Patent Number: 051494957
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 a perspective view of a portion of a reactor core is provided. FIG. 1 depicts four fuel bundles 12, 14, 16, and 18, positioned as they would be in a nuclear reactor between a lower core plate 19 and an upper "top" guide 20. The fuel bundles 12, 16 are shown in partial cutaway view, without upper tie plates, exposing the interior of the bundles. A cruciform control rod 21 is depicted in a partially inserted position. Each of the four fuel bundles 12, 14, 16, and 18 contain 81 lattice positions in a regular square 9.times.9 array as defined, for example, at a top plate 22. As best seen in FIG. 3, the 81 lattice positions of the array are defined by imaginary lines 24, which are spaced apart by a distance or pitch 26. Each of the 81 lattice positions represents a potential site for a fuel rod. In each fuel bundle construction, certain of these sites are occupied by tie rods (not shown) for holding together top and bottom plates for structural purposes. Most of the lattice positions are substantially occupied by basic fuel rods 28. In most fuel bundle constructions, the fuel rods are tangentially separated by a space 32. As described above, this spacing of fuel rods is preferably maintained by means of spacer devices, described more fully below. Typically, in a boiling water reactor, using a 9.times.9 array, the pitch 26 equals about 11 mm and the spacing 32 equals about 3 mm. In the following, a new water-rod efficiency parameter is described, and water-rod efficiencies for a number of previous water rods are shown. Next, three particular water-rod configurations having a high water-rod efficiency, according to new efficiency parameters, are described. Finally, the way in which the water rods can be provided, and a manner in which they can joined to spacers, is described. According to the present invention, a water rod is provided which affords a desired amount of moderating effect without over moderating. Over-moderation means providing so much moderation that overall efficiency of the reactor is undesirably diminished because of absorption of thermal neutrons. It has been found that, for these purposes, a water rod should be configured to occupy greater than four lattice positions, preferably at least five lattice positions. The water rod should also occupy less than nine lattice positions, preferably about seven or fewer lattice positions. It has been found that by providing water rods occupying this range of lattice positions, the moderating effect of the water in the water rod is sufficient for providing a desirable increase in reactor efficiency without over-moderating the reaction. As described above, an indicator of efficiency is provided which is termed "water rod efficiency." This quantity has been found to be usefully provided by calculation as follows: ##EQU1## Referring now to FIG. 2, the net water rod cross-sectional area divided by the area of a single lattice position is shown as a function of the number of sacrificed lattice positions. TABLE 1 ______________________________________ Previous Device Number of or Sacrificed Point Present Invention Lattice Positions Shape Figure ______________________________________ 34 Previous Device 1 Round* -- 38 Previous Device 1 Round** -- 42 Previous Device 4 Round -- 48 Previous Device 4 Square -- 50 Previous Device 9 Square -- 46 Present Invention 7 Figure-8 9, 10 52 Present Invention 7 Peanut 3, 4 54 Present Invention 7 Rectangle 5, 6 56 Present Invention 5 Cruciform 7, 8 ______________________________________ *diameter = fuel rod diameter **diameter &gt; fuel rod diameter The water rod efficiency, defined by Equation No. 1 above, can be graphically seen in FIG. 2 as corresponding to the slope of a line connected to a point to the graph origin (0,0). Line 64, for example, depicts efficiencies of water rods configured as that shown for water rod 34. The efficiency for such configuration is about 0.33. Line 66 depicts the efficiency for water rods configured as water rod 38. The efficiency of such water rods is about 0.65. Line 68 depicts an efficiency of 100% or 1.0. As can be seen from FIG. 2, a number of previous water rod configurations, such as water rods of lines 64 and 66 have been limited to a water rod efficiency of less than about 0.6. For the understanding of the following graphical representation, reference will first be made to certain prior art water rod constructions. Specific points will be plotted on the graph so that the parameter of water rod efficiency can be evaluated in terms of the prior art. Thereafter, the designs herein developed with the help of this tool will be evaluated on the graphical representation. It will be shown that for water rods occupying more than four lattice positions but less than nine lattice positions that the graphical representation of FIG. 2 constitutes a valuable design tool. Consider the simplest prior example of a water rod. This water rod has the same shape and inner diameter of a fuel rod. Naturally, it occupies one lattice position and is round. In order to distribute the supplemental moderator throughout the bundle, ten evenly spaced rods are considered distributed in a 9.times.9 array. Assuming that one such water rod only is used, such a water rod displacement will appear at location 34 on the graphical representation of FIG. 2. Now take the same rod and multiply its location to a total number of 10 locations in the same array. Such a after rod distribution will be found to be at point 58 on the graphical representation of FIG. 2. The connection of the points 34 and 58 by a straight line through the origin of the graph is instructive. It will be found that intermediate numbers of water rods all adhering to the same configuration, will plot at the corresponding "Lattice Positions Sacrificed" location on line 64 of the graph. Consider the same round shape but expand the diameter of the water rod. Expansion of the water rod continues to and until the water rod reaches a maximum diameter without interfering with adjacent lattice position boundaries. Presuming that one such water rod is placed within a fuel bundle, the generation of the point 38 will be plotted. Expanding the total number of water rods to 10 in number, the point 60 will be generated. Connecting these two points by a straight line 66 extending through the origin of the graph as before defines further the efficiency of this water rod design. Further, it will be found that intermediate numbers of water rods all adhering to the same configuration, will plot at the corresponding "Lattice Positions Sacrificed" located on line 66 of the graph. We then have defined on FIG. 2, two lines 64 and 66 whose slope readily define relative boundaries of efficiencies for round prior art water rod and the maximum diameter of a water rod occupying single lattice positions. The plotting of two additional prior art water rod constructions can be instructive. Consider a round water rod. Have this water rod occupy four (4) lattice positions. Such a water rod will plot at point 42 on the graph. Now take the water rod shape and make the water rod section square instead of round. Further have the water rod occupy four lattice positions. Such a water rod will plot at point 48. Finally, take the same square shaped water rod. Continue to expand the dimension of the water rod until nine lattice positions are occupied. A plot of the point 50 will result. There is, however, a drawback to the configurations of the water rods of points 48 and 50. Water rod 48 has been found to occupy less than the desired number of lattice positions; water rod 50 has been found to occupy more than the number of desired lattice positions. It has been found that water rod configurations which occupy four or fewer lattice positions, and have been found to provide too little moderation for the desired efficient reaction in the bundle resident. Other configurations occupying nine or more lattice positions, and have been found to undesirable over-moderate the reaction or sacrifice too many fuel rods. Remembering that the slope of the lines connecting the point of origin (0,0) and a particular point on the graph of FIG. 2, it will be understood the two configurations of points 48 and 50 had highly desirable efficiencies. However, through either occupying too few lattice positions (four for point 48) or too many lattice positions (nine for point 50), respective under moderation or over moderation was present. Therefore, despite the apparent high efficiencies these designs of the prior art are not preferred. The present invention thus includes providing a water rod configuration which both efficiently uses the lattice positions that must be sacrificed, such as providing a water rod efficiency greater than about 0.6, preferably greater than about 0.7, and provides for efficiency of nuclear reactor operation by producing moderation in a desired range. Referring now to FIGS. 3 and 4, water rod 52 contains two topologically concave regions 74a and 74b. A topologically concave region is one in which there is at least one line segment connecting two points of the region which must pass outside of the boundary of the water rod 52. For example, taking points 100, 102, and connecting them by line 103, it can be seen line 103 passes outside of the water rod boundary. The water rod 52 occupies seven lattice positions, and is configured to define two round-cornered triangular regions 76a and 76b, continuously connected at their respective bases 78a and 78b by a constricted region 82. This constricted region 82 is defined by two inwardly-extending longitudinal projections 84a and 84b, as best seen in FIG. 4. As seen in FIG. 4, the inwardly extending longitudinal projections define two grooves 86a and 86b in the exterior of the water rod, which are configured to accommodate portions of fuel rods 87a, 87b. Referring now to FIGS. 5 and 6, a water rod 54 is provided, which is a topologically convex shape. A shape is topologically convex if a line segment connecting any two points does not pass outside the boundary of the water rod. For example, it can be seen that no two points connected by a line will cause the line to pass outside of the water rod. The cross-sectional region of the water rod 54 is substantially rectangular in shape. As best seen from FIG. 5, the water rod is substantially adjacent to a least ten fuel rods positioned in the fuel rod bundle. For this purpose, a fuel rod is adjacent if its lattice position has at least an edge in common with a displaced lattice on position. The water rod 54 occupies seven lattice positions. Turning now to FIGS. 7-8, a water rod 56 is shown, having four interiorly extending longitudinal projections 96a-96d which define therebetween four exteriorally projecting lobes 98a-98d. The interior extending projections 96a-96d define grooves 102a-102d which are configured to accommodate at least a portion of fuel rods 104a-104d within each. The water rod 56 occupies five lattice positions, and includes four topologically concave regions. Referring now to FIGS. 9-10, a water rod 46 is shown having two substantially circular portions. The water rod is substantially adjacent to at least ten fuel rods positioned in the fuel bundle. The water rod 46 occupies seven lattice positions. The water rod 46 can be conceptually viewed as two adjacent and contracting round tubes. In this view, each of the round tubes occupies three and one-half lattice positions. This is one advantage of providing tubes in closely adjacent positions, rather than in spaced-apart positions. If the ground tubes were isolated, each would occupy four lattice positions. By positioning two tubes adjacent to form a single "FIG. 8" water rod, only seven positions are occupied in total, for a savings of one lattice position. Plotting of the designs in the graphical representation of FIG. 2 can be instructive. First, it can be seen that the graphical plot of embodiment of FIGS. 3 and 4 plots at point 52 on the graph. This point yields an efficiency of over (0.91) and has the highest efficiency of the designs developed herein. Accordingly, it is preferred. Plotting of the design of FIGS. 5 and 6, plots at point 54 on the graph. This point yields a lower efficiency (0.77) than the design of FIGS. 3 and 4, but shows structurally a design that is easy to fabricate. This design is not as preferred as the embodiment of FIGS. 3 and 4, but is nevertheless highly advantageous. The four leaf or "clover" design of FIGS. 7 and 8 has high efficiency. It also includes occupation of 5 lattice positions, with the requisite range of lattice positions occupied to produce sufficient moderation. The efficiency of this design is (0.83). This design, because of its complexity of manufacture, is subordinate in preference to the design of FIGS. 3 and 4. Finally, the two adjacent round rods of FIGS. 9 and 10 exhibits high efficiency. It is noted, however, this design is truncated by a chord-presenting a difficulty of manufacture. This design has an efficiency of (0.76). As noted above, the water rods and fuel rods are maintained in a spaced-apart configuration using spacers. In previous configurations, such as circular and square configurations, some amount of rotation about the longitudinal axis of a water tube was possible without interfering with adjacent fuel rods. This characteristic was used to attach or latch a water rod to a spacer to prevent relative axial motion. In such method, a tab was provided on an exterior surface of the water tube. The water tube was moved longitudinally with respect to the spacer until the tab was aligned with, but offset from, an engaging or latching portion of the spacer. The tube was then rotated about its longitudinal axis to bring the tab into engagement with the engaging or latching portion of the spacer. The present invention includes providing a different method for maintaining the axial position of a water rod with respect to a spacer. Although this method can be used with a variety of water rods, it is especially useful when rotation of a water rod about its longitudinal axis is impractical or impossible. According to the present invention, a recess is defined on a portion of an exterior surface of a water rod. The recess can be defined by protrusions extending from the water rod, either integrally formed or attached by welding, brazing, adhering, and the like. The configuration of the water rod is such that the position of the recess with respect to the spacer can be changed by resilient deflection. In the preferred embodiment, the portion of the sidewall of the water tube adjacent to the recess has an amount of resilience. There is enough resilience that a portion of the sidewall near the recess can be inwardly deflected to effect movement of the recess portion. The resiliency also permits later springing back of the sidewall to substantially its original shape for engagement of the recess portion with a portion of the spacer. Referring now to FIGS. 11A-11C, protrusions 110 are integrally formed on an exterior surface of a water rod 112. The water rod protrusions 110 define a recess 114. The spacer 116 has a structure with a shape complementary to the recess. The water rod is axially slid with respect to a spacer assembly 116 until the protrusion 110 contacts at least a portion of the spacer assembly 116. Continuing axial movement of the water rod results in the water rod 112 being resiliently deflected, such as by engagement with a camming surface of the spacer assembly. This causes inward deflection of the sidewall of the water rod 112, as best seen in FIG. 11B. Further axial movement of the water rod with respect to the spacer assembly permits the recess 114 to become aligned with the engaging of the spacer assembly 116. Such engagement permits the water rod sidewall 112 to resiliently return to substantially its original position, as seen in FIG. 11C. The water rod 112 is thus in engaging or latching position with respect to the spacer, thereby maintaining the axial position of the spacer assembly 116 with respect to the water rod 112. Preferred circumferential positions for water rod protrusions are depicted in FIGS. 3, 5, and 7. In FIG. 3, a protrusions 120 is provided on the exterior surface of the water rod 52 in one of the grooves 86b. In FIG. 5 a protrusion 122 is provided on an exterior surface of one of the long sidewalls of a rectangular water rod 54. In FIG. 7 a protrusion 124 is provided in one of the grooves 102c of the water rod 56. The water rod depicted in FIG. 9 is preferably attached to spacer configurations by rotation around longitudinal axes of the round tubes, as described above. Although a number of possible water rod configurations are conceivable, only some configurations can be, in a practical sense, accurately and economically manufactured in the quantities needed. The water rod configurations 52, 54, 56, and 46 depicted in FIGS. 3, 5, 7 and 9, respectively, can be accurately and economically made in quantity. One method of manufacture involves beginning with conventionally-shaped, preferably thin-walled (e.g., 30-35 mils or about 0.75-0.85 mm), tubular bodies, such as circular or square cross-sectional bodies. The bodies are shaped as needed by cold-drawing through one or more dies to provide the desired configurations. The shaping can include the formation of grooves 86a, 86b, 102a-102d, lobes 98a-98d, or corners, as shown in FIGS. 3-8. The water rod 54 depicted in FIGS. 5-6 can also be made by joining, such as by welding, two U-shaped channels. The configurations depicted in FIGS. 3-10 have been found to represent viable configurations in the sense that they provide the desired efficiencies and moderation, and are manufacturable in a practical sense. The present invention includes a method of designing water rods to provide a water rod configuration which is practical and provides for a desired efficiency and desired moderation. Previous substantially empirical methods involved selecting a water rod configuration, typically without knowing the efficiency thereof. Many other reactor designs considerations are dependent upon the choice of water rod configuration. Thus, once a choice was made, redesign was so prohibitively expensive that the process often involved commitment to a design before the pertinent characteristics of the design could be empirically determined. In contrast, the present invention includes calculation of water rod efficiencies and lattice position displacements for two or more designs, and selecting a design using the calculated efficiencies and displacements. In this manner, water rods can be designed with knowledge of their efficiencies and displacements. The likelihood of later difficult and expensive redesign, dependent on the choice of water rod configuration, is, therefore, lessened. As will be apparent to those skilled in the art, a number of modifications and variations of the disclosed embodiments can also be practiced. Other cross-sectional configurations of water rods can be used, provided they produce a water rod efficiency greater than about 0.6, preferably greater than about 0.7, and produce the desired range of moderation. More than one water rod could be provided in a single fuel rod bundle, and different shapes of water rods can be provided in different bundles, although, preferably, the same shape is used in all bundles. Water rods can be provided which combine characteristics of various disclosed water rods. Because fuel for fuel rods a typically produced with a standard cross-sectional configuration, fuel rods are typically integral in that they either occupy all of a fuel rod position, as depicted in FIG. 2, or are entirely absent. However, it is also possible to provide an increased size axially or a changed-configuration of fuel rods to accommodate water rods of different shapes, with appropriate modifications to the calculation of efficiency, degree of moderation, and displaced flow area. Other methods of manufacture of water rods can be used, including casting, milling, rolling, hot-drawing, and the like. The attachment of a water rod to a spacer can be achieved by deflection of a tab without substantial sidewall deflection of a water rod, or by deflection of a portion of the spacer assembly without requiring deflection of the water rod, or some combination thereof. Although the description of the present invention has included a description of the preferred embodiments, other modifications and variations are included within the spirit and scope of the invention as limited only by the appended claims.