Patent Number: 048329010
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and, more particularly, to a method for repairing bent mixing vanes found in the fuel assemblies of nuclear, pressurized-water reactors. As described in co-assigned U.S. Pat. No. 3,719,560 issued to MAYERS et al., located within a conventional nuclear, pressurized-water reactor is a plurality of parallel fuel rod supporting grids. These grids are made up of a plurality of thin bands or straps arranged in a lattice configuration to form individual fuel cells. Each fuel rod containing nuclear fuel pellets is inserted through a fuel cell within a grid. Each fuel rod is held in a fixed relationship within the fuel cell by a plurality of springs (see, e.g., co-assigned U.S. Pat. No. 3,379,618 issued to FRISCH), and punched, metal dimples arranged along the inner faces of each fuel cell. Each fuel rod is supported by being lightly pushed against the dimples via one or more of the springs. The grids also include metal fins or "mixing vanes" at the corners thereof, perpendicular to the grid straps, which may lightly abut, but do not mechanically support, the fuel rod. Mixing vanes are intended to disturb fluid flow, i.e., cause a swirling action to improve heat transfer and reduce the potential for hot spot temperatures at the fuel rods. During individual fuel rod removal and reassembly, e.g., at scheduled maintenance, the mixing vanes can be damaged, i.e., bent. A bent mixing vane interferes with normal fuel rod reinsertion, i.e., exerts bending moment on the fuel rod, thus causing the fuel rod to deflect and bow out of its intended path. Deflection causes the fuel rod to "hang-up" on the first grid below the grid with the bent mixing vane. If a fuel rod is hung up, it has been usual practice to remove the fuel rod and not to replace it. As a result, a void is left where fluid can leak through. The performance of the fuel assembly is degraded as a result of deleting one or a plurality of fuel rods which cannot be reinserted. In light of the above, a method for straightening bent mixing vanes is desired so that fuel rods can be reliably reinserted. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a nuclear reactor fuel assembly mixing vane repair method which is capable of straightening bent mixing vanes efficiently and quickly, without having to dismantle the nuclear reactor and without having to introduce a man in the area of the bent mixing vane. It is another object of the present invention to provide a nuclear reactor fuel assembly mixing vane repair method which causes little interruption of nuclear reactor scheduled maintenance, is relatively simple in construction and steps, respectively, and does not significantly increase costs related to nuclear reactor operation or maintenance. To achieve the foregoing and other objects of the present invention, and in accordance with the purpose of the invention, there is provided herein a nuclear reactor fuel assembly mixing vane repair method preferably using a unique tool and a related tool handling device. The tool is made up of an elongated tube with a housing containing two sets of high-strength blades remotely movable between closed and opened positions. The tool is inserted into a fuel cell detected as having a grid with a bent mixing vane therein via the tool handling device to a location below the damaged grid with the blades in the closed position. The blades are then remotely opened via a cable and rod combination within the apparatus, and the tool is withdrawn. As the tool is withdrawn, a blade abuts the bent mixing vane and bends it back close to its original position. After straightening of the bent mixing vane, the blades are closed and the tool is fully extracted from the fuel cell. Once the tool is fully extracted, the fuel rod can again be reliably inserted into the repaired fuel cell. The method includes the steps of: introducing the apparatus with the blades closed below the damaged grid; opening the blades as the apparatus is withdrawn to abut and straighten the bent mixing vane; closing the blades; and fully extracting the apparatus from the fuel cell.