Patent Number: 047284900
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending U.S. patent applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Nuclear Fuel Spacer Grid With Improved Outer Straps" by E. E. DeMario, assigned U.S. Ser. No. 473,515 and filed Mar. 9, 1983 (W. E. 51,064). PA0 2. "A Low Pressure Drop Grid For A Nuclear Reactor Fuel Assembly" by David J. Sperhac et al, assigned U.S. Ser. No. 567,448 and filed Dec. 30, 1983 (W. E. 51,417). 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 at least some fuel rod spacers with perimeter scoops configured to redistribute liquid coolant flow from outer to interior spacer cell locations just upstream of the locations of the spacers so as to avoid dryout along the interior fuel rods and thereby improved overall critical heat flux (CHF) performance of the fuel rod bundle. 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 a typical boiling water reactor (BWR) fuel assembly, a bundle of fuel rods are subdivided into four separate mini-bundles by a central water cross and each mini-bundle is supported in laterally spaced-apart relation by a plurality of axially displaced grids or spacers, for example six in number, axially spaced apart along its fuel rods. The spacers are needed to maintain the desired fuel mini-bundle configuration. Then, all four mini-bundles of the fuel assembly are 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 assesmbly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. Such BWR fuel assembly is illustrated and described in U.S. Pat. No. 4,560,532 to Barry et al. The critical heat flux (CHF) performance of a BWR fuel assembly is of paramount importance in the safe operation of a BWR. Upon occurrence of a CHF condition, the surface heat transfer coefficient drops by a large amount, leading to an exponential rise in fuel rod cladding temperature. This can cause rupture and release of fission fragments into the coolant. In order to avoid this condition, regulatory guidelines restrict BWR operation to a low enough level to avoid the CHF condition. With respect to a BWR fuel assembly like that of the aforementioned patent, it has been realized that CHF performance is highest for fuel rods in the corner and side, or perimeter, spacer locations and lowest for fuel rods in the interior locations. This is due primarily to the hotter steam of the two-phase coolant tending to drift towards the interior fuel rod locations while the cooler liquid of the two-phase coolant accumulates at the channel along the perimeter fuel rod locations. As a result, premature dryout and degradation of CHF performance occurs at the interior fuel rod locations. Consequently, a need exists to improve CHF performance at the interior fuel rod locations so as to enhance the overall CHF performance of the fuel assembly. While turbulence generating mixing vanes have been employed heretofore on PWR fuel assembly spacers to achieve mixing and homogenization of flowing coolant, for example as disclosed in the above cross-referenced applications and in U.S. Pat. Nos. to Andrews et al (3,379,619) and Tong et al (3,395,077) being assigned to the assignee of the present invention, none are viewed as particularly adapted to provide the improvement being sought herein. SUMMARY OF THE INVENTION The present invention provides features which are designed to satisfy the aforementioned needs. Underlying the present invention is the realization that a more even distribution of liquid coolant within the mini-bundles, especially to the interior rod locations for the top three CHF limited spacers, would lead to the interior rods exhibiting enhanced CHF performance similar to the rods at corner and side, or perimeter, spacer locations. Specifically, a plurality of scoops are positioned upstream of the spacers by about three to six inches. The scoops extend in inclined inward relationships between the fuel rods at the locations of the perimeter cells of the spacer. With such an arrangement, the scoops acts to "scoop" or divert liquid coolant flow from the cold walls of the tubular channel to the interior heated rods, where it is needed the most. Due to the improved liquid coolant flow distribution in each mini-bundle, the interior rods can be expected to experience better cooling characteristics. Also, the extensions on the outer strap of the spacer which form part of the scoops increase the longitudinal height of the spacer outer strap so as to impart greater structural rigidity to the spacer. The seismic characteristics of the fuel assembly would thus tend to be improved. Accordingly, the present invention is directed to an improved fuel rod spacer for use in a nuclear reactor fuel assembly containing a plurality of elongated fuel rods. The improved spacer includes: (a) inner means defining a plurality of inner cell openings for receiving therethrough respective ones of the fuel rods in spaced apart and generally parallel extending relation, the inner means having outer spaced apart terminal end portions; (b) outer peripheral means attached to the respective outer terminal end portions of the inner means to define a number of perimeter cell openings for receiving therethrough other ones of the fuel rods in spaced apart and generally parallel extending relation, the perimeter cell openings being arranged to encompass the inner cell openings as a group, the inner means defining a border which surrounds the inner cell openings as a group and separates them from the perimeter cell openings; and (c) a plurality of coolant flow diverting scoops mounted on the outer peripheral means in spaced apart relation and along a common side thereof so as to extend inwardly along and in spaced relation from a common side of respective ones of the inner means terminal end portions and within the respective spaces between the other ones of the fuel rods when received trough the perimeter cell openings, the scoops terminating at inner ends being disposed generally above the border defined by the inner means. More particularly, the inner means is a plurality of inner interleaved straps having the terminal end portions and arranged in an egg-crate configuration to define the plurality of inner cell openings. The outer peripheral means is an outer peripheral strap attached to the respective terminal end portions of the inner straps to define the perimeter cell openings. The inner straps define the border which surrounds the inner cell openings as a group and separates them from the perimeter cell openings. The plurality of coolant flow diverting scoops are mounted on the outer peripheral strap. Each scoop includes a mounting portion and a flow deflecting portion. The mounting portion of each scoop is in the form of an extension of the outer peripheral strap which extends from an upstream side thereof generally parallel to the fuel rods when received in the perimeter cell openings. The flow deflecting portion of each scoop defines the inner end of the scoop and is connected at an outer end to the mounting portion. The outer end of the flow deflecting portion is spaced farther from a respective one of the inner strap terminal end portions than the inner end thereof such that the flow deflecting portion extends in an inclined relation thereto and across a portion of the coolant flow path through the perimeter cell openings. The flow deflecting portion also has a longitudinally-extending tapered and arcuate shape. 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.