Westinghouse Electric Corporation

Claude M Mildrum

Rusi P Taleyarkhan

Pratap K Doshi

Thomas R Freeman

William L Orr

A BWR fuel assembly has a bundle 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, and a hollow central water cross with opposed walls, a lower flow inlet end, an opposite upper flow outlet end, and an open inner cruciform flow channel for subcooled moderator fluid flow through the fuel assembly. The water cross extends through the outer flow channel and is interconnected with the outer channel so as to divide it into separate compartments and the bundle of fuel rods into a plurality of mini-bundles thereof. Also, a pair of upper and lower tie plates are connected to the opposite ends of the fuel rods in each mini-bundle thereof so as to provide a separate fuel rod subassembly in each of the compartments. The tie plates have flow openings defined therethrough for allowing the flow of the coolant/ moderator fluid into and from the separate fuel rod subsasembly. The BWR includes an improvement which allows selected flow into said water cross and cross flow of fluid between the lower ends of the respective fuel rod mini-bundles of the separate fuel rod subassemblies and in such manner minimizes maldistribution of flow between the mini-bundles. The improvement comprises cross flow inlet means in the form of opposed holes located in the opposed walls of the water cross above the lower tie plates of the mini-bundles.

Orificing of water cross inlet in BWR fuel assembly

A fuel assembly has a bundle of elongated fuel rods disposed in a 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 of the fuel rods includes an outer cladding tube with an inner clad surface and a plurality of fuel pellets contained within the tube. Each spacer is composed of a plurality of interleaved inner straps and an outer strap encompassing the inner straps. The interleaved inner straps and the outer strap have respective protrusions formed thereon and together define spacer cells into which the respective protrusions extend to maintain the fuel rods received through the spacer cells in laterally spaced relationships in respective corner, side and interior cells defined by the interleaved inner straps and the outer straps. The BWR fuel assembly includes several improvements which enhance its thermal-hydraulic performance. First, 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 extend a greater distance into the corner cells than the distance through which the protrusions in the side and interior cells extend into those 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. Second, perforations are formed in the outer strap at the locations of the corner and side cells of the spacer for reducing the amount of strap area adjacent the fuel rods received in the corner and side cells and thereby increasing coolant flow to the corner and side fuel rods. Third, a generally uniform poison coating, such as boron, is applied within at least a majority of the fuel rods, the poison coating being applied to either the fuel pellets or the inner clad surface of the cladding tube of each fuel rod in the majority. Additionally, a predetermined pattern of fuel enrichment is provided with respect to the fuel rods of each mini-bundle thereof which together with the uniform poison coatings 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 spacers.

BWR fuel assembly with improved spacer and fuel bundle design for

A fuel rod for a nuclear reactor fuel assembly includes a cladding tube and a plurality of fuel pellets contained in the tube and composed of fissile material, such as uranium dioxide, having a single enrichment of U-235. At least some of the fuel pellets have an annular configuration. Some of the annular fuel pellets have an annulus of a first size, whereas other of the annular fuel pellets have an annulus of a second size different from the first size. In such manner, graduation of axial enrichment loading is provided between the annular fuel pellets of the fuel rod. Also, a nuclear fuel assembly includes a multiplicity of fuel rods with a plurality of fuel pellets contained in each of the fuel rods and being composed of fissile uranium having a single U-235 enrichment. Each of the fuel pellets in a majority of the fuel rods has a solid configuration, whereas each of the fuel pellets in a minority of the fuel rods has an annular configuration. Each of the annular fuel pellets of some of the fuel rods has an annulus of a first size. On the other hand, each of the annular fuel pellets of other of the fuel rods has an annulus of a second size, whereby graduation of enrichment loading is provided between the solid and annular fuel pellets of the respective majority and minority of fuel rods. The value of the single U-235 enrichment is at the level of the maximum enrichment loading of the fuel assembly.

Fuel rod with annular nuclear fuel pellets having same U-235 enrichment

A control rod assembly arrangement is disclosed for a light water nuclear reactor. Absorber rods are combined with stainless steel rods to achieve a more homogeneous poison distribution effect which reduces power depression within associated fuel assemblies and thereby flattens the overall power distribution within the core. In addition to improved load follow performance, safety analysis margins are improved due to the reduction in control rod reactivity worth.

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Monroeville, PA, United States of America

Claude M Mildrum