Patent Number: 047737999
Section: description

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Referring to FIG. 1, a fuel assembly comprises a bundle of fuel elements 11 retained by grids 12 distributed along the bundle. The grids 12 define passages the majority of which are traversed by fuel elements 11. The other passages receive tubes 15. Each tube 15 is fixed to the lower end-piece (not shown) by connecting means, generally constituted by a screw to be dismountable. Each tube 15 is fixed to the upper end-piece 14 through a sleeve 16. As shown in FIG. 1, sleeve 16 is welded to the adaptor part of the end-piece 14. However, other connections are possible, for instance by radial expansion into a counterbore of the adaptor parts. The sleeve passes through the upper grid 12, to which it may be fixed, and terminates immediately under the grid. The tube 15 is fixed to the sleeve 16 by crimpings 19 at intermediate locations. As shown in FIG. 2, crimping is in the form of a plurality of local expansions. Additionally, the upper end of tube 15 may be crimped in the sleeve. The tubes 15 are typically of zirconium-based alloy, whilst the sleeves are typically of stainless steel. Cutting off and removal of a longitudinal section of the assembly of a tube 15 and its connection to the upper end-piece 14 involve, first, cutting out a passage opening in the upper end-piece 14, second, sectioning the assembly constituted by the tube 15 and the sleeve 16 for separating it from the balance of the tube. Cutting out of an opening for passage of a tool The opening may be cut out with a cutting tool similar to that described in European No. 138,711 to which reference may be made. FIG. 2 shows the lower part of the cutting tool 20, mounted on a mask 22 which guides the tool. The mask comprises a base plate 24 provided with pins 26 insertable in indexing and centering blind holes formed in the end-piece 14. The tool 20 comprises a sleeve 28 provided with retainer pins 30 for locking it in the base plate 24 of the mask 22 and a unit rotatably mounted in the sleeve 28 and connected to the shaft of a motor (not shown). The lower element of the rotary unit consists of a tubular spindle 32 internally threaded for receiving a bladed milling cutter 34, such as a hand wheel, or provided for moving the rotary mechanism downwards and upwards with respect to the sleeve 28. The milling tool may be formed with a suction passage (not shown) for drawing the chips and dust which collect in the annular space between the sleeve 28 and the spindle 32. The opening for passage of the section to be extracted is cut out conventionally: the spindle 32 carrying the cutter 34 is rotated and progressively lowered until it has passed fully through the plate of end part 14. The outer diameter of the cutter must be large enough to permit removal of the section. If the connection between the tube and sleeve include projecting zones, as illustrated in FIG. 2, the opening formed by the cutter must have a sufficient diameter to permit these zones to pass. Cutting off the tube and the sleeve The tube 15 and sleeve 16 may be cut off with the milling tool of FIG. 2, after the cutter 34 has been replaced with an assembly as shown in FIGS. 3 and 4. This assembly comprises a fixed structure comprising a fastening ring 36 which is connected, for example by screws (not shown), to the nose 38 of the stationary part of the tool. The structure also comprises tie rods 40, for example four in number, which connect the ring 36 to a supporting sole plate 42. The sole 42 has a downwardly extending tubular projection which constitutes a guide tube 44 and carries a ball bearing 46 coaxial with the spindle for receiving the rotary unit of the tool. The rotary unit of the tool comprises a casing 52 rotating freely in the bearing 46 and having a key 54 engaging into an axial groove of a drive tube 56 for mutual sliding connection. An upper end piece is securely connected to tube 56, for instance by screws and has a threaded portion for connection with the spindle 32 in substitution for the cutter 34 of FIG. 2. An upper end portion of casing 52 is tubular and defines a chamber receiving a bulged portion of a shank or shaft 60. A bearing 65 carried by the sleeve 52 supports the shank for mutual rotation about an axis 50 radially offset with respect to the axis 48 of the spindle 32, for a purpose which will be apparent below. An end shoulder of the bulged portion is maintained in abutting contact with the bearing 65 by a ring 59 fixed to casing 52 by a radial pin 58. The lower end portion of shank 60 projects out of the casing 52 and a cutter element 62 is secured thereto by a nut 64. The rotary unit of the tool further comprises an adjustment shaft 65 whose axial displacement controls the angular portion of the cutter element 62 and consequently its amount of radial projection with respect to axis 48, since it rotates about the offset axis 50. For that purpose, a helical connection is provided between shaft 66 and shank 60. As shown in FIG. 3, it comprises helical grooves formed in shaft 66 and splines or fingers 67 of the bulged portion protruding into the grooves. The distance from the end fitting at which the spacer tube 15 and the sleeve 16 will be cut off is determined by the length of the spindle 60 from the support represented by the sole 42. The sole is supported by a respective mask (not shown) which may be similar to those described in the above-mentioned patents. The sequence of operations for cutting a sample is as follows. The nose of the tool is located on the fastening ring 36 of the stationary structure and the upper end piece of the drive tube 56 is fixed to spindle 32, while the spindle is in its uppermost portion. In that position, shaft 66 retains the cutter element 62 in retracted condition, as shown in dash-dot lines in FIG. 4. A return spring 70 may be provided for biasing the shaft and spindle upwardly. Then the motor of the tool is energized for driving the rotary unit and the rotary spindle of the tool is progressively lowered. Downward movement of the spindle 32 and shaft 66 causes the shank 60 to rotate about its axis due to the helical coupling between shaft 66 and the shank 60. Rotation of the shank 60 moves the cutting element 62 towards the position in which it is shown in solid lines in FIG. 4. If the rotary unit of the cutting tool is maintained in rotation while a downward pressure force is maintained on spindle 66, the spacer tube 15, and then the sleeve 16 are progressively cut off. Referring to FIG. 4, the amount of angular travel .alpha., and hence the maximum radial projection of the cutter element 62, may be adjusted with an abutment ring 68 threadedly received on drive tube 56. Downward movement of the unit consisting of spindle 32, shaft 66, drive tube 56 and ring 68 is limited by abutment of ring 68 onto a radial flange of casing 52, as shown in FIG. 3. Preliminary adjustment of the final position of the cutting element eliminates the risk of damaging an adjacent fuel element. During the cutting step, tube 15 and sleeve 16 are retained against rotation by the usual fastening of the sleeve 16 to the upper grid 12 and possibly the connections of tube 15 with other grids and the lower end-piece (not shown). The cut-off section can then be removed, possibly along with the tool since it is supported by the cutter element 62 held in its position of maximum projection, and then introduced into a closable handling and protection container. Then the container may be transported to a laboratory for analysis. Once the sampling has been carried out, the fuel assembly can again be handled, for example for transfer to a deactivation pool or for containment in a container, with an apparatus which may be as described in European No. 138 711 already mentioned.