Patent Number: 
Section: claims

1. Apparatus for assembling a spacer grid of a fuel assembly for a nuclear reactor, the grid being constituted by a first set of metal straps of generally rectangular shape that extend parallel to one another, and a second set of metal straps that are rectangular, parallel to one another, and that cross the strap of the first set of straps in planes perpendicular to the straps of the first set, the straps being assembled together by interengagement via slots that are spaced apart at a constant distance in a length direction of the straps, the slots occupying at least half a width of the straps so as to constitute a regular array of rectangular cells, each cell being defined by four walls, each wall being constituted by a portion of a strap that lies between two successive slots, each strap carrying a set of springs projecting from at least one face of the strap over at least a fraction of the portions between the slots of the strap, the assembly apparatus comprising a frame configured to support the straps of the first set of straps so that they are disposed parallel to one another, clamping and engagement arrangement configured to clamp on each of the straps of the second set of straps in succession and to engage them with the straps of the first set of straps in position in the support frame, and at least one comb movable between a disengaged position and an engaged position with each of the straps of the first set when in position in the support frame, the comb occupying a position adjacent to a slot configured to engage a strap of the second set of straps, wherein: the support frame configured to support the straps of the first set of straps is secured to a fixed structure;  the clamping and engagement arrangement and the comb are mounted to move in respective planes perpendicular to the straps of the first set of straps in position in the support frame and in a direction parallel to the straps of the first set of straps, on a first carriage itself mounted on a first guide arrangement secured to the fixed structure to move in a direction parallel to a longitudinal direction of the straps of the first set of straps;  said first carriage carrying a second guide arrangement configured to guide a second carriage and a third guide arrangement configured to guide a third carriage, on respective first and second sides of the support frame, in planes perpendicular to the straps of the first set of straps and in directions parallel to straps of the first set of straps;  said clamping and engagement arrangement being fixed on said second carriage and said comb on said third carriage for being moved towards and away from the support frame in opposite directions, on the first and second sides of the support frame;  the clamping arrangement comprises two jaws movable in translation parallely to each other in opposite directions under the control of a cam and studs for retracting springs of a strap of the second set of straps engaged in the clamping arrangement by pressing them into the thickness of the strap. 2. The assembly apparatus according to  claim 1 , wherein the comb comprises successive teeth separated by slots for engaging the comb with practically no clearance on the straps of the first set of straps in a position that is adjacent to a plane for engaging a strap of the second set of straps. claim 1 3. The assembly apparatus according to  claim 1 , wherein the fixed structure comprises a base configured to stand on a plane horizontal surface, and a vertical frame including a window configured to receive the straps of the first set of straps so that their longitudinal direction is vertical. claim 1 4. The assembly apparatus according to  claim 3 , wherein a top guide and a bottom guide are disposed along two horizontal edges of the window in the frame, the guides having notches configured to receive longitudinal end portions of the straps of the first set of straps, the notches being spaced apart at regular intervals along the horizontal edges of the window in the support frame. claim 3 5. The assembly apparatus according to  claim 4 , wherein the carriage is mounted to move in the vertical direction parallel to the longitudinal direction of the straps of the first set of straps on two vertical slideways disposed along two side uprights of the frame. claim 4 6. The assembly apparatus according to  claim 5 , wherein the frame and the carriage have complementary indexing arrangements enabling the carriage to be placed in a plurality of assembly positions corresponding to a plurality of positions for engaging straps of the second set of straps on the straps of the first set of straps. claim 5 7. The assembly apparatus according to  claim 6 , wherein the indexing arrangement is constituted by two indexing units and two indexing fingers carried respectively by the carriage and by the frame, the indexing units having aligned openings, and the indexing fingers being actuatable for insertion into the openings and being disposed laterally on either side of the carriage. claim 6 8. The assembly apparatus according to  claim 1 , wherein the clamping arrangement is constituted by a clamp wherein clamping jaws have actuating surfaces facing each other, at least one of the actuating surfaces having projecting studs configured to retract the springs of a strap of the second set of straps inserted between the actuating surfaces of the jaws of the clamp by pressing against them. claim 1 9. The assembly apparatus according to  claim 8 , wherein the cam for controlling displacement of the jaws of the clamp is connected to a control lever that can be placed in at least one of a first position in which the clamp is wide open for inserting a strap of the second set of straps in the clamp, and in a position in which the clamp is closed for clamping on the strap of the second set of straps, for retracting the springs of the strap of the second set of straps, and for engaging the strap of the second set of straps in the straps of the first set of straps, and in a position between the wide-open position and the closed position to enable the clamp to be extracted after engaging the strap of the second set of straps on the straps of the first set of straps. claim 8 10. A method of assembling a spacer grid of a fuel assembly for a nuclear reactor, the grid being constituted by-a first set of metal straps of generally rectangular shape that extend parallel to one another, and a second set of metal straps that are rectangular, parallel to one another, and that cross the strap of the first set of straps in planes perpendicular to the straps of the first set, the straps being assembled together by interengagement via slots that are spaced apart at a constant distance in a length direction of the straps, the slots occupying at least half a width of the straps so as to constitute a regular array of rectangular cells, each cell being defined by four walls, each wall being constituted by a portion of a strap that lies between two successive slots, each strap carrying a set of springs projecting from at least one face of the strap over at least a fraction of the portions between the slots of the strap, by using an assembly apparatus comprising a frame configured to support the straps of the first set of straps so that they are disposed parallel to one another, a clamping and engagement arrangement configured to clamp on each of the straps of the second set of straps in succession and to engage them with the straps of the first set of straps in position in the support frame, and at least one comb movable between a disengaged position and an engaged position with each of the straps of the first set when in position in the support frame, the comb occupying a position adjacent to a slot for engaging a strap of the second set of straps, wherein the support frame for supporting the straps of the first set of straps is secured to a fixed structure, the clamping and engagement arrangement and the comb are mounted to move in respective planes perpendicular to the straps of the first set of straps in position in the support frame and in direction parallel to the straps of the first set of straps, on a first carriage itself mounted on a first guide arrangement secured to the fixed structure to move in a direction parallel to a longitudinal direction of the straps of the first set of straps, said first carriage carrying a second guide arrangement configured to guide a second carriage and a third guide arrangement configured to guide a third carriage, on respective first and second sides of the support frame, in planes perpendicular to the straps of the first set of straps and in directions parallel to straps of the first set of straps, said clamping and engagement arrangement being fixed on said second carriage and said comb on said third carriage for being moved towards and away from the support frame in opposite directions, on the first and second sides of the support frame, said clamping and engagement arrangement having two jaws movable in translation parallely to each other in opposite directions under the control of a cam and studs for retracting springs of a strap of the second set of straps engaged in the clamping arrangement by pressing them into the thickness of the strap, wherein the springs are retracted by being pressed into the thickness of each of the straps of the second set of straps by using the clamping and engagement arrangements prior to engaging the strap of the second set of straps on the straps of the first set of straps.