GB1593578A

GB1593578A - Nuclear reactor fuel element

Info

Publication number: GB1593578A
Application number: GB19385/78A
Authority: GB
Original assignee: Interatom Internationale Atomreaktorbau GmbH
Current assignee: Interatom Internationale Atomreaktorbau GmbH
Priority date: 1977-05-14
Filing date: 1978-05-12
Publication date: 1981-07-22
Also published as: GB1593579A; DE2721869A1; IT1095142B; FR2390805B1; FR2390806A1; BE866950A; IT7823266A0; IT1095143B; FR2390806B1; FR2390805A1; IT7823267A0; BE866951A

Description

(54) A NUCLEAR REACTOR FUEL ELEMENT (71) We, INTERATOM, INTERNATION ALE ATOMREAKTORBAU GmbH, of 5060 Bergisch Gladbach 1, Germany (Fed. Rep.), a German company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be pårticularly described in and by the following state ment :- This invention relates to a nuclear reactor fuel element ot the type comprising a plurality of fuel rods disposed parallel to one another within a casing and fixedly held in relation thereto. More particularly this invention relates to fuel elements of such type in which the fuel rods are secured in a so-called rod-holding plate at the lower end of the casing and are allowed to freely expand upwards whilst being laterally guided by spacers. The necessary mechanical strength of the element is provided by the casing, and coolant flowing around and along the fuel rods is guided by the casing. It is desirable for the means used to secure the fuel rods in relation to the casing to be as simple as possible not only with regard to the manufacture and assembly of the element but also with regard to subsequent dismantling of the spent element which, as is known, can be carried out only by means of remote controlled tools because of intense radiation.

Since a large number of fuel elements are required for loading an individual nuclear reactor, and since requirements with regard to the quality thereof are most stringent, even a small saving in the manufacture and assembly of individual parts has an appreciable effect on total costs.

Various methods have already been proposed for securing the fuel rods in the rodholding plate. For example each rod end extending through the plate may be secured thereto by means of a nut screwed onto the rod end. The nut must, however, be secured against loosening during operation, for example by spot welding. The rod end may alternatively be made in resilient form so that parts pressed together for passage through the bore in the rod-holding plate spread apart again thereafter and thus secure the fuel rod, it being possible for the locking action of the springing parts to be overcome during withdrawal by a sharp tug on the opposite end of the rod, just as it is overcome by pressure during their insertion (see German Offenlegungsschrift 25 20 233, U.S.

Patent Specification 3 945 885). However, owing to the fact that the fuel rods are very thin and have small wall thickness, application of the appreciable pulling and pushing forces which are necessary for the assembly and dismantling of such fuel rods is not desirable.

Securing means are also known in which one end of the fuel rod is shaped in the form of a simple key barrel and is pushed into a corresponding key-hole shaped aperture in the rod-holding plate and is locked by rotation about its longitudinal axis (Germany Offenlegungsschrift 25 20 234). This form of construction tends to lead to jamming in spme cases and hence it may be necessary to a ply undue force whereby the strength of the rods is endangered. A further proposal is to form the ends of the fuel rods with keyhole like openings which extend transversely in relation to the longitudinal axis of the rod and through which correspondingly shaped holding members are passed. These members also engage with corresponding a er- tures in the casing there being mounted on each of the holding members a row of fuel rods (German Offenlegungsschrift 25 31 641, U.S. Patent Specification 3 890 197).

Apart from the fact that the manufacture of fuel rods with ends of complex shape involves considerable cost, these constructions have the disadvantage that even when the rod-holding plate has, in addition to the bores receiving the fuel rods, further openings for the passage of coolant, the crosssectional area of flow available within the casing is reduced to a greater extent than by the fuel rods themselves in the further course of the flow. In other words the rod-holding plate causes a pressure loss which necessitates the installation of more powerful and costly pumps in the coolant circuit.

According to the present invention, there is provided a nuclear reactor fuel element comprising a plurality of fuel rods disposed substantially parallel to one another within a casing, a plurality of rod-holding plates each having a pair of opposed faces and being disposed between the fuel rods transversely thereto, thereby dividing them into a plurality of rows, and one or more elongate members extending through the rod-holding plates and releasably connected to mounting means fixedly disposed in the casing, wherein each fuel rod has, at one end, projections engaged in corresponding apertures formed in each rod-holding plate over a part of its width.

Each fuel rod is desirably formed at said one end with two parallel faces each of which has a length equal to the combined width of the corresponding aperture and the width of the rod-holding plate above the aperture, said faces being undercut over said width of the rod-holding plate.

The rod-holding plates are formed with apertures through which the elongate members can be passed. These apertures are so distributed along the length of the rodholding plates that when a plate is disposed within the casing the apertures are aligned with those in the other rod-holding plates.

When the elongate members are passed through these apertures the individual rodholding plates and with them the fuel rods are combined to form an assembly.

Rows of fuel rods and rod-holding plates are alternatively assembled in the correct orientation in relation to one another, the rod-holding plates are secured against slipping in relation to one another by means of the mounting means, and the whole bundle of fuel rods thus produced is pushed into the casing which prevents the elongate members from falling out and thus ensures that the bundle is held together. The spent fuel element is taken apart in inverse sequence, in which process, as in the assembly, no fixed connections or connections which may have become fixed in the course of the use in the reactor such as weld seams, screw connections and the like have to be broken. No special forces have to be applied to dismantle the assembly into individual parts because the bundle breaks up into its individual parts after having been withdrawn from the can.

No single rod-holding plate as such is employed, it being replaced by a plurality of rod-holding prates and elongate members which have only a small cross-section in the direction of flow of coolant, so that the available cross-sectional area of flow is less restricted than in the region in which most heat is emitted by the rods.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 shows a horizontal section through part of a nuclear reactor fuel element according to the invention taken along the line EF of Figure 2; Figures 2 and 3 show vertical and horizontal sections respectively through part of a nuclear reactor fuel element according to the invention the section of Figure 2 being taken along the line GH of Figure 3, and the section of Figure 3 being taken along the line IK of Figure 2; Figure 4 shows in more detail the end of a fuel rod and associated rod-holding plate in section along the line Z of Figure 3.

The illustrated part of the fuel element shown in the accompanying drawings comprises a plurality of fuel rods 1 in the form of long cylindrical pins disposed in a hexagonal pattern within a casing 2. These rods engage at the lower ends as shown in closure or rod-holding plates 4 which in turn are held together by elongate members 5 passed therethrough. The members 5 are fixed in corresponding slots in mounting means which takes the form of a retaining ring 6.

The assembly is put together outside the casing 2 by pushing together the parts 1,4 and 5.

The finished bundle is pushed into the casing 2 and prevented from falling out by a sleeve 8 which is connected to the retaining ring 6 by means of screws 7 and which is secured to the casing 2 by further screws (not shown here) or by welding.

The structure of the ends of the fuel rods will be described in more detail with reference to Figure 4. Each rod 1 has an end piece 14 comprising two parallel faces each cut into the cylindrical form of the rod and extending to the end of the rod. Lateral undercuts 141 are formed, for example by turning, so that projections 142 remain. These projections engage in apertures 41 in rod-holding plates 4. Adjacent to the line of apertures 41, each rod-holding plate 4 comprises upper and lower strips 42 and 43 respectively of full cross-section. In the region of the strip 42, the rod-holding plate 4 engages in the undercuts 141 (see also Figure 3). In the region of the strip 43, the rod-holding plate 4 is formed with apertures 131 through which the elongate members 5 are pushed during assembly.

The apertures 131 in individual rod-holding plates are each offset from the centre of the plate to the same extent so that the apertures are aligned with one another when the fuel rod assembly is put together. At its upper end, the end piece 14 is adjoined by a hollow can 12 which is for the greater part filled by fuel pellets.

WHAT WE CLAIM IS: 1. A nuclear reactor fuel element comprising a plurality of fuel rods disposed substantially parallel to one another within a casing, a plurality of rod-holding plates each having a pair of opposed faces and being

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. comprising a plurality of fuel rods disposed substantially parallel to one another within a casing, a plurality of rod-holding plates each having a pair of opposed faces and being disposed between the fuel rods transversely thereto, thereby dividing them into a plurality of rows, and one or more elongate members extending through the rod-holding plates and releasably connected to mounting means fixedly disposed in the casing, wherein each fuel rod has, at one end, projections engaged in corresponding apertures formed in each rod-holding plate over a part of its width. Each fuel rod is desirably formed at said one end with two parallel faces each of which has a length equal to the combined width of the corresponding aperture and the width of the rod-holding plate above the aperture, said faces being undercut over said width of the rod-holding plate. The rod-holding plates are formed with apertures through which the elongate members can be passed. These apertures are so distributed along the length of the rodholding plates that when a plate is disposed within the casing the apertures are aligned with those in the other rod-holding plates. When the elongate members are passed through these apertures the individual rodholding plates and with them the fuel rods are combined to form an assembly. Rows of fuel rods and rod-holding plates are alternatively assembled in the correct orientation in relation to one another, the rod-holding plates are secured against slipping in relation to one another by means of the mounting means, and the whole bundle of fuel rods thus produced is pushed into the casing which prevents the elongate members from falling out and thus ensures that the bundle is held together. The spent fuel element is taken apart in inverse sequence, in which process, as in the assembly, no fixed connections or connections which may have become fixed in the course of the use in the reactor such as weld seams, screw connections and the like have to be broken. No special forces have to be applied to dismantle the assembly into individual parts because the bundle breaks up into its individual parts after having been withdrawn from the can. No single rod-holding plate as such is employed, it being replaced by a plurality of rod-holding prates and elongate members which have only a small cross-section in the direction of flow of coolant, so that the available cross-sectional area of flow is less restricted than in the region in which most heat is emitted by the rods. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 shows a horizontal section through part of a nuclear reactor fuel element according to the invention taken along the line EF of Figure 2; Figures 2 and 3 show vertical and horizontal sections respectively through part of a nuclear reactor fuel element according to the invention the section of Figure 2 being taken along the line GH of Figure 3, and the section of Figure 3 being taken along the line IK of Figure 2; Figure 4 shows in more detail the end of a fuel rod and associated rod-holding plate in section along the line Z of Figure 3. The illustrated part of the fuel element shown in the accompanying drawings comprises a plurality of fuel rods 1 in the form of long cylindrical pins disposed in a hexagonal pattern within a casing 2. These rods engage at the lower ends as shown in closure or rod-holding plates 4 which in turn are held together by elongate members 5 passed therethrough. The members 5 are fixed in corresponding slots in mounting means which takes the form of a retaining ring 6. The assembly is put together outside the casing 2 by pushing together the parts 1,4 and 5. The finished bundle is pushed into the casing 2 and prevented from falling out by a sleeve 8 which is connected to the retaining ring 6 by means of screws 7 and which is secured to the casing 2 by further screws (not shown here) or by welding. The structure of the ends of the fuel rods will be described in more detail with reference to Figure 4. Each rod 1 has an end piece 14 comprising two parallel faces each cut into the cylindrical form of the rod and extending to the end of the rod. Lateral undercuts 141 are formed, for example by turning, so that projections 142 remain. These projections engage in apertures 41 in rod-holding plates 4. Adjacent to the line of apertures 41, each rod-holding plate 4 comprises upper and lower strips 42 and 43 respectively of full cross-section. In the region of the strip 42, the rod-holding plate 4 engages in the undercuts 141 (see also Figure 3). In the region of the strip 43, the rod-holding plate 4 is formed with apertures 131 through which the elongate members 5 are pushed during assembly. The apertures 131 in individual rod-holding plates are each offset from the centre of the plate to the same extent so that the apertures are aligned with one another when the fuel rod assembly is put together. At its upper end, the end piece 14 is adjoined by a hollow can 12 which is for the greater part filled by fuel pellets. WHAT WE CLAIM IS:

1. A nuclear reactor fuel element comprising a plurality of fuel rods disposed substantially parallel to one another within a casing, a plurality of rod-holding plates each having a pair of opposed faces and being

disposed between the fuel rods transversely thereto, thereby dividing them into a plurality of rows, and one or more elongate members extending through the rod-holding plates and releasably connected to mounting means fixedly disposed in the casing, wherein each fuel rod has, at one end, projections engaged in corresponding apertures formed in each rod-holding plate over a part of its width.

2. A nuclear reactor fuel element as claimed in claim 1, wherein each fuel rod is formed at said one end with two parallel faces, each of which has a length equal to the combined width of the corresponding aperture and the width of the rod-holding plate above the aperture, said faces being undercut over said width of the rod-holding plate.

3. A nuclear reactor fuel element substantially as hereinbefore described with reference to, and as shown, in the accompanying drawings.