GB2133918A - Fuel sub-assemblies for nuclear reactors - Google Patents

Abstract

A fuel sub-assembly for a fast liquid metal cooled nuclear reactor has a bundle of fuel pins (10) located by grids (11) in an outer wrapper (12). The grids are held in supports (13) insertable into the wrapper and end secured to the wrapper end (16) by way of a skirt (14). Preferably the supports (13) are in two parts (13A, 13B) insertable from opposite ends of the wrapper and secured to opposite ends of the wrapper so that the sub- assembly can be taken apart by remote control involving cutting at the centre region of the wrapper. <IMAGE>

Description

SPECIFICATION Fuel sub-assemblies for nuclear reactors This invention relates to fuel sub-assemblies for nuclear reactors, such as liquid metal cooled fast breeder reactors, comprising a bundle of spaced elongate fuel pins contained within a tubular sleeve or wrapper.

The invention is concerned primarily with the problem of constructing such assemblies so that, after use in a nuclear reactor, they may be taken apart by remote control operations to release the fuel pins for reprocessing of the fuel.

One approach to this problem is disclosed in GB 1,597,189. In summary, this document shows a fuel sub-assembly, comprising an elongate outer wrapper with pin locating grids secured at points along the length of the wrapper, and elongate fuel pins in the grids secured against axial movement at an end securing grid (commonly called a "holddown" grid). To release the fuel pins from the wrapper, the wrapper is cut near the hold-down grid, this grid is clamped, and the wrapper and the locating grids can then be withdrawn axially as a whole by moving them axially away from the clamped hold-down grid. In this action, the locating grids slide over the fuel pins and the fuel pins are left exposed, rather like bristles, standing out from the hold-down grid. They can then be cut away from this grid.

With the construction disclosed in GB 1,597,189 and with the method disclosed for taking the sub-assemblies apart for reprocessing there are, nevertheless, further problems envisaged. One of these problems stems from the fact that the fuel pins bow and both the pins and wrappers swell in their central region under irradiation in a nuclear reactor. Bowing of the pins has the effect of swelling the bundle within the swollen region of the wrapper and to draw an unswollen end of a wrapper over the swollen region of the bundle is, of course, difficult. The swollen regions of the pins are also required to pass through unswollen cells of the locating grids creating further difficulty.

In accordance with the present invention, a fuel sub-assembly for a nuclear reactor comprising a bundle of spaced elongate fuel pins located in locating grids in an outer wrapper is characterised in that the grids are held in a support means insertable with the wrapper and end secured to a wrapper end.

Preferably the support means comprises two support structures which are insertable from respective opposite ends of the wrapper and are secured to respective wrapper ends.

Such a construction is considered to present the following advantages: a. To take the fuel assemblies apart, the wrapper can be cut in the centre region and when the two resultant parts are drawn axially apart, movement takes place away from the region of maximum swelling of the fuel pins and hence there is less likelihood of jamming of the fuel pins in the grids.

b. As the support structures are only secured to the wrapper at their ends there is introduced an added measure of compliance at the locating grids remote from those ends. This compliance aids the introduction of fuel pins with the sub-assembly when the sub-assembly is being made up and aids their withdrawal when the sub-assembly is being taken apart.

c. An added degree of freedom is allowed as between the locating grids and the wrapper so that the wrapper can expand or contract relative to the locating grids and, probably more important, can twist reiative to locating grids and thereby minimise jamming forces of pins in grids which could arise from a relative twist.

d. An opportunity arises to remove the wrapper by a cutting technique involving longitudinal slitting as the pin locating grids need no longer be tied to the wrapper.

e. Fuel pins may be fitted into their grids at a location remote from the wrapper and then inserted as a completed group with the wrapper.

The invention will now be described further with reference to the accJmpanying drawings in which: Figure it and 1 B show diagrammatically two forms of the invention; Figure 2 is an underside perspective view in the region indicated in Figure 1 A by the reference il; Figure 3 is a fragmentary section on the plane indicated by reference Ill-Ill of Figure 2; Figure 4 is a fragmentary perspective view illustrating a locating means for locating a compliant grid on a support structure; Figure 5 is a section on the line V-V of Figure 4.

Figure 1 A shows a fuel sub-assembly for a fast breeder reactor comprising a bundle of spaced elongate fuel pins 10 (represented by a single pin -typically there are 325 fuel pins) located in locating grids 11 in an outer wrapper 12. The grids 11 are held by a support structure 1 3 inserted into the wrapper 12 one end being secured to the wrapper 12 at a skirt 14 spot welded at welds 1 5 to one end 1 6 of the wrapper.

Although in Figure 1A the skirt 14 is shown secured to the upper end of the wrapper it could, alternatively, be secured to the lower end. An upper neutron shield 1 7 and lower assembly locating spike 1 8 with coolant entry apertures 1 9 are identified to complete the assembly.

The structure 13 and wrapper 12 have been hatched to give them emphasis. In practice the wrapper 12 would be a hexagonal section sleeve and the structure 1 3 would be constructed as six axial legs (see numeral 30 in Figure 3) fitting in the corners of the hexagonal sleeve section.

Figure 1 B shows a structure having many elements in common with Figure 1 A but differing in that the grids support structure inside the wrapper 12 is now in two parts 1 3A and 13B which have been inserted into the wrapper 1 2 from respective opposite ends 1 6A, 1 6B of the wrapper and are secured respectively at welds 15A, 15B.

The constructions illustrated diagrammatically in Figures 1 A and 1B will now be described in more detail with reference to the remaining diagrams.

In Figure 2 the upper end of one leg 30 of a grid support structure 13 (or 1 3A or 13B) is shown.

The leg, of which there are six, is shown attached to a skirt 14 (or 14A or 14B) at a recess 31 and supporting a fuel pin hold-down grid 11 at a recess 33. Fuel pin and coolant apertures 34 are shown in the grid 11. One fuel pin 10 is shown in the grid (the grid 11 and the fuel pin 10 are not shown in full detail).

The skirt 14 has narrow regions 35 for locating in the recess 31 and broad regions 36 which are suitable for welding to the fuel sub-assembly wrapper.

The grid 11 has lug parts 37 for locating in recesses 33. This grid 11 shown is a fuel pin hold down grid and can be heavily stressed and is of solid construction. The method of locating other grids 11 on the support structure 1 3 is shown in Figure 4. The grid shown may be tack welded to the legs 30. Similarly the skirt 14 may also be tack welded to the legs 30.

Figure 3 shows one corner 38 of a hexagonal wrapper 12 with one leg 30 of the support structure located in that corner. The hold down grid 14 supported in the recess 31 of the leg 30 is also shown.

In Figures 4 and 5 one leg 30 of a grid support structure is shown. This leg has a number of cutaways 30a (only one being shown) spaced apart along the length of the leg. The cut-aways leave an approximately rectangular part 30b (best seen in Figure 5).

The spacing of the recesses determines the spacing of grids 11. Each grid 11 has six clip parts 41, one for each leg 30. The ciip parts 41 each have two flaps 42 which can be deflected into the recesses 30a to locate the grids 11 on the legs 30.

The parts 41 have a degree of compliance which complements the compliance inherent in the grids 11.

The clip parts 41 are bolstered by a grid corner post 44 which is edge welded at 45 to the flaps 42. There is a small clearance 46 between post 44 and part 30b. This further gives a degree of freedom in the whole assembly.

For dismantling the assembly above described with reference to Figures 1 B, 2 to 5 above the wrapper 12 is cut in the region marked 50 in Figure 1 B which lies approximately at the centre of the length of the wrapper 12 where the ends of structures 1 3A and I 3B (Figure 1 B) face each other. With this cut made the bottom half of the wrapper 12, the structure 1 3B, and the grids 11 carried in structures 1 3B are withdrawn downwardly leaving the pins 10 projecting from the structure 1 3A as they are held in the uppermost grid 11 which is a pin holding grid. The pins 10 are now released from this grid 11 and can be separated from it.

Circumstances may arise when it is prudent to support the pins 10 as the part of the wrapper 12 is removed from the other part. For this purpose a sliding clamp can be provided which supports the fuel pins once the wrapper part has been partly removed and can follow the wrapper part as it is removed. This clamp can also be used by increasing clamping force to pull the pins from the hold down grid 11 once released.

In the event that the construction of Figure 1 A is used then the assembly can be dismantled by longitudinal cutting of the wrapper 12 so that the cut wrapper remains attached to the structure 1 3 and opening the cut wrapper to expose the structure 13. The legs 30 of the structure 13 can now be cut together with cutting of fuel ends to release them for their hold down grid. The pins 10 can then be released from the grids 11 surrounding them.

It is also to be observed that a fuel assembly is provided by the invention which can be free of riveting and hence avoids structural weaknesses and stress concentrations arising from drillings.

This not only helps to provide a potentially more reliable assembly in use but also provides a stronger assembly to withstand forces which may have to be applied on dismantling.

Claims (9)

1. A fuel sub-assembly for a nuclear reactor comprising a bundle of spaced elongate fuel pins (10) located in grids (11) in an outer wrapper (12) characterised in that the grids (11) are held in a support means (13) insertable into the wrapper and end secured to a wrapper end (16).

2. A fuel assembly as claimed in claim 1 in which the support means (13) comprises two support structures (12A, 13B) insertable into the the wrapper from respective opposite ends of the wrapper and secured to respective wrapper ends.

3. A fuel assembly as claimed in claim 1 or 2 in which the wrapper is of hexagonal section and the support means comprises six legs fitting in the corners of the hexagonal section.

4. A fuel assembly as claimed in any preceding claim in which the support means (13) is secured to a wrapper end (16) through the agency of a skirt (14).

5. A fuel assembly as claimed in claim 4 in which the skirt (14) is welded to the wrapper end (16) and the support means (13) is attached to the skirt at recesses (31).

6. A fuel assembly as claimed in claim 5 in which a hold down grid (11) is attached at iug parts (37) to the support means (13) at recesses (33) in the support means.

7. A fuel assembly as claimed in claim 4, 5 or 6 in which fuel pin locating grids (11) are attached at flaps (42) deflected into recesses (30a) in the support means (13).

8. A fuel assembly as claimed in claim 2 having the wrapper cut (50) where the two support structures (13A, 13B) come together.

9. A fuel assembly substantially as hereinbefore described with reference to Figures 1 A and Figure 1B taken with the remaining figures of the drawings.