GB2207798A - Dismantling of nuclear fuel sub-assemblies - Google Patents

Abstract

Apparatus for dismantling nuclear fuel sub-assemblies comprises a support 10 for mounting a nuclear fuel sub-assembly 12, a pin-engaging unit 16 mounted on a carriage 18, and piston and cylinder assemblies 20, 34 for moving the carriage 18. The pin-engaging unit 16 comprises an assembly 32 of pin-engaging chucks each of which is independently movable in the z-direction, and is biased forwardly to an extended position but retracts if the chuck encounters a fuel pin or other structural component which projects beyond a datum position. The carriage 18 is capable of movement in a direction transverse to the Z - direction. <IMAGE>

Description

Dismantling of nuclear fuel sub-assemblies This invention relates to apparatus for use in dismantling nuclear fuel sub-assemblies and, in particular, irradiated sub-assemblies of light water reactors in which the fuel pins are arranged generally parallel to each other in a square array - typically a x 17 pin array.

According to the present invention there is provided apparatus for use in dismantling nuclear fuel sub-assemblies comprising a support for mounting a fuel sub-assembly, a pin-engaging unit mounted on a carriage, and means for moving the carriage in a direction (z-direction) towards and away from one end of a sub-assembly mounted on the support and also in, a plane (x-y plane) transverse to the z-direction, said pinengaging unit comprising a plurality of pin-engaging chucks each of which is independently movable in the z-direction and is biased forwardly to an extended position but retracts if the chuck encounters a fuel pin or other structural component which projects beyond a datum position.

Each chuck conveniently comprises an outer sleeve receiving a cam-operated gripping collet, the sleeve and the collet being scalloped to allow the sleeve and collet assembly to engage with a single fuel pin without being impeded by the neighbouring pins.

The collet may be displaceable axially within the sleeve and the sleeve and collet conveniently have co-operating surfaces which exert a camming action in response to displacement of the collet in one direction whereby the collet is caused to retract radially to produce a gripping action on a fuel pin received in the collet.

The chucks are preferably spaced laterally from each other by a multiple (ie two or more) of the pitch between adjacent fuel pins. In the case of fuel sub-assemblies comprising a square array of 17 rows x 17 rows of pins, there may be nine chucks spaced with a pitch corresponding to twice the pitch between adjacent pins in a row.

To promote further understanding of the invention reference is now made to the accompanying drawings illustrating by way of example only a fuel pin-pulling machine according to the invention. In the drawings: Figure 1 is a schematic perspective view of the machine; Figure 2 is side elevation in section of the multi-chuck pin-pulling unit of the machine; Figure 3 is a front elevation of the unit; Figure 4 is an overhead view, partly sectioned, in the direction 4 in Figure 3 showing the collet of one of the chucks; Figure 5 is a sectional view in the direction 5-5 in Figure 2 of the collet; Figure 6 is a detailed view of part of the collet; Figure 7 is a view similar to that of Figure 4 but showing the sleeve of the chuck; and Figure 8 is a view similar to that of Figure 5 but showing the sleeve.

As shown in Figure 1, the machine comprises a mounting structure 10 for clamping a fuel element 12 by means of a main chuck 14 with one end of the fuel element cut-away to expose the ends of the fuel pins. The fuel element may typically comprise a light water reactor sub-assembly having a square array of 17 x 17 rows of fuel pins and the mounting arrangement is adjustable, eg by rotation of the chuck 14, to locate the fuel element axially and angularly in such a way that the sets of fuel pin rows extend substantially vertically and horizontally respectively. In practice, at the exposed end, the fuel pins, as a result of irradiation may project to differing axial extents with respect to a datum position and in addition to fuel pins other components such as guide tubes may project beyond the datum position.

A pin-pulling unit 16 is movable towards and away from the exposed end 13 of the clamped fuel element 12 and comprises a carriage 18 connected to the piston of a rodless piston and cylinder assembly 20 which is connected at its opposite ends by universal joints 22 to a rear stand 24 and to a cross-slide 26. The cross-slide 26 is movable in an X-Y plane parallel to the exposed end of the fuel element by means of stepper motors 28 and leadscrews 30. The pin-pulling unit 16 comprises a multiple chuck assembly 32 connected to the carriage 18 by a piston and cylinder 34 so that axial movement (z direction) of the chuck assembly 32 is subject to coarse control by cylinder 20 and fine control by cylinder 34.

The chuck assembly 32 has a limited universal coupling with the carriage 18 to compensate for the different degrees of tilt which the cylinder 20 undergoes during X-Y positioning of the cross-slide 26. To ensure that the assembly 32 is square-on to the exposed end of the fuel element 12 the assembly 32 has, at opposite sides, dowels 36 for registry with corresponding sockets 38 in the cross-slide 26 so that, irrespective of the tilt of the cylinder 20, the assembly 32 on engagement with the cross-slide 26 is brought into parallel relation with the fuel element end 13.

Referring to Figures 2-8, the chuck assembly 32 comprises a support bracket 38 which is coupled to the piston rod 40 of the cylinder 34 and which is mounted slidably on rods 42. The bracket 38 in this instance mounts three chuck units 44 each comprising a collet 46 and a sleeve 48. Each sleeve 48 extends slidably through a cross piece 50 and a keyed connection to prevent rotation of each collet/sleeve assembly 46, 48 is provided by pins 52 and slots and grooves 54, 56 in the collets and sleeves respectively. The sleeves 48 are each connected to a respective plate 58 which is springbiased into the position shown in Figure 2 by a pair of coiled springs 60 providing a substantially constant spring force of, for example, 60N.Each plate 58 mounts a collet-actuating cylinder 62 coupled to the respective collet 46 through a lever system comprising a lever 64 anchored pivotally at its upper end to the plate 58 and a connecting link 66 pivoted to lever 64 at pivot 68 and also coupled to the trailing end of the collet 46.

Each collet 46 is provided with a slidable ejector 70 which is biased forwardly by spring 72 housed in bore 74 and bearing against an enlarged section 76 of the ejector. Forward movement of the ejector 70 is limited by a stop 78 threaded into the bore 74. The spring 72 biases the ejector 70 with a spring force, eg 45N, lower than that of the springs 60. It will be seen that each collet 46 is displaceable relative to its associated sleeve 48 by means of the cylinder 67 and lever system.

Thus, irrespective of the axial position of the sleeve 48, the collet 46 can be shifted forwardly into a gripping position in which it is caused to contract radially by a camming action between frusto-conical surfaces 76, 78 of the collet and sleeve, the collet 46 being slit axially (see slits 79) at equidistant points around its periphery to allow such radial contraction.

The collet/sleeve assemblies are spaced at intervals corresponding to twice the pitch of the pins in the rows of the 17 x 17 array and both the sleeve and the collet of each assembly are formed with scallops 80 at four equispaced positions around their periphery to enable them to encircle a pin without being impeded by the neighbouring pins. The leading end of each sleeve 48 is provided with an entry cone 82 to act as a lead-in for the pins and typically the arrangement is such that the entry cone can tolerate pin misalignment of up to 2mm.

To increase the gripping force, the collets 46 are provided with a serrated or saw-tooth profile 84 (see Figure 6). The sleeves 48 are designed to allow entry of fuel pins but not the larger diameter guide tubes located at different positions in the 17 x 17 array.

When the sleeve 48 engages a guide tube during extension of cylinder 34, it is pushed back (along with the associated plate 58) against the biasing force of the springs 60 while the other chucks are free to continue forwardly for registry with the fuel pins. This independent movement of the chucks relative to one another also allows the chucks to engage with pins which project to differing extents. For example, if one pin projects to a greater extent than its neighbours, the respective chuck will engage it first and when the pin bottoms in the associated collet, that chuck and the associated plate may be pushed back relative to the others. In this way, the degree of engagement between each chuck and the pin with which it registers can be substantially the same even though the pins project to differing extents.

Although a three chuck assembly is shown, there may be for example nine chucks for dismantling of subassemblies with a 17 x 17 fuel pin array. In this case, pin removal operations would involve indexing the chuck assembly to pull out 9 alternate pins on the top horizontal row. The assembly is then indexed a single pin pitch so that the remaining 8 pins in that row can be pulled. Subsequently each horizontal row of pins can be pulled by suitably indexing the chuck assembly vertically and horizontally.

A typical operation sequence involves the following steps: a. The primary Z axis cylinder 20 drives the unit 16 to the end of its stroke. The unit 16 engages the location sockets 38 and is driven to its pin pulling location by the X-Y stepper motors 28.

b. The second Z axis cylinder 34 typically has a stroke of 75 mm and a thrust of 225 N. This is energised and the chucks move forward to engage the fuel pins.

c. The spring loaded ejection plunger 70 contacts the end of the pin as the pin enters the sleeve and collet and retracts a distance of 30 mm, generating an ejection force of 45 N. The remaining secondary Z axis motion is accommodated by the sleeve 48 sliding within the crosspiece 50 against the constant spring force of 60 N provided by springs 60.

d. The cylinders 62 are actuated and the lever systems push the collets 46 into the female cones 78 within the sleeves 48 and apply a gripping force of 500 N on the end caps of the fuel pins.

e. Energisation of the primary Z axis cylinder 20 retracts the fuel pins from the assembly. At full stroke the gripper actuation cylinders 62 release the collets 46 and the preloaded plungers 70 eject the fuel pins.

If there is a guide tube present then, at the position of the guide tube on stage b), the larger diameter guide tube cannot enter the sleeve. The sleeve is pushed oninternal slides by the guide tube into the chuck crosspiece 50 against the constant force springs 60 by a distance of up to 50 mm depending on the cut length of the guide tube. The chuck assembly is therefore able to discriminate between guide tubes and fuel pins and the remaining chucks are unaffected by the fact that one chuck has engaged a guide tube.

Claims (6)

Claims

1. Apparatus for use in dismantling nuclear fuel sub-assemblies comprising a support for mounting a fuel sub-assembly, a fuel pin-engaging unit mounted on a carriage, and means for moving the carriage in a direction (z-direction) towards and away from one end of a sub-assembly mounted on the support and also in, a plane (x-y plane) transverse to the z-direction, said pin- engaging unit comprising a plurality of pin-engaging chucks each of which is independently movable in the z-direction and is biased forwardly to an extended position but retracts if the chuck encounters a fuel pin or other structural component which projects beyond a datum position.

2. Apparatus as claimed in Claim 1, wherein each chuck comprises an outer sleeve receiving a cam-operated gripping collet, the sleeve and the collet being scalloped to allow the sleeve and collet assembly to engage with a single fuel pin without being impeded by the neighbouring pins.

3. Apparatus as claimed in Claim 2, wherein the collet is displaceable axially within the sleeve, and the sleeve and the collet have co-operating surfaces which exert a camming action in response to displacement of the collet in one direction whereby the collet is caused to retract radially to produce a gripping action on a fuel pin received in the collet.

4. Apparatus as claimed in any one of the preceding claims, wherein the chucks are spaced laterally from each other by a multiple of the pitch between adjacent fuel pins.

5. Apparatus as claimed in Claim 4, wherein nine said chucks are provided spaced with a pitch corresponding to twice the pitch between adjacent pins in a row in a fuel sub-assembly having a square array of 17 rows x 17 rows of pins.

6. Apparatus for dismantling nuclear fuel subassemblies, substantially as hereinbefore described with reference to Figures 1 to 8 of the accompanying drawings.

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