GB2221787A - Nuclear fuel sub-assemblies - Google Patents

Abstract

A device for closing an upper end of a channel 44 provided in a nuclear fuel assembly, comprises a rod-like body 60 provided with catches 82 for engaging behind a lip 52 around the channel. The catches are moveable from a first position in which the engagement takes place to prevent removal of the device from the channel and a second position in which the catches are held clear of the lip to permit removal of the device from the channel. The device may be coupled to a tool for installing the device and removing it from the channel (fig. 3, not shown). The channel 44 replaces an upper fertile pin in the assembly to allow a fuel pin to be removed. In figure 2, the device is coupled by rod 87 to the fuel pin, but in an alternative (figure 8, not shown) it may be uncoupled. In either case a limited amount of upward movement of the fuel pin is allowed, but excessive movement is prevented by the device. <IMAGE>

Description

Nuclear fuel sub-assemblies This invention relates to nuclear fuel subassemblies and is particularly, but not necessarily exclusively concerned with nuclear fuel sub-assmblies for liquid-cooled fast neutron reactors.

In one type of liquid-cooled fast neutron reactor nuclear fuel pins containing fissile fuel are arranged in bundles so that the fuel pins are vertical, or near vertical, and are spaced apart to allow coolant flowing upwardly past the fuel pins to come into contact with the fuel pins and hence take heat from them. Each fuel pin bundle is housed within a tubular wrapper to constitute a fuel sub-assembly and the fuel pins within the wrapper are spaced apart by means of spacer grids provided at intervals along the length of the wrapper. Typically, the wrapper is hexagonal in cross-section and the spacer grids hold the fuel pins in a hexagonal array comprising, for example 325 fuel pins.Upward movement of the fuel pins, under, for example the influence of coolant flow, is limited by an upper support grid, whilst the pins are supported on a lower support grid and are also usually secured to this lower grid. Further pins containing fertile fuel are often disposed above the upper support grid and these fuel pins are usually of larger cross-section than those containing fissile fuel and are spaced apart by means of wire wrapped around the fuel pins in a helical fashion. As with the fissile nuclear fuel pins, the fertile fuel pins are supported between an upper support grid and a lower support grid.

According to one aspect of the present invention there is provided a device for closing an upper end of a channel provided in a nuclear fuel assembly (whilst permitting flow of coolant through the channel), said device comprising a body provided with means for engaging behind a lip around the channel, the engaging means being moveable from a first position in which said engagement takes place to prevent removal of the device from the channel and a second position in which the engaging means is held clear of the lip to permit removal of the device from the channel, and means for coupling the device to a tool used to install and remove the device in and from the channel.

The engaging means may comprise legs provided on the device and which have shaped end portions which engage behind the lip when the engaging means is in the first position. Preferably the legs are outwardly biased so that a force has to be applied to the legs to effect inward movement of the legs into the second position and hence permit removal of the device.

It is also preferable that the legs converge to a point lying above the opening of the channel when the legs are in their first position in which they engage behind the lip around the opening of the channel. The legs advantageously comprise at least a pair of angularly spaced legs and may comprise a plurality of angularly spaced legs.

The coupling means may comprise a profiled portion adapted to be clamped by the tool, for example the coupling means may comprise a recessed portion provided on the device.

Preferably, the device has a bore throughout its length.

The device may be provided with a collar which is arranged so as, in use, to rest on a surface of the channel such as a ledge to prevent movement of the device downwards through the channel.

The device may be coupled to the pin. For example the device may be provided with a chamber in which a collar on the pin is retained and preferably the collar is slidable within the chamber to allow for longitudinal expansion of the pin.

According to a second aspect of the present invention there is provided a tool for use in installing and/or removing the device of the first aspect of the invention from the channel, second tool comprising a tubular member provided with means for effecting movement of the engaging means between the first and second positions, means for clamping onto the device, and means for moving the tool towards and away from the device.

The means for effecting movement of the engaging means when the engaging means comprises legs may comprise an end portion of the tubular member which is of such diameter that when the end portion is pushed over the legs the legs are joined towards each other into their second position.

The clamping means may comprise opposing arms provided inside the tubular member, the arms having end parties which are engagable with the coupling means of the device. The arms may be provided with projections at positions intermediate the ends of the respective arms and which extend inwardly towards each other and the tool may be provided with a piston having a portion with a diameter greater than the spacing between the projections so that when, in use, this portion resides between the projections the arms are held apart and free of the coupling means on the device and when the portion is removed from between the projections the arms return to their normal positions in which they can couple with the coupling means in the device.

The piston may also comprise a portion which can penetrate the bore in the device and push the pin in the channel.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a part cut away, part fragmentary diagrammatic-perspective view of a nuclear fuel assembly; Figure 2 is a sectional side view showing a device for closing a channel in a nuclear fuel assembly; Figure 3 is a sectional side view showing a tool for installing and removing the device of Figure 2 in and from the channel; Figures 4 to 6 show the stages involved in removing the device from the channel using the tool illustrated in Figure 2; Figure 7 is a sectional side view of the device of Figure e in the channel; and Figure 8 is a sectional side view of a modified version of the device of Figure 7.

Figure 1 shows a fuel sub-assembly 10 comprising a plurality of elongate fuel pins 12 (only one shown) containing fertile fuel disposed lengthwise within an elongate wrapper 14 of hexagonal cross-section. The fuel pins 12 are spaced apart and held in an hexagonal array by means of honeycomb spacer grids 15 which are disposed at intervals along the length of the wrapper 14. The spacer grids 15 are supported by six grid legs 16 each of which is located in a respective corner of the wrapper 14 and extends along the length of the wrapper 14. The fuel pins 12 are supported on a support plate 18 disposed near to the lower end of the wrapper 14 and the pins 12 locate in a hold-down grid 20 joined to the upper face of the support plate 18 so as the render the pins 12 axially captive with the hold-down grid 20 and plate 18.A spike 22 is provided at the lower end of the sub-assembly 10 for locating the sub-assembly in a nuclear reactor core and a gag 24 is provided in the wrapper at a position intermediate the spike 22 and the plate 18 so that coolant entering the sub-assembly 10 through perforated regions 26 in the spike 22 passes through the gag 24 before travelling upwards through the wrapper 14 and around the fuel pins 12. The sub-assembly 10 also contains a plurality of breeder fuel pins 28 containing fertile fuel. These breeder pins 28 are disposed above the fissile pins 12, are of a larger diameter than the fissile pins 12 and are spaced apart in a hexagonal array by means of a length of wire wrapped around each pin 28 in a helical fashion. The breeder pins 28 are located between a mixer upper end grid 30 and a mixer lower end grid 32 on a support grid 34. A space 36 is provided between the support grid 34 and the top of the fissile pins 12 so that any longitudinal growth of the pins 12 due to radiation or increase in temperature is taken up in the space 36 but any upward movement of the pins 12 beyond such expansion is arrested by engagement of the pins 12 with the support grid 34. For example, if the fissile pins 12 become detached from the hold-down grid 20 then upward movement of the pins 12 under the influence of coolant flow is prevented by the support grid 34. The sub-assembly 10 is also provided with lifting lugs 38, equipment 40 for detecting burst fuel pins and an orientation bar 42.

In the arrangement access to selected fissile pins 12 or selected fissile pin positions is gained by replacing one or more of the breeder pins 28 with a channel member 44 (not shown in Figure 1). As shown in Figure 7 the channel member 44 extends between the mixer upper end grid 30 and the support grid 34 and has its ends located in respective co-axial apertures provided in the mixer upper end grid 30 and the mixer lower end grid 32/support grid 34.

Referring now to Figure 2. A tube 50 extends upwardly from the breeder upper end grid 30 and has an annular lip 52 disposed around its inside wall at its upper opening so as to define a chamfered entry 54 to the tube 50 of smaller diameter than the internal diameter of the tube 50. The channel member 44 fits inside the tube 50 with the upper end 56 of the channel member 44 above the level of the breeder upper end grid 30. Figure 2 also shows a closure device 58 comprising a rod-like body 60 having a bore 61 running through its length and four angularly spaced legs 62 arranged in opposing pairs.The rod-like body 60 comprises an upper portion 64 of frustro-conical shape with its smaller end at the upper end of the body 60, a recessed portion 66, a straight portion 68 and a lower tubular portion 70 which extends from a position intermediate the radially inner edge and the radially outer edge of the straight portion 68 so that the bore 61 at the lower tubular portion 70 is of greater diameter than at the straight portion 68. A sleeve 72 fits around, and is secured to, the lower tubular portion 70 and extends downwardly therefrom. The lower end of the sleeve 72 has an annular lip 74 so that the inner wall 75 of the sleeve 72, the lower end 76 of the lower tubular portion 70 and the lip 74 define a cylindrical chamber 78 having openings at both ends which are of smaller diameter than the internal diameter of the chamber 78.An annular collar 80 is provided on the sleeve 72 at a position intermediate its ends and the collar 80 rests on the upper end 56 of the channel member 44. The legs 62 extend from angularly spaced positions on the straight portion 68 near to the recessed portion 66 initially downwardly along the sides of the straight portion 68 and then outwardly. Each leg 62 has a foot 82 on its end and the feet 82 are engageable behind the lip 52. The legs 62 are formed from spring steel or the like so that on application of a force they can be pushed together to bring the feet 82 clear of the lip 52 and when the force is removed they move outwards to return to their original position. Thus the legs 62 are outwardly biased and when in the position shown in Figure 2 the feet 82 engage the walls of the tube 50 and lie behind the lip 52.

As shown in Figure 7, the pin 12 is coupled to an extension bar 84 which extends upwardly through the channel member 44 and into the bore 61. The extension bar 84 comprises a first portion 86 which extends part way into the channel member 44, a second portion 88 whose longitudinal axis is offset from the first portion 86 and which extends through the channel member 44, a third portion 87 which extends part way into the bore 61 at the lower tubular portion 70 of the body 60 and is a sliding fit therein, and a fourth portion 90 which extends part way into the bore 61 at the straight portion 68 and is a sliding fit therein. A collar 92 is provided on the second portion 88 of the extension bar 84 and is a sliding fit within the chamber 78.The first portion 86 of the bar 84 is of smaller diameter than the internal diameter of the channel member 44 except at two regions 94 where the bar 84 contacts the channel member 44 to centralise the bar 84 in the channel member 44.

Referring now to Figure 3. A tool 102 for installing the device 58 in the tube 50 and removing it therefrom comprises a tubular guide 104 having a first section 106, a second section 108 with a thinner wall than the first section 106 so as to provide it with a greater internal diameter than the first section 106, and a third section 110 which tapers from the second section 108 to a mouth 112 of a diameter slightly greater than that of the straight portion 68 of the device 58 and the legs 62 so that when the tubular guide 104 is pushed over the device 58 the legs 62 are pushed together and as they pass through the mouth 112 they are held against the side of the straight portion 68.

The first section 106 of the guide 104 is provided with a pair of straight opposing arms 114 which extend into the second section 108 and are spaced from the wall of the second section 108. Each arm 114 has an inwardly projecting lug 116 at its end and the lugs 116 are spaced apart by a distance approximately equal to the diameter of the recessed portion 66. A wedged-shaped projection 118 projects inwardly from a position intermediate the ends of each arm 114. Figure 3 also shows a piston 120 comprising a first portion 122 tapering into a second portion 124 which in turn tapers into a third portion 126 which can pass between the lugs 116 and penetrate the bore 61 in the body of the device 58.The diameter of the second portion 124 is greater than the spacing between the opposed wedge-shaped projections 118 so that as the piston 120 is moved so that the second portion 124 moves between the projections 118 the arms are deflected outwards.

In use, the device 58 is plaCed in the tube 50 as shown in Figure 2 with the collar 80 resting on the upper edge 56 of the channel member 44 and the feet 82 in engagement with the inside wall of the tube 50 but spaced from the lip 52. If the pin 12 increases in length, as a result of, for example thermal expansion or radiation induced growth, the collar 92 on the bar 84 slides upwardly through the chamber 78. Any upward movement of the pin 12 above that due to thermal expansion or radiation induced growth causes the collar 92 to engage the lower end 76 of the lower tubular portion 70 and push the device 58 upwards until the feet 82 engage the underside of the lip 52 and thus preventing any further upward movement of the pin 12.

To remove the device 58 from the tube 50 the tool 102 is positioned above the tube 50 with the tool 102 and tube 50 substantially co-axial. The piston 120 is moved downwards through the tubular guide 104 towards the third section 110 of the tubular guide 104 until the second portion 124 of the piston 120 resides between the wedgeshaped projections 118. During this operation the tapered portion between the third and second portions 126 and 124 respectively engages the projections 118 first and as the piston 120 is moved further the projections 118 and hence arms 114 are deflected outwardly until the arms 114 engage the wall of the guide 104 and the space btween the lugs 116 on the arms 114 is greater than the diameter of the upper portion 64 of the device 58. The tool 102 is then lowered so that the upper portion 64 of the device 58 enters the guide 104. When the third portion 110 of the guide 104 engages the legs 62 the legs 62 are forced inwards until ultimately they lie against the side of the straight portion 68 (see Figure 5). Movement of the tool 102 is arrested when the tool 102 is in the position shown in Figure 5 in which the end of the third portion 110 of the guide 104 resides in the end 52. In this position the feet 82 on the legs 62 are clear of the lip 52 on the tube 50 and the lugs 116 on the arms 114 are level with the recessed portion 66 of the device 58. The piston 120 is now actuated again and moved away from the third portion 110 of the guide 104 until the third section 126 of the piston again resides between the wedge-shaped projections 118.In this position (see Figure 6) the arms 114 move inwards until they are straight again and the lugs 116 on the ends of the arms 114 engage below the frustro-conical upper portion 64 and in the recessed portion 66 to latch the tool 102 to the device 58. The tool 102 is now moved upwards away from the tube 50, and since the tool 102 is latched to the device 58 and the feet 82 on the legs 62 are clear of the lip 52 the device is pulled free of the tube 50. Also, since the device 58 is coupled to the pin 12 via the collar 92 located in the chamber 78 the pin 12 is pulled free of any attachment means provided to secure it at its lower end and pulled out of the channel member 44. Once removed, the pin 12 and device 58 can be de-coupled, a different pin coupled to the device 58 and the new pin and device 58 replaced in the channel member 44 and the tube 50 respectively.

The replacement operation is the reverse of the removal operation. With the device 58 and tool 102 in the positions shown in Figure 6 the tool is lowered so that the pin enters the tube 50 and thereafter the channel member 44. Should the pin 12 encounter resistance when, for example, it meets spacer grids 15 after passing through the channel member 44 the tool 102 moves relative to the device 58 so that the lugs 116 on the arms 174 which are engaged in the recessed portion 66 more to engage against the upper end of the straight portion 68.

The tool 102 then pushes the device 58 and hence the pin 12 through the channel member 44 and spacer grids 15.

When the pin 12 is in position the piston 120 is moved downwards so that the end of the piston 120 engages the bar 84 and pushes the bar 84 and pin 12 downwards to cause the pin 12 to engage a snap-fitting (not shown) in the hold-down grid 20.

Figure 8 shows a modified version of the device 58.

In this embodiment the pin 12 is coupled to an extension bar 130 which extends part way into the channel member 44. The channel member 44 is provided with an inwardly projecting collar 132 which is eccentric so as to slidably receive the bar 130 whose longitudinal axis is offset from that of the channel member 44. The device itself is similar to that of the first embodiment but the rod-like body does not have the chamber 78 and, the pin 12 is not coupled to the device 58. In the embodiment the lower end 34 of the body 60' of the device 58' is spaced from the upper end of the bar 130 such that longitudinal growth of the pin 12 is taken up in that space but any upward movement that might occur if the pin 12 becomes detached from its hold-down grid 32 results in the bar 130 engaging the lower end 134 of the device 58'.

The device 58' is moved upwards until the feet 82 engage behind the lip 52 when movement of the device 58' and pin 12 is arrested.

The removal and installation of the modified device 58' is similar to that of the unmodified device of the first embodiment. However, as the device 58' and pin 12 are not coupled a clamp arrangement (not shown) is used to remove the pin 12 from the sub-assembly 10 following removal of the device 58' using the tool 102.

The pin 12 can be a conventional fuel pin which contains fissile fuel, a dummy fuel pin or experimental fuel pin which carries monitoring equipment. For example, the pin can contain material such as a uranium-nickel alloy used to calibrate for a delayed neutron safety system.

It will be appreciated that the numbers of legs on the devices 58 and arms 114 on the tool 102 can be more or less than described above.

In some cases bar portions 86, 88 can be coaxial.

Claims (17)

Claims

1. A device for closing an upper end of a channel provided in a nuclear fuel assembly the device comprising a body provided with means for engaging behind a lip around the channel, the engaging means being moveahle from a first position in which said engagement takes place to prevent removal of the device from the channel and a second position in which the engaging means is held clear of the lip to permit removal of the device from the channel, and means for coupling the device to a tool used to install and remove the device in and from the channel.

2. A device as claimed in Claim 1, in which the engaging means comprises legs provided on the device and which have shaped end portions which engage behind the lip when the engaging means is in the first position.

3. A device as claimed in Claim 2, in which the legs are outwardly biased so that a force has to be applied to the legs to effect inward movement of the Jeqs into the second position and hence permit removal of the device.

4. A device as claimed in Claim 2 or Claim 3, in which the legs converge to a point lying above the openinq of the channel when the legs are in their first position in which they engage behind the lip around the opening of the channel.

5. A device as claimed in any of Claims 2 to 4, in which the coupling comprises a profiled portion adapted to be clamped by the tool.

6. A device as claimed in Claim 5, in which the profiled portion comprises a recessed portion.

7. A device as claimed in any preceding Claim, having a bore throughout its length.

8. A device as claimed in any preceding claim, having a collar arranged in use to rest on a surface of the channel to prevent movement of the device downwards through the channel.

9. A device as claimed in any preceding claim, having a chamber for retaining a collar on a fuel pin.

10. A device as claimed in Claim 9, in which a collar received in the chamber is slidable in the chamber.

11. A device for closing an upper end of a channel provided in a nuclear fuel assembly substantially as hereinbefore described with reference to and as shown in Figure 2, or Figure 8, of the accompanying drawings.

12. A tool for use in installing and/or removing a device as claimed in any preceding claim, the tool comprising a tubular member provided with means for effecting movement of the engaging means between the first and second positions, means for clamping onto the device, and means for moving the tool towards and away from the device.

13. A tool as claimed in Claim 12, in which the means for effecting movement of the engaging means when the engaging means comprises legs may comprise an end portion of the tubular member which is of such diameter that when the end portion is pushed over the legs the legs are joined towards each other into their second position.

14. A tool as claimed in Claim 12 or Claim 13, in which the clamping means comprises opposing arms provided inside the tubular member, the arms having end parties which are engagable with the coupling means.

15. A tool as claimed in Claim 14, in which the arms are provided with projections at positions intermediate the ends of the respective arms and which extend inwardly towards each other and the tool is provided with a piston having a portion with a diameter greater than the spacing between the projections so that when, in use, this portion resides between the projections the arms are held apart and free of the coupling means and when the portion is removed from between the projections the arms return to their normal positions in which they can couple with the coupling means.

16. A tool as claimed in Claim 15, in which the device has a bore and the piston comprises a portion which can penetrate the bore and push the pin in the channel.

17. A tool for use in installing and/or removing a device for closing an upper end of a channel provided in a nuclear fuel assembly substantially as hereinbefore described with reference to and as shown in Figure 3 of the accompanying drawings.