Patent Number: 048204790
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a guide pin assembly for aligning the upper hold down plate of the top nozzle of a fuel assembly with the upper core plate of a pressurized water nuclear reactor. Fuel assemblies for pressurized water nuclear reactors are supported within the reactor core by upper and lower core plates. The upper and lower core plates are supported by a core support barrel, which in turn surrounds the reactor core and extends between the ends thereof. The number of fuel assemblies within the reactor core varies according to the size of the reactor core. Such fuel assemblies include a plurality of guide tubes through which the control rods are inserted and withdrawn from the reactor core. The guide tubes are supported between a top nozzle and a bottom nozzle. The top nozzle includes an upper hold down plate, a lower adapter plate, and an enclosure forming a sidewall and extending between the upper hold down plate and the lower adapter plate. Generally, the top nozzle has a square cross-section, and includes a plurality of guide tubes, the number of which varies according to the size of the fuel assembly. The upper hold down plates of a plurality of top nozzles are aligned with, and secured to, the upper core plate to support a plurality of fuel assemblies below the upper core plate. The upper core plate and upper hold down plates of the top nozzles include aligned bores therethrough for the guide tubes. As further described in U.S. Pat. No. 4,534,933 to Gjertsen et al., and assigned to the assignee of the present invention, guide pins are disposed between the upper hold down plate of the top nozzle of each fuel assembly and the upper core plate to properly align the upper core plate and the upper hold down plate of the top nozzle of the fuel assembly so that the guide tubes extend vertically. U.S. Pat. No. 4,534,933 is incorporated herein by reference. These guide pins must sometimes be replaced. The replacement of missing, broken or damaged guide pins is difficult without risking the exposure of plant personnel to dangerous levels of radiation. It is an object of the invention to develop a remotely installable guide pin assembly for aligning the upper core plate of the reactor with the upper hold down plate of the top nozzle of a fuel assembly. SUMMARY OF THE INVENTION The present invention is a guide pin assembly for aligning the upper hold down plate of the top nozzle of a fuel assembly with the upper core plate of a pressurized water nuclear reactor. The guide pin assembly includes a guide pin, a nut and a locking cup. The guide pin has a generally circular cross-section and includes a nose, a threaded portion, a shaft, and an end. The nose has engagement means thereon. The threaded portion, adjacent the nose, has a threaded outer surface. The shaft, adjacent the threaded portion, has an upper section, a lower section, and a radial alignment section between the upper and lower sections. The radial alignment section of the shaft has a diameter that is approximately the diameter of a first bore in the upper core plate of a nuclear reactor, and the lower section of the shaft has a diameter less than that of the radial alignment section. The end, adjacent the shaft, includes a top having a diameter greater than that of the diameter of the first bore in the upper core plate. The guide pin is adapted to be inserted within a second bore in the upper hold down plate of the top nozzle of the fuel assembly and through the first bore in the upper core plate of a nuclear reactor from the bottom of the first bore in the upper core plate so that the nose and at least a portion of the threaded portion of the guide pin protrude above te upper core plate. The nut is adapted to be threaded onto the threaded surface of the guide pin, and includes a top and a bottom. The locking cup is securable about the nut and is adapted to engage the engagement means on the nose of the guide pin when the nut has been tightened onto the threaded surface of the threaded portion of the guide pin so that the bottom of the nut abuts the upper surface of the upper core plate. The guide pin assembly of the invention is remotely installable within the second bores and first bores of the upper hold down plate of the top nozzle and the upper core plate, respectively, in an operating nuclear reactor without the risk of exposing plant personnel to dangerous levels of radiation.