BWR fuel assembly with improved spacer and fuel bundle design for enhanced thermal-hydraulic performance

摘要

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.