Patent Number: 047388196
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Nuclear Fuel Assembly" by Robert F. Barry et al, assigned U.S. Ser. No. 368,555 and filed Apr. 15, 1982. 2. "Coolant Flow Paths Within A Nuclear Fuel Assembly" by Pratap K. Doshi, assigned U.S. Ser. No. 602,089 and filed Apr. 19, 1984, a continuation of U.S. Ser. No. 368,552, filed Apr. 15, 1982 and now abandoned. 3. "Cross Brace For Stiffening A Water Cross In a Fuel Assembly" by C. K. Lui, assigned U.S. Ser. No. 672,042 and filed Nov. 16, 1984. 4. "Improved Boiling Water Nuclear Reactor Fuel Assembly" by Rusi Taleyarkhan, assigned U.S. Ser. No. 726,602 and filed May 2, 1985. 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 incorporating features which eliminate large cross-flows between fuel rod mini-bundles at upper spacer locations to avoid degradation of critical heat flux (CHF) performance. 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 within 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. 3,689,358 to Smith et al and 3,802,995 to Fritz et al and Canadian Pat. No. 1,150,423 to Anderson et al, as well as in the patent applications cross-referenced above. In a fuel assembly of this type the fuel rods in the central region of the bundle thereof may be undermoderated and overenriched. In order to remedy this condition by increasing the flow of moderator water through this region of the assembly, several arrangements have been proposed. In the Fritz et al patent, one or more elongated empty rods are substituted for fuel rods in the central region of the assembly. In the Anderson et al patent, an elongated centrally-disposed stiffening device with vertical water passageways is used in the assembly. In the above cross-referenced Barry et al, Doshi and Lui patent applications, an elongated centrally-disposed water cross is used in the assembly. As disclosed in the aforementioned latter four cross-referenced applications, the water cross has a plurality of four radial panels, forming a cruciform water flow channel, which divide the fuel assembly into four, separate elongated compartments, with the bundle of fuel rods being divided into mini-bundles supported by axially displaced grids or spacers and upper and lower tie plates disposed in the respective compartments. The cruciform water flow channel provides a centrally-disposed cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of, but separate from, adjacent fuel rods in the mini-bundles thereof. In the above cross-referenced applications, the radial panels of the water cross are interconnected to the sides of the outer flow channel to support the water cross. However, it is considered advantageous to provide some means to permit coolant flow transversely between the separate minibundles of fuel rods of the fuel assembly to provide hydraulic pressure equalization therebetween. For example, in the Taleyarkhan application, pressure equalization openings permitting cross-flow between the compartments are defined between vertically spaced ribs formed in the sides of the outer flow channel which connect to the outer edges of the water cross panels. Notwithstanding the improvements fostered by the cruciform water cross flow channel of the above cross-referenced applications with respect to hydraulic pressure equalization between the fuel rod mini-bundles in the separate compartments, other problems have recently been recognized which, if left unresolved, will cause degradation in CHF performance. These problems relate to certain of the openings which allow cross-flow communication of two phase (steam/water) coolant between the mini-bundles in order to achieve the desired flow stability and pressure equalization. At the locations of the fuel rod mini-bundle spacers, this cross-flow between the compartments can rise to large values and degrade CHF performance. The CHF phenomenon in BWRs is of paramount importance in characterizing the power rating of the plant. Hence degradation of this margin should be minimized. Consequently, the need exists for further improvement of the BWR fuel assembly to eliminate or minimize CHF margin penalties and uncertainties. SUMMARY OF THE INVENTION The present invention provides features which are designed to satisfy the aforementioned needs. These features block portions of the coolant cross-flow between mini-bundles so as to eliminate degradation of CHF performance and the penalties and uncertainties associated therewith, without adversely affecting the benefits to be gained from the degree of cross-flow that remains. It has been discovered that cross-flows between mini-bundles are relatively small at all places except around the spacer locations and that CHF occurs predominantly at the top three spacers locations. The solution of the present invention is to provide lateral flow blockage of pressure equalization openings around the top three spacer locations. This will eliminate sudden or large cross flows at these locations, essentially eliminating or minimizing CHF margin penalties while simultaneously preserving the overall benefits of cross-flow communication between mini-bundles (i.e., stability, reduction of flow maldistribution, pressure equalization). The solution of the present invention would also permit one-dimensional thermal hydraulic analysis thereby leading to further substantial reduction in uncertainties and savings in licensing complexity over multi-dimensional analysis which was required but has not been possible to carry out. Also, in addition to vastly improved thermal hydraulic design allowing a single one-dimensional analysis, improved structural integrity and reduced flow induced vibration result. Accordingly, the present invention sets forth improved features in a BWR fuel assembly. The fuel assembly includes an array of spaced fuel rods, an outer tubular flow channel surrounding the fuel rods so as to direct flow of coolant/moderator fluid therealong, and a water cross extending through the channel and having a plurality of radially extending members dividing the array of fuel rods into a plurality of separate fuel rod mini-bundle subassemblies. Each radial member is connected at multiple segments along its outer longitudinal edge to the channel so as to define a series of spaced apart openings between the member and channel which allow cross-flow of coolant/moderator between adjacent subassemblies for pressure equalization therebetween. Each subassembly has a plurality of spacers being displaced from one another along the length of the subassembly between the bottom and top thereof and surrounding the fuel rods thereof so as to maintain them in side-by-side spaced relationship. The features of the present invention incorporated in the BWR fuel assembly relate to means interconnecting each water cross radial member with the channel for closing predetermined ones of the openings at upper ones of the spacers for blocking portions of the coolant/moderator cross-flow between the separate mini-bundles so as to eliminate or at least minimize degradation of CHF perforamnce and resulting penalties and uncertainties associated therewith. More particularly, the cross-flow blocking means is in the form of a solid continuous structure impervious to cross-flow and interconnecting each radial member with the channel. In one embodiment, the blocking means is a continuous rib formed in the channel and connected to a respective one of the water cross radial members. The rib extends along the channel through a distance which encompasses the regions of the adjacent subassemblies occupied by the upper ones of the spacers, which preferably are three in number. In another embodiment, the blocking means is a continuous weld or bar interconnecting the channel with a respective one of the water cross radial members. The weld or bar extends along the channel through a distance which encompasses the regions of the adjacent subassemblies occupied by the upper ones of the spacers. 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 wherein there is shown and described an illustrative embodiment of the invention.