Patent Number: 043326404
Section: claims

1. A method for loosening a stuck fuel assembly in the core of a reactor and increasing the withdrawal capability without exceeding an allowable axial force safety limit exerted on a fuel assembly while removing same from a reactor core comprising the steps of: attaching a grapple mechanism to a fuel assembly to be removed, transmitting a vibrational force to the fuel assembly for loosening the assembly in a reactor core, and applying axial force to the fuel assembly for removing same from a reactor core without exceeding an allowable axial force safety limit. 2. The method defined in claim 1, wherein the step of transmitting the vibrational force is carried out by providing the grapple mechanism with a pneumatic vibrator mechanism, producing a centrifugal force by actuation of the pneumatic vibrator mechanism which is transmitted through the grapple mechanism to an attached fuel assembly. 3. The method defined in claim 2, wherein the step of producing a centrifugal force is carried out by positioning a rotatable ball in an annular track, and driving the ball around the track by directing gas under pressure thereagainst, thereby creating a centrifugal force. 4. In a grapple assembly for removing spent fuel assemblies from a reactor core which includes a latch mechanism for attaching the grapple assembly to a fuel assembly to be removed, and an actuator for the latch mechanism, the improvement comprising: means for loosening a stuck fuel assembly such that axial force applied for removing the fuel assembly does not increase beyond a safe limit, said means including a pneumatic vibrator mechanism positioned in said grapple assembly adjacent said latch mechanism, and means for activating said vibrator for producing a centrifugal force which is transmitted through said latch mechanism to produce a vibration of an associated fuel assembly causing loosening thereof within an associated reactor core. 5. The improvement defined in claim 4, wherein said pneumatic vibrator mechanism comprises a housing defining therein an annular track within which is located a rotating ball, gas passage means positioned peripherally of and tangentially with respect to said track, whereby said activating means allows gas under pressure to be directed through said passage means for driving said rotatable ball around said track producing said centrifugal force. 6. The improvement defined in claim 4, additionally including seal means positioned to prevent leakage of said gas into said latch mechanism. 7. The improvement defined in claim 5, wherein said track has a tapered configuration with an annular groove in the outer center section thereof in which said tangential gas passage means terminates. 8. The grapple assembly defined in claim 4, wherein said latch mechanism includes at least one latch means adapted to be engaged with an associated fuel assembly for removing such an assembly from an associated reactor core, and push-pull actuator means for moving said latch means, and wherein said pneumatic vibrator mechanism includes a housing surrounding a portion of said actuator means, and additionally including seal means positioned around a portion of said actuator means intermediate said vibrator mechanism and said latch means. 9. The assembly defined in claim 8, wherein said housing of said vibrator mechanism has a central annular opening therein defining an annular, tapered track, a rotatable ball positioned to travel in said annular track, a gas passage means positioned to direct gas under pressure against said ball for rotating same around said track, whereby centrifugal force created by the ball as it travels around said tract is transmitted through said housing to said latch mechanism for producing vibration of an associated fuel assembly when engaged with said latch means of said latch mechanism. 10. The assembly defined in claim 9, wherein said gas is composed of argon or other reactor cover gas.