Patent Number: 050248073
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 of the fuel assembly. BACKGROUND OF THE INVENTION Metallic debris in the coolant which collects or is trapped in fuel rod spacer grids adjacent the fuel containing cladding of the active region is believed responsible for a significant percentage 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 adjacent to a grid in less than 1,000 hours in a test simulating reactor operation. Since most cladding failures in reactors due to debris have occurred either within or below the lowermost spacer grid, a conventional grid apparently provides a rather effective screen for collecting debris. In order to prevent damage in the active area of the reactor, applicant set out to design an improved spacer 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; 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. An earlier debris catching grid of Combustion Engineering, Inc., the assignee of the instant invention, was issued as U.S. Pat. No. 4,781,884 on Nov. 1, 1988. That invention was a separate debris catching strainer grid with no rod support function. A more traditional and prior art Combustion Engineering, Inc. design of fuel assembly (FIG. 1) has sustained a known distribution of debris-induced failures which shows clearly that the lowest spacer grid represents a very effective filter for debris. Unfortunately, the short lower end cap on the fuel rod of that fuel assembly ensures that the hollow cladding tube is adjacent to the trapped debris, and that any flow-induced motion of the debris can wear through the thin wall of the tube and create a failed rod. Based on available knowledge, conventional fuel from all vendors has experienced about the same distribution relationship between failures near the bottom of the assembly and those higher up. In the traditional or prior art fuel assembly, the lowest spacer grid is some distance up from the bottom of the fuel rod, since, in the absence of a positive axial capture device for the rod, the grid needs to be located at an elevation where it will always laterally capture a "lifted" rod. Rods could potentially lift in response to coolant flow (FIG. 2) during abnormal conditions. Taking into account the known distribution of debris-induced failures mentioned above, one choice for a debris-resistant fuel assembly design is one in which the solid end cap is merely lengthened such that it extends up through the bottom spacer grid. This simplistic solution, however, has several drawbacks, as follows: a. zirconium alloy bar stock used for end caps is very expensive and, therefore, there is a strong incentive to minimize end cap length; and b. void volume within the fuel rod and/or the active fuel length will be affected negatively. SUMMARY OF THE INVENTION The present invention is an "egg crate" type spacer grid which is the lowermost spacer grid adjacent the lower end fitting. The grid is made up of "wavy" strips which provide bends or arched portions to cooperate with springs to provide support and positive capture for the fuel rods with a minimum of pressure drop across the grid. The solution provided by the "spring detent spacer grid" of the instant invention is to move the grid down, to thereby reduce the solid zirconium alloy material length required. To preclude the condition depicted in FIG. 2, the grid includes the provision of a fuel rod capturing spring detent device. This spring device engages a circumferential groove with tapered sides in the fuel rod end cap which creates enough axial restraint to prevent or minimize "rod lift" under all flow conditions, but not enough restraint to significantly affect fuel rod reconstitution. In addition, to further reduce the amount of potentially harmful debris passing by the first spacer grid (which typically has flow areas the same size as those of the spacer grids at higher elevations), integral leaves substantially symmetrically arranged on either side of the strip intersections have been added to greatly increase the likelihood that debris that passes the novel first or bottom "spring detent spacer grid" is too small to become trapped at a higher grid where it can damage the cladding of the active fuel region. The particular advantages of this spacer grid over other debris-catching concepts are: a. Compared to a separate screen-type grid: there is no cost of an extra component and there is a much lower incremental increase in pressure drop; b. Compared to a lower end fitting type filter (e.g., small holes in casting or integral screen): tests showed that with the spacer grid-type device there is a large amount of retained debris (probably because of tapered surfaces into which debris becomes "wedged") when the coolant flow is turned off, instead of with the other devices allowing debris to drop back into the region below the core for multiple attacks and chances to get through to the cladding. Tests also showed that the turbulence present in the region just below the lower end fitting caused the debris in many cases to "bounce" along the lower surface of the lower end fitting type filters and to eventually move over to the edge where it could pass up the gap between the fuel assemblies to get caught in a higher grid. With the "spring detent spacer grid", the debris passes through large lower end fitting holes, hits the grid edges, and even if it does not immediately become wedged, it is blocked from escaping into the assembly-to-assembly gap by the perimeter strip. c. Compared to a long end cap and conventional grid: there is less cost and less negative impact on stack length or rod void volume.