Patent Number: 063273225
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a device for transporting poison rod assemblies between fuel assemblies in a nuclear fuel storage facility and, more specifically, to a device for transferring a poison rod assembly which uses an overhead crane as its only lifting mechanism. 2. Description of the Prior Art Fuel for a nuclear reactor used to create steam and, ultimately, electricity, generally is in the form of fuel rods containing a fissile material. When fuel rods are being stored, the fuel rods are typically supported in nuclear fuel assemblies arranged as spaced parallel arrays. Fuel assemblies are stored in racks in a protective medium, such as water containing boric acid. In addition to the fuel rods, poison rods are disbursed throughout the fuel assemblies to control the fission process. Poison rods generally include a plurality of elongated rods, each containing a neutron absorbent material, which fit in longitudinal openings, or thimbles, defined in the fuel assemblies. The top end of each poison rod is attached to a web thereby forming a poison rod assembly. A T-shaped bar is affixed to the top of the web, creating an easily accessible handle for lifting the poison rod assembly so that it may be transferred from one fuel assembly to another. As shown in Hornak et al., U.S. Pat. No. 5,325,408, prior art transfer devices use a winch located at the top of the transfer device to lift the poison rod assembly. Movement of the transfer device itself between fuel cells, is accomplished by a gantry crane in the fuel cell storage facility. Thus, prior art lifting devices consist of at least two lifting means, one to lift the transfer device itself, and one to lift the poison rod assembly within the transfer device. Lifting of a poison rod assembly could be performed more efficiently by the crane used to move the transfer device. Additionally, elimination of the winch would reduce the cost of the transfer device and eliminate unnecessary parts that are subject to mechanical failure. Therefore, there is a need for a poison rod transfer device that does not require the use of a winch. There is a further need for a poison rod transfer device that uses a single lifting mechanism to remove the poison rod assembly from a fuel cell and transfer it to another fuel cell. SUMMARY OF THE INVENTION The present invention satisfies the above referenced needs and others by providing a poison rod assembly transfer device having a poison rod assembly lifting device which may be actuated by the overhead crane used to move the transfer device. Lifting of the poison rod assembly is accomplished by a lifting assembly coupled to the overhead crane. The lifting assembly consists of an elongated outer member, an inner member, a gripper, assembly and an interlock device. The crane is attached to the top of the inner member. The gripper assembly is connected to the bottom of the inner member. The inner member is slidably disposed within the elongated outer member. The elongated outer member is seated on the fuel cell from which the poison rod assembly will be lifted. When the interlock device is not engaged, raising or lowering the crane will slide the inner member within the elongated outer member between an upper locked position and a lower locked position. When the interlock is engaged, the inner and elongated outer members are coupled so that the inner member cannot slide within the elongated outer member and raising the crane will lift the entire transfer device. Thus, when the interlock is not engaged the crane may lift a poison rod assembly out of the fuel cell, when the interlock is engaged the crane will lift the entire transfer device. Accordingly, only a single lifting mechanism is required. The interlock device utilizes a pair of releasable latch members which pass through openings in the outer and inner members. The interlock device is designed to automatically lock the inner member in place each time the inner member is brought to the upper position. The interlock device is also designed to automatically lock the inner member in place when brought into the lower position if the gripper is not engaged with a poison rod assembly. When the gripper has engaged a poison rod assembly, a shield device prevents the interlock device from operating. Thus, when the inner member is in its lower position and the gripper has engaged a poison rod assembly, the inner member will not be locked in the lower position and, therefore, can be lifted without the operator having to release the interlock, thus simplifying the lifting operation. Typically, removal of a poison rod assembly will begin with the inner member locked in the lower position. When the inner member is locked in the lower position, the gripper is located near the bottom of the outer member. After the transfer device is seated on a fuel cell, the operator will release the interlock allowing the gripper to be lowered further to engage the T-bar on the poison rod assembly. When the gripper is rotated into the latched position, the shield device is engaged and will prevent the interlock device from reengaging when the inner member reaches the lower position. When the operator raises the overhead crane without the interlock engaged, the inner member slides vertically within the outer member, lifting the poison rod assembly out of the fuel cell. When the inner member reaches the upper position, the interlock device engages, preventing the inner member and poison rod assembly from moving relative to the outer member. At this point, as the crane continues to lift, the entire transfer device will be lifted off the fuel cell. When the transfer device is seated on another fuel cell, the operator can only lower the poison rod assembly by releasing the interlock device. When the interlock device is released, the inner member and poison rod assembly may move vertically relative to the outer member. When the crane is lowered, the inner member slides downwardly in the outer member as the poison rod assembly is lowered into the new fuel cell. When the poison rod assembly is seated within the fuel cell, the operator may unlatch the gripper, thereby also disengaging the shield device. When the inner member is raised to the lower position, the interlock assembly once again locks the inner member in the lower position. When the crane is raised, the entire device will lift with the inner member locked into the lower position.