Patent Number: 044118627
Section: summary

The present invention relates to a spacer grating for a fuel assembly used in a nuclear reactor constituted by intersecting plates in square-mesh lattice form. Fuel assemblies in nuclear reactors, particularly light water nuclear reactors, are generally constituted by a bundle of fuel pencils, i.e. tubes, made of material which sparingly absorbs neutrons, filled with pellets of fissile matter, the bundle being assembled with the aid of pieces disposed at each of the ends of the assembly. For transversely holding the fuel pencils in the bundle, spacer gratings are generally used, constituted by plates intersecting in square-mesh lattice form, the fuel pencils passing in the cells of the grating. In this way, the pencils are in regular arrangement in the transverse directions imposed by the grating. Certain of the places arranged by the spacer gratings for the fuel pencils are occupied by guide tubes which are connected at their ends to the plates of the assembly to ensure cohesion thereof. Each cell defined inside the spacer gratings receives either a fuel pencil, a guide tube, or a tube allowing passage of instruments for automatic control of the core of the reactor. To ensure transverse holding of the fuel pencils inside the cells of the spacer gratings, it is necessary to exert transverse forces on these pencils and to this end to provide devices borne by the walls of the cells of the grating extending inwardly of the cell, these cells being of such dimensions that a considerable clearance is made between the pencil and the walls of the cell. This considerable clearance is in fact necessary to allow the flow of the coolant and to facilitate installation of the assembly. The devices for holding the pencil inside the cells of the grating must in particular allow a sufficient semi-rigid fit of the pencils in the cells of the grating to avoid displacements and deformations of the pencils of too great amplitude and an exaggerated bending under the effect of the deformations of thermal origin and caused by irradiation. Deformations of thermal origin are generally caused by temperature gradients between guide tubes, guide tubes and pencils and pencils between themselves, the deformations due to the effect of irradiation resulting from a certain dispersion in the elongations due mainly to the flux gradients and to the stress gradients. In fact, if the structure of the assembly is too strongly disturbed during operation in the reactor, the circulation of the coolant fluid of the reactor risks being modified and a certain asymmetry of this flow may occur locally, so that the conditions of exploitation of the reactor diverge from the optimal conditions. On the other hand, a semi-rigid fit of the pencil which employs too considerable tightening forces on the tubular cladding of the pencil, excessive stresses of abutment on this cladding as well as axial stresses in this cladding produced by the differential thermal explansions which risk destroying it at the semi-rigid fit, must be avoided. A mode of fixing the pencils in the cells of the grating must also be avoided which risks introducing exaggerated wear and tear of the cladding of this pencil under the effect of the vibrations to which the assembly in operation is subjected by exerting in this case a relatively great tightening force ensuring the best moment of semi-rigid fit. Devices have therefore been proposed for holding the fuel pencils in the cells of spacer gratings, whose object was to provide an acceptable compromise between these contradictory requirements. It has, for example, been proposed to make the spacer gratings of an elastic alloy such as an alloy of nickel or a stainless steel, springs and rigid stops being obtained by cutting out and pressing certain portions of plates constituting the grating to form devices for retaining the pencils. Such a solution presents the drawback of introducing in the assembly gratings of a relatively large volume made of a material which strongly absorbs the neutrons. It has also been proposed to make the plates constituting the cells of the grating of a material which sparingly absorbs the neutrons, such as an alloy of zirconium, elements for holding the fuel pencils being added to these zirconium alloy gratings. These holding elements may be constituted by springs made of elastic alloy and by rigid stops disposed on the plates constituting the different walls of the cells of the grating, so that, in each cell, the springs are on faces opposite the faces of these cells on which the rigid stops are disposed. To make this distribution of the springs and the rigid stops added to the plates of the grating, it has been proposed to use springs comprising two active parts on either side of the wall on which the spring is disposed to allow the action of the spring on two different pencils inside two adjacent cells. Such springs may, for example, be constituted by a leaf made of elastic material surrounding the plate and closed on itself. In each cell of such a known embodiment with cells of a single type, double springs are associated with two of the walls of the cell and rigid stops with the other two walls. However, such devices do not enable a grating comprising an odd number of cells to be entirely equipped on each of its sides and neither do they take into account the disturbances caused, in the distribution of the springs equipping the cells containing the fuel pencils, by guide tubes disposed in the lattice whose mode of fixation on the cell is rigid. It is an object of the invention to provide a spacer grating for a fuel assembly used in a nuclear reactor, constituted by intersecting plates in square-mesh lattice form, in the cells of which the fuel pencils pass for transverse holding thereof; these pencils constitute a bundle where certain pencils are replaced by guide tubes, the plates which constitute the walls of the cells bearing elements for holding the fuel pencils projecting inside the cells, of two types, namely, on the one hand, rigid stops, and, on the other hand, springs adapted to exert transverse forces on the fuel pencils, added to the grating and disposed around the plates so that each of the faces of each cell containing a pencil bears on element of type different from that borne by the opposite face, this grating having to allow an efficient holding of the pencils without leading to a deterioration of the claddings of these pencils. According to the invention, the adequate distribution of the springs and the rigid stops is obtained despite the presence of guide tubes and an odd number of cells on each of the sides of the grating, whilst avoiding the use of too numerous holding elements associated with the walls of the grating and too large a quantity of neutron-absorbing material for constituting these holding elements. To this end, the spacer grating according to the invention comprises: on the greater part of the walls equipped with springs, double springs comprising two active parts on either side of the wall on which the double spring is placed, allowing the symmetrical action of this spring on two different pencils inside two adjacent cells; on the other walls equipped with springs, single springs comprising one active part only, disposed in one of the two cells separated by the wall on which is placed the spring of which the other part is not capable of coming into contact with the fuel pencil, the rigid stops being arranged on the plates of the grating independently of the springs. In a variant embodiment of the invention, single springs are found on the other walls equipped with springs and/or rigid stops, which comprise one active part only, disposed in one of the two cells separated by the wall on which is placed the spring which is connected to rigid stops coming into contact with the fuel pencil passing through the cell located on the other side of said wall, and rigid stops located on one side of a wall of the cell and constituted by added pieces made fast with the single springs or the rigid stops placed on the other side of the wall. According to a particular embodiment of the invention, the springs comprise two arms disposed on either side of the wall on which they are added and welded together at their two ends, so that the moments and shearing strains exerted on one of the arms are balanced to a large extent by those exerted on the other arm and are virtually not transmitted to the plate constituting the wall of the grating. According to a likewise preferred embodiment of the invention, the springs have on their active parts a section comprising two parts projecting inwardly of the cells of the grating coming into contact with one of the generatrices of the pencil at two points spaced apart at maximum along the length of this pencil and the faces of the cells opposite these springs with double projection comprise two rigid stops disposed substantially at the level of the projections of the springs in the transverse direction perpendicular to the faces of the cells in question. It is also an object of the invention to provide a spacer grating, wherein the walls of cells equipped on one side with springs and on the other side with rigid stops, comprise a spring with two projections and three rigid stops added and fixed to the wall due to two welded end shoulders and to a likewise welded median sliding plate, each housed in one of the three rectangular openings made in alignment on the wall, and to two fixing orifices disposed near the shoulders, so that the transverse thrust forces exerted on each side of the wall are directly balanced via the shoulders and the median sliding plate, without stress of the wall in flexion, the balancing of the longitudinal thrusts on the two projections being effected by sliding of the median sliding plate mounted to this end with a longitudinal clearance in its opening, and the fixing of the end shoulders in their respective opening being obtained with the aid of bent edges of the shoulders tightening on the wall. In a variant embodiment, the preceding spacer grating is characterised in that the walls of cells equipped with rigid stops on their two faces, comprise a pair of rigid stops back to back at each end, each rigid stop being ovoid in form, obtained by cold-forming a strip of Inconel, each pair of rigid stops being added and fixed to the wall via two welded shoulders, each mounted with a longitudinal clearance in a rectangular opening in the wall, to which they are fixed by bent edges tightening on the plate. In another variant embodiment, the walls of the cells of the spacer grating are equipped on each side with two pairs of springs constituted by two continuous strips of Inconel welded to each other in three zones through three aligned rectangular openings in the wall, namely two end zones or shoulders fixed by bent edges to the wall and a central zone or median sliding plate mounted with a longitudinal clearance in its corresponding opening. In an advantageous embodiment, the outer wall of a cell containing a guide tube is equipped with an added rigid stop formed in a strip crimped at its two ends in a slot made to this end in said wall. Furthermore, to promote the guiding of the plates forming the walls of the cells during assembly, said plates may be provided with embossed portions having a bevelled section in the direction of introduction of said plates.