IL30949A - Fabrication of nuclear fuel assemblies and resultant product - Google Patents

Description

Vapnori isiarn ,»3*Ϊ?ΤΑ f»>t ηιτ»η» Fabrication of nuclear fuel assemblies? and resultant prpduot TOOTaHOTJSE ELECTRIC CORPORATION C: 29158 This Invention relates to nuclear fuel assemblies and it has particular relation to apparatus and processes for fabricating nuclear fuel assemblies which are pressurized.

In nuclear reaotdrs it is the practice to employ a plurality of nuclear fuel assemblies each comprising a(tube containing nuclear fuel, the ends of the tube being seied, by plugs. Due to the pressure of the environment to which ihe assembly is located it io subject to high creep stresses and even to collapse* Heretofore it has been proposed that a fuel assembly be evacuated to a pressure of the order of 0.1 torr and that helium be admitted at a low pressure thereafter to the fuel assembly. While it was in the low pressure helium environment the fuel assembly was then sealed. The purpose of the helium wae to increase the heat transfer between the nuclear fuel and the exterior of the fuel container and to enable any leak through the container to be deteoted by spectrometer inspection. The final sealing was effected by an arc welding step which produced a circular weld uniting the periphery of a plug to the associated tube.

It is the principal object of the invention to provide a nuclear fuel assembly having a high internal gas pressure and a simple method for filling a fuel assembly with gas* According to the present invention there is provided a method of constructing a sealed nuclear fuel element from a tube having a plugged end, and a small cross-section axial opening formed in the oute end thereof, characterized by the steps: farming a fusible collar around the outer end of said opening, evacuating the tube and again pressurizing it with inert gas, disposing nuclear fuel within the tube, positioning an electrode adjacent said opening of the 30949/2 - the collar to fuse the collar sufficiently to bridge and close the opening* and terminating the arc discharge to permit sealing solidification ot the material closing said opening, The invention will become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only, in the accompanying drawings in which: Figure 1 Is a schematic view with parts shown in elevation and with parts broken away of a nuclear fuel assembly and apparatus for pressurizing such assembly; and Figure 2 is a detail view in elevation with parts broken away showing one end of a nuclear fuel assembly and an associated electrode as employed in the system of Figure 1.

Referring to the drawing Figure 1 shows a nuclear fuel assembly 1 comprising an elongated thin-walled tube 3 which contains nuclear fuel, such as fuel pellets of uranium oxide enriched with U-235. The tube 3 has a plug 5 sealing its lower end and a plu 7 located adjacent its upper end. The plug 5 may be secured to the tube 3 by means of a circular weld in a manner well understood in the art. The plug 7 is welded similarly to the tube 3· The tube and plugs may be constructed of a zirconium-containing alloy. Such an alloy may contain by weight 1.2 to 1.7$ tin, 0.07 to 0.20 iron, 0.05 to 0.15 chromium, 0.03 to 0.08 nickel and 97.87 to 98 zirconium. A specific alloy of this type may contain 1.5 tin, 0.12 iron, 0.10 chromium, 0.05 nickel with the balance being zirconium.

A suitable zirconium alloy containing a low nickel content may contain 1.2 to 1.7 tin, 0.l8 to 0.24 iron, 0.07 to 0.13 chromium, 0.007 nickel .with the balance being zirconium.

Stainless steel may also be employed for the tube and plugs.

In accordance with the invention the tube 3 and the plugs 5 and 7 are sealed while containing gas under a high pressure. Pressures in the range of 500 pounds per square inch to 1900 pounds per square inch (gauge) have been employed with good results. Such pressurizing is particularly suitable for fuel assemblies to be employed in pressurized water reactors.

Under the conditions of use the contents of the tube 3 tend to expand. Preferably an initial pressure is selected such that at the end of the life of the nuclear fue.-l assembly the internal pressure does not exceed 2000 to 2200 pounds per square inch.

As shown in Figure 2 the plug 7 is provided with an axial small cross-section bore 9 which establishes communication between the exterior and interior of the fuel assembly. The bore may have a small diameter such as 0.04 inch, and the end of the plug 7 is provided with a circular groove 11 concentric with the bore 9. This groove may have a depth of the order of 0.025 to 0.03 inches and a width of the order of 0.020 to O.03O inches. The groove defines a collar 13 which In order to pressurize the fuel assembly the assembly is placed in a pressurizing enclosure or chamber which includes a steel sleeve 15. The sleeve at. its lower end is welded in sealed relationship to a steel disc 17. At its upper end the sleeve 15 is welded around the inner opening of a steel flange 19. As an additional safety measure an outer steel sleeve 21 may surround the sleeve 15 and may be welded to the disc 17 and the flange 19.

The enclosure is completed by a steel ring 23 which is bolted to the flange 19 with a suitable seal 25 located therebetween. The seal 25 may take the form of a highly-polished stainless-steel ring. The ring 23 has its center closed by a steel cup 27.

The enclosure is suitably connected to an inert-gas supply 29 and a vacuum pump 31. In the specific embodiment of Figure 13 a nipple 33 located in an opening provided in the cup 27 is connected by piping through a valve 35 to the Inert-gas supply 29. A gauge 37 is provided for indicating the pressure within the enclosure.

In a similar manner the cup 27 is connected through a valve 39 to the vacuum pump 31.

The fuel assembly 1 is positioned concentrically within the sleeve 15 by means of suitable insulators 41. These insulators may be constructed of any suitable material such as porcelain or a fluorocarbon resin which is available under the trademark "Teflon". The plug 5 is releasably positioned in an electrical socket 3 which is secured to the disc 17 and which is connected through a suitable conductor to the positive terminal of a source of direct current 5.

An electrode 47 extends through an insulating bushing electrode 47 is connected to the negative terminal of the source 5. In order to assure retention of the electrode in position under the conditions of high pressure to which it is subjected a stainless steel collar 1 is welded to the electrode. In service the collar 51 is located adjacent the lower end of the bushing 49.

It will be noted that the electrode 47 is aligned with the fuel assembly 1. The electrode may be constructed of a thoriated tungsten electrode having a diameter of 0.0β2 inch and may contain for example two per cent by weight of thorium. The electrode is located adjacent the collar 13 to establish an arc gap between the electrode and the collar. As an example the electrode may have a tapered end positioned above the collar by a distance of the order of 10 mils.

The pressurizing gas should provide a good atmosphere for welding. Argon and helium are examples of suitable gases and helium is preferred. Helium has the additional advantage that it may be detected in a known manner by spectrometer inspection for the purpose of detecting leaks in the fuel assembly.

A procedure for pressurizing a nuclear fuel assembly now may be described. A loaded fuel assembly is placed within the sleeve 15 with the plug 5 inserted in the electric socket 17. The ring 23 then is bolted to the flange 19 for the purpose of compressing the seal 25 and establishing a completely sealed enclosure for the fuel assembly. If the electrode 47 is threaded through the bushing 49 or is otherwise adjustable it is then adjusted to establish a suitable arc gap between its tip and the adjacent collar 13. It will be assumed for present purposes that the electrode 7 is preset in the bushing pump 31 Is operated to evacuate the enclosure to a pressure which may be of the order of 0.1 torr. The valve 39 is now closed and the valve 35 is opened to supply helium under pressure to the enclosure. The pressure is built up to the desired value which for example may be of the order of 1000 pounds per square inch. At this stage the valve 35 i closed.

An arc next is formed between the electrode k and the collar 13 » The arc may be stuck or started in a conventional manner as by the application of a high frequency alternating current from the energy source 5 across the arc gap. Thereafter the direct current supplied by the energy source maintains the arc and heats the collar 13 to a temperature sufficient to fuse or melt the material of the collar. The melted metal of the collar bridges and seals the adjacent end of the bore 9° The current may be supplied to the arc for. a predeter-minded time sufficient to assure completion of the sealing opera tion. As an example a current of the order of 60 amperes for one second has been found sufficient to provide a good seal. The completed fuel assembly now may be removed from the enclosure.

Claims (8)

1. A method of constructing a sealed nuclear fuel element from a tube having a plugged end, and small cross-section axial opening formed in the other end thereof, characterized by the steps: forming a fusible collar around the outer.end of said opening, evacuating the tube and again pressuri,&ng it wit inert gas, disposing nuclear fuel within the tube, positioning an electrode adjacent said opening of the tube spaced from said outer end to establish an arc gap to the collar, establishing an arc discharge between the electrode and the collar to fuse the collar sufficiently to bridge and close the opening, and terminating the arc discharge to permit sealing solidification of the material closing said opening.

2. A method as claimed in Claim 1, characterized in that the gas is an inert-gas maintained at a pressure in excess of 500 pounds per square inch.

3. A method as claimed in Claim 1 or 2, characterized in that said element is placed in a gas-tight housing and helium gas unde pressure is introduced into said housing*

4. pressurizing unit for pressurizing a nuclear fuel element,, said pressurizing unit comprising a cylindrical housing having closed end portions, characterized by an electrode passing through one of said end portions concentrically with the axis of said housing and insulated from the housing, and valve^controlled gas supply means for supplying gas under pressure to said cylindrical housings said one of said end portions forming a lid to permit insertion and removal of a fuel element relative to the housing, said electrode being 30949/2 — 8 disposed opposite an opening formed i said housing thereby to establish when energized an arc between said electrode and the area of said fuel element around said opening whereby the material around said opening is melted to close said opening. -

5. A pressurizing unit as claimed in Claim 4 characterized in that said electrode has a collar secured thereto for restraining movement of the electrode out of said enclosure.

6. A pressurizing unit as claimed In Claim 4 or 5» characterized in that a second housing surrounds the first* named housing*

7. A method of constructing a sealed nuclear fuel.l element substantially as hereinbefore described with referenoe to, and as shown i t the accompanying drawings.

8.. ' A pressurizing unit for pressurizing a nuclear fuel element substantially as hereinbefore described with referenoe to, and as shown in, the accompanying drawings. IS:CB