Patent Number: 048715093
Section: summary

BACKGROUND OF THE INVENTION This invention relates to fuel rods. More particularly, a spring clamp for maintaining fuel pellets compressed within a fuel rod during both manufacture and shipment is disclosed. SUMMARY OF THE PRIOR ART Fuel rods having fuel pellets contained therein are basic elements in a nuclear reactor. Typically, the so-called "fuel rod" includes a Zircaloy cylinder loaded with cylindrical uranium or plutonium fuel pellets. The cylindrical fuel pellets have an outside cylindrical diameter less than the inside diameter of the cylindrical and hollow Zircaloy rod into which they are placed. A fuel rod is typically on the order of 165-inches long and, dependent upon the designed fuel load, has between 140 and 150-inches of its total length occupied by the fuel pellets. The fuel rod is typically plugged and sealed at both ends. Usually, one end of the rod is plugged first. Thereafter, the fuel pellets are installed. A retainer spring is then inserted to bias the fuel pellets to the plugged end of the fuel rod. A cylindrical canister filled with Zirconium alloy chips and having holes in its ends is also installed. The Zirconium alloy chips absorb any hydrogen which may be present in the fuel rod. If free hydrogen were present it would be absorbed by the fuel rod tubing and would embrittle the tubing. The canister and the Zirconium alloy chips are referred to as a "getter". Next, the second plug is held in place and welded to the fuel rod tube. This second plug compresses the spring, so that an axial force is required to hold it in place during the welding operation. Next the gases present in the fuel rod are pumped out through a small hole and the fuel rod is back filled with helium. Helium pressure is typically between one and ten atmospheres (14.7 to 147 pounds per square inch). The fuel rod is then sealed. The sealed fuel rod contains the fuel pellet column, the retainer spring, and the getter. The remaining volume is initially occupied by the helium fill gas. During reactor operation, inert gases xenon and krykton are produced as a byproduct of the nuclear fission process. These gases must also be contained in the volume initially occupied by the helium fill gas. As the inert gases accumulate, the pressure rises. Typically, end of life pressures are in the range of 1,000 to 3,000 pound per square inch. To minimize this pressure, it is desirable to minimize the volume occupied by the retainer spring. In anticipation of the pressure ultimately to be encountered, the fuel rods are dimensioned and plugged at both ends. The rods are given a cylindrical side wall dimension that will not burst or crack under pressure of the accumulated gas. Moreover, Zircaloy plugs are welded at each end of the rods. These plugs are welded in gas tight relationship to prevent gas escape. In short, a fuel rod nearing the end of its life in a nuclear reactor is, among other things, a pressure vessel constructed to contain gases under high pressure and temperature. When the fuel rods are initially fabricated they are loaded with their pellets while horizontally disposed. Further, when the loaded rods are moved, they are also horizontally disposed. The rods on their side are moved from place to place while the steps required for the ultimate fabrication are completed. During such horizontally disposed movement, there is always a danger that the relatively heavy pellets will move from a compacted position (to and towards the bottom of the fuel rod) into a non-compacted position occupying the gas space at the top of the rod. During shipment from the site of fabrication to the nuclear reactor where the rods are ultimately placed in a fuel bundle and utilized, the rods are typically placed on their side. If the fuel pellets are not restrained, movement of the pellets from a compacted disposition into the gas space can take place. When the rods are installed within a reactor, they are disposed vertically. If the fuel column has gaps, these gaps will not close, because of pellet wedging and friction. The presence of axial gaps would lead to undesirable consequences. Specifically, reactors operate under pressure in the range of 1,000 psi. It will be remembered that initially the pressure on the inside of the rods was in the order of 200 psi. If a pellet or pellets are out of their required stacked and end to end relationship when a fuel rod is installed, the rod in the area of pellet separation can neck down under the pressure of the reactor and hold the pellet out of place. Such a phenomenon can occur where one pellet when moved out of place becomes canted at an angle. The pellet can then wedge itself to the inside diameter of the rod and remain spatially separated from its adjacent pellets. In such a case, the unoccupied volume on the inside of the fuel rod can neck down initially under the pressure of the reactor. Two consequences follow from a rod with fuel pellets out of place. First, the axial power distribution is altered to give high local powers near the axial gap in the fuel column. The high local power cause local overheating of the fuel rod. Second, overstressing or cracking of the fuel rod can occur. In view of these difficulties, the prior art has included a spring in the fuel assemblies. The spring bears against the welded end plug at one end of the rod. The spring also bears against the adjacent fuel pellet at the end of the fuel rod. Typically, the spring is configured to either contain or bear against the getter. In any event, the spring exerts a force tending to keep the fuel pellets compacted towards one end of the fuel rod after the ends of the rod are sealed. Statement of the Problem This type of spring assembly--after having been closely analyzed in an effort to achieve this improved design--has been noted to include at least five problems. First, the spring extends from the last fuel pellet to the end plug of the fuel rod. This occupancy of the full length--in the order of 12-inches--adds to the volume of the spring and reduces the volume available for the fuel column and for the gases produced by nuclear fission events in the fuel pellets. Second, the spring is a neutron absorber. Neutrons--which might otherwise be used in the desired chain reaction--are absorbed and lost when they come into contact with the excess mass of material of the spring. Third, the full volume between the end plug and the fuel pellets varies in length. Consequently, the length configuration of the springs used for various lengths of pellet loading must vary. In short, different fuel designs require different springs--a complicating factor in the production of fuel rods. Fourth, the spring installation is one of the last steps in the fuel rod fabrication. During earlier fabrication steps, the spring is not present, and there is a possibility of opening gaps between pellets in the fuel column. Finally, the end of the spring adjacent the end plug bears against the end plug while welding occurs. An axial force is required to hold the end plug in position. Further, this portion of the spring is in the area highly heated by the weld. The heat of the weld can change the spring characteristics. It will be understood that recognition of a problem can constitute invention. Insofar as this recognition constitutes invention invention is claimed. SUMMARY OF THE INVENTION A spring retainer is disclosed for use in retaining fuel pellets in a fuel rod both during fabrication and shipment to prevent the fuel pellets from being moved from their design location before installation within a reactor. The cylindrical and solid nuclear pellets containing the reactor fuel are placed within the fuel rods and have an outside diameter sightly less than the inside diameter of the fuel rod. Once the pellets are in place, a two-part spring holder is inserted into the end of the fuel rod. A first compression spring part of the coil spring holder is a conventional coil spring which, acting in compression, bears against the fuel pellets with a preselected force typically forcing the pellets when in the horizontal position into a compacted disposition when the fuel rod is horizontal. This conventional coil spring has a diameter which is less than the inside diameter of the fuel rod. A second locking spring part of the coil spring holder is a coil spring having a diameter which exceeds the inside diameter of the fuel rod. This helical locking spring is spirally wound down to an outside diameter less than that of the inside diameter of the fuel rod for insertion into the rod, and then released to key to the inside diameter of the rod. Winding occurs through a special tool. By winding one end of the locking coil spring relative to the other end of the locking coil spring, spiral winding of the helical spring to a diameter less than the inside diameter of the rod occurs. Installation includes winding the locking spring, inserting the wound locking spring, compressing the installed fuel pellets with the compression spring, and releasing the spirally wound helical locking spring to unwind the locking spring so that it keys to the inside walls of the fuel rods. The disclosed design accommodates conventional fuel getters and is shown with a preferred one piece construction where the compression and locking spring sections are fabricated from the same single piece of wire. Other Objects, Features and Advantages An object of this invention is to disclose a simplified two-part spring clamp for retaining fuel pellets under compression within a fuel rod. A spring having two sets of helical coils is disclosed. The first set of coils is a compression spring bearing against the fuel pellets. The second set of coils is a locking spring having a diameter which exceeds the inside diameter of the fuel rod. This helical locking spring is spirally wound down to an outside diameter less than the inside diameter of the fuel rod. As wound, it is inserted with compression spring disposed to and toward the fuel pellets. The fuel pellets are compressed by the compression spring. When the fuel pellets come under the desired compression, the locking spring is released, and keys to the inside rod walls with securing of the fuel pellets in place under compression occurring. An advantage of the disclosed two-part spring clamp is that it occupies the minimal volume within the fuel rod. No longer need the spring extend between the last pellet and the end plug. Instead the spring can occupy a relatively short distance on the order of 2 to 4 inches. Second, the new spring introduces a minimum amount of metal into the fuel rod. Not only is the spring clamp less expensive than those contacting the end plug but additionally a smaller volume of metal is present for the undesired adsorption of neutrons. Third, the new spring accommodates variation in the total length of pellets used within the fuel rod. One spring design can be used for all loads of fuel pellets--short or long. Fourth, the new spring is inserted immediately after insertion of the fuel pellets. Then the pellets are restrained during subsequent fabrication steps. Finally, the clamp keying to the side walls of the fuel cladding or rod, no longer bears against the end plug. As no force is now exerted against the end plug, it is no longer necessary to overcome a force during welding to permit sealing of the end plug. Further, the clamp keys to the side walls of the fuel rod remote from the site of the welding. This being the case, the heat of the weld does not adversely affect the metallic properties of the clamp. A further object to this invention is to disclose a process and apparatus for the installation of the clamp. According to this aspect of the invention, at least the second part of the coil spring which has a diameter which exceeds the inside diameter of the fuel rod, is wound to a smaller diameter while the spring is inserted into the fuel rod. This helical spring is spirally wound down to an outside diameter less than the inside diameter of the fuel rod by a tool exerting a spirally winding torsional force. Thereafter, the wound spring and its depending conventional compression coil spring member are inserted into the rod. Forced insertion is continued to achieve the designed compressive force on the fuel pellets--a force in the order of 7 pounds. Once this force is achieved, the wound spring is released. The released spring keys to the inside diameter of the cylindrical cladding walls. Securing of the pellets occurs. An advantage of the disclosed process is that it is simple and remote. It is easily done by those having minimum skill. A further advantage of the process is that the force of spring compression on the pellets is readily measured. Thus, precise precompression of the pellets can occur when the clamp keys to the inside diameter of the fuel rod before the rod is sealed. Yet another advantage of the process is that the spring, once in place, secures the pellets. This securing occurs long before the ends of the rod are sealed. Not only is there an absence of a force tending to push on the end plug as it is welded, but from the moment of time that the spring clamp is placed, the pellets are under the desired compressive load.