Patent Number: 048636819
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The numeral 10 in FIG. I generally designates a fuel assembly for a pressurized water nuclear reactor. The fuel assembly 10 includes an upper end fitting 12 and a lower end fitting 14 connected by a plurality of guide tubes or thimbles 16 for receipt of control elements, in known manner. Cell-defining spacer grids 18, 20 and 22 have fuel or poison rod support features in the form of arches 24 and opposing springs 26, in known manner, and are secured at spaced intervals to the guide tubes 16 with their cells in register. The fuel rods or poison rods 30 are dotted in in FIGS. 2 and 3, shown in cross-section in FIGS. 4 and 11 and are shown partially in full in FIGS. 5-7 and 12-15. The grid 22' of FIG. 2 is simpler than the grid 22" of FIG. 3 in that the former includes no mixing vanes in the coolant flow paths while the latter includes vanes 32. The critical problems involved in inserting replacement rods are more severe in the case where the fuel assembly 10 includes grids with vanes 32 like the grid 22" in FIG. 3. However, replacement rods of the novel structure of the invention will provide an improved ease of insertion in the fuel assembly even for a FIG. 2 type of grid 22'. The enlarged portion of grid 22" as shown in FIGS. 4 and 11 includes vanes 32. Since vanes 32 extend partially into the cells of grid 22", when it is viewed in the plan views of FIGS. 4 and 11, it will be clear that there is a potential for grid damage when a symmetrical prior art replacement rod 34 is inserted into a cell of the grid 22". The preferred embodiment of the invention is a novel replacement rod 40, as shown in FIGS. 8 to 10. It is designed for use with spacer grids like grid 22" with mixing vanes 32 in that it minimizes damage to the vanes during the rod installation process. The replacement rod can be a pure fuel rod or may include "poison" for fuel moderating and management. Alternately, it could be a "dummy" constructed from a solid metallic bar or hollow tubing with welded end plugs. FIGS. 4-7 depict the problem with inserting rods which have conventional end cap designs. If the rod 34 is approximately centered in the array as it approached a grid, it will feed itself through the grid properly regardless of the grid design. However, it can be seen that a prior art off-center rod 34 will guide itself into grids which do not contain mixing vanes 32, but will not do so in all cases when vanes 32 are present. It is likely that the mixing vane 32 will be damaged, and possibly the adjacent rod loosened in its grid cell, because of the wedging action inherent in the contact geometry between the vane 32 and rod 34. Also, the bending action depicted in FIG. 7 could fracture vane 32, in which case it would become potentially damaging debris within the reactor system. FIGS. 11-14 demonstrate how the new rod 40 prevents damaging interference with the vanes 32. (For this discussion, it will be assumed that any grid cells adjacent to the open cell re filled with rods or guide thimbles, since this is the standard practice for reconstitution.) When the tip 42 of the rod 40 approaches any grid other than the uppermost one in the fuel assembly, two special features of rod 40 result in two separate actions. First, rod 40 has a tip offset feature or permanently diverging portion 44 between the tip 42 and the shank 46; this is the portion of rod 40 between the arrows A and B in FIG. 8. Its length is approximately equal to the distance between grids 18, 20 and 22. The diverging portion 44 has a centerline 48 which diverges from the centerline 50 of shank portion 46 to locate the end of diverging or bowed portion 44 adjacent the tip 42 out of alignment with the shank portion 46 by an amount "x" (FIG. 8) greater than the rod separation "y" (FIG. 11) within the fuel assembly 10. In combination with the grids through which the rod has already passed, the tip offset feature 44 ensures that the rod 40 slides along the surface of one or more often two rods 30 (or one rod 30 and a thimble 16) during insertion. The centerline 48 diverges from centerline 50 to locate the tip out of alignment with the shank portion by the amount "x" which is less than an amount which permits the rod 40 to be improperly guided into a grid cell location in lower grids 20 and 22 out of register with the grid cell location of grid 18 in which the shank portion 46 is located. Second, an elongated curved surface 52 at the tip 42 and an opposing chamfered portion 54 at the tip 42, together make up an asymmetrically beveled tip 42 such that the centerline of the elongated curved portion 52 has an intersecting centerline 58 tilted with respect to the centerline 48 of the diverging portion and defines a plane of symmetry with it. The chamfer 54 is greater in distance in the direction of the cell interior (see FIGS. 11-14) than are the rod support features 24 and 26. This feature of the replacement rod (the curved, eccentric tip) comes into play as the rod contacts a mixing vane 32 or horizontal grid strip. As seen in FIGS. 11-14, if the rod is located in a corner of the cell with a mixing vane and the eccentric cladding tip 42 of the rod is diagonally inboard in the grid cell, the rod will guide itself over the vane and into the cell without damage. Obviously, if the rod is located in a non-vane corner or over a straight strip at the cell centerline, it will also guide itself into the cell. Under the procedure described above, no special rotational control of the rod is necessary. For some fuel assembly designs, it may be desirable to control the cell corner into which the rod is being inserted. As a first step in these cases, the replacement rod 40 is positioned with its two special features in a known orientation. The equipment operator then grips the rod with a rod handling tool, maintaining some reference as to the orientation. As the rod is inserted into the fuel assembly, the operator has the capability of rotating the tool and thus preferentially directing the rod features. Since the operator has knowledge of the direction of the rod offset to begin with, and can rotate the rod, he has control over the specific cell corner where initial contact will occur. Because the asymmetric or eccentric tip and rod offset features have an established relationship, the operator can prevent damage by properly controlling rod orientation during insertion into each grid. (In most fuel assembly designs, it would be unnecessary to rotate the rod between one grid elevation and the next since the vane configuration does not change.) A side benefit of the new rod 40 comes about in the case of rod insertion into peripheral grid cells. In several past instances, insertion of rods 30 into peripheral cells in assemblies has resulted in the rods snaking out of the assembly. This may have been due to misalignment between the rod handling tool and the assembly, or to a combination of radiation-induced rod bow and the absence of constraint on all sides by surrounding rods. The new rod 40 prevents this problem since the replacement rod can be oriented in such a manner as to tilt the tip 42 into one of the two interior corners of each peripheral cell. Another advantage is the minimization of damage to a vane 32 if the tip 42 offset feature somehow fails to locate the tip properly. FIG. 15 demonstrates how the eccentric tip, if it becomes wedged behind a mixing vane, will create less damage to the vane than that when a conventional tip is involved (FIG. 7). The replacement rod 40 described will eliminate a major disadvantage associated with mixing vane grids. While it does not cover the situation where a given fuel rod must be removed an then returned to its original location, future cases where this is necessary are expected to be relatively rare since devices are available to identify failed and non-failed rods prior to any removal operation. Thus, in most cases only failed rods will be removed. These rods must be replaced with substitute rods since they will continue to leak fission products from the nuclear reaction if they are returned to the reactor.