Patent Number: 052215156
Section: claims

1. A method for manufacturing a grid for a nuclear fuel assembly, said grid having two sets each consisting of mutually parallel inner plates, the plates of one set being at an angle with the plates of the other set, and a belt welded to end portions of said inner plates and forming side faces of said grid, comprising the steps of: placing said inner plates and said belt of a grid in assembled condition in a holding frame having passages for access to points to be welded on two major faces and on the side faces of the grid;  gripping the holding frame containing the grid with a first device placed in a first closable chamber having a transparent window and rotatable about an axis parallel to said side faces;  closing said chamber and creating an inert gas atmosphere therein;  executing all welds on one face of the grid parallel to said axis using a laser beam orthogonal to said axis of rotation and originating from a laser generator placed outside the chamber and delivering a beam which penetrates into the chamber through said transparent window, said laser beam being moved along two directions orthogonal to each other and to the direction of said beam while the frame is held stationary in the chamber for executing said welds on said one face;  repeating the welding operations on at least another of said faces of the grid which are parallel to the axis after angularly repositioning the device for exposing said another of said faces;  removing the frame and grid from said first chamber and gripping said frame with a second device placed in a second closable chamber having a transparent window and rotatable about a axis parallel to at least two faces of said grid other than said four of said major faces and side faces;  creating an inert gas atmosphere in said second chamber;  executing all welds on one face of the grid parallel to the axis of said second device using a laser beam orthogonal to said axis of said second device and originating from said laser generator placed outside the chamber and delivering a beam which penetrates into the chamber through said transparent window, said laser beam being moved along two directions orthogonal to each other and to the direction of said beam while the frame is held stationary in the chamber for executing said welds on said one face; and  repeating the welding operations on other of said faces until all welds have been completed.  placing said inner plates and said belt of a grid in assembled condition in a holding frame having passages for access to points to be welded on two major faces and on said side faces of said grid;  gripping said holding frame containing said grid with a first device placed in a first closable chamber having a transparent window and rotatable about an axis parallel to said side faces;  closing said chamber and creating an inert gas atmosphere therein;  locating a carriage carrying said chamber in a working area of said laser generator;  executing all welds on one face of said grid faces using a laser beam directed orthogonally to an axis of rotation of the device parallel to said one side face, originating from a laser generator placed outside said chamber and delivering a beam which penetrates into said chamber through said transparent window, said laser beam being moved along two directions orthogonal to each other and to the direction of said beam while said frame is held stationary in said chamber for executing said welds on said one face for executing said welds on said one side face;  repeating the welding operations on the other ones of said side faces of said grid which are parallel to the axis after angularly repositioning said device for exposing said other faces in succession;  moving said carriage out of the laser working area in a direction transverse to said beam;  removing said frame and grid from said first chamber and gripping said frame with a second device placed in a second closable chamber having a transparent window, carried by said carriage and rotatable about an axis parallel to at least said two major faces of said grid;  creating an inert gas atmosphere in said second chamber;  moving said carriage to bring said second chamber into said working area; and  executing all welds on one major face of said grid parallel to the axis of said second device using a laser beam orthogonal to said axis of said second device and originating from said laser generator placed outside said chamber and delivering a beam which penetrates into said chamber through said transparent window, said laser beam being moved along two directions orthogonal to each other and to the direction of said beam while said frame is held stationary in said chamber for executing said welds on said major face and repeating the welding operations in said second of said faces after angularly repositioning said device for exposing said other face. 2. Method according to claim 1, wherein one frame carrying a grid is unloaded from one of the chambers, another grid to be welded is loaded in the chamber and an inert gas atmosphere is created in the chamber while another grid in the other, chamber is being welded. 3. Method according to claim 2, wherein some of said welds are carried out with laser shots focussed in two points located on two parallel lines at a short distance from each other. 4. Method according to claim 3, wherein the optical path of the laser beam is defined by optical means which are vibrated in synchronism with the pulses of the laser. 5. Method according to claim 2, wherein the inert gas atmosphere is created by pumping out the chamber until a primary vacuum is obtained, then filling with inert gas while the window is protected from the alternating pressure stresses by lids. 6. Method for manufacturing a grid for a nuclear fuel assembly, said grid having two sets each consisting of mutually parallel inner plates, the plates of one set being disposed at an angle to the plates of the other set, and a belt welded to end portions of said inner plates and forming side faces of said grid, said method comprising the steps of: