Patent Number: 047088451
Section: summary

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. "Water Tubes Arranged In Cross-Like Pattern In A Fuel Assembly" by Carl A. Olson et al, assigned U.S. Ser. No. 642,844 and filed Aug. 20, 1984. 4. "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. 5. "Improved Boiling Water Nuclear Reactor Fuel Assembly" by Rusi Taleyarkhan, assigned U.S. Ser. No. 726,602 and filed May 2, 1985. 6. "BWR Fuel Assembly With Water Flow Mixing Chamber At Fuel Bundle/Water Cross Entrance" by Rusi Taleyarkhan, assigned U.S. Ser. No. 746,619 and filed Jun. 19, 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 employing improvements in spacer and fuel bundle design for enhanced thermal-hydraulic 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. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. 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, an elongated centrally-disposed water cross is frequently used in the assembly, as disclosed in the above cross-referenced Barry et al, Doshi and Lui patent applications. The central water cross has a plurality of four radial panels which together form a cruciform water flow channel which divides the fuel assembly into four, separated elongated compartments, with the bundle of fuel rods being divided into mini-bundles disposed in the respective compartments. The water cross thus provides a centrally-disposed cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of, but separated from, adjacent fuel rods in the mini-bundles thereof. The fuel rods of each mini-bundle extend in laterally spaced apart relationship between an upper tie plate and a lower tie plate and connected together with the tie plates to comprise a separate fuel rod subassembly within each of the compartments of the channel. The water cross has approximately the same axial length as the fuel rod subassemblies, extending between the upper and lower tie plates thereof. Additionally, a plurality of grids or spacers, for example six in number, are disposed at axially displaced positions along the fuel rods of each fuel rod subassembly to maintain the fuel rods in their laterally spaced relationships. The spacers are introduced to maintain the desired fuel bundle configuration and to prevent excessive fuel rod bow and flow induced vibrations. Thus, the spacers provide significant benefits from a structural standpoint. However, the spacers are undesirable from a thermal-hydraulic performance standpoint. Specifically, they cause increased pressure drop (consequently higher coolant pumping powers are needed) and reduce the bundle critical power ratio (CPR) due to turbulence generated. The critical heat flux (CHF) deteriorates because the flow turbulence, induced by the spacers, strips away the liquid film from the heated fuel rod. This causes the bundle heat transferred to the coolant to drop significantly. Consequently heat builds up in the fuel rod causing it to overheat, and in some cases "burnout", releasing radioactivity into the coolant. This imposes a safety constraint. In order to avoid this possibility (i.e., overheating), safety analysis calculations are performed for each cycle to demonstrate that adequate margin to the CPR is always available, even under the worst postulated transient. Dedicated computer hardware performs continuous on-line surveillance of the fuel thermal margins to insure that adequate CPR margin is always maintained during reactor operations. It is well known that different spacers exhibit variable CHF performance. Further, this performance varies radially with the fuel rod location, and in general correlates with the spacer pressure drop. The smaller the value of the pressure drop, the better is the CHF performance. However, the spacer pressure drop characteristics vary radially within the fuel rod bundle. Due to this, spacers can either be so-called "corner", "side" or "interior" limiting. If a rod peaking occurs in the limiting location, CHF occurs much earlier, and thus causes deterioration in CPR margins. It should also be noted that not all spacers for substantially identical fuel rod array configurations (having the same N.times.N array of fuel rods) will behave identically. Consequently, a need exists for some design modifications to fuel assembly spacers in order to avoid or minimize the detrimental effects of CPR limiting locations within the spacers. SUMMARY OF THE INVENTION The present invention provides a BWR fuel assembly incorporating spacer and fuel bundle modifications which are designed to satisfy the aforementioned needs. Underlying the present invention is the recognition that it is necessary to know a spacer's basic characteristics and compensate via design modications for the detrimental effects of any CPR limiting locations. An objective analysis of the grid or spacer for its CPR characteristics has resulted in a design modification thereof which, together with a modification to the nuclear design of the fuel assembly, minimize the detrimental effect of rod peaking at CHF limited locations. The impact of these modifications on other operational and safety parameters are expected to be largely minor or beneficial in nature. Accordingly, the present invention sets forth improved features in a BWR fuel assembly. The fuel assembly includes a bundle of elongated fuel rods disposed in side-by-side relationship so as to form an array of spaced fuel rods, an outer tubular flow channel surrounding the fuel rods so as to direct flow of coolant/moderator fluid along the fuel rods, a hollow water cross extending centrally through and interconnected with the outer flow channel so as to divide the channel into separate compartments and the bundle of fuel rods into a plurality of mini-bundles thereof being disposed in the respective compartments, and a plurality of spacers axially displaced along the fuel rods in each of the mini-bundles thereof, each spacer being composed of inner and outer means which together define spacer cells at corner, side and interior locations of the spacer and have respective protrusions (e.g. spacer dimples and springs) formed thereon which extend into the cells so as to maintain the fuel rods received through the spacer cells in laterally spaced relationships. The improved features of the present invention comprise: (a) a generally uniform poison coating within at least a majority of the fuel rods; and (b) a predetermined pattern of fuel enrichment with respect to the fuel rods of each mini-bundle thereof which together with the uniform poison coatings, such as of boron, within the fuel rods ensures that the peaking powers of fuel rods in the corner and side cells of the spacers are less than the peaking power of a leading one of the fuel rods in the interior cells of the mini-bundle. More particularly, each of the fuel rods includes an outer tubular member having an inner clad surface and a plurality of fuel pellets contained within the tubular member. The uniform poision coating is applied to either one of the fuel pellets or the inner clad surface of the tubular member of the fuel rods in the majority thereof. Another feature of the present invention is that each of the fuel rods received through the corner cells of each spacer has a diametric size smaller than that of the fuel rods received through the side and interior cells of each spacer. Also, each of the protrusions in the corner cells extends a greater distance into the corner cells than the distance through which the protrusions in the side and interior cells extend into the side and interior cells, whereby increased coolant flow space is provided through the corner cells as compared to the side and interior cells so as to increase heat transfer from the corner fuel rods to the coolant. Still another feature of the present invention is that perforations are formed in the outer means (e.g. straps) at the locations of the corner and side cells of the spacer for reducing the amount of spacer area adjacent the fuel rods received in the corner and side cells and thereby increasing coolant flow to the corner and side fuel rods.