Patent Number: 052689484
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to a locking assembly used in conjunction with nuclear reactors to normally lock in place an upper support plate disposed at the top of a fuel bundle, and to selectively permit the easy removal of such support plate when it is necessary or desirable to remove one or more of the fuel rods in the fuel bundle. There are several types of nuclear reactors used to generate power, one of which is the pressurized water reactor (PWR). In a PWR the reactor core contains an array of fuel bundles or assemblies comprised of fuel rods containing uranium. PWR units in the United States generally operate for approximately 12 to 18 months after which the plant is routinely shut down for refueling so that all systems and components can be checked to ensure safety and reliability. Occasionally, fuel will fail during the 12 to 18 months of power operation. Fuel is considered failed when a fuel rod wall is breached and radioactive isotopes are released into the water which cools the fuel during power operation. Failure can result in a number of ways, such as debris in the cooling water fretting the fuel rod in a localized area, or flaws introduced during fuel fabrication. In the past, a small percentage of failed fuel was acceptable, but, recently, emphasis has been placed on eliminating the continued operation of any known failed fuel. When fuel fails, radioactive isotopes are released into the coolant water and should the level of these radioactive isotopes indicate that there is failed fuel, such failed fuel should be promptly removed. Rather than remove an entire fuel assembly from further operation, a more economical solution is to replace only the failed fuel rods with structurally sound replacement rods and return the assembly back to operation. The fuel rods are replaced by disconnecting an upper support plate or upper end fitting from guide tubes forming part of the fuel assembly, thereby allowing the fuel rods to be removed. The replacement or removal of failed fuel rods occurs during the outage when time to perform such work is limited, and, therefore, a need exists for fuel assembly features which allow replacement to be performed as quickly as possible. Early fuel assembly designs did not provide a means for replacement since some failed fuel was acceptable. However, as emphasis was placed on removal of failed fuel, designs were developed to allow replacement. For example, Long et al U.S. Pat. No. 4,064,004 discloses an assembly mechanism which permits the upper support plate to be removed from guide tubes and which includes a pair of cooperating movable members, one of which is operable to be moved to one position at which the upper support plate is normally maintained in place on the guide tubes by lugs, and to be selectively rotated to a second position at which a slot formed in the support plate can be aligned with such lugs to permit removal of the support plate. The other movable member is axially movable and is urged by a coil spring to a normal position at which it prevents the rotatable member from being moved to its aligned position with respect to the support plate slot. In this system, the cooperating locking members are locked together at the support plate release position but they are not locked to the guide tube after the support plate is removed. Therefore, there is no assurance that the locked members will remain at the same orientation relative to the guide tube after the support plate is removed, and they are free to move to a different position on the guide tube which will require that all of the locked members be carefully aligned with the slots on the support plate when the support plate is repositioned on the ends of the guide tubes. In accordance with the present invention, a simplified locking assembly is provided which overcomes the aforesaid drawback of known locking mechanisms, and which provides an arrangement that is easy to operate while providing a secure lock for the upper support plate. SUMMARY OF THE INVENTION Briefly summarized, the present invention provides a locking assembly for permitting the easy removal of fuel rods from a nuclear power reactor fuel bundle and this locking assembly includes a plurality of control rod guide tubes extending upwardly from the fuel bundle, and a support plate that is removably mounted on the guide tubes to permit removal of the fuel rods when the support plate is removed from the guide tubes, this support plate being formed with openings for receiving the extending ends of each of the guide tubes and being formed with a slot extending outwardly from each such opening. A collar assembly is mounted at the extending end of each guide tube for permitting selective removal of the support plate from the guide tubes, and this collar assembly includes a base portion and first and second locking portions mounted on the base portion for movement relative to one another, and one of the locking portions is formed with lugs or projections that correspond generally in shape to the aforesaid slot in the support plate, and one of the locking portions is arranged for movement between a first position at which the aforesaid projection is openly aligned with the slot in the support plate to permit removal of the support plate, and a second position at which the projection is not openly aligned with the slot whereby the plate is locked in place on the guide tubes. A resilient member is associated with the collar assembly for resiliently locking the movable locking portion against movement with sufficient force to maintain it at its second locking position, and for selectively releasing the movable locking portion when a predetermined torsional force is applied to the movable locking portion to thereby permit movement of the movable locking portion to its first aligned position at which the support plate can be removed from the guide tubes. Preferably, the resilient means resiliently locks the movable locking portion against movement by resiliently engaging an annular engagement surface on the second locking portion, and the resilient element is capable of locking the movable locking portion in place at both its first aligned or removal position and at its second locking position. In one embodiment of the present invention, the movable locking portion is arranged for rotational movement with respect to the base portion, and it is formed with a body portion and a slot therein which corresponds in shape to the slot in the support plate. A second locking portion is fixed to the base portion and includes a projection extending outwardly therefrom, and the rotatable locking portion is disposed intermediate this projection and the support plate and is rotatable between a first position at which the slot therein is openly aligned with the slot in the support plate to permit the support plate to pass over the projection for removal, and a second position at which the slot in the movable locking portion is out of alignment with the slot in the support plate and the body portion of the movable locking portion is disposed between the projection and the support plate to block removal of the support plate over such projection. Preferably, the rotatable locking portion includes an exterior annular engagement surface, and the resilient element resiliently engages the annular engagement surface by presenting a spring biased ear that is resiliently engaged at either one of two detents formed in the annular engagement surface. In a second embodiment of the present invention, the movable locking portion is rotatably mounted on the base portion and is formed with a projection that corresponds generally in shape to the support plate slot, and a retaining member is fixed to the guide tube for maintaining the movable locking portion in place on the base portion and permitting it to move between its first and second positions, and, preferably, the fixed element includes an exterior annular wall portion disposed within an adjacent interior annular wall portion on the movable portion, and the fixed element includes at least one, and preferably two, detents formed in the exterior annular wall portion, and a spring biased ear is formed at the interior annular wall of the movable locking portion. In a third embodiment, the collar assembly includes a base portion and a locking portion that is rotatably mounted on the base portion and formed with a projection corresponding generally in shape to the slot in the support plate, and this locking portion is rotatable between first and second positions for releasing and locking in place the support plate, respectively. In this embodiment, the resilient element includes a spring biased element disposed within and adjacent an interior annular engagement surface on the rotatable locking portion, and movable therewith, for resiliently engaging a fixed element on the base with a sufficient force to normally maintain the locking portion at its first position and for releasing the locking portion when a predetermined torsional force is applied thereto. Preferably, the annular engagement surface of the movable locking portion has an inwardly extending protrusion, and the spring biased element is a snap ring positioned within, and movable with the movable locking portion and formed with a spring biased ear for resiliently engaging the fixed element.