Patent Number: 050323512
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear fuel bundles and more particularly to spacers used with the nuclear fuel bundles. 2. Brief Description of the Relevant Art Nuclear reactors have fuel bundle arrays constructed of individual fuel bundles. These individual fuel bundles have a plurality of side-by-side fuel rods. The individual fuel bundles typically include a lower fuel rod supporting tie plate, an upper tie plate for maintaining the fuel rods in upstanding relation, and a fuel bundle channel for enclosing the fuel rods between the tie plates. Water flows into the fuel bundle through the lower tie plate, is confined to a flow path immediately about the fuel rods by the bundle enclosing channel, and flows out the top of the bundle in a steam water mixture. In the assembly, transport, and operation of the fuel bundle, there is a tendency of the individual fuel rods to move from their vertical side-by-side disposition along the 160" length fuel bundle. When the fuel bundles are installed and operating within a boiling water nuclear reactor, such movement causes unwanted vibrations, interferes with the designed nuclear efficiency and is otherwise generally undesirable. In order to prevent such movement of the fuel rods, five to eight spacers are utilized at selected elevations along the length of the fuel bundle. It is the function of such spacers to confine the fuel rods with respect to the channel as well as to maintain the designed side-by-side spacing of the fuel rods along the total length of the fuel bundle. There have been developed over the years many kinds of spacers. While these spacers are desirable for controlling flow along the fuel rods, the spacers constitute neutron absorbing materials. This absorption of neutrons is detrimental to the nuclear economy and efficiency of the fuel within the fuel bundle. Consequently, it is always desired to maintain the least amount of neutron absorbing material in the spacers and yet have the spacers still function to perform their desired fuel rod separation function. Spacers of the prior art have commonly been constructed of two materials. One material is the metal Inconel.RTM., a registered trademark of the International Nickel Company. The metal is an alloy of iron, chromium and nickel. This metal is strong, elastic and forms an ideal spring material. Unfortunately the metal has a relatively high neutron absorbing cross section. The remaining material from which spacers are constructed is Zircaloy. Zircaloy is an alloy of zirconium with small amounts of iron, tin and other alloy metals. This metal has lower neutron absorbing cross sections but does not have the same strength of material properties and is unsuitable as a spring material. Consequently, most Zircaloy spacers have Inconel.RTM. springs incorporated to their structure. The present invention is an improvement to a prior art spacer sometimes known as a "cross point spacer." This spacer has the majority of its structure fabricated from Zircaloy with Inconel.RTM. springs for the biasing of the fuel rods to the desired positions within the matrix of the spacer. The cross point spacers are formed out of a lattice of vertically aligned tube members interconnected by strips. The tubes and strips fastened together form a continuous grid. The tube members typically form stops against which the fuel rods can be biased. This grid has individual spring members fastened to the tube interconnecting strips. The springs bias the fuel rods extending through the spacer onto tube stops formed integrally from the cylinders. The combination of the grid with its matrix of tube stops and strip fastened springs assures the uniform spacing of the fuel rods at selected elevations within a fuel bundle. So-called cross point spacers have been proposed in a variety of configurations. In one configuration, the spatial relation or pitch between all of the fuel rods within the fuel bundle is maintained uniform. In another configuration, spacing or pitch between the individual fuel rods is varied. In either case, the improved method of spacer construction set forth herein is applicable. It is also known to construct spacers with so-called swirl vanes incorporated to the spacers. In such construction, twisted pieces of material-twisted in the order of at least 90.degree., and more frequently in the order of 180.degree. to 360.degree., function to partially separate water and steam passing through the spacer. Specifically, water and steam rising through the swirl vane portion of the spacer have rotating momentum imparted. Water is thrown outwardly and onto the fuel rods where the more efficient generation of steam can occur. Steam within the rising steam water mixture maintains a more central flow path. For an example of a spacer incorporating such so-called swirl vanes, attention is invited to U.S. patent application Ser. No. 323,075, filed Mar. 14, 1989 entitled Swirl Vanes Integral with Spacer Grid, now U.S. Pat. No. 4,913,875, issued 4/3/90. SUMMARY OF THE INVENTION An improved spacer and method of making a spacer is disclosed for use in a nuclear fuel bundle wherein a plurality of fuel rods enclosed within a channel are maintained in parallel side-by-side relation by a plurality of the spacers. Each spacer is placed within the fuel bundle at selected elevations between upper and lower tie plates. The improved spacer is a member of the class of spacers wherein solid strips of material are welded at interstitially placed tube members between the fuel rods to form the continuous spacer grid. The improvement constitutes forming separate upper and lower reduced section grids from separate, normally aligned, first and second parallel sets of grid members. One grid is formed for the top of the spacer; the remaining grid is formed for the bottom of the spacer. Tube members placed interstitially between the fuel rods are used to interconnect the grids. The tube members themselves are in turn notched; the notches are at the upper portion of the tube members to receive the upper grid and at the lower portion of the tube members to receive the lower grid. Grids are placed within the notched tube members and fastened, typically by welding to the top and bottom of the tubes to form a unitary spacer structure. Thereafter, the excess material of the grid crossing the interior of the tube members is drilled out of the tube members to eliminate excess neutron absorbing material, and to reduce the hydraulic resistance of the spacer. There results in the disclosed spacer, two interconnected grid members extending at the top and bottom of the spacer having less material than the single and continuous grid of the prior art. At the same time, the assembly of what is otherwise a difficult member to construct is simplified. Provision is made for applying the improved spacer to fuel bundles having differing pitch between the separated fuel rods. Additionally the incorporation of so-called swirl vanes in some grid locations in substitution for the tube members is disclosed. OTHER OBJECTS, FEATURES AND ADVANTAGES An object to this invention is to disclose a simplified technique for the fabrication of a cross point spacer for use in a nuclear fuel bundle. According to this aspect of the invention, paired upper and lower grids are constructed from normally aligned parallel arrays of confronting strips of metal. These upper and lower grids are fastened to tube members, the tube members functioning to form the point of interconnection between the upper and lower grids. Each tube member is notched at the upper and lower ends so as to receive the grids. Thereafter, the grids are fastened to the notches of the tube members. Once attachment of the upper and lower grid members has occurred, the interior of the respective tube members is drilled out and excess neutron absorbing material removed. An advantage of the disclosed spacer construction is that less Zircaloy material is used for the spacer. The spacer is not constructed of a solid grid extending the full depth between the tube members forming the vertical interconnecting members of the spacer. Instead, the upper and lower grid members extend only partially the thickness of the spacer. In the medial region of the disclosed spacer construction, the spacer is open. As a result, less neutron absorbing material is present. A further object to this invention is to utilize the disclosure construction to form a variable pitch spacer of the type known in the prior art. According to this aspect of the invention, the upper and lower grids are formed with varied pitch. In the preferred variation of pitch here used within a 9.times.9 matrix of fuel rods, eight subgroups of nine fuel rods each are clustered about a central water rod. The individual rods of the subgroup have a first relatively close spacing. The spacing between the subgroups is at a second and relatively more distant dimension. The resultant spacer thus conforms to fuel designs having variable pitch between the spaced fuel rods. An additional object to this invention is to incorporate swirl vanes into the constructed spacer. According to this aspect of the invention, certain of the tube members are replaced with twisted strips of metal. These respective twisted strips of metal cooperate at their upper and lower portions to become integral with the disclosed grid construction.