Patent Number: 051788250
Section: summary

This invention relates to fuel bundles for boiling water nuclear reactors utilizing the prior art suggested concept of tapered fuel rods having relatively large diameter bottom tubes and relatively smaller diameter top tubes placed in the fuel rod array within nuclear fuel bundles. More particularly, a nuclear fuel bundle and improved upper fuel rod spacer for use within the fuel bundle is disclosed which enables tapered fuel rod removal from the top and replacement from the top of the fuel bundle. SUMMARY OF THE PRIOR ART Ueda Japanese Kokai Patent No. Sho 52-50498 of Apr. 22, 1977 discloses a fuel bundle having partial length fuel rods. These partial length fuel rods are arrayed is side-by-side relation so as to define large central voids in the center of fuel bundles within a boiling water reactor. The large central void defined by the partial length rods is in the upper two phase region of the fuel bundle and has the shape of a cone. In a first embodiment of Ueda, the cone in the upper portion of the fuel bundle is filled with a correspondingly large central conical water rod. In a second--and apparently preferred embodiment of Ueda--the large central conical void is vacant. This large central conical void permits the discharge of steam within the fuel bundle. The purpose of the design of Ueda is to provide a kind of fuel assembly that can suppress "output peak", that is the tendency of portions of the fuel rods in the upper two phase region adjacent the control rod to heat to levels approaching the thermal limit of the fuel rods. The function of the above mentioned disclosed designs "is characterized by the feature that the volumetric ratio of themmoderator (water) to fuel is made to change in the axial direction in the channel box, so that the above-mentioned peak caused by the distribution of axial voids can be inhibited." The Ueda disclosure continues. It points out that in addition to the conical voids, other expedients can be used. Specifically the suggestion is made as follows: "In addition, similar effects as above can be achieved by using fuel rods with changed diameter in the axial direction, i.e., larger outer diameter of the fuel rods in the lower portion of the fuel assembly, and small diameter of the fuel rods in the upper portion of the fuel assembly." No practical design of the suggested fuel rods is disclosed. SUMMARY OF THE INVENTION In a nuclear fuel bundle for a boiling water reactor, the prior art suggested concept of tapered fuel rods forming the bundle array is disclosed in a practical design. This design includes a fuel bundle and an improved associated spacer. A tapered fuel rod is utilized having a large diameter bottom tube for holding corresponding large diameter fuel pellets at the fuel rod bottom, a smaller diameter top tube for holding correspondingly smaller diameter pellets at the top of the fuel rod, and a bell reducer situated between the large diameter lower fuel tube and the smaller diameter upper fuel tube. This bell reducer tapers from the large outside diameter of the lower fuel tube to the smaller outside fuel diameter of the upper fuel tube and forms a smooth transition between the large and small diameter sections of the fuel rod. Current spacer designs can easily be adapted to the upper small diameter portion of the fuel rods, and to the lower large diameter portions of the fuel rods, resulting in one spacer design for the upper portion of the fuel bundle and a different spacer design for the lower portion of the fuel bundle. However, a major problem arises with such a fuel bundle; the large diameter portion of the fuel rods cannot pass through the upper spacers. Current practice in fuel bundle assembly is to insert fuel rods into the bundle from the top. Insertion of fuel rods from the bottom of the bundle would greatly complicate the fuel bundle assembly. In addition, it is sometimes necessary to replace fuel rods in fuel bundle after operation in a nuclear reactor. Current practice is to remove and insert from the top of the fuel bundle. Insertion from the bottom would require major changes in reactor servicing equipment, would be a more complex procedure, and would require more time. Improved spacer designs are disclosed which allow insertion and removal of tapered fuel rods. An improved fuel rod spacer is disclosed for use with the upper and smaller tube portions of the fuel rods. This upper spacer defines a matrix of rod holding positions having sufficient dimension for enabling both the upper smaller diameter tube and the lower large diameter tube to pass through the cells defined for each fuel rod. A spring having a relatively wide flexure range protrudes into each cell. When the fuel bundle is conventionally assembled, the spring in each cell biases the upper smaller diameter tubes in their deigned side-by-side relation against stops defined at each cell. At the same time, the spring is provided with sufficient flexibility to allow the lower large diameter tube and the transition bell reducer to pass through the individual cells of the upper spacer. With the improved upper spacer, a fuel rod can be lifted vertically upward and out of the fuel bundle or replaced vertically downward into the fuel bundle. In the upper spacers the stops are smaller than in a conventional spacer. The reduced stop dimension provides additional space in the cell, allowing the larger diameter portion of the fuel rod to pass through. In the assembled condition with the small diameter portion of the fuel rod in the cell, and with the fuel rod biased against the stops by the spring, the fuel rod is off-center with respect to the cell. All of the cells and stops in the spacer have the same orientation, so that a uniform array of fuel rods results. Two embodiments are disclosed for the lower spacers. In the preferred embodiment, the cells are identical to those of the upper spacers. When the large diameter portion of the fuel rod is biased against these stops, the fuel rod is off-center with respect to the cell, in the direction opposite to that of the upper portion of the fuel rod in the upper spacers. All of the cells and stops of the lower spacers have the same orientation, resulting in a uniform array of fuel rods. In an alternate embodiment, the stop dimensions are different from those of the upper spacers, and are chosen to center the fuel rods in the cells. There results a fuel bundle which has a uniform distribution of fuel and coolant with resultant uniform critical power response, no regions of non uniform vapor flow, and improved flow area in the upper two phase region of the bundle. OTHER OBJECTS, FEATURES AND ADVANTAGES An object of this invention is to disclose a practical design for a fuel bundle utilizing an array of vertically upstanding tapered fuel rods. The prior art suggested concept of a tapered fuel rod is provided with an upper smaller diameter portion, a lower larger diameter portion, and a reducer providing a smooth transition there between. An improved spacer is disclosed. The spacer--like all prior art spacers--defines a plurality of individual cells with stops for the bias of the fuel rods to a designed limit of side-by-side spacing. The spacer--unlike all prior art spacers--has special characteristics allowing the removal and replacement of the tapered fuel rods from the top of the fuel bundle. Accordingly, each cell of the spacers in the upper portion of the fuel bundle is of sufficient dimension to permit withdrawal of the lower large diameter portion of the fuel rods. At the same time, springs within each of the cells of the upper spacers have two discrete design functions. In the first design function, the springs act to bias the upper smaller diameter portions of the fuel rods to their designed side-by-side spacing. In a second design function, the springs deflect sufficiently to permit serial passage of the reducer and large diameter portion of the fuel rod. An advantage of the disclosed fuel bundle and improved spacer is that the fuel bundle thus permits conventional removal and replacement of the individual fuel rods from the top of the fuel bundle without requiring inversion of the fuel bundle. A further object of the invention is to disclose a cell arrangement which preserves a uniform rod to rod spacing while each fuel rod center is offset from the center of the cell surrounding it. In this arrangement of cells the orientation of spring and stops is the same in all cells. The direction and magnitude of the fuel rod center displacement relative to the cell center is the same for all cells. In this arrangement each spring can act on only one fuel rod. The prior art for ferrule spacers where one spring acts on two adjacent fuel rods cannot be used. A further object of this invention is to disclose a spacer format for maintaining both the upper and lower portions of the tapered rods in perfect verticality. According to this aspect of the invention, the orientation of the cell stops and springs is the same in the upper and in the lower spacers. In the upper spacers all the fuel rods are displaced relative to the cells by a constant amount, and are displaced in the same direction. There is a uniform array of cell centers and a uniform array of rod centers, and the rod array is offset relative to the cell array. In one embodiment of the lower spacers, the cell center array is coincident with the array of fuel rod centers. In another embodiment, the arrays are offset. In current practice the spacer is located in the bundle channel, and is centered in the channel. If this practice were followed with the spacers disclosed here, the fuel rod centers in the upper spacers would be offset relative to the fuel rod centers in the lower spacers. In the invention disclosed here, the upper and lower spacers are offset from their centered positions so the array of fuel rod centers is centered in the channel. In either embodiment, the cells on the lower spacers are offset with respect to the cells of the upper spacers so that the centers of the upper portions of the fuel rods lie directly above the centers of the lower portions of the fuel rods. There results a spacer system that maintains top to bottom designed verticality of the tapered rods within the fuel bundle. An advantage of this aspect of the invention is that the disclosed spacer displacement scheme can be utilized for all spacer designs having cell stops identically arrayed. Accordingly, the disclosed scheme will work with either spacers having Zircaloy ferrules or lighter Inconel grids.