Patent Number: 054904188
Section: description

DETAILED DESCRIPTION The invention is described in its application to measurement of the pressure force exerted by grid springs supporting fuel rods at the nodes of a square array. As shown in FIG. 1, such a grid 10 may be constituted by two sets of mutually crossed plates 11 assembled together via interfitting half-depth slots. Once they have been engaged in one another, the plates are held in place e.g. by punched protrusions 13 and by weld spots 12 at the cross-points. The crossed plates define cells most of which receive fuel rods (not shown) and some of which are occupied by guide tubes belonging to the skeleton of a fuel assembly. Each of the cells that receives a rod includes means for holding the rod in two orthogonal directions. In general, for each direction, these means comprise rigid abutment elements on one of the walls of the cell. In the example shown, the abutment elements are constituted by projections 16 located near the top and near the bottom of the wall. The holding means also comprise a spring 15 facing the projections and designed to cause the rod to exert a determined thrust force against the projection. The force must be large enough to prevent the fuel rods vibrating, but small enough to allow a fuel rod to slide in the event of thermal expansion. The spring 15 shown in FIG. 1 comprises two "active" branches, i.e. branches designed to exert a holding force on a rod. However, the spring could have only one active branch, e.g. when mounted on a wall between a cell occupied by a rod and a cell designed to receive a guide tube, for example, as shown in FIG. 2. In either case, each rod is held at three points in one direction, and at three points in the direction orthogonal thereto. The disposition described above is conventional. Embodiments may be found, for example, in documents EP-A-280 595 and U.S. Pat. No. 5,091,145. The device shown diagrammatically in solid lines in FIGS. 2 to 4 is designed to measure the pressure force exerted by a single spring 15. The device is in the form of an elongate body 20 that is generally cylindrical in shape, and connected by two flexible blades 22 to two respective beams 24 having an outside surface that substantially reproduces a fraction of the surface of a fuel rod. The beams are of a length that is not less than the distance between the two abutment projections of a cell. To guarantee that the beams are accurately located in the grid, the body is advantageously fixed on a positioning cap 26 designed to bear against one of the major faces of the grid. A unit that is movable in the elongate direction of the body enables the beams to be moved relative to each other between the position shown by solid lines in FIG. 2 and the position shown by chain-dotted lines. The moving equipment is designed to keep the beams parallel during such displacement. In the example shown in FIGS. 2 and 3, the movable unit is actuated by fluid pressure means. In a simplified embodiment, those means could be manual. As shown, the fluid pressure means comprise a piston 30 mounted in a bore 32 formed in the body. The piston 30 is subjected to the action of a return spring 34 urging it against the end wall 35. The piston 28 is coupled by a pin 38 to a connecting rod 36 which extends between the blades 22. The connecting rod 36 is hinged to a slider 40 connected to each of the beams 24 by a set of arms 42 and 44 constituting a deformable parallelogram. In the example shown in FIGS. 2 and 3, the arms are mounted on the slider 40 about pins 46 which are also engaged in side plates 48. The two side plates are fixed to each other by screws 50. Pegs 52 may also be used to hold the side plates 48 in position relative to each other. When the piston 30 contacts the end wall of the bore in which it moves, the arms 42 retain the beams inwardly so that they take up a volume such that the set of beams can be inserted in a cell without friction. When fluid pressure is applied against the piston, e.g. via a nozzle 53, the movable unit moves downwards and deforms the parallelogram in a direction that causes the beams 24 to move apart from each other. The spacing imparted to the beams by the piston should correspond to the size of a fuel rod. The device shown by way of example in FIGS. 2 and 3 includes adjustment means enabling that result to be achieved. The adjustment means comprise a bush 54 capable of sliding on a small diameter length of the connecting rod 36, serving to fix the amplitude d of displacement available to the piston starting from its rest position. The longitudinal position of the bush 54 which receives the pressure from the return spring 34 is set by a wedge 56 that is displaceable transversely to the rod 36 by means of an adjustment screw 58 passing through the body and capable of being locked by means of a nut 60. The body 20 is generally made up of a plurality of mutually assembled parts which, in the examples of FIGS. 2 and 3, comprise a cylinder for receiving the piston and a base which are fixed together by screws 61. These screws may also serve to secure the stop-forming cap 26 that is positioned relative to the base by pegs 63. The beam 24 designed to come into contact with the spring 15 caries a measuring element 62 such as a piezoelectric gauge or a strain gauge, connected by wires (not shown) to a connector 64 for coupling to an outlet cable 66. Operation of the device can be seen directly from the above description. While the piston is not subjected to a fluid pressure force, the device is inserted with appropriate orientation into a cell fitted with a spring whose force is to be measured. Since the beams are close together, insertion takes place without significant friction. The length of the cap 26 is such that when its spacer comes into contact with the large face of the grid, the measurement element 62 is facing the portion of the spring 15 that projects furthest. Fluid is then fed to the nozzle 53, thereby causing the piston rod 28 to move downwards until a shoulder thereof comes into abutment against the bush 54 which has been set so that the spacing of the beams is then equal to the diameter of a fuel rod. A measurement is taken. Fluid is then allowed to discharge through the nozzle 53 and the device is withdrawn. When the device is designed to measure the force exerted by a spring that has two active branches, like the spring shown in FIG. 1, and in chain-dotted lines in FIG. 2, two sets of blades 22 and beams 24 are provided which are actuated simultaneously by the same drive means, as shown in chain-dotted lines in FIG. 2. Tests performed with a device of the above kind have shown that all of the operations required for measuring the force of the springs in a complete grid require no more than half an hour for a grid that needs 472 measurements. In the modified embodiment shown in FIG. 5, where members corresponding to those described above are given the same reference numerals, the arms 42 and 44 constituting a virtual deformable parallelogram in the example of FIG. 2 have been replaced by a set of cams and ramps. The slider 40 connected to the connecting rod 36 via a fork and a pin 68 has two sloping ramps 70 facing each beam 24. Each beam carries two pins 72 constituting cams for following the profiles of the ramps. When the ramps are lowered from the rest position shown in FIG. 5, the beams are moved apart against the action of resilient return means that are constituted in the example shown by a tweezer-shaped spring urging the beams towards each other. As in the preceding example, moving the movable unit downwards causes the beams to move apart while keeping them parallel to themselves, providing only that the resilient blades 22 are long enough to take up an S-shape while leaving all of the cams in contact with the corresponding ramps. Although the embodiments shown in FIGS. 2 to 5 provide good results in cells that have two springs situated face to face, they can be less satisfactory when that condition is not satisfied. Under such circumstances, there is a danger of the beams taking up a skew position in the cell. FIGS. 6 and 7 show such a disposition. In each of two cells in FIG. 7, each spring 15 faces a rigid projection 16. Because the springs bend, the beams of a single-cell device run the risk of taking up an oblique position, as shown by a dashed outline. This risk is eliminated with a two-cell device as shown in FIG. 6, where elements corresponding to those already described are given the same reference numerals. The device of FIG. 6 can be considered as having two sets of beams and flexible blades, each secured to a body. The two bodies are carried by a common housing 76 made up of a plurality of assembled-together parts, and having an extension 26 that constitutes a stop and that bears against a grid plate between the two assemblies. The bush 54 can rest against the wedge 56 via slideways 77 that reduce friction. Each of the assemblies is connected to the housing 76 by a peg 78 which constitutes an axis of rotation orthogonal to the axes of the pins 38 and 46, also designated 78 on FIG. 7. In other words, the assemblies can pivot about respective axes placed close to their top ends and parallel to the beam-separation direction. The assemblies can also be offset angularly, each to one side of the vertical, in such a manner that the beams take up the position shown in solid lines in FIG. 7 and therefore provide an accurate measurement regardless of the bending of the springs 15. In the embodiment of FIG. 6, the outer blade 22a of each assembly is thicker than the inner blade 22b and is less flexible. When the slider 40 is pushed in, then the inner beam bears against the double spring 15 before the outer beams begin to move away so as to press against the projections 16. The dimensions of the device are such that the beams are inserted into the cells without friction when the slider is raised. The device shown in FIG. 6 can be used as shown for measuring the force of a double spring, or only one of its sets can be used in conjunction with a single spring.