Patent Number: 043269222
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there is shown an exemplary pressurized nuclear reactor utilizing composite fuel assemblies 10 in accordance with this invention. The reactor includes a core 12 of the fuel assemblies 10 arranged to approach the configuration of a right circular cylinder. The assemblies 10 are supported within a vessel 14 between an upper core plate 16 and lower core plate 18 both of which are perforated to allow the flow of a coolant fluid therethrough. The reactor coolant fluid, preferably a liquid such as water, enters the vessel 14 through an inlet 20, flows downwardly through an annulus 22, is turned in a plenum 24, and passes upwardly through the core 12. In similar reactor configurations the coolant enters the vessel 14 below the core and passes upwardly. The coolant absorbs energy as it flows upwardly through and about the fuel assemblies 10, is discharged from the vessel 14 through an outlet 26, and discharges energy to apparatus (not shown) typically for the ultimate purpose of electrical power generation. The power generated in the core 12 can be controlled in various well-known manners including use of a neutron absorber, such as boron, flowing with the coolant together with neutron absorbing control elements 28, top mounted in accordance with this invention, reciprocatingly positionable within or about the fuel assemblies 10 by drive apparatus 30. While a multitude of core and fuel assembly configurations exist, this invention is particularly beneficial in thermal neutron cores having elongated vertically positioned fuel assemblies through which coolant flows in a generally upward direction. A preferred fuel assembly 10 in accordance with the invention is shown in FIG. 2. It includes a plurality of upper fuel rods 40 and a plurality of lower fuel rods 42 arranged in a regular preferably rectangular array. The rods 40, 42 are peferably cylindrical and the upper rods 40 are of smaller diameter than the lower rods 42. Accordingly, the number of upper fuel rods is greater than the number of lower fuel rods. Each rod 40, 42 includes an hermetically sealed metallic cladding 44 within which is disposed fissonable nuclear fuel preferably in the form of cylindrical pellets 46. The fuel can be of various types well known in the art, such as enriched uranium, and in one embodiment advantageously includes enriched uranium in the upper rods 40 and plutonium fuel in the lower rods 42. Where, for example, both the upper and lower rods include uranium of similar enrichment, although differing enrichments can be utilized, the power generated per unit length of each fuel rod is higher in the lower rods 42 than in the upper rods 40. Thus, coolant flowing in an upward direction through and about the assembly 10 is initially heated in the lower portion of the assembly and further heated to a higher temperature in the upper portion. Departure from nucleate boiling (DNB) concerns are, however, alleviated and a higher coolant core discharge temperature attained as a result of the lower energy output per unit length of upper fuel rod measured, for example, in kilowatts per foot. Table I presents exemplary parameters resulting from utilization of the inventive fuel assembly in a pressurized water reactor core utilizing enriched uranium fuel in upper rods 40, approximately six feet in length, and in lower rods 42 approximately six feet in length. The base for comparison is a core of assemblies having twelve foot long fuel rods in a fifteen by fifteen array, and is compared to composite assemblies 10 having lower rods 40 in a fifteen by fifteen array and upper rods 42 in, respectively, a twenty by twenty array and a thirty by thirty array. TABLE I ______________________________________ Composite Composite Base Lower: 15 .times. 15 Lower: 15 .times. 15 15 .times. 15 Upper: 20 .times. 20 Upper: 30 .times. 30 ______________________________________ No. of Fuel 204 316 816 Rods Average Linear Power, kw/ft 7.06 4.6 1.76 Peak Linear Power, kw/ft 18.8 12.1 4.7 Peak UO.sub.2 Center-Line .about.4200 .about.3000 .about.1600 Temperature, .degree.F. Increase in Coolant Outlet -- 20 50 Temperature, .degree.F. ______________________________________ As seen from Table I the increase in coolant temperature is greater where the number of upper rods is greater. Additionally, the number of fuel rods is less than that of a complete array as a result of the fabrication of the assembly 10 and incorporation of additional components such as guide thimbles discussed below. For example, a complete 15.times.15 array would have 225 fuel rods; however, only 204 fuel rods are utilized, the balance of locations being occupied by guide thimbles. In addition to the fuel rods 40, 42 the assembly 10 includes (FIG. 2) a top nozzle 48 and a bottom nozzle 50 affixed by guide thimbles 52 to form a skeletal load transmitting unit. The thimbles 52 can be used merely for assembly support of for guiding the control element rods 54 (FIGS. 1 and 6) into and out of the assembly or for locating other core components. Upper 56 and lower 57 lattice grid structures, shaped generally as an "egg crate", are affixed to the thimbles 52 at selected elevations. The grids 56, 57 form a cell 58 about each rod or thimble, shown best in FIGS. 3 through 5. The cells 58 for the fuel rods 40, 42 provide lateral support while allowing axial rod expansion. The grids 56, 57 include, in addition to typical flow mixing vanes 59 and spring acting supports 61, outer straps 60 which form a peripheral boundary about the rod array, and inner straps 62 together forming the individual cells 58. The peripheral dimensions of the upper 56 and lower 57 grids, defined by the outer straps 60, are the same. The grids 56, 57 must be arranged not only to provide support for the rods 40, 42 of differing diameter, but also to provide cells 58a, 58b for attachment to the thimbles 52 which are, in the preferred embodiment, of constant, preferably circular cross section. Since the upper and lower grid cells are of different dimensions, provision must be made for attachment of the guide thimbles. Accordingly, the lower fuel rods 42 are preferably of substantially the same cross section as the thimbles 52 so that the cells 58 and 58a of the lower grids 57 are all of the same dimensions. The cells 58 of the upper grids 56, however, include cells 58b which receive guide thimbles 52 and are enlarged in comparison to the cells 58 supporting the upper fuel rods 40. As shown in FIG. 3, in the exemplary composite fuel assembly 10 the cells 58b each represent a combination of four upper grid fuel rod receiving cells. In a modification, the thimbles 52 can also be larger than the lower fuel rods 42, so that cells are also provided in the lower grids 57 which are larger than the cells receiving the fuel rods 42. It can also be seen from a comparison of FIGS. 3 and 4 that the cells 58b, which receive guide thimbles, are larger than the cells 58 of the lower grid 57. As the grids are affixed to the thimble 52 within a cell, a means for affixing the thimbles to both upper and lower grids must be provided. One means for such attachment is shown in FIG. 6. Here a sleeve 64 is provided and brazed 66, welded, or otherwise attached to the straps of the cell 58b of the upper grid 56 at, for example, four locations. The inner diameter of the sleeve 64 can be the same as the across flats dimension of a lower grid cell 58, 58a so as to maintain alignment of the thimble. In this case, the guide thimble can be brazed or otherwise directly attached to straps on the lower grid. Alternatively a large braze 66a, (FIG. 3) weld, or other attachment between the innerstrap 62 and the thimble outer periphery can be made thus eliminating use of the sleeve 64. The thimble can also be bulged or expanded above and below the sleeve 64 for the attachment. Or, the cell 58b can be formed with a strut 68 to which the thimble is attached. Further, the thimble 52 can have a cross section smaller than lower fuel rods 52 and accordingly the lower grid cells, and also smaller than the large cells 58b of the upper grid. In this configuration sleeves 64 can be incorporated separately in both the upper and lower grid cells wherein the sleeve have the same inside diameter for receiving the thimble but the separate sleeves will have differing outer diameters matched to the respective size of an upper grid cell 58b and a lower grid cell 58a. Further, as shown in FIG. 7, the thimble 52 can include a different diameter along its length so long as the inner aperture 53 is sufficiently large throughtout its length to allow passage of a control element rod 54. The aperture 53 can also have a varying size, for example, being larger at the upper portion and smaller at the lower portion, thereby additionally providing a dashpot effect for the control rod 54 at the lower portion of the thimble 52. The outer cross sections of the varying sized thimble can be sized to most compatibly fit within the grid cells, with or without sleeves. It is preferable that a space 70 (FIGS. 2 and 5) be provided between the bottom of the upper fuel rods 40 and the top of the lower fuel rods 42. Space 70 can alleviate a pronounced change in power distribution which otherwise could occur at the interface of the lower and upper fuel rods, particularly where mixed oxide fuel is utilized. The space preferably is no larger than about two percent of the sum of the lengths of the upper and lower fuel rods. Too large of a space could allow an excessive area of moderating coolant in the center of the core, undesirably causing flux peaking. This effect, however, can be counteracted by fabricating the fuel rod sealing end caps in the interface area, or an attachment thereto, of a material and size to absorb excessive neutrons. It can also be counteracted by placing plenums 2, 4 in the fuel rods, which allow for a buildup of fission product gases, at the bottom of the lower fuel rods and at the top of the upper fuel rods. A support for the fuel pellets of the upper fuel rods, such as a spring 6 or other support means, can be utilized to maintain a lower fuel rod plenum. Similarly, depleted fuel in pellet or other form can be included at the bottom of the upper fuel rods and the top of the lower fuel rods or, inert ceramic spacers 8 can be positioned at these locations. Across the space the lowermost upper grid 56 is preferably rigidly affixed to the uppermost lower grid 57 through their outer straps 60. This attachment can take many forms, including use of thin peripheral straps 72 or a larger peripheral strap 74, shown alternatively in FIG. 5. The straps 72, 74 are disposed externally of the rod array and can include structure as utilized in typical lattice grids, including flow mixing vanes 59, support springs 61, and flow openings 80. The lowermost upper grid is rigidly affixed to the uppermost lower grid in order to strengthen the assembly at its central region where relatively high stresses and deflections can occur in the event of a seismic occurrence. The space 70 between the upper and lower fuel rod arrays is particularly useful where plutonium fuel is utilized in the lower rods and uranium in the upper rods to alleviate a power peak which occurs at the interface when the two fuels are used in other than a homogeneous mixture. The composite assembly 10 is further suited to utilization of mixed oxide fuel. The relative quantities of, for example, uranium and plutonium fuel can be adjusted by varying the length of the upper 40 and lower 42 fuel rods. It is preferable that plutonium bearing lower fuel rods be disposed over one half to two thirds of the fuel assembly and uranium bearing rods over the remaining one half to one third of the assembly. The inventive composite assembly is economically advantageous regarding plutonium utilization since plutonium fuel, for example in the form of plutonium oxide, is appreciably more expensive to fabricate than uranium as, for example, uranium dioxide, due to the high plutonium toxicity which requires remote fabrication. Placing the plutonium in larger diameter rods reduces the number of plutonium bearing rods per assembly, and accordingly the cost. Disposition of plutonium in the lower portion of the core is also beneficial in terms of nuclear control characteristics. By locating the plutonium near the bottom of the core where the worth of top-mounted control elements (as shown in FIG. 1) is lessened, the impact of the high neutron capture cross section of plutonium is reduced. Further, the effect of the strong moderator temperature coefficient of reactivity of plutonium on control requirements is reduced since the full load to no load coolant temperature swing is lessened in the plutonium zone where coolant enters the core 12. These advantages are discussed further in U.S. Pat. No. 4,096,933 in the name of R. F. Barry entitled "Core For A Nuclear Reactor". It will also be noted that the disclosed assembly is beneficially responsive under assumed accident conditions where the core is temporarily drained of coolant. Upon such conditions a plurality of redundant systems operate to refill the reactor vessel from the bottom to the top. Accordingly, the more reactive lower rods are covered with coolant faster than the less reactive upper fuel rods, adding additional safety margin as compared to cores generating generally uniform power along their length. There has therefore been described a composite fuel assembly useful in achieving higher reactor coolant temperatures and better overall reactor plant efficiency. The assembly is further beneficial for mixed oxide fuel utilization, as well as providing added flexibility in all types of core designs. It will be apparent that many modifications and additions are possible in view of the above teachings. It therefore is to be understood that within the scope of the appended claims the invention may be practiced other than as specifically described.