ASSEMBLY OF NUCLEAR FUEL RODS BUNDLE

Abstract: A puller rod (1) is provided for use in assembling a nuclear fuel element by loading nuclear fuel rods containing nuclear fuel pellets into a support skeleton in the form of spaced apart grids. The puller rod (1) has a tubular body in which at least a forward or leading portion of the body is made of carbon fibre.

Description: ASSEMBLY OF NUCLEAR FUEL RODS BUNDLE FIELD OF THE INVENTION This invention relates to apparatus for building nuclear fuel elements in which a plurality of nuclear fuel rods, containing nuclear fuel pellets, are loaded into support skeletons in the form of spaced apart grids. By way of example, this invention relates to the assembly of fuel elements for use in a Pressurised Water Reactor (PWR) which makes use of fuel rods that are both long and slender. If such rods were to be pushed into the skeleton, they would tend to buckle due to their inherent weakness. Accordingly, they are in fact loaded into the skeleton by pulling them. In order to achieve this, a device known as a puller rod is used. The puller rod is passed through the grids of the skeleton and the lead end is caused to engage an end of the fuel rod. The puller rod is then pulled back through the skeleton, thereby pulling the fuel rod into position within the skeleton. The present invention is particularly concerned with puller rods. BACKGROUND OF THE INVENTION A puller rod is known in the form of a sleeve carrying a locking rod down its centre. Both the sleeve and the locking rod are long enough to reach through a fuel skeleton and attach via a collet to the bottom end plug of a fuel rod. The collet is an externally profiled cylinder which is axially split from one end to provide three prongs or tines. These tines are normally close together allowing them to enter the end cap on a fuel rod. They are then forced apart and held open inside the end plug by the locking rod being moved forward down the centre of the collet, thereby stopping it closing again. Once the puller rod has pulled the fuel rod into position within the skeleton, the locking rod is pulled away from the collet allowing the tines to close together again and the puller rod to be removed from the fuel rod. The puller rod typically comprises a steel tube of metallic material. In terms of dimensions, a typical puller rod might be 4m in length and about 8mm in diameter and, because of its length and slenderness, it has a large inherent deformation. This deformation continues throughout its period of use due to the significant deflection forces during loading. During the passage of the puller rod through the skeleton, in order to link up with the fuel rod, the lead end of the puller rod is unsupported for a distance which, at its maximum, is the distance between adjacent grids of the skeleton. This distance may be of the order of 0.5m and can be sufficient for the leading end of the puller rod, which is unsupported, to droop sufficiently for it to become misaligned and, therefore, for its front end not to enter the hole or cell in the next grid. Instead, the front end of the puller rod will strike the cell wall and be damaged, or damage the grid, or both. STATEMENTS OF INVENTION According to the present invention there is provided a puller rod having a tubular body, at least the forward or leading portion of said body being made of carbon fibre. A puller rod of the present invention may include a tubular body made entirely of carbon fibre. However, it is preferred that only the leading or forward section of the tubular body be made of carbon fibre since it is the leading end, carrying the collet which is cantilevered as the rod advances from one grid in the skeleton to the next. More preferably, the length of the tubular body of the puller rod made of carbon fibre is at least the length between adjacent grids in the skeleton. For instance, the carbon fibre length may be approximately equal to the separation of the adjacent grids. It is also preferred to coat the outer surface of the carbon fibre portion of the puller rod with a protective coating in order to reduce wear on the rod while it is in use and to prevent carbon being transferred from the puller rod to the skeleton grids during the passage of the puller rod through the skeleton. Preferably, the protective coating is a metal plating such as electro-less nickel plating or chromium plating. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings further illustrate the present invention and are as follows :- Figure 1 shows the forward or leading end of a puller rod; and Figure 2 illustrates the forces acting on a puller rod while it is in use. DESCRIPTION OF THE EMBODIMENT SHOWN IN THE DRAWINGS There is illustrated in Figure 1 the leading or forward end of a puller rod 1 which is in the form of a tube 3 having a collet 5 provided with tines 7. Located within tube 3 is a rod 9 which, in use, is moved forward relative to tube 3 in order to force apart tines 7 when the puller rod has engaged the end of a fuel rod. The tube 3 of puller rod 1 is formed partly of steel and partly of carbon fibre. The carbon fibre section is the forward or leading end of the puller rod and extends over about one third of its length. This carbon fibre section is chromium plated on its outer surface. Referring to Figure 2 of the accompanying drawings, there are illustrated the forces acting on a puller rod which, in use, may be described as a propped cantilever with an end load. The deflection at the end of the puller rod is a function of the length of the puller rod and the position of the prop during operation. It has been found that, by the addition of a downward force over the critical section on the top of the puller rod, the deflection at the end of the rod is minimised, allowing the threading of the puller rod through the skeleton without manual intervention. The provision of a forward carbon section to replace the normal steel tubular rod over this forward portion achieves the same effect, namely, the reducing to a minimum of the deflection of the end of the rod.

Claims: CLAIMS 1. A puller rod for use in assembling a nuclear fuel element by loading nuclear fuel rods containing nuclear fuel pellets into a support skeleton in the form of spaced apart grids, the apparatus comprising a puller rod having a tubular body in which at least a forward or leading portion of said body is made of carbon fibre. 2. A puller rod as in Claim 1 in which the portion of the puller rod made of carbon fibre has a length at least equal to the length between adjacent grids in the support skeleton. 3. A puller rod as in Claim 1 or Claim 2 in which the outer surface of the carbon fibre portion of the puller rod is coated with a protective coating. 4. A puller rod as in Claim 3 in which the protective coating is a metal plating. 5. A puller rod as in Claim 4 in which the metal plating is an electro-less nickel plating or chromium plating.

Inventors: BRIDGES MICHAEL KEVIN, DESBOROUGH PAUL HUGH, WIGG GRAEME MICHAEL, BRIDGES, MICHAEL, KEVIN, DESBOROUGH, PAUL, HUGH, WIGG, GRAEME, MICHAEL
Applicants: BRIDGES MICHAEL KEVIN, BRITISH NUCLEAR FUELS PLC, DESBOROUGH PAUL HUGH, WIGG GRAEME MICHAEL, BRIDGES, MICHAEL, KEVIN, DESBOROUGH, PAUL, HUGH, WIGG, GRAEME, MICHAEL
Application year: 1997
Main classification: G21C3
IPC codes: G21C3, G21C21