Patent Number: 053393414
Section: description

DESCRIPTION OFT HE PREFERRED EMBODIMENT FIGS. 1 and 2 illustrate a known mixer grid for a nuclear reactor fuel assembly wherein the mixer grid comprises a grid structure, generally designated 10, comprising a plurality of strips 12 which are stamped and welded and assembled in an egg crate configuration. The arrangement of inner strips 12 provides for a plurality of individual cells 17 for receiving a fuel rod 20 of a nuclear reactor fuel assembly. This particular grid will also have the fuel rods 20 contacting each side of each cell. Each cell 17 contains a soft spring stop which the fuel rod compresses. This provides support for the rods. Each cell 17 also has a plurality of hard stops 40 arranged along the inner strips 12 of the cells 17 for holding a fuel rod. In addition to the hard stops 40, mixing vanes 30 are provided in each cell 17 for directing the flow of coolant through the grid 10 for cooling the nuclear fuel rods and hot spots 50. It is common to have an outer strip 15 on the periphery of the grid 10 for enclosing the grid 10. Outer strip 15 provides lead-in and coolant flow to the hotter spots 50 of the peripheral cells 17. These lead in tabs however will not provide the same amount of mixing as the stoppers. The periphery cells only have one vane directing flow whereas there are two (2) stoppers in each of the periphery cells of the invention. FIG. 1 also illustrates that each cell 17 has both the hard stops 40 and mixing vanes 30 contained within each individual cell 17 which support the fuel rods and direct coolant flow. FIG. 3 illustrates that the present invention comprises a mixer grid 10 having strips 12 arranged in interlocking rows and columns for forming individual cells 17' similar to the grid structures found in the prior art. The present invention allows for efficient loading and unloading of the mixer 10 by using guide thimble cells 35 for connection with the nuclear reactor fuel assembly. A plurality of support arches 60 are fixed within each guide thimble cell 35 for facilitating welding to the nuclear reactor fuel assembly. FIG. 3 also illustrates that stoppers 45 are fixed at each corner of each cell 17' for increasing the level of coolant mixing while the chamfered edges provide a surface to contact in case of rod bow, shifting or misalignment. The flatness of the chamfer allows the fuel rod to wear at a much slower rate because of its lengths compared to the older style hard stops. Unlike the mixer grids found in the prior art, the present invention does not use an outer strip for confining the peripheral cells. The peripheral cells 17' are left open and the grid 10 is positioned in-board a distance, for instance 0.25 inches, reducing the risk of the grid 10 contacting adjacent fuel assemblies during loading, unloading and faulted conditions. Even without the outer strip, the present invention allows for the peripheral cells 17' to receive more coolant because of the stoppers 45. In known devices there are two (2) corner cells that do not have vanes as shown in FIG. 1. The lead in tabs may provide for some mixing but the actual amount of surface area engaged by coolant makes the mix very small. By way of contrast each corner cell of the current design has one stopper for providing increased mixing. FIG. 4 illustrates a stopper 45 fixed at each corner of each cell 17'. A fuel rod 20 is located within each cell 17'. Stoppers 45 force the coolant into the hot spots of the fuel assembly seen in FIG. 4 increase the level of mixing in the coolant and thus increases the heat transfer between the fuel rods 20 and the coolant for the reactor. The increased mixing of the coolant allows for an increase in the overall power output of the nuclear reactor. The increased mixing results from the stopper more effectively providing coolant to the hot spots. The hot spots are located radially between each fuel rod, which is the area where the rods are closest together. The stopper forces the water into each hot area as well as mixing the coolant. The present invention allows for the stopper 45 to perform both the coolant flow diverting and mixing and to provide a nondamaging contact surface. Thus, the problems with the mixing vane 30 and the hard stop 40, shown in FIG. 1, previously mentioned, is avoided. FIG. 5 illustrates a stopper 45 having a cone-shaped or conical configuration for directing coolant flow 80 to hot spot areas 70 shown in FIG. 4. Conical stopper 45 has a chamfer 47 which is a flat, smooth beveled edge for accepting fuel rod 20 contact and reducing fretting and damage to the fuel rods 20 as illustrated in FIG. 4. Edge 43 of conical stopper 45 assists in directing coolant flow 80 over the grid 10 as shown in FIG. 5. FIGS. 6-7 illustrate a stopper 45' having a cylindrical configuration for channeling the coolant flow 80 over the grid 10. Cylindrical stopper 45' has radiused edges 49 which direct the coolant flow 80. The conical stopper is more efficient than the cylindrical as it reduces the drag on the coolant. However, it would probably be the more expensive to manufacture. Both could be manufactured by known techniques such as EDM or separate stoppers may be welded to an egg crate configuration. Stoppers 45 (FIG. 4), 45' (FIG. 6) both direct the flow of coolant 80 (FIGS. 5 and 7) to hot areas 70 (FIGS. 4 and 6) based on two principles: 1. the stoppers 45, 45' provide a path of least resistance due to the water channel dimensions of the stoppers and 2. the stoppers provide a flow diversion due to the water channel blockage provided. Due to the chamfer 47 of the stopper 45, 45' (FIGS. 4 and 6), fuel rods 20 can rest or contact the chamfer 47 which prevents damage or fretting to the rod 20 should the rod 20 shift its position. The present invention provides for a stopper 45, 45' in each corner of each cell 17' resulting in a uniform configuration for each cell 17' which is unlike the uneven pattern of mixing vanes used in conjunction with known mixer grids. The present invention provides for a more even distribution of coolant flow 80 (FIGS. 5 and 7) and may minimize the lateral movement of the fuel assembly. The present invention allows for better heat transfer over the mixer grid 10 than currently found on mixer grids employing the standard mixing vane design. The increased heat transfer ability of the present invention also provides for an increase power output for the nuclear reactor. Additionally, by not employing an outer strip, the present invention does not interfere with or cause damage to adjacent assemblies during loading or unloading of the mixer grid. While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.