Patent Number: 055219500
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to tooling and a process for inserting, rearranging and/or removing the control rods in a boiling water reactor pressure vessel. The control rods in a boiling water reactor contain an absorbent material that when positioned in the reactor core can be used to slow the fission rate of the nuclear fuel. However, the absorbent material is subject to degradation after extended use. Therefore, it is periodically necessary to replace the control rods. Since different regions of the reactor core have different levels of irradiation fluence, in order to reduce expenses, it is common to periodically reposition the control rods within the core to maximize, their useful life. In order to pull a control rod out of its associated core location, it is also necessary to pull out the associated fuel support since the control rod's velocity limiter cannot slide past an installed fuel support. Therefore, the tools commonly used to reposition and/or replace the control rods include a first grapple for lifting the fuel support. A second grapple is then used for uncoupling the control rod from its associated control rod drive and lifting the uncoupled control rod. Operators that use just these two tools have been known to occasionally drop the control rod. Therefore, a third grapple tool is often used to grab the top bail handle of the control rod when the control rod is being lifted. When the control rods are being rearranged, it is customary to set the fuel support on the reactor pool floor in a sheltered area. The control rod is then taken out of the pressure vessel and placed in a fuel pool located adjacent the pressure vessel. When the control rod is to be reinserted, it must be transferred back from the fuel pool to the pressure vessel. When the fuel support is to be reinserted, it must be picked up from the floor. These transferring motions take a relatively long time and in the case of the control rods result in the undesirable exposure of a radioactive component. SUMMARY OF THE INVENTION Accordingly, it is an objective of the present invention to provide an improved tool set and an improved method for rearranging and/or replacing control rods in a boiling water reactor. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a tool set and a method of using those tools is disclosed that is especially well adapted to the function of rearranging and/or replacing the control rods and fuel supports in a boiling water reactor. In a first aspect of the invention a grapple is disclosed that is capable of simultaneously picking up both a control rod and its associated fuel support. The grapple includes a frame, a slidable control rod holding mechanism that is adapted to grab the bail handle of a control rod, and a fuel support holding mechanism adapted to grasp a fuel support. In a second aspect of the invention, an unlatching tool is disclosed for pulling the control rod's release handle in order to decouple the control rod from a control rod drive. The unlatching tool includes a housing having a positioning projection adapted to engage the fuel support associated with the control rod being detached in order to help position the unlatching tool during use. An unlatching finger is pivotally mounted to the housing near its distal end and an actuating mechanism is provided to actuate the unlatching finger. The actuating mechanism is adapted to move the finger between a withdrawn position wherein the finger is substantially parallel with the housing and an extended position wherein the finger extends substantially perpendicular to the housing such that it can readily engage the control rod's release handle. In a third aspect of the invention a storage rack suitable for mounting within the reactor pressure vessel is provided for storing a fuel support and a pair of control rods when the control rods are being shifted about the reactor core area. The storage rack is designed such that it can be temporarily installed in the reactor pressure vessel and includes a frame, a pair of control rod holders and a fuel support storage seat for holding a fuel support. The control rod holders each include a control rod seat that is shaped to receive the socket of a control rod velocity limiter. In a method aspect of the invention, a method of removing and/or rearranging the position of control rods and fuel supports is disclosed. The method includes the step of pulling the control rod release handle using an unlatching tool. Thereafter, the control rod and its associated fuel support are lifted together using a single grapple tool that is separate from the unlatching tool. When removing the control rod, the control rod can be simply lifted out of the pressure vessel. However, when rearranging the control rods, the first control rod and its associated fuel support are placed a storage rack that is temporarily positioned within the reactor pressure vessel. Then the unlatching and lifting out steps are repeated for a second control rod/fuel support assembly. The second control rod is placed in the storage rack without releasing the second fuel support. Thereafter, the first control rod is removed from the storage rack and placed together with the second fuel support in the position formerly occupied by the second control rod.