Patent Number: 053717686
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a typical fuel bundle B having the spacer of this invention is illustrated. The fuel bundle includes lower tie plate L having nose piece N. A plurality of full length fuel rods R extend from lower tie plate L to upper tie plate U. In the usual embodiment, one or more of the fuel rods R is a part length fuel rod R.sub.p. These fuel bundles are surrounded by a channel C extending from lower tie plate L at least to the vicinity of upper tie plate U. As is known in the prior art, the bands of the spacers can include flow tabs T. (See FIGS. 1 and 2). The reader will understand that the fuel bundle B is shown only for a portion of its length. Specifically, the fuel bundle is sectioned at surrounding channel C so that the spacers S.sub.1, S.sub.2, and S.sub.3 can be seen. The reader will understand that the invention here is especially applicable to spacers S.sub.2 and S.sub.3. Referring to the perspective view of FIG. 2, the construction of a spacer S.sub.2 and S.sub.3 without the placement of the fuel rods within the spacer is illustrated. The spacer is a two level spacer including preferably lower ferrule layer F and upper swirl vane layer V. Lower ferrule layer F includes conventional side-by-side ferrules 14 having shortened height and annular walls of minimum thickness. Normally, such ferrule spacers have heights in the order of 1.2 inches. The preferred spacer of this invention has a preferred height in the order of one-half to three-quarters of the normal value. That is to say, the ferrule height is held to a limit of 0.9 inch or less. A similar modification has been made to the thickness of the spacer. Normally, ferrule spacers have wall thickness in the order of 0.020 inches. In the construction here utilized, the wall thickness of the material of the ferrule is reduced below 0.020 inches--preferably in the range of 0.015 inches. These reductions in material dimensions are required for designs which normally have a high inherent bundle pressure drop. The remainder of the construction of the ferrule layer F is conventional. The spacer is surrounded by band 16 and includes stops 18 and loop springs 20 interior of the ferrules (See FIG. 3B). Vane portion V is just as easily understood. Referring to FIG. 4A, a material which is preferably Zircalloy is shown with generally "I" shaped upstanding metal cut out sections connected at the bottom by continuous band 42. At the top, sections 40 have an interrupted band 44. Centrally of the generally "I" shaped bands are central tabs 46. The width of sections 40 is generally the minimum width of the subchannel region 48 formed between adjacent ferrules 14 (See FIG. 3D). This minimum width is utilized so that when the respective members 40 are twisted, the resulting twisted structure only overlies the subchannel region at 48. Continuous arm 42 includes tabs 43. Tabs 43 are spaced for keying to the tops of the assembled ferrules 14. These tabs 43 may conveniently serve as points of attachment. Preferably, vanes 30 include at least 90.degree. twists from top to bottom. As here shown, the channels include the illustrated 180.degree. twists. Such twisting enables arms 44 to form a grid parallel to continuous arm 42 at the bottom with arms 46 being joined at 90.degree. to form a continuous interval. Preferably continuous arm 42 is fastened to the top of ferrule array F. Returning to FIG. 2, the paths along which the swirl vanes V are attached is shown. Preferably, a row R.sub.1 of vanes V is placed between the first and second rows of fuel rods. Likewise, a row R.sub.2 of vanes V is placed between the second and third row of fuel rods. It will be observed that the matrix illustrated is a 10 by 10 array of fuel rods having water rods W.sub.1 and W.sub.2 each displacing four fuel rods from the matrix. In this event, partial rows R.sub.3 of swirl vanes V can be used between the fuel rods of the third and fourth rows. Having set forth the construction of spacers S.sub.2 and S.sub.3, the operation of the spacer can now be set forth. It will be understood that when a reactor operates under normal power loads--in excess of 80% of available power--the upper two phase region of bundle B constitutes a region where rods R must be provided with a desired liquid film coating for the generation of steam. This will be the region where spacers S.sub.2 and S.sub.3 will be located. It will further be understood, that spacer S.sub.1 is not a candidate for this spacer construction. Simply stated, nuclear loading of fuel rods R is designed so that at the top end of the active fuel region, fuel rods R do not have the heat output that threatens the "dryout" conditions on the surfaces of the fuel rod cladding. It has been set forth that the spacer construction of spacers S.sub.2 and S.sub.3 has minimized the pressure drop effect of adding swirl vanes to an existing spacer design. This minimized pressure drop is at least due to not decreasing the flow area through the axial length of the spacer. Therefore, the more mobile, lower density steam will tend to avoid the region in favor of the higher density, less mobile liquid water. Regarding ferrule layer F, it will be understood that the interstitial volume between each ferrule 14 and each fuel rod R is an area of relatively high flow resistance. Regarding the vane layer V, it will be understood that the high velocity flow exiting from the subchannels between the spacer ferrules will immediately impact upon the swirl vanes. There the liquid (being the higher density fluid component) will be centrifugally thrust toward the surrounding fuel rods while vapor will continue relatively unobstructed through the vane region. At the same time, the subchannel region 48 between ferrules 14 will define a flow path of relatively low resistance. Thus, steam vapor passing the level of ferrule layer F can in large measure be bypassed to flow in subchannel region 48. It will be understood that it is preferred to place vane layer V overlying ferrule layer F. There is a reason for this order. Specifically, it is the function of swirl vanes 40 to permit steam to rise directly upwardly along vanes 40. At the same time, water particles will be centrifugally impelled from vanes 40 to surrounding fuel rods. Presuming vane layer V was below ferrule layer F, such impelling of water to and toward fuel rods R would cause water to contact directly the ferrules 14 and not the rods R. This does not appear to be as desirable as permitting impelled water to impact otherwise unobstructed fuel rods R--as where vane layer V overlies ferrule layer F.