Patent Number: 054904188
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a device for measuring the force exerted by a spring of a supporting grid on a nuclear fuel rod passing through the grid. The fuel assemblies used in nuclear reactors that are cooled and moderated by water include support grids for supporting fuel rods and/or for holding them at the nodes of a regular array, generally a square array. The grid is often constituted by two sets of mutually crossed plates defining rod-receiving cells. Two facing walls in a given cell are organized so that one of them includes a rigid cell-engaging projections and the other includes a spring, often in the form of a hairpin, straddling a plate and intended to force the rod against the projections. The springs may be single, i.e. they may have a single resilient branch acting on one rod; or they may be double, i.e. they may have two branches each acting on a respective rod placed on a respective side of the plate carrying the spring. It is important for the springs to exert forces on the rods that do not depart too far from a set value. Various devices have already been proposed for measuring the force exerted by a spring that is single or double, for the purpose of verifying that the force exerted by each spring of a grid lies within a predetermined range of values prior to incorporating the grid in an assembly. For example, document EP-A-0 501 663 describes a device enabling the force exerted by a double spring to be measured; that device includes a gauge designed to be forced into a cell and including a feeler for transmitting the force exerted by a spring. The gauge is fixed to a centering peg which is designed to engage in the cell adjacent to that in which measurement is being performed and to retain the double spring therein. Forced insertion of the gauge and of the centering peg runs the risk of damaging the spring. Document DE-A-3 242 407 describes another measuring device having a gauge of a diameter that is substantially equal to that of a fuel rod. A bore is provided in the gauge transversely to its axis at a location that faces the thrust point of the spring when the gauge is pushed home. The bore contains a piston whose outside face carries a measuring element. To measure the force of a spring, the gauge is inserted and hydraulic pressure is exerted on the piston to bring it into a determined position such that the width of the gauge level with the spring thrust point is equal to the nominal diameter of a fuel rod. Given that the piston is hardly capable of projecting from the gauge, it is not possible to completely eliminate friction while the gauge is being inserted and removed. SUMMARY OF THE INVENTION It is an object of the invention to provide an improved device for measuring the force exerted by a grid spring on a fuel rod. It is a more specific object to reduce the friction forces exerted on the spring whose characteristics are to be measured. To this end, the invention there is provided a measurement device comprising: an elongate body connected by two flexible blades that are parallel to the long direction, to two respective beams designed to be inserted in a cell, at least one of the beams carrying a force sensor designed to be pressed against the spring; a unit slidable in the body along the body between a position in which it moves the beams apart while keeping them parallel to give them a spacing that corresponds to the nominal diameter of a fuel rod, and a position in which it allows the flexible blades to move the beams towards each other. Displacement means are provided for moving the unit at will between the two positions. The means for displacing the moving equipment may be manual; but they will generally be motor driven. In particular they may be actuated by a pressurized fluid. The movable unit may be of various different structures. In particular, it may comprise a connecting rod that is longitudinally displaceable by the displacement means and that is connected to the beams by arms in such a manner as to constitute a virtual deformable parallelogram that obliges the beams to remain parallel to themselves as they move apart or towards each other. In another embodiment, the movable unit carries at least two sets of ramps co-operating with thrust means provided on the beams and constituting cams. Whichever embodiment is adopted, the fact that the thrust surfaces of the beams remain parallel to themselves guarantees good contact between a force sensor carried by at least one of the beams and the spring whose force is to be measured. The device may be doubled up, i.e. it may have two sets of beams so as to make it possible to perform measurements on double springs.