at the nodal points of a given lattice, thus avoiding any risk of jamming during extraction thereof. Means similar to those which have been described could be used for dismantling an assembly having a hexagonal rather than square cross-section and rods distributed at the nodal points of a triangular lattice. The method of using the cell is clear from the above description and therefore need not be described in detail. The fuel assembly to be dismantled is lowered into the cell while suspended from a handling tool, the jaws of the different mechanisms and the combs being spaced apart. The lower end piece 4 is centered on studs 64 at the end of its downward movement. Jacks 74 of mechanism 20 are actuated for moving jaws 66 close toward each other and gripping the end piece 4 on two faces. The grid holding mechanisms 22 and 44 are then brought into action; these mechanisms prevent the grids from being deformed during extraction of the fuel rods by facilitating sliding thereof. The guide tubes are severed below the level of the top grid 8a and the sub-assembly comprising this grid and the upper end piece 3 is removed. Finally, the combs of mechanism 18 are moved toward each other so as to hold the upper ends of the fuel rods in position at the nodes of their nominal distribution lattice.