Patent Number: 052215148
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is generally related to guide pins for a nuclear reactor fuel assembly and more particularly to the replacement of existing guide pins. 2. General Background In commercial nuclear reactors fuel guide pins extend down from the upper core plate into guide tubes in the fuel assemblies This aids in maintaining proper alignment of the fuel assemblies with the pressure vessel so that the control rods operate properly. During initial construction of the reactor each guide pin is inserted into a bore in the upper core plate from the bottom of the plate and threadably secured with a nut at the top of the core plate. The nut is then welded to the pin to prevent loosening during service. After construction is completed and operation begun, fuel guide pins may need to be replaced due to a defect or the fuel guide pin becoming bent. A problem encountered in replacing fuel guide pins is that the top of the upper core plate is very difficult to access. This makes removal of the nut from the top of the core plate very difficult and time consuming. SUMMARY OF THE INVENTION The present invention addresses the above problem in a straightforward manner. What is provided is a method and apparatus where the existing pin is removed by cutting it near the bottom of the upper core plate. A removal process such as EDM (electrical discharge machining) is then used to remove that portion of the pin remaining within the plate to a level slightly below its retaining nut. The removal process is continued to bore a hole through the remaining part of the pin. A replacement pin is then inserted into the plate from the bottom and through the hole in the remaining part of the original pin. The replacement pin has a collet lock with a tapered shoulder at its upper and that allows it to go through the hole in the original pin but not to come back out. The replacement pin body is provided with a bore to receive the collet lock. A bolt fits through the replacement pin body and threadably engages the collet, spreads the tapered shoulder, and secures the replacement pin and the remaining portion of the original pin and its retaining nut in position.