GB2163593A - Improvements in nuclear fuel assembly sleeves - Google Patents

Abstract

The graphite sleeve (10) of a nuclear fuel assembly or reflector element for a stringer mounts a number of grids (12, 14, 16) via mounting assemblies (24, 28, 38) installed in grooves (18, 20, 22) formed in the interior wall surface of the sleeve. The bore of the sleeve is of reduced cross-section between two successive grooves (18, 20) such that the internal diameter of the sleeve is substantially the same as the inner diameter of the radially innermost extremity of the mounting assemblies (24, 28, 38) whereby the coolant pressure loss at each transition between the reduced diameter bore section and the mounting assemblies is reduced. Each mounting assembly may be of radially contractable split ring construction to permits its placement in the groove. and may carry burnable poison material (60). <IMAGE>

Description

SPECIFICATION Improvements in nuclear fuel assembly sleeves This invention relates to nuclear fuel assembly cages and has particular application to the sleeves of fuel assemblies for gas cooled nuclear reactors.

As used herein, the terminology "nuclear fuel assembly cage" refers to a tubular enclosure and associated structural components locating the nuclear fuel elements (usually fuel pins) and/or a guide tube within the enclosure.

The cage may house fuel elements and a guide tube or alternatively a guide tube only in which case the cage constitutes a reflector rather than a fuel sub-assembly. In a known gas cooled reactor, the fuel-containing cages are stacked one on top of the other to form a fuel stringer and reflector cages are located at the top and bottom ends of the stringer. The guide tubes of all of the cages are axially aligned with one another so that a common tie bar can be passed through them and used to take the weight of the stringer during handling. Thus, as used herein, the term "nuclear fuel assembly cage" is to be construed as referring to a fuel bearing cage and a reflector cage.

According to the present invention there is provided a nuclear fuel assembly cage comprising a sleeve composed of moderating material and at least one grid mounted by mounting means installed in at least one groove extending around the inner peripheral surface of said sleeve, characterised in that the bore of the sleeve is of stepped diameter such that the sleeve bore diameter is less on one side of the groove than on the opposite side.

Thus, the bore of the sleeve is stepped in such a way that at the transition point between the bore wall and one side of at least one of the mounting means, the bore diameter is substantially equal to the inner diameter of the innermost radial extremity of the mounting means so as to reduce coolant pressure loss at such transition.

To promote further understanding of the invention, examples thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section of a single sleeve fuel assembly cage in accordance with the invention, the fuel pins being omitted for clarity; Figure 2 is an enlarged view showing the construction of one of the mounting assemblies for the support and bracing grids; Figure 3-6 are similar views to that of Fig.

2 illustrating the installation of a mounting assembly within the sleeve; and Figure 7 is an underside perspective view, partly broken away, illustrating an alternative form of a liner for mounting of the grids.

Referring now to Figs. 1-6, Fig. 1 illustrates a fuel cage comprising a single graphite sleeve 10 and three grids 12, 14 and 16 of which the grids 12 and 14 are spacer or bracing grids for maintaining the fuel pins in generally parallel spaced relation and grid 1-6 is a weight-bearing support grid to which the pins are secured. In use, the sleeve 10 is disposed generally vertically with the weight-bearing grid 16 lowermost. The inner periphery of the sleeve 10 is formed with three peripheral grooves 18, 20, 22 which are of generally similar configuration. The inside diameter of the sleeve 10 is substantially uniform over its length except in the region between the grooves 18 and 20 where its inside diameter is reduced, as will be explained hereinafter.

Each grid 12, 14, 16 is secured in position along the sleeve by means of mounting assemblies accommodated by the grooves 18, 20 and 22. The mounting assemblies and the manner in which they co-operate with the grids is substantially the same in each instance and will therefore be described with reference to the mounting assembly for the grid 12 (see Figs. 2-6). Each mounting assembly comprises a split ring 24 of U-section to one flange 26 of which there is secured an L-section member or members 28 (eg. by spot welding at points 30 around the periphery of the ring). The L-section member may also be of split ring configuration (with for example castellations in that limb of the Lsection which extends axially) or, if desired, a plurality of L-section brackets may be secured to the ring 24 at spaced positions around its periphery.In either case, the L-section members(s) 28 serves to form with the ring 24, a pocket or pockets 32 (see Fig. 3) for reception of the leading edge 34 of the rim 36 of the grid 12. The mounting assembly comprises a further split ring 38 of L-section separate from the ring 24 and the L-section member(s) 28.

Fig. 2 illustrates the mounting assembly installed into the groove 18 (whose side walls are depicted by reference numerals 1 8a and 18b). Installation will now be described with reference to Figs. 3-6. Initially, the ring 24 and the attached L-section member(s) 28 are radially contracted to enable them to be displaced along the sleeve bore. Radial contraction may be effected by axially offsetting the free ends of the ring 24 (and L-section member 28 if in the form of a ring) so as to obtain a generally helical configuration and then causing the free ends to overlap in the peripheral direction and thereby reduce the outside diameter of the ring 24 until it is a free sliding fit in the sleeve bore.The ring 24 (with attached L-section member(s)) is then displaced along the sleeve bore towards the desired groove (ie. groove 20 or 22 if the assembly is installed from the bottom end of the sleeve 10 or groove 18 if installed from the top end). In practice, the ring 24 associated with groove 20 will be installed before that associated with groove 22.

As the ring 24 begins to register with the groove (eg. groove 18), it is manipulated so as to eliminate the axial offset of its ends and allow it to expand elastically and radially into the groove. To prevent rotation of each mounting assembly relative to its respective groove, the assembly is provided with a series of peripherally spaced projections 40 for engagement in a similarly spaced series of key-ways 42 adjacent each groove. Preferably there are at least three such keyways each for reception of a corresponding projection 40 as a close fit so that co-operation between the projections 40 and keyways 42 affords accurate radial location of the ring 24 and hence the grids when the latter have been installed.

The ring 24 is so dimensioned that after it has been allowed to expand radially into its groove, the pocket(s) 32 project into the sleeve bore. Thus, the associated grid, eg.

12, may then be displaced along the sleeve bore until the leading edge 34 of its rim 36 engages in the pocket(s) 32. It will be understood that the outside diameter of the rim 36 will be such as to provide a small degree of clearance within the larger diameter regions of the sleeve bore.

Once the rim edge 34 has been engaged with the pocket(s) 32, the ring 38 can be installed by radially contracting it in the same fashion as described above and then allowing it to radially expand into the gap 44 (see Fig.

4) so as to trap the trailing edge 46. At this time, the arrangement of the mounting assembly and the grid rim 36 will be as illustrated in Fig. 5 from which it will be seen that the overall axial dimensions of the compo nents 24, 28, 38 is such that the assembly is a snug fit within the respective groove. Finally, the grid rim 36 is welded to the components 28 and 38 at a plurality of peripherally spaced points 48 by means of spot welding heads 50, 52. Each pair of axially spaced welds 48 may be made simultaneously be passing the welding current along a conduction path com prising head 50, L-section ring 38, rim 36, L section member 28 and head 52.

In practice, the centre bracing grid 14 is installed first via the bottom end of the sleeve followed by the top bracing grid 12 and then the lower support grid 16 is installed from the lower end of the sleeve. In the case of the grid 16, the rim may be smaller in the axial direction such that its edges do not project axially as in the case of bracing grids 12, 14.

In this event, the L-section member 28 may be suitably cut-away at the grid web positions to allow the resulting castellations to overlap the grid rim. The ring 38 will be similarly cut away at the grid web position. In addition, during installation of the grid 16, after installation of the ring 38 and before welding, the grid 16 is pushed downwards, eg. with the aid of guide tube 54 (see Fig. 1), to ensure that the grid 16 is firmly seated against the ring 38 and welding is executed with the grid 16 in this position. In this way, it can be ensured that the weight of the fuel pins carried by the grid 16 in use is primarily transmitted to the sleeve via abutment between the grid rim and the ring 38 rather than via the spot welds 48 therebetween.

At least one of the grooves 18, 20, 22 may also accommodate burnable poison elements for absorbing neutrons during the early stages of fuel burn-up. Such elements may conveniently be attached to the mounting assemblies, eg. within the channels formed by the rings 24. Fig. 2 illustrates two such elements 60 which may be in the form of cables incorporating gadolinia.

As mentioned previously, the inside diameter of the sleeve bore is reduced between the grooves 18 and 20. The reduction is such that the sleeve bore in this region has substantially the same inside diameter as the radially innermost extremities of the adjacent mounting assembly. In this way, the loss in coolant pressure as the coolant traverses the transition between the mounting assembly in groove 18 and the adjacent sleeve bore and the transition between the sleeve bore and the assembly installed in groove 20 may be reduced. The direction of coolant flow is depicted by arrow 61 in Fig. 1.

In a modification, at least one of the mounting assemblies may omit one or other of the components 28, 38. For example, the mounting assemblies for the grids 12, 14 may omit the rings 38 in which event, the ring 24 and member 28 will be axially dimensioned so as to be a close fit within the respective groove.

In another modification which is illustrated in Fig. 7, the pockets afforded by the mounting assembly may be provided by radially projecting, closely spaced pairs of fins 70 spaced around the periphery of a U-section split ring 72 similar to that employed in the embodiment of Fig. 1-6. In this case, the rims 76 of the support and/or bracing grids (a support grid is illustrated) is provided with axial lugs 74 for engagement between each pair of fins 70.

The foregoing applies to fuel-bearing cages.

In the case of a reflector cage, the grids may be of a simpler construction comprising a rim with a number (eg. six) of inwardly extending arms connected to a small diameter ring at the centre for locating the guide tube. The grids in this case are mounted in the same manner as described above in relation to the fuel-bearing cage grids.

Claims (3)

1. A nuclear fuel assembly cage comprising a sleeve composed of moderating material and at least one grid mounted by mounting means installed in at least one groove extending around the inner peripheral surface of said sleeve, characterised in that the bore of the sleeve is of stepped diameter and that the sleeve bore diameter is less on one side of the groove than on the opposite side.

2. A cage as claimed in Claim 1 in which the reduced bore diameter is substantially equal to the inner diameter of the innermost radial extremity of the mounting means whereby coolant pressure loss is substantially reduced at the transition between the mounting means and the reduced diameter bore section of the sleeve.

3. A cage as claimed in Claim 1 or 2 in which the reduced diameter bore section extends between successive grooves in the sleeve whereby coolant pressure loss is reduced at each transition between the reduced diameter bore section and each of the successive grooves.