Patent Number: 048184713
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. IN GENERAL Referring now the the drawings, and particularly to FIGS. 1 to 3, there is shown a nuclear fuel assembly, generally designated 10 for a boiling water nuclear power reactor (BWR), in which the improvement of the present invention is incorporated. The fuel assembly 10 includes an elongated outer tubular flow channel 12 that extends along substantially the entire length of the fuel assembly 10 and interconnects an upper support fixture or top nozzle 14 with a lower base or bottom nozzle 16. The bottom nozzle 16 which serves as an inlet for coolant flow into the outer channel 12 of the fuel assembly 10 includes a plurality of legs 18 for guiding the bottom nozzle 16 and the fuel assembly 10 into a reactor core support plate (not shown) or into fuel storage racks, for example in a spent fuel pool. The outer flow channel 12 (also see FIG. 4) generally of rectangular cross-section is made up of four interconnected vertical walls 20 each being displaced about ninety degrees one from the next. Formed in a spaced apart relationship in, and extending in a vertical row at a central location along, the inner surface of each wall 20 of the outer flow channel 12, is a plurality of structural ribs 22. The outer flow channel 12, and thus the ribs 22 formed therein, are preferably formed from a metal material, such as an alloy of zirconium, commonly referred to as Zircaloy. Above the upper ends of the structural ribs 22, a plurality of upwardly-extending attachment studs 24 fixed on the walls 20 of the outer flow channel 12 are used to interconnect the top nozzle 14 to the channel 12. For improving neutron moderation and economy, a hollow water cross, as seen in FIGS. 1, 2 and 4 and generally designated 26, extends axially through the outer channel 12 so as to provide an open inner channel 28 for subcooled moderator flow through the fuel assembly 10. The hollow water cross 26 has a plurality of four radial panels 30 which extend in a cruciform configuration to divide the fuel assembly 10 into four separate elongated compartments 32. The water cross 26 is mounted to the angularly-displaced walls 20 of the outer channel 12. Preferably, outer elongated lateral ends of the water cross panels 30 are connected such as by welding to the structural ribs 22 along the lengths thereof in order to securely retain the water cross 26 in its desired central position within the fuel assembly 10. Further, the inner ends of the panels 30 together with the outer ends thereof define the inner central cruciform channel 28 which extends the axial length of the hollow water cross 26. Also, the water cross 26 has a lower flow inlet end 34 and an opposite upper flow outlet end 36 which each communicate with the inner channel 28 for providing subcoolant flow therethrough. Disposed within the channel 12 is a bundle of fuel rods 38 which, in the illustrated embodiment, number sixty-four and form an 8.times.8 array. The fuel rod bundle is, in turn, separated into four mini-bundles thereof by the water cross 26. The fuel rods 38 of each mini-bundle, such being sixteen in number in a 4.times.4 array, extend in laterally spaced apart relationship between an upper tie plate 40 and a lower tie plate 42. The fuel rods 38 in each mini-bundle are connected to the upper and lower tie plates 40,42 and together therewith comprise a separate fuel rod subassembly 44 within each of the compartments 32 of the channel 12. A plurality of grids 46, such being six in number, are axially spaced along the fuel rods 40 of each fuel rod subassembly 46 and maintain the fuel rods in their laterally spaced relationships. The lower and upper tie plates 42,40 of the respective fuel rod subassemblies 44 have flow openings (not shown) defined therethrough for allowing the flow of coolant/moderator fluid into and from the separate fuel rod subassembly 44. Also, coolant flow paths provide flow communication between the fuel rod subassemblies 44 in the respective separate compartments 32 of the fuel assembly 10 through a plurality of openings 48 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 48 serves to equalize the hydraulic pressure between the four separate compartments 32, thereby minimizing the possibility of thermal hydrodynamic instability between the separate fuel rod subassemblies 44. The above-described basic components of the BWR fuel assembly 10 are known in the prior art, being disclosed particularly in the above-cited U. S. patents to Barry et al and Taleyarkhan, and have been discussed in sufficient detail herein to enable one skilled in the art to understand the improvement of the present invention presented hereinafter. The BWR fuel assembly 10 just described is manufactured and sold by Westinghouse Electric Corporation, the assignee of the present invention and of the Barry et al and Taleyarkhan patents. LPRM CALIBRATION STRIPS ON FUEL ASSEMBLY OUTER CHANNEL Referring now to FIG. 5, there is seen a schematic representation of a group of four Westinghouse BWR fuel assemblies 10 and a LPRM string 50 located centrally therebetween and spaced from the adjacent corners 52 of their rectangular channels 12. Also shown in FIG. 5 (and in FIGS. 1-4 as well) is the improvement of the present invention in the form of a plurality of angle-shaped strips 54 composed of neutron absorber material and located about and attached such as by welding to the adjacent corners 52 of the fuel assembly channels 12. The strips 54 are placed on the outer tubular channels 12 of the above-described Westinghouse BWR fuel assemblies 10 at the axially spaced locations shown in FIG. 6 for facilitating calibration of Local Power Range Monitor (LPRM) neutron detectors 56 contained within a hollow tube 58 of the LPRM string 50. More specifically, as is conventionally known, the detectors 56 are adapted to provide local power monitoring information during reactor operation. Also, the hollow tube 58 of the string 50 is adapted to receive a conventional neutron flux sensitive probe 60 at selected times for calibrating the detectors 56. Also in FIG. 6 is a schematic representation of a GE BWR fuel assembly 62 with the axially spaced Inconel sacers 64. The axial positions of the neutron absorber strips 54 on the Westinghouse fuel assembly channel 12 are in alignment with the axial positions of the Inconel spacers 64 in the GE BWR fuel assembly 62. The spacers or grids 46 of the Westinghouse fuel assembly 10 by being composed of a material incapable of producing a localized change in neutron flux and by being located at different axial positions than the Inconel spacers 64 of the GE BWR fuel assembly 62 are thus not capable nor adaptable for serving the purpose of the Inconel spacers 64 with respect to the probe 60 when the GE BWR fuel assembly 62 is replaced in a reactor core by the Westinghouse fuel assembly 10. However, the improvement in the form of the strips 54, being composed of neutron absorber material, such as material containing boron, hafnium and/or silver, does adequately serve such purpose. By each strip 54 being composed of a material capable of producing a localized change in neutron flux, then upon passage of the probe 60 through the hollow tube 58 of the string 50 and past the strips 54, the probe 60 will sense the neutron flux change being produced by each strip and thereby the position of the probe can be tracked as it is moved through the string tube. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the forms hereinbefore described being merely a preferred or exemplary embodiment thereof.