Patent Number: 051788250
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1A, a fuel bundle of this invention is illustrated. The fuel bundle includes a lower tie plate 14, an upper tie plate 16 and a handle 18 for manipulating the bundle. As is common in the prior art, a channel 20 extends between the upper and lower tie plates and surrounds a group of fuel rods R. Fuel rods R are typically arrayed in rows and columns. By way of example, the arrays can includes matrices of rods from 7 by 7 to 12 by 12 arrays. Indeed, the preferred embodiment includes in FIGS. 5 and 6, a 9 by 9 array and in FIGS. 7 and 8, a 10 by 10 array. Following the suggestion of the prior art, a detailed construction of the fuel rods at their points of transition can be best seen with respect to FIG. 1B, a section taken at the point of transition between fuel rods having larger, bottom diameter fuel tubes and upper smaller diameter fuel tubes. Those having skill in the art will realize that rods R are sealed top and bottom. Bottom tubes 24 contain a column of relatively large diameter pellets. Top tube 22 contains a column of smaller diameter pellets. Reducers 23 form a smooth surface lacking discontinuities for transition between the large diameter tubes 24 and the smaller diameter tubes 22. Referring further to FIG. 1B, a lower spacer L is illustrated. Lower spacer L is shown schematically functioning to keep the large diameter tubes 24 spaced apart, one from another and from the channel structure 20. Upper smaller diameter tubes 22 are shown with a spacer U. It is the function of spacer U to keep the smaller diameter tubes spaced apart one from another and from the channel 20. As will hereinafter be seen, upper spacers U and lower spacers L constitute an array of ferrules (illustrated in FIGS. 2A-2B, 3) or alternately an Inconel grid structure (illustrated in FIG. 4). In either case, a problem at the spacers is created by the tapered rods R. Simply stated, it is from time to time necessary in the life span of a fuel bundle B to remove, inspect and/or replace fuel rods R. Such removal requires partial disassembly of the fuel bundle including removal of upper tie plate 16 and handle 18. Thereafter, the rods are lifted upwardly from the fuel bundle array. In such lifting, it can be seen that upper spacers U have a dual function. First, the springs in the upper spacer U must be sufficiently resilient to bias the smaller diameter fuel tubes 22 against stops in the spacer. Secondly, the spacers must permit the larger diameter rods 24 to be removed. It will be appreciated that fuel bundle B could be inverted for the removal, inspection and/or replacement of fuel rods. Lower tie plate 14 could be removed followed by all fuel rods being withdrawn in an ordinary fashion from the bottom of the fuel bundle. It is the purpose of this invention to obviate the necessity of such inversion during fuel rod removal. Referring to FIGS. 2A-2B, a dimensional analysis with respect to round ferrules F is illustrated. Referring to FIG. 2A, a ferrule F having a nominal thickness of 0.020 is shown containing a fuel rod having a diameter of 0.460 inches. The rod illustrated is shown at large tube 24. A uniform clearance of 0.033 is shown. As is common in the art, ferrule F is provided with stops 30 and a spring 32 which biases rod 24 into the stops. Uniform matrix spacing of the large diameter portion 24 of the fuel rod R occurs. With respect to FIG. 2B, a ferrule F is illustrated having larger stops 31. Larger stops 31 are configured so as top center a 0.420 inch diameter fuel rod R at smaller diameter portion 22. Such biasing occurs via a spring 32 and results in a 0.053 inch clearance uniformly around the ferrule. Referring to FIG. 2C, the ferrule F of FIG. 2B is shown with the large diameter tube 24 placed within it. Specifically, the large diameter tube (which is 0.460 inches in diameter) ends up with only 0.0055 inches of clearance with respect to the ferrule F. Furthermore, those having skill in the art can understand that there is virtually no space available for spring 32. As a practical matter, the design of FIG. 2C is not feasible. Given manufacturing tolerances, both directed to the ferrule F, stops 31 and the diameter of the fuel rod R at the large fuel diameter tube 24, it would be expected by those having skill in the art that interfering contact would occur with such an arrangement. Finally referring to FIG. 2D, a compromise is illustrated. Stops 34 having a 0.0439 inch radial dimension are illustrated with respect to rod R at larger diameter portion 24. Adjacent spring 32 it can be seen that a clearance of 0.0180 inches in clearance is allowed. Removal of a large diameter rod portion 24 from such a ferrule is practicable against the bias of spring 32. The question then becomes what net effect does a ferrule having the dimensions of 2D have with respect to the larger diameter rod portions 24 and the smaller diameter rod portions 22. This is illustrated with respect to FIG. 3. Referring to FIG. 3, 2 side-by-side ferrules F are illustrated. It will be seen that the orientation of the spring and stops is the same in the two ferrules. Specifically referring to upper ferrule F, it can be seen that stops 34 are at the top. Referring to lower ferrule F, stops 34 are identically and symmetrically placed with respect to the top. This orientation is used for all the spacer cells in the preferred embodiment of this invention. In the preferred embodiment, the upper and lower spacer cells are identical. In FIG. 3 the small diameter portion of the fuel rods in the upper spacers are shown as continuous lines. The large diameter portions of the fuel rods in the lower spacers are shown as dashed lines. When the lower large diameter portion of a fuel rod is inserted or withdrawn through the upper spacers, its position is also shown by the dashed lines. It is known that ferrule F surrounding fuel rods can have beneficial flow effects as to passing coolant. Specifically, the ferrules F can function to channel water to flow against the outside surface of the fuel rods R so that boiling within the fuel bundle optimally occurs. The design clearances illustrated in FIG. 2D will produce this desirable effect. Referring to FIG. 4, an Inconel grid-type spacer G is illustrated. The Inconel grid spacer G includes stops 60 and spring portions 62. In the upper grid G, rod R at small diameter portion 22 is shown biased by spring 62 against stops 60. In FIG. 4, at the lower grid G, large diameter rod R at large tube 24 is schematically shown at broken lines 24. The necessary movement of springs 62 to permit passage of such a rod is illustrated. Again, offset of the centers of the rods at the large diameter tube 22 and small diameter tube 24 is illustrated. As before, it can be seen that the respective centers 51, 52 of the large diameter rod portion and small diameter rod portion 22 are offset, one with respect to the other. Referring to FIGS. 5A and 6A the solution is illustrated for a spacer consisting of circular ferrules. FIG. 5A illustrates the upper spacers, and FIG. 6A the lower spacers. The channel cross section 20 is the same for both locations. The spacer band 40 is shown together with some of the ferrules. An enlarged view of a ferrule and fuel rod is also shown in FIGS. 5B and 6B illustrating the offset X of the ferrule center relative to the fuel rod center in each case. For both the lower and upper spacers, the array of fuel rods is centered in the channel. The centers of the upper small diameter part of the fuel rods are directly above the centers of the lower large diameter part of the fuel rods. Because the fuel rod centers are offset relative to the spacer cells, the spacers must be offset with respect to the channel. The spacers are located in the channel by spacer band stops 70, 71, 72. Since the relative displacement of rod centers an cell centers is in the vertical direction of the figure, stops 70 have the same height on the upper and lower spacers. In FIG. 5A, the fuel rod diameter is small, and the ferrules are displaced downward (in the plane of the Fig.) relative to the fuel rod centers. The spacer band stops 71 at the top are larger than the stops 72 at the bottom. In FIG. 6A, the situation is reversed for the lower spacers, and the upper band stops 73 are smaller than the lower band stops 74. With such a scheme, the respective centers of the large diameter tubes 24 and the small diameter tubes 20 exactly overly one another. It will be realized that this precise overlying cannot occur while a fuel rod is being withdrawn. Specifically, when the larger diameter portion 24 of rod R is in the upper spacer U, displacement of the center 51 in an upper spacer U must inevitably occur with respect to a lower spacer L. It will be remembered, however, that such rods are flexible. Specifically, and during rod removal for inspection, flexibility of the rods will accommodate the movement described herein. Referring to FIGS. 7A and 8A, the solution for the Inconel grid-type spacer can be seen to be precisely identical. Referring to FIGS. 7A and 8A, stops 70 on sides have equal heights for the upper and lower spacers. In FIG. 7A the upper spacer stops 91 are larger than stops 92, while on FIG. 8A the upper spacer stops 93 are smaller than stops 94. An enlarged view of a cell and fuel rod is shown for each case in FIGS. 7B and 8B respectively illustrating the offset X of the cell center relative to the fuel rod center in each case. DESCRIPTION OF THE ALTERNATIVE EMBODIMENT In the alternate embodiment the upper spacers are identical to those of the preferred embodiment. The lower spacers are centered in he channel, and the lower portions of the fuel rods are centered in the cells, as in the prior art. FIG. 9A shows a ferrule type spacer with the fuel rods centered in the ferrules, and FIG. 10A shows an Iconel grid spacer with the fuel rods centered in the cells. In both cases the stops on the bands are of equal size on all four sides.