Patent Number: 055725600
Section: description

DETAILED DESCRIPTION Referring to FIG. 1, a boiling water reactor nuclear fuel assembly is generally shown at 10 having elongated fuel rods 12 each of which generally includes a zirconium alloy tube 12a within which are nuclear fuel pellets 12b. Fuel rods 12 have a uniform diameter along their length. The fuel rods are supported between a lower tie plate 14 and upper tie plate 16. The lower and upper tie plates can also or alternatively function to position the ends of the fuel rods in a spaced relationship. Fuel rods 12 pass through apertures or support cells in spacer grids 18, only two of which are shown in this fragmentary view. Spacer grids 18 provide intermediate support of fuel rods 12 over the length of fuel assembly 10 and position them in a spaced relationship while restraining them from lateral vibration. The fuel rod pitch or distance between the centerlines of adjacent fuel rods is maintained by the spacers. Outer square channel 11 is shown around the fuel rods 12 and spacers 18. Although a central water channel 44 is shown disposed in the center of the array of fuel rods 12 and replaces in this example the innermost three by three array of fuel rods, the present invention is not limited to fuel assemblies with central water channel(s) or water rod(s). Assembly 10 houses an 11.times.11 fuel array although most of the fuel rods 12 are not shown for clarity of illustration. Although reference is made in the specification to an 11.times.11 fuel rod array with each fuel rod having an equal diameter, such an array has been selected for illustrative purposes only. The present invention can be used with other arrays including, but not limited to 8.times.8, 9.times.9, and 10.times.10. Central water channel 44 is shown in this example with a square cross-sectional area which varies along the height of the fuel assembly. In accordance with the present invention, in order to accommodate the changing cross-sectional area of central water channel 44 but without changing the diameter of fuel rods 12 and without increasing the size of outer channel 11, the position and the distance between fuel rods 12 in the example shown in FIG. 1 varies along the height of the fuel assembly. Referring to FIG. 2 which is a cross-sectional view taken along line 2--2 of fuel assembly 10 shown in FIG. 1, central water channel 44 is at the center of the fuel assembly and the arrangement of the fuel rods is square with the distance between each fuel rod being the same. The pitch or distance between the centerlines of each fuel rod at the elevation shown in FIG. 2 is uniform and is designated by "P". Referring to FIG. 3 which is a cross-sectional view taken along line 3--3 of fuel assembly 10 shown in FIG. 1, the cross-sectional area of central water channel 44 is enlarged from that shown in FIG. 2. The cross-sectional area of central water channel 44 varies along the height of the fuel assembly and in the view shown in FIG. 3 has increased from its position in the lower portion of the fuel assembly (FIG. 2) toward the top of the fuel assembly where it achieves in this particular example its maximum cross-sectional area. Each of the four corner areas of the assembly shown in FIG. 3 has a 4.times.4 square array of fuel rods with a fuel rod pitch of P.sub.1. Between each of the four corner areas are regions called flats where the fuel rods are arranged in a 3.times.4 or 4.times.3 rectangular array with fuel rod pitches of P.sub.2 and P.sub.3. In an array of fuel rods other than a square array such as the 3.times.4 or 4.times.3 rectangular array shown in FIG. 3., the pitch or distance between the centerlines of two adjacent fuel rods which extend in the radial direction away from the center of the fuel assembly is referred to as a radial pitch. In the 3.times.4 or 4.times.3 array, the four fuel rods in a row extend in the radial direction. An example of radial pitch is shown in FIG. 3 as P.sub.3. Similarly, in an array of fuel rods other than a square array, the pitch or distance between the centerlines of two adjacent fuel rods which extend tangentially or circumferentially from the center of the fuel assembly is referred to as a tangential pitch. In the 3.times.4 or 4.times.3 array, the 3 fuel rods in a row extend tangentially. An example of a tangential pitch is shown in FIG. 3 as P.sub.2. In order to accommodate the larger cross-sectional area of the central water channel but without decreasing the fuel rod diameter and without using short or part length fuel rods, the fuel rod pitch changes from the lower elevation (FIG. 2) to the upper elevation (FIG. 3) of the fuel assembly. The pitch P.sub.1 of each of the fuel rods in the 4.times.4 array of fuel rods in each of the four corner areas of fuel assembly 10 shown in FIG. 3 is smaller than the pitch P of the fuel rods in the corners at the elevation shown in FIG. 2. of the fuel assembly. The complete array shown in Fig. 3 is a combination square/rectangular array in which P.sub.1 has been chosen to equal P.sub.3, and P.sub.2 has been chosen to be greater than P.sub.1 Other values of the pitches P.sub.1, P.sub.2 and P.sub.3 can be chosen. In the embodiment shown in FIGS. 1-3, the fuel rod pitch at each of the intermediate elevations between the view shown in Fig. 2 and that in FIG. 3 varies from the pitch at the lower tie plate to the pitch of the upper spacer. Thus, the fuel rods extend from their positions in the uniform square array in the view shown in FIG. 2 to their positions in the combination square/rectangular array shown in FIG. 3. More particularly, the 4.times.4 square array in each of the corners of fuel assembly 10 and the 3.times.4 and 4.times.3 rectangular arrays in the flats of the assembly shown in FIG. 3 progress from the uniform square array in the view shown in FIG. 2. Thus, the pitch of the fuel rods in fuel assembly 10 can vary in the radial direction and/or in the tangential direction at each elevation of the fuel assembly (e.g. the corners and the flats) and can thus vary vertically or axially along the height of the fuel assembly. Although in the embodiment of the present invention shown in FIGS. 1-3 the fuel rod pitch varies from the lower portion of the assembly to the upper portion of the assembly, the pitch(es) could vary abruptly or may alternate from an increasing to a decreasing pitch (or vice versa) at selected elevations determined by the position of a spacer (or spacers) as shown for example in the region above the uppermost spacer in FIG. 1. Varying the fuel rod pitch along the fuel assembly height permits positioning the fuel rods to take advantage of local moderator distribution so as to provide closer to the optimum local water to fuel ratios at different axial locations. In addition to improvements in local water to fuel ratio, rod positioning and pitch may be varied along the fuel assembly height to accommodate fuel assembly geometry changes which in turn enables selective control of the water to fuel ratio or the coolant flow areas. Such modifications could include increases or decreases in the center water channel cross-sectional area along the height of the assembly, or increases in the flow area within the outer channel which is accomplished by adjusting the rod pitch at selected axial locations. As the coolant density decreases as a function of the height of the fuel assembly, there is an associated velocity increase which causes a proportionately higher pressure drop in the two phase flow region of the fuel assembly. A high two phase to single phase pressure drop ratio can be detrimental to core stability. For this reason strategies to increase coolant flow area and reduce pressure drop at the top of the assembly are sometimes employed. A change in the lateral rod positioning toward the top of the assembly may be beneficial, for example, by more effectively using the water channel exit flow or the flow from part length rods to improve the cooling of fuel rods at the top of the assembly. The changes in fuel rod pitch may be accomplished by flexing the fuel rods laterally in the spans between grid spacers. Flexure of the fuel rod, for example, from one cell position in a square array in one span to an adjacent cell in the next span can be achieved without exceeding the yield strength of the fuel rod cladding. Such flexure can also be achieved without interference from the pellets as the relatively short pellet length and large pellet to clad diametral gap can accommodate the necessary clad curvature without pellet-clad interference. By varying the fuel rod pitch radially, tangentially and axially in the fuel assembly, one can accommodate an inner or central water channel that varies in cross-sectional area as well as shape along the height of the fuel assembly without having to remove any fuel rod from the assembly. Furthermore, according to the invention, the size of the center water channel can be optimized axially by selectively changing its cross-sectional area as a function of the height of the assembly which can be accomplished by selectively changing the pitch of the fuel rods as a function of the height of the assembly. With prior art fuel assemblies having uniform fuel rod pitch, increasing the size of the central water channel necessitates the removal of those fuel rods which occupy the space or volume into which the enlarged center water channel would extend, thereby decreasing the number of fuel rods in the fuel assembly. With non-uniform pitch, the center water channel size can be increased without necessarily removing any fuel rod. With prior art fuel assemblies having uniform pitch, if the fuel rods were not removed, then the shape of the central water channel which could be accommodated would have to be changed because of the physical position of the fuel rods. Referring to FIG. 4, an alternative embodiment of the present invention is shown in which a boiling water reactor nuclear fuel assembly is shown at 110 having elongated fuel rods 112 each of which generally includes a zirconium alloy tube 112a within which are nuclear fuel pellets 112b. Fuel rods 112 have a uniform diameter along their length with each fuel rod having an equal diameter. The fuel rods are supported between a lower tie plate 114 and an upper tie plate 116 and pass through apertures or support cells in spacer grids 118, only two of which are shown in this fragmentary view. The lower and upper tie plates can also or alternatively function to position the ends of the fuel rods in a spaced relationship. Spacer grids 118 provide intermediate support of fuel rods 112 over the length of fuel assembly 110 and position them in a spaced relationship while restraining them from lateral vibration. The fuel rod pitch is maintained by the spacers. A central water channel 144 is disposed toward the center of the array of fuel rods 112 and replaces a three by three fuel rod array disposed toward the center of the fuel assembly. Outer channel 111 is shown around the fuel rods 112 and spacers 118. In those reactor assembly designs in which a structural connection is formed by the inner or central water channel to the upper and the lower tie plates, the spacers provide support for the fuel rods over the length of the assembly and position the fuel rods in an array with the fuel rods having a predetermined pitch or pitches. Although assembly 110 houses a 10.times.10 fuel array, such an array has been selected for purposes of illustration only. The embodiment shown in FIG. 4 can be used with other arrays including, but not limited to 8.times.8, 9.times.9, and 11.times.11. Referring to FIG. 5 which is a cross-sectional view taken along line 5--5 of fuel assembly 110 shown in FIG. 4, the array of fuel rods is uniform and square, and their pitch is constant and designated by P.sub.4. Referring to FIG. 6 which is a cross-sectional view taken along line 6--6 of fuel assembly shown 10 in FIG. 4, central water channel 144 is enlarged from that shown in FIG. 5. The cross-sectional area of central water channel 144 has increased eccentrically from its position in the lower portion of fuel assembly 110 (FIG. 5) toward the top of the fuel assembly. The eccentrically expanding 3.times.3 central water channel 144 is more centrally located within the interior of the fuel assembly thereby enabling the selective positioning of increased moderation in the center as well as the upper portions of the fuel assembly. In order to accommodate the larger cross-sectional area of the eccentrically expanded central water channel 144 but without decreasing the diameter of the fuel rods and without substituting short or part length fuel rods, the fuel rod pitch changes from a uniform pitch p.sub.4 at a lower elevation of the fuel assembly (FIG. 5) to non-uniform pitches P.sub.i, P.sub.j, P.sub.k, P.sub.L, etc. at a higher elevation of the fuel assembly (FIG. 6). The non-uniform pitches of the fuel rods shown in FIG. 6 varies from a square pitch P.sub.i in the bottom right corner of the assembly to smaller pitches (e.g. P.sub.j, P.sub.k) of combination square/rectangular arrays of fuel rods, to an even smaller pitch (P.sub.L) of the square array of fuel rods shown in the opposite top left diagonal corner of the fuel assembly. In this example, P.sub.i has been chosen to equal p.sub.4. Although the non-uniform pitches shown in FIG. 6 are shown to vary substantially continuously from one corner of the fuel assembly to the diagonally opposite corner, the variation and degree of variation can depend upon other factors or design choices. For example, selecting the radial pitch of the rods closest to the inner and/or outer channels to be smaller than the radial pitch of the intermediate rows of fuel rods results in a more uniform moderator to fuel ratio for all the rods in the assembly at the upper more voided region of the fuel assembly. Stated more broadly, since moderation is a function of the ratio of the amount of fuel to the amount of moderator, changing the fuel rod pitch changes the moderation. Thus, selectively changing the fuel rod pitch while maintaining uniform rod size allows changing or tailoring the moderation along the axial position of the fuel assembly. While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.