Patent Number: 051732521
Section: summary

BACKGROUND OF THE INVENTION This invention relates to spacers for use in nuclear fuel bundles, the spacers maintaining individual fuel rods or tubes containing fissionable materials in their designed spaced apart relation. More particularly, a spacer is disclosed which has a spring that can be placed within and removed from the previously assembled spacer. Further, provision is made for the insertion of fuel rods through the spacers of a fuel bundle with the spring of the spacer in the compressed, partially inserted state for avoiding scratching of inserted fuel rods on fuel bundle assembly. SUMMARY OF THE PRIOR ART Modern boiling water nuclear reactors include a core composed of many discrete fuel bundles. Water circulates from the bottom of each fuel bundle, is heated in passing upward through each fuel bundle, and passes out the top of each fuel bundle in the form of heated water and steam. The fuel bundles are composed of discrete groups of fuel rods--sealed tubes which contain nuclear fuel. Typically, the fuel rods are supported upon a lower tie plate and held in side-by-side vertical relation by an upper tie plate. Water flow is confined within a fuel bundle channel extending from the lower tie plate to the upper tie plate. In addition to supporting the fuel rods, the lower tie plate admits water into the interior of the fuel bundle. The upper tie plate--in addition to maintaining the fuel rods upright--permits the heated water and generated steam to exit the fuel bundle. The fuel bundles are typically about 160 inches in length. Consequently, the individual fuel rods within the fuel bundles are flexible along the length of the fuel bundle. If unsupported, the individual fuel rods could easily move out of their intended side-by-side spacing responsive to the forces of flow induced vibration and metallic creep. The reader will understand that metallic creep is a well known phenomenon resulting from both pressure and radiation within the reactor. Preservation of the intended side-by-side spacing of fuel rods within a fuel bundle is important. Specifically, if the fuel rods are not maintained within their desired side-by-side spacing, the designed nuclear reaction and concurrent heat generation with steam production does not occur efficiently. To maintain the required spacing between the individual fuel rods and to prevent unwanted vibration, it has long been the practice of the nuclear industry to incorporate spacers along the length of the fuel bundles. Typically, anywhere from five to ten spacers--usually seven--are placed within the each fuel bundle. The spacers are preferably placed at varying elevations along the length of the fuel bundle to brace the contained fuel rods in their designed location. Design considerations of such fuel rod spacers are well known. They include retention of rod-to-rod spacing; retention of fuel assembly shape; allowance for fuel rod thermal expansion; restriction of fuel rod vibration; ease of fuel bundle assembly; minimization of contact areas between the spacer and fuel rods; maintenance of structural integrity of the spacer under normal and abnormal (such as seismic) loads; minimization of reactor coolant flow distortion and restriction; maximization of thermal limits; minimization of parasitic neutron absorption; and minimization of manufacturing costs including adaption to automated production. Spacer construction is easily understood. Each spacer has the task of maintaining the precise designed spacing of the particular matrix of fuel rods at the spacer's elevation within a fuel bundle. It has been a common practice to provide each spacer with a matrix of ferrules for surrounding each fuel rod of the matrix of fuel rods. Each ferrule is provided with at least one stop. The fuel rods when biased into the stop(s) of their ferrules have their precise designed side-by-side spacing preserved. The necessary biasing of the fuel rods within the spacers has been accomplished by individual springs. In the prior art it has been a common practice to have two side-by-side ferrules share the same spring at a common aperture defined between the ferrules. Typically the shared spring is of the loop configuration having two spring legs joined together at the top and at the bottom to form a continuous and elongated loop spring. One spring leg protrudes through the common aperture into a first ferrule of a ferrule pair and biases the fuel rod in the ferrule against the stops of the first ferrule of the ferrule pair. The other spring leg protrudes through the common aperture into the other ferrule of the ferrule pair and biases the other fuel rod in the second ferrule against the stops of the second ferrule of the ferrule pair. Maintaining the loop springs of the prior art within the side-by-side ferrule pairs has been difficult. The common aperture between adjacent ferrules has been defined by configuring an aperture in each ferrule and confronting the ferrules at these defined apertures. The confronted apertures define the common aperture. These confronted apertures have been configured with irregular shapes having protruding internal surfaces--for example apertures of the "E" variety have been used. By the expedient of either overlapping or confronting protruding portions of the confronted apertures between the loops of the prior art springs, capture of the springs into the common aperture between the spacers has resulted. With the loop springs confined in the common aperture between the metal walls of a ferrule pair, the required spring biasing in two ferrules requires only a single confined loop spring. Unfortunately, modern fuel bundle design has complicated the design of spacers and spacer springs. Fuel bundles are now more densely packed with smaller diameter fuel rods. As a consequence, the space available for both spring movement and capture of the spring to the spacer is vastly reduced. As fuel bundles become more dense, the number of springs required across a spacer has increased. Unfortunately, the required movement of the springs in either maintaining the fuel rods in alignment or permitting assembly of the fuel bundle in the first instance has remained unchanged. The practical effect of having denser fuel bundles is the need to redesign the springs within fuel bundle spacers. The assembly of fuel bundles has further complicated this problem. Specifically, the biasing springs of individual spacers have a tendency to scratch fuel rods when fuel rods are inserted into the spacers. These scratches can be the location where corrosion of the fuel rods starts during their in-service life. This being the case, it has been desirable to encase fuel rods in protective plastic sheaths during their insertion into the spacers. Once insertion is complete, the plastic sheaths are removed. The use of the plastic sheaths can prevent scratches. Unfortunately, the same plastic sheaths require additional spring flexure during fuel bundle assembly. This additional flexure is necessary to permit the plastic protective coating to be temporarily inserted along with the fuel rods into the fuel bundle. In some fuel bundles requiring the use of these plastic sheaths, the existing spring flexure is not within design tolerance when two plastic covered fuel rods are placed simultaneously within the ferrules of a ferrule pair. As a consequence, construction of some fuel bundles requires a complex procedure for inserting the fuel rod. Given a ferrule pair and spring, a first fuel rod with a plastic sheath is inserted into one ferrule of the pair, and the sheath is removed. Then a second fuel rod with a plastic sheath is inserted into the remaining ferrule and its sheath is removed. This procedure is required because the prior art springs cannot deflect far enough to accommodate both fuel rods and both plastic sheaths simultaneously. If this alternating insertion procedure must be followed over a 9 by 9, 10 by 10, 11 by 11 or 12 by 12 matrix in a carefully controlled sequence, it can be understood that a spring design which permits the elimination of these plastic sheaths without increasing the risk of undesirable scratches is desirable. The springs of the prior art are also difficult to replace if they become damaged. Replacing known springs requires that the ferrule pair be cut apart, along with the damaged spring. Then a new ferrule pair with a spring must be inserted and the ferrules rewelded. A spring design which could be more easily replaced would be a substantial improvement. Finally, those familiar with mechanical design and mechanical design tolerances will realize that exact dimensions and perfect alignment are never achieved. Instead, a tolerance range is specified. The cost of manufacture increases as the tolerance range is narrowed. In prior art spacers, and to a greater degree in new designs, a very tight tolerance range is required for the springs and ferrules. If a spring can be designed with greater flexibility, and a mounting method which allows more spring deflection, the tolerances can be less restrictive. Because of at least the given design considerations, designing springs having improved flexibility for use in spacers has become a high priority. A standard method for providing increased flexibility is to vary the width of the spring, using a lesser width in regions of low stress. Unfortunately, the width of the current loop spring is not easily varied. The loop spring starts out as a continuous circular loop of constant width and is then bent into its final shape. The circular loop, or the final spring could be machined to a varying width, but the cost would be high. COPENDING PATENT APPLICATION Not Prior Art In my copending patent application Ser. No. 07/623,828 filed Dec. 6, 1990 entitled Self Locating Springs for Ferrule Spacer, now U.S. Pat. No. 5,078,961 issued Jan. 7, 1992, I set forth an improved spring construction which is self centering with respect to confronted ferrules. In this invention, a ferrule spacer is disclosed with each of the discrete ferrules surrounding a fuel rod within the fuel rod matrix. Ferrule pairs are used for capturing a spring between the ferrules. Each ferrule defines an aperture for confrontation with the corresponding aperture in the adjacent ferrule of the ferrule pair. The ferrules at their respective ferrules define two types of apertures. A common aperture opening to the center of each of the ferrules is defined for the capture of the spring between the ferrules. Paired side apertures opening to the outside of each of the ferrules are defined. It is into these paired side areas that portions of the springs protrude to cause the self centering feature of this disclosure. Loop springs are used in this disclosure. One portion of the loop protrudes into one ferrule for biasing one fuel rod passing through that ferrule. Another portion of the loop protrudes into the other ferrule for biasing another fuel rod passing though that ferrule. The loops springs have tabs. These tabs protrude out of the paired side apertures. These tabs in combination with the side apertures cause the centering feature of the springs. SUMMARY OF THE INVENTION In the present disclosure, the prior art practice of having two side-by-side ferrules share the same biasing spring for two adjacent fuel rods is followed. Consequently, paired ferrules are each provided with apertures for capturing a single spring between the ferrules. The springs are provided with a continuously looping main body having protruding tabs on opposite sides of the springs. The paired ferrules are confronted at their respective apertures for the capture of the springs at their main body and to provide defined side apertures between the confronted apertures for permitting protrusion of spring tabs for holding and centering the springs within the confronted apertures. Modification of the confronted apertures occurs to permit insertion of the spring in a compressed disposition from the side of the ferrules. Once the spring is inserted fully between the ferrules it expands. And once the spring expands, it is captured. As a consequence, spacer construction can be substantially completed prior to the insertion of the springs. In the construction of the disclosed spacer, at least two ferrules are welded together to form the confronted apertures and side apertures between the ferrule pair. After the ferrules have been welded together, a spring is compressed with a tool, such as a needle-nosed pliers, inserted vertically into the interstitial space formed by the two ferrules, and then moved horizontally into the central aperture between two ferrules. After the spring is fully inserted it springs open and is captured in the slots between the two ferrules. The spring herein consists of two identical halves which are welded together. The manufacture of the spring begins with flat strip material. A punching operation provides the variation in width required for optimum spring design and provides the locating tabs. The tab portions are included at either end of the spring, are used for entrapping the springs within their respective ferrule apertures, and are incorporated into the spring legs to produce a spring having longer spring legs with a resulting lesser range of spring force over the designed range of spring deflection. OTHER OBJECTS, FEATURES AND ADVANTAGES An object of this invention is to mount a loop type spring between a ferrule pair in a spacer assembly without having the material of the ferrules intrude within the loop of the spring. According to this aspect of the invention, paired ferrules are provided with confronting apertures. These apertures when confronted provide two functions. First, they trap between the ferrules the main body of the loop type spring. Second, they provide confining slots defined between the respective ferrule pairs. To mate with these confining slots, tabs protrude from the main body of the loop springs on either side of the loop springs. The tabs extend from the trapped main body of the loop spring within the confronted apertures into the confining slots. As a result, the loop springs are held to the confronted apertures of the ferrule pair by the tabs. An advantage of the disclosed spring is that it is self centering with respect to the ferrule pair. Under the forces of compression exerted on the fuel rods, the spring seeks and maintains its designed position with respect to the ferrule pair. An additional advantage of the disclosed spring design is that the material of the ferrules is no longer required to penetrate in between the discrete legs of the loop springs. This being the case, the spring legs are permitted a relatively greater movement--this compression permitting movement of each leg toward the remaining leg until contact of one spring leg with its opposed spring leg occurs. No longer is spring leg movement limited by the structure of portions of the ferrules invading the interstitial space between the discrete spring legs of the loop spring. A further advantage of the disclosed spring and ferrule construction is that assembly of the spacers is simplified. In the past the loop springs have had to be individually threaded to portions of the ferrules and thereafter trapped in place by manipulation of the confronting ferrules. With the design here disclosed, simple trapping of the spring between confronted ferrules is all that is required. An additional advantage of the spring construction here disclosed is that the spring can be partially inserted into the ferrule apertures, allowing fuel rods to be inserted through the ferrules without contacting the springs. This eliminates the need to use plastic sheaths to protect fuel rods as they are inserted. An additional object of this invention is that the spring can be inserted into and removed from the ferrules after the ferrules and their bands are welded together to form the spacer unit. As no disassembly of the spacer unit is necessary to remove a defective or failed spring, repair and replacement of springs and/or fuel rods is greatly simplified.