Patent Number: 050646079
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments according to the present invention will now be described with reference to the Figures. In this description certain dimensions are indicated, but it is to be understood that these dimensions are merely illustrative to enable a person of ordinary skill in this field to make and use the invention. The dimensions indicated, however, are not intended to limit the invention. Referring to FIG. 1, there is shown an elevational view of a conventional "17.times.17" PWR fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. As suggested above, the present invention is particularly suited for such a "17.times.17" fuel assembly wherein the rod outer diameters are about 0.4". However, the present invention is also applicable to other than a 17.times.17 fuel assembly, but may provide lesser benefits (or may not be required) for designs which utilize large diameter control/grey rods (i.e., diameters of 0.8 to 1.0 inch). Fuel assembly 10 basically includes: a bottom nozzle 12 for supporting the assembly 10 on a lower plate (not shown) in the core region of a reactor (not shown); a number of longitudinally extending guide thimbles 14 projecting upwardly from the bottom nozzle 12; a plurality of transverse grids 16 axially spaced along the guide thimbles 14; an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16; an instrumentation tube 20 located in the center of the assembly 10; and a top nozzle 22, attached to the upper ends of the guide thimbles 14, to form an integral assembly 10 capable of being conventionally handled without damaging the assembly components. The top nozzle 22 includes a transversely extending adapter plate 24 having upstanding sidewalls 26 (the front wall being partially broken away) secured to the peripheral edges thereof to define an enclosure or housing. An annular flange 28 is secured to the top of the sidewalls 26. As shown in FIGS. 1 and 2, disposed within the opening defined by the annular flange 28 of top nozzle 22 is a spider assembly 30, having radially extending flukes 32 connected to the upper ends of grey rods 34 which are adapted to be inserted down through the guide thimbles 14 of the fuel assembly 10. The spider assembly 30 is connected to a control/drive mechanism (not shown) which is operable in a well known manner to move the rods 34 in and out of the guide thimbles 14. As best seen in FIG. 2, each grey rod 34 includes an elongated, metallic tube or cladding 36. Preferably, the cladding 36 is stainless steel or INCONEL having an outer generally constant diameter. The grey rod 34 also includes means in the form of respective upper and lower end plugs 38, 40 for closing or sealing the opposite ends of the cladding 36. The upper end plug 38 has an upwardly extending integrally formed stem section 39 with an externally threaded end for connection to the outward end 41 of the radial fluke 32 of the spider assembly 30. The lower end plug 40 is cone-shaped. Slidably disposed within the cladding 36 and resting on the lower end plug 40 is a stack or plurality of closely packed pellets 50 (the specific arrangement and characteristics thereof will be described shortly hereafter) which only partially fill the cladding 36, leaving a space or axial gap between the top of the pellets 50 and the upper end plug 38 defining a plenum chamber 42 for receiving gases generated during use. A coil spring 44 is disposed within this plenum chamber 42 and held in a state of compression between the upper end plug 38 and the top pellet to thereby maintain the plurality of pellets 50 in their closely packed arrangement during use of the grey rod 34. The inner diameter of the guide thimbles 14 is usually chosen to be the maximum permitted by the fuel assembly lattice in order that the maximum possible diameter grey rod 34 can be inserted therein. It is desirable to maximize the diameter of the plurality of pellets 50 in the grey 34 rod because the absorption effectiveness of the rods is very strongly dependent, particularly in thermal neutron reactors, on the surface area of the pellets 50. For this reason, and to promote heat transfer, there usually are narrow clearances between the pellets 50 and the cladding 36, and between the rod 34 and its guide thimble 14. The gap between the pellets 50 and the cladding 36 must be large enough, however, to accommodate any swelling which the pellets 50 may experience when they are irradiated while in the reactor core. It is very important that the swollen pellets 50 not press too strongly against the cladding 36 inner wall because significant cladding deformation can result, causing the grey rod 34 to jam in its guide thimble 14. However, if the gap is too large, chips that are dislodged from the pellets 50 as a result of the grey rod 34 reciprocation will settle in the gap in the lower tip of the rod 34 and quickly deform the cladding 36 as the pellets near the lower end plug 40 swell. The present inventors have found that ZBCLF is attainable by increasing the relative worths of each grey rod to a level approximately intermediate that of conventional grey rods and control rods. In order to achieve this goal, several criteria had to be satisfied in order to provide an efficient design: (a) The grey rod worth is approximately 40-60% greater than the conventional grey rod in order to obtain ZBCLF. PA0 (b) The stiffness of the grey rods is not significantly increased, if at all, over conventional grey rods. PA0 (c) The weight of grey rod clusters are not greater than that of conventional clusters. The stiffness requirement is based upon the desire to prevent additional drag forces which, as noted above, could be detrimental to stepping performance and could adversely effect the wear characteristics of the grey rod clusters. The weight restriction is also based upon the desire to limit stepping forces during grey rod clusters withdrawal and insertion. The present invention uses hybrid grey rods to obtain the required reactivity worth for ZBCLF. These grey rods include combinations of a strong absorber material, such as hafnium, and a weak absorber material, such as stainless steel, INCONEL or zirconium, in proportions to achieve the desired reactivity worth. Since the worth of the strong absorber materials is significantly greater than that of e.g. stainless steel, INCONEL or zirconium, the volume fraction of hafnium or other strong absorber material required to increase the overall reactivity worth of the grey rods to the desired level is relatively small. FIGS. 3-6 show alternate ways to vary the volumetric concentration of the strong absorber material within the present 0.381 inch O.D. of a 1733 17 configuration fuel assembly. Based upon the reactivity worth requirements, one can determine the volume fraction of hafnium or other strong absorber material required according to conventional analytical methods. One of the configurations shown in FIGS. 3-6 would then be selected based upon economic and manufacturing considerations. FIG. 3 shows a first embodiment of the grey rod 34 according to the present invention. In this embodiment, the cladding 36 is, e.g. a stainless steel or INCONEL tube of about 0.344" I.D. and about 0.381" O.D. The plurality of pellets 50, includes a first group of weak absorber material (e.g. stainless steel or INCONEL) solid pellets 52, each of which is interposed between two strong absorber material (e.g., hafnium) solid pellets 54. The hafnium or other strong absorber material pellets 54 can be of shorter height than the stainless steel or INCONEL pellets 52 to obtain the worth desired. Alternatively, hollow or annular pellets could also be used in this embodiment to adjust the volume fraction of the absorber. A space 56 is longitudinally provided between the outer diameter of the plurality of pellets 50 and the inner diameter of the cladding 36. FIG. 4 illustrates a second embodiment including a relatively thick stainless steel cladding 36 of about 0.381" O.D. and approximately 0.26" I.D. The cladding I.D. is not necessarily a fixed value but could vary between 0.26 to 0.30 inch. The plurality of pellets 50 includes relatively small (approximately 0.25" O.D.), preferably solid hafnium or other strong absorber material pellets 60. Again, a longitudinal space 62 exists between the stack of pellets 50 and the I.D. of the cladding 36. FIG. 5 shows a third embodiment in which the plurality of pellets 50 includes a group of stainless steel or INCONEL pellets 70. Each pellet 70 is an annular or hollow pellet positioned inside the cladding 36. Each pellet 70 receives at the central hole thereof a relatively small diameter strong absorber material (such as hafnium) pellet 72. Alternatively, the holes of the group of pellets 72 could be filled by a continuous wire made of hafnium or another strong absorber material. The cladding 36 is again about 0.381" O.D., 0.344 I.D., as with the first embodiment, and a space 74 exists between the cladding 36 and the plurality of pellets 50. FIG. 6 illustrates a fourth embodiment, wherein the plurality of pellets 50 includes a group of pellets 80 made up of a homogeneous alloy of a weak absorber material, such as zirconium, and a strong absorber material, such as hafnium. The parent metal is Zirconium ZIRCALOY (the trademark for an alloy composed mostly of zirconium with small amounts of various additives such as Tin, Iron, Chromium and Nickel for corrosion resistance. There are currently two basic ZIRCALOY alloys, Zirc-2 and Zirc-b 4, which differ to a minor extent in Nickel and Iron/Chromium content into which a small fraction of hafnium is blended. Hafnium is, in fact, a trace element of naturally occurring zirconium and can be readily combined with the zirconium in a vacuum melting process. That is, materials such as ZIRCALOY and hafnium are susceptible to high temperature oxidation when exposed to air. In the vacuum melting process, all of the air is evacuated (pumped) from the melting chamber before the materials are heated, thus preventing oxidation. The process is widely used and well known in the materials field. The percentage of hafnium is adjusted to obtain the desired reactivity worth of the grey rods 34. The nuclear designer would specify the desired mass fraction of hafnium required to obtain the reactivity worth and a small ingot of zirconium or ZIRCALOY with the desired concentration of hafnium would then be prepared. This ingot would then be formed into rods of proper diameter, cut into pellets 80 of the proper length, and centerless ground. That is, a rod is passed through a grinder with floating heads and a rod of a smaller diameter with very close tolerance is produced without the necessity of knowing the true center of the initial rod. The pellets are then inserted into the stainless cladding 36. As in the case of the embodiment shown in FIG. 3 described above, hollow or annular pellets 80 could be used. Again, a space 82 exists between the plurality of pellets 50 and the cladding 36. The method used by the nuclear designer would be the same regardless of which of the mechanical or alloy embodiments described above were used. More particularly, depending upon a particular application and the desired load follow strategy to be used, the nuclear designer would determine the number of grey rods needed and the reactivity worth requirement of each rod. The reactivity worth defined above would then be used to determine the relative amount of strong neutron absorber material required in each grey rod cluster assembly (i.e., 24 rods in a 17.times.17 fuel assembly). The grey rod mechanical designer would then determine what size components would produce the desired volume fraction of the strong absorber material. For example, if the embodiment shown in FIG. 3 were used, the designer would determine the cladding thickness and pellet heights. Or, if the alloy embodiment shown in FIG. 6 were used, he would determine the volume fraction of hafnium which would be added to the zirconium to produce the desired worth. More specifically, a grey rod cluster in a 17.times.17 fuel assembly (i.e, 24 rods) has a total volume of 394 cubic inches (i.e., 24.times..pi..times.0.381.sup.2 .times.144/4). If the nuclear designer required 100 cubic inches of a strong absorber to achieve the desired grey rod worth, the grey rod mechanical designer would find the optimum configuration which would contain 294 cubic inches of stainless steel and 100 cubic inches of either hafnium, silver-indium cadmium, boron carbide or other strong absorber material. As described above, according to the present invention, each grey rod 34 can have a particular worth by choosing a particular combination of a strong absorber material like hafnium and a weak absorber material like stainless steel or zirconium. In this way, the cluster of grey rods can include rods of the same or of varying worths, thereby providing a cluster of a particular worth. Studies have indicated that, even with a reduced number of grey rods, about a 40-60 percent increase in individual grey rod worth is sufficient to achieve ZBCLF capability. It appears that 20 grey rod clusters, instead of the conventional 28, would be sufficient to obtain the desired load follow characteristics. The elimination of eight control rod clusters and associated equipment results in a significant capital cost reduction. This includes the cost of the grey rod clusters, the drive rods, rod grinders, head penetrations and control rod drive mechanisms. Further, the improvements in power distribution control strategy to accomplish ZBCLF described herein can be incorporated with no adverse impact on load follow behavior. Finally, these benefits are made possible without exceeding the stiffness and weight parameters currently followed for grey rods. The foregoing is considered illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention and the appended claims.