GB2129187A - Method for wire wrapping nuclear fuel pins - Google Patents

Abstract

A method for wrapping wire (5) around a nuclear reactor fuel pin (2) wherein the wire (5) is preformed into a coil (7), the pin (2) is placed into the coil (7), the coil is brought into contact with said pin (2) by expansion along the axis of the pin (2), and the wire (5) is attached to opposite ends of the fuel pin (2).

Description

SPECIFICATION Method for wire wrapping nuclear fuel pins This invention describes a method for wire wrapping fuel pins for nuclear reactors. Many designs for nuclear reactors, especially fast reactors, utilize wire which is wrapped in a helix around the individual fuel pins. This wire serves to provide spacing between the pins, thus permitting the reactor coolant to flow through the pin array.

In a typical fast reactor design, the reactor core which contains the fuel material that generates heat from the nuclear reaction, is comprised of a number of fuel assemblies. These assemblies are positioned in a compact array that maximizes nuclear and thermal efficiency. The assemblies are usually very long relative to the cross section, and normally are positioned with the long axis vertical.

The coolant, which may be a liquid metal such as sodium, flows through the fuel assemblies to remove the heat resulting from the nuclear reaction. This heat is generated within the fuel pins which comprise the fuel assembly. These pins usually are enclosed in a duct which serves to contain the fuel pins and direct the flow of the coolant.

The fuel pins contain the fissile, or fuel, materials which are necessary for the nuclear fission reaction to occur and thus for the reactor to operate. These pins are long, thin-walled tubes that are sealed at both ends to contain the fuel.

The pins may also contain non-fuel internal components to position the fuel and to collect fission products. The ends of these pins are designed to support the pin in the assembly.

Because there are extreme amounts of heat generated during the fission process, it is imperative that this heat be removed by the reactor coolant quickly and efficiently. This heat must be transferred from the fuel through the cladding to the coolant. Adjacent pins must be positioned such that coolant can flow between them and remove this heat. Wire helically wrapped around the pins provides this spacing between pins but does not impair the essential heat removal process.

Prior art methods of accomplishing the wire wrapping process all have specific disadvantages which are eliminated by the invention disclosed herein. These prior art methods all utilize the same basic technique.That technique is to 1) fasten the wire to one end of the pin (usually by combination of mechanical and welding techniques); 2) with the wire held in tension, rotate the pin to achieve the desired pitch, and 3) fasten the wire to the opposite end of the pin. Disadvantages of these prior art methods are: 1) The fuel pin to flex and bow as it is being wrapped with the wire. Depending upon the design of the pin, this flexing and bowing action can cause mechanical stresses in the fuel and in the cladding. These stresses may have unknown effects during subsequent use of the pins during subsequent reactor operations.

2) Manual operation handling is required which results in personnel exposure to ionizing radiation due to the proximity to the fuel pin while attaching the wire to the pins. The projected use of fuel which has higher radiation exposure rates could cause a dramatic increase in the magnitude of this problem.

It is therefore the principal object of the present invention to provide a method for wrapping coolant spacer wire around fuel pints which is well adaptable to automated operations and which can be remotely controlled, and which will not result in undesirabie stresses in the pins.

With this object in view, the present invention resides in a method of mounting a wrapping wire around a nuclear reactor fuel pin wherein the wire is preformed into the shape of coil characterized in that the fuel pin is placed inside the wire coil, said wire coil is axially expanded thereby causing radial contraction of said coil until it is in contact with said fuel pin substantially along the contiguous lengths of said wire and said pin and the ends of said wire are attached to the opposite ends of said fuel pin.

Preferably, the wire is welded to the end plugs of the fuel pin.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein: Figure 1 is a part sectional view in elevation showing a nuclear reactor fuel assembly, the design of which utilizes wire wrapped fuel pins, and Figure 2 is a schematic of a helical wire prior to installation on a fuel pin.

In Figure 1 the reference numeral 1 designates the outer duct of a fuel assembly. This duct encloses a cluster of parallel fuel pins 2. Fuel pins 2 are closed by top end caps 3 and bottom end caps 4. Typically, pins 2 of the fuel assembly are placed in a uniform hexagonal planar lattice.

Spacing between pins 2 within duct 1 is achieved by means of a spacer element or wire 5 which is wound in a coarse pitch helix configuration over the external surface of each fuel pin 2. Wire 5 extends over the entire length of fuel pins 2. Wire 5 is rigidily fixed to the ends of fuel pin 2 by welding, but may be alternatively fixed by crimping or other means. Various modifications to the spacer element may involve changing wire 5 to a tube, a wire within a tube, or any other configuration which will achieve the desired spacing.

The specific method of this invention is as follows: 1. Wire 5 is prewrapped into a coil 7 (see Figure 2) 2. Fuel pin 2 is centered over the wire coil 7 and wire 5 is attached to one of end caps 3 or 4.

Various means of attachment to one of end caps 3 or 4. Various means of attachment of the wire to the pin are known to the prior art, including mechanical attachments and welding. Use of a lay-along TIG weld or resistance weld is proposed to facilitate automating the total process.

3. With pin 2 held stationary, axial tension (see arrow 6 in Fig. 2) is applied to wire 5 which radially contracts coil 7 in the direction of arrow 8 in Fig. 2 and brings it into intimate contact with the pin cladding. The geometry of the pin and wire coil will cause practically simultaneous contact along the entire length of the pin, as shown in Fig.

1.

4. Wire 5 is then attached to the opposite end of fuel pin 2.

A significant feature of this invention is that the described method can be used to wrap pins of any diameter and at any helical wire pitch without extensive modifications to the wire wrapping machine (within obvious machine dimensional limitations). This would be accomplished merely by varying the diameter and pitch of the prescribed wire. Minor adjustments to the location of the welds at one or both ends might be required to accommodate different pin diameters or wire pitches. This process is amenable to automated remote operation, reducing the necessity for handling of pins by operating personnel and thus reducing the potential for radiation exposure problems.

This invention was conceived during performance of a contract with the United States Government designated DE-AC14-76FF021 70.

Claims (2)

1. A method of mounting a wrapping wire (5) around a nuclear reactor fuel pin (2) wherein the wire (5) is preformed into the shape of a coil (7) characterized in that the fuel pin (2) is placed inside the wire coil (7), said wire coil (7) is axially expanded thereby causing radial contraction of said coil (7) until it is in contact with said fuel pin (2) substantially along the contiguous lengths of said wire (5) and said pin (2) and the ends of said wire (5) are attached to the opposite ends of said fuel pin (2).

2. A method as claimed in claim 1, characterized in that said wire (5) is welded to said fuel pin (2).