Patent Number: 047818840
Section: summary

FIELD OF THE INVENTION This invention relates to nuclear reactor fuel assemblies and in particular those assemblies which include spaced fuel rod support grids mounted in a reactor core as a unit. The fuel rods are held between an upper end fitting and a lower end fitting by means of spacer grids. The reactor coolant flows upwardly from holes in the lower end fitting along the fuel rod lower end caps and upwardly along the fuel rod cladding and through the spacer grids in the active region of the fuel assembly. BACKGROUND OF THE INVENTION Debris in the coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 30% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. The size and shape of the debris capable of causing severe damage is quite variable. In fact, metal fragments which can only be picked up with tweezers have been known to "drill" a hole in fuel rod cladding in less than 1,000 hours of reactor operation. Since most failures occur either within or below the first spacer grid, the grids apparently provide a rather good screen for collecting debris. In order to prevent damage in the active area of the reactor, Applicant set out to design a grid structure for straining debris which: has a good probability of filtering out particles that could cause cladding damage: does not significantly increase fuel assembly uplift; does not significantly alter existing proven designs: will not jeopardize fuel rod support: will not hinder fuel assembly reconstitutability; will not significantly compromise fuel performance; has high mechanical integrity; is cost effective considering the risk/benefit; will not significantly infringe fuel rod plenum volume; and, does not require unplanned out-of-reactor flow testing. SUMMARY OF THE INVENTION The invention is an "egg crate" type strainer grid which is welded to the upper surface of the lower end fitting. The grid is only approximately 5/8 of an inch high and is placed in the region of the fuel rod lower end caps but does not provide any fuel rod support. The individual grid strips are pierced and stamped to include arch-type leaves at various elevations. The leaves are alternately stamped 180.degree. opposite each other. The leaf with the largest projection is on the bottom of the strip with each succeeding leaf projecting lesser than the one below. The design has the leaves minimizing pressure drop by presenting an edge to the coolant flow and being in rather close proximity to the contour of the end caps so as to provide a means for trapping debris against the end caps. Since the end caps rre of solid material they are capable of accepting a great deal of wear without penetrating the fuel barrier. While one embodiment shows only four leaves per strip, more or less are possible. The grid strip thicknesses is related to the number of leaves but may be as thin as 12 mils to minimize overall fuel assembly pressure drop. The novel strainer grid is more effective in collecting debris than a standard spacer grid because the largest diameter in flat shaped openings through a strainer grid made according to the principles of the invention are typically 0.090 inches and 0.16 inches, respectively. This compares with similar openings in a standard fuel rod spacer grid complex of 0.14 and 0.41 inches, respectively. Furthermore, when debris collected in the strainer grid and contacts fuel rod end caps, no ensuing fuel rod failures will occur because the wear takes place in the solid region of the fuel rods, namely its end cap, as opposed to in the hollow clad portion in the active region of the fuel assembly. The shape and orientation of the strainer grid leaves can be important in optimizing collection efficiency and pressure and maintaining relatively low pressure drops. The calculated pressure through the strainer grid of the invention as described above is about 0.4 psi which represents a 3% increase in the total pressure drop across a fuel assembly. A typical screen or perforated metal plate with even larger openings (0.187 inches diameter) results in an overall increase in fuel assembly pressure drop of more than 17%. While the strainer grid of the invention will not entirely eliminate debris related failures, it has the potential for greatly reducing the number of failures. It also provides a side benefit of breaking up jets eminating from the flow holes in the lower end fitting flow plate, thereby further reducing the probability of fuel rod failures due to fretting or vibration in the critical entrance region of the fuel assembly.