Patent Number: 048184713
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a boiling water reactor (BWR) fuel assembly having localized neutron absorber strips placed on its outer tubular channel for facilitating calibration of Local Power Range Monitor (LPRM) neutron flux detectors. 2. Description of the Prior Art Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work. In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable between the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core. A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. Examples of such fuel assemblies are illustrated and described in U.S. Pat. Nos. (3,349,004) to Lass et al, (3,689,358) Smith et al, (3,802,995) Fritz et al, (4,560,532) Barry et al and (4,649,021) Taleyarkhan and in a Canadian Pat. No. (1,150,423) to Anderson et al. A BWR core typically includes several LPRM strings dispersed throughout the core. These strings are located inbetween the corner locations of four fuel assemblies. Each string includes a hollow tube with four neutron detectors located at discrete axial locations. During reactor operation these detectors provide crucial local power monitoring information. However, the detectors need to be calibrated at specific time intervals with a movable tip probe that is inserted from the bottom of the core, into selected detector string tubes. This calibration is necessary for maintaining the accuracy/fidelity of the LPRM readings on the control console. In a BWR core made up of General Electric (GE-8.times.8) fuel assemblies as the tip probe is inserted in the string tube, its relative position is evaluated from the location of neutron flux dips caused by Inconel fuel rod spacers located axially along the fuel assembly. The Inconel spacers, usually seven in number, act as neutron absorbers and hence such dips occur. In reload situations where Westinghouse Electric (W-QUAD+) BWR fuel assemblies are used to replace selected General Electric BWR fuel assemblies, it is highly likely that Westinghouse fuel assemblies will end up replacing GE fuel assemblies at one of the LPRM string locations. The Westinghouse fuel assembly, designed for optimized fuel cycle cost benefits, employs an all-Zircaloy spacer design. However, these Westinghouse Zircaloy spacers will not produce local neutron flux dips like the GE Inconel spacers. Furthermore, the six Zircaloy spacers in the adjacent fuel rod subassembly of the Westinghouse fuel assembly are located at axial positions different from the axial positions of the seven Inconel spacers in the GE fuel assembly. Hence, proper positioning of the tip probe for calibration purposes becomes impossible with current plant setup. Such a situation could lead to NRC-imposed uncertainty penalties in the form of plant derates. Consequently, a need exists for an effective means of providing an indicator for locating the LPRM detectors in plants where GE BWR fuel assemblies are replaced with Westinghouse BWR fuel assemblies. SUMMARY OF THE INVENTION The present invention provides an improvement which is designed to satisfy the aforementioned needs. The technique underlying the present invention relates to providing an improvement in the form of a plurality of local neutron absorber strips, for instance made of a material containing boron, hafnium and/or silver, at axial locations on the exterior of the outer channel of the Westinghouse BWR fuel assembly which correspond to the axial positions of the Inconel spacers of the GE BWR fuel assembly. This ensures compatibility of the Westinghouse BWR fuel assembly design with the existing GE fuel assembly design in a reload core. The above technique provides effective positioning of the tip probes for calibration purposes and eliminates uncertainty-related penalties for the Westinghouse BWR fuel assembly design in reload BWR cores. An additional benefit attributed to the above technique is the assurance of proper fuel assembly orientation. That is, the corner where absorber strips are attached can be used for assuring proper orientation of the Westinghouse BWR fuel assembly in a reload BWR core. The relatively small amount of neutron absorber strips used is estimated to cause a negligible impact on nuclear fuel cycle cost. Further, structural, thermal-hydraulic and LOCA performance areas would also remain unaffected. Accordingly, the present invention is set forth in the combination of at least one Local Power Range Monitor (LPRM) string and a plurality of fuel assemblies arranged in side-by-side spaced positions about the string. The LPRM string has a hollow tube and a plurality of neutron detectors located therein at spaced axial locations and being adapted to provide local power monitoring information. The hollow tube of the string is adapted to receive a neutron flux sensitive probe for calibrating the detectors. Each of the fuel assemblies has a plurality of spaced fuel rods, an outer hollow tubular channel surrounding the fuel rods and a plurality of spacers disposed within and axially along the channel and about the fuel rods so as to maintain them in side-by-side spaced relationship. The spacers of at least one of the fuel assemblies is composed of a material incapable of producing a localized change in neutron flux. The feature of the present invention is an improvement comprising a plurality of elements attached to the at least one fuel assembly and located axially at different known positions therealong and adjacent to the hollow tube of the string. Each of the elements is composed of a material capable of producing a localized change in neutron flux such that, upon passage of the probe through the hollow tube of the string and past the elements, the probe will sense the neutron flux change being produced by each of the elements and thereby the position of the probe can be tracked as it is moved through the string tube. More particularly, the elements are attached to the exterior of the at least one fuel assembly channel at the different known positions therealong. Preferably, each element is in the form of a strip of the material. Further, the fuel assembly channel is rectangular in cross-section and has a corner located adjacent to the string tube. Each of the plurality of elements is attached about the corner of the fuel assembly channel. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings .