Patent Number: 047284870
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also, in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. IN GENERAL Referring now to the drawings, and particularly to FIG. 1, there is shown an elevational view of a fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 20. The fuel assembly 20 is the type used in a pressurized water reactor (PWR) and basically includes a lower end structure or bottom nozzle 22 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 24 which project upwardly from the bottom nozzle 22. The assembly 20 further includes a plurality of transverse grids 26 axially spaced along the guide thimbles 24 and an organized array of elongated fuel rods 28 transversely spaced and supported by the grids 26. Also, the assembly 20 has an instrumentation tube 30 located in the center thereof and an upper end structure or top nozzle 32 attached to the upper ends of the guide thimbles 24. With such an arrangement of parts, the fuel assembly 20 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 28 in the array thereof in the assembly 20 are held in spaced relationship with one another by the grids 26 spaced along the fuel assembly length. Each fuel rod 28 includes nuclear fuel pellets (not shown) and the opposite ends of the rod are closed by upper and lower end plugs 34, 36. The fuel pellets composed of fissile material are responsible for creating the reactive power of the PWR. A liquid moderator/ coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract some of the heat generated therein for the production of useful work. STANDARDIZED REDUCED LENGTH BURNABLE ABSORBER RODS In the operation of a PWR it is desirable to prolong the life of the reactor core as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, provide means to maintain the reactivity relatively constant over its lifetime. The present invention relates to such means in the form of a cluster of standardized reduced length burnable absorber rods 38, as seen in FIG. 1, inserted in the guide thimbles 24. The cluster of standardized rods 38 are stationarily supported by a holddown assembly 40 in the guide thimbles 24 of some of the fuel assemblies 20 to assist the movable control rods (not shown) in the guide thimbles 24 of other fuel assemblies (not shown) in maintaining a substantially constant level of neutron flux or reactivity in the core throughout its operating cycle. Heretofore, burnable absorber or poison rods have lately been of a custom-designed reduced length construction for maximization of the peaking factor margin in the reactor core. However, these latter reduced length rods have created other problems in their manufacture and handling, as discussed earlier. The standardized reduced length rod 38 of the present invention has substantially eliminated the drawbacks of the prior art rod while retaining its advantages. Referring now to FIGS. 2 and 3, in addition to FIG. 1, the holddown assembly 40 which supports the standardized rods 38 in spaced side-by-side relationship includes a lower flat perforated support plate 42 which fits within the fuel assembly top nozzle 32 and rests on the adapter plate 44 of the top nozzle. The holddown assembly 40 also includes a sleeve 46, being attached at its lower end within a central opening 48 in the support plate 42 and extending upwardly therefrom, and an upper holddown plate 50 which receives the sleeve 46 and is slidable vertically along it. Further, a holddown coil spring 52 is disposed about the sleeve 46 and extends between the lower support plate 42 and the upper holddown plate 50. Thus, the support plate 42 is held down against the top nozzle adapter plate 44 by the coil spring 52 which is compressed by the upper core plate (not shown) acting through the upper holddown plate 50 which abuts the upper core plate. This arrangement assures that the standardized rods 38 which are attached to the support plate 42 cannot be ejected from the reactor core by coolant flow forces while any thermal growth of the rods is accommodated. Turning next to FIG. 4, there is shown one embodiment of the standardized reduced length burnable absorber rod 38 of the present invention. The standardized rod 38 basically includes an upper end plug 54 having a threaded upper end 56 by which means the rod is connected to the support plate 42, a lower end plug 58, an elongated middle tubular rod section 60 having opposite upper and lower ends 62, 64 and a chamber 66 defined therein between the opposite ends 62, 64, an upper end spacer section 68 in the form of an elongated tubular extension extending between and rigidly interconnecting the upper end plug 54 and the upper end 62 of the middle tubular section 60, and a lower end spacer section 70 in the form of an elongated tubular extension extending between and rigidly interconnecting the lower end plug 58 and the lower end 64 of the middle tubular section 60. The rigid interconnection can take any suitable form, for instance, screw threaded, welded or crimped connections. By way of example, the sections 68, 60, 70 are shown welded together at W in FIGS. 4-7. Also, the standardized rod 38 has a burnable absorber material 72 disposed in the chamber 66 of the middle tubular section 60 and a pair of upper and lower end caps 74, 76 which seal the chamber 66. Specifically, the upper end cap 74 is spaced below the upper end plug 54 and attached in any suitable manner, such as by welding, to the upper end 62 of the middle tubular section 60 so as to seal the chamber 66 at the upper end 62, while the lower end cap 76 is spaced above the lower end plug 58 and attached also in any suitable manner, such as by welding, to the lower end 64 of the middle tubular section 60 so as to seal the chamber 66 at the lower end 64. Also, preferably a coil spring 78 is disposed in the chamber 66 of the middle tubular section 60 between the upper end cap 74 and the burnable absorber material 72 contained in the chamber 66 for maintaining the material in a stationary position within the chamber. In the one embodiment of the standardized rod 38 of FIGS. 4 and 8, the burnable absorber material 72 is in the form of a borosilicate glass tube 80 disposed in the chamber 66 of the middle tubular section 60 between its upper and lower end caps 74, 76. The upper end spacer section or tubular extension 68 of the standardized rod 38 has a hollow chamber 82 formed therein which defines an upper space extending on the upper end 62 of the middle tubular section 60 of the rod 38. Similarly, the lower end spacer section or tubular extension 70 of the standardized rod 38 has a hollow chamber 84 formed therein which defines a lower space extending between the lower end plug 58 and the lower end cap 76 of the middle tubular section 60 of the rod 38. The purpose of the spacer sections 68, 70 will become clear hereinafter. Another embodiment of the standardized rod 38' is illustrated in FIGS. 5 and 9. It only differs from the construction of the rod of FIGS. 4 and 8 in the particular burnable absorber material 86 being used therein, such being in the form of a stack of pellets 88 containing a boron compound, B.sub.4 C, and the specific axial lengths of the three sections of the rod. All of the remaining parts of the rod of FIG. 5 which are substantially the same as those of FIG. 4 are identified with the same reference numeral followed by a single prime mark. The embodiments of FIGS. 6 and 7 are identified similarly with double and triple prime marks being used in the respective figures. With respect to the standardized rods 38, 38', 38", 38"' of FIGS. 4 to 7, their upper and lower end spacer sections 68,70; 68',70'; 68",70"; and 68"',-70"' and the middle tubular sections 60,60',60",60"' each has one of a plurality of known different standard axial lengths being selected so that the upper and lower tubular extensions and middle tubular sections when rigidly interconnected together to form any one rod will have a combined axial length which is the same from rod to rod. However, in view that the axial lengths of the middle sections 60,60', 60",60"' of each rod can differ, the rod in effect simulates the prior art custom-designed reduced length rod. The upper and lower end spacer sections are selected to have certain axial lengths in view of the length of the middle section in order to ensure that the overall axial length of the rod will meet the desired standard length. Thus, the end spacer sections merely define empty space of varying axial lengths at the opposite ends depending on the length of the burnable absorber material in the middle section. In FIGS. 4 to 8, none of the axial lengths a, d, g, j of the upper end spacer sections 68,68',68",68"' of the standardized rods are equal. On the other hand, the axial lengths b and e of the middle sections 60, 60' are equal as are the axial lengths h and k of the middle sections 60", 60"'. Finally, the axial lengths c and i of the combined lower end spacer sections and lower end plugs 70,58 and 70",58" are equal as are the axial lengths f and i of the combined lower end spacer sections and lower end plugs 70',58' and 70"',58"'. With such variability in the axial lengths of the individual sections making up the rods 38 in a cluster thereof, it is readily seen that the axial location of the burnable absorber material can be different from rod to rod, although not necessarily different for every one of the rods. Each rod can be fitted with upper and rigidly lower end spacer sections of appropriate axial lengths selected from a number of different standard axial lengths available so that together with a middle section of any one of several different standard axial lengths, an overall rod is assembled having the same length as every other rod regardless of its absorber length or placement. With respect to adjacent rods, the upper and lower ends of the middle sections which contain the absorber material do not normally align with one another and thus provide a "ragged edge" at both the upper and lower ends. The "ragged edge" of the standardized rods of the present invention provides a smoother transition of material characteristics if the maximum peaking factor should occur at such interfaces than in the case of the aligned edge of the custom-fit reduced length rods of the prior art, which provides a significant reduction in the peaking power. FIGS. 10 and 11 show two different patterns of mixed clusters of standardized rods having burnable absorber middle sections of two different axial lengths. In FIG. 10, circles marked with an "x" signify rods having absorber material of one reduced length, such as 104 inches (or 264.16 centimeters), and circles marked with a dot "." signify rods having absorber material of another reduced length, such as 134 inches (or 340.36 centimeters), whereby the effective average axial length of the absorber material across the cluster is 108 inches (or 274.32 centimeters). In FIG. 11, circles marked with an "x" signify rods having absorber material, for instance, of an axial length of 108 inches (or 274.32 centimeters), whereas circles marked with a dot "." signify rods having absorber material of 120 inches (or 304.8 centimeters) in axial length, for an effective average axial length of 110 inches (or 279.4 centimeters) of absorber material across the cluster. It is thought that the standardized reduced length burnable absorber rod of the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.