Patent Number: 047537698
Section: summary

FIELD OF THE INVENTION The invention relates to devices and methods to safely retain in place a linearly movable element, typically a drive shaft of a control cluster, in the upper internals of a nuclear reactor. BACKGROUND OF THE INVENTION Light water cooled and moderated reactors, and particularly PWRs, are generally provided with clusters of neutron absorbing rods, for controlling operation of the reactor. Each cluster is arranged for being securely connected to a drive shaft extending upwardly from the cluster. The drive shaft is associated with a drive mechanism suitable for linearly moving the shaft, whereby the cluster may be introduced into the core by an adjustable amount and removed from the core. The drive mechanism is typically placed above the cover of the pressure vessel of the reactor and is associated with a stationary sleeve projecting through the cover and accomodating the drive shaft, which is connected to the cluster by a disconnectable coupling. In most PRWs, the vessel contains a stationary structure for supporting and holding the core and for guiding the clusters and their drive shafts. The latter function is fulfilled by an assembly belonging to the reactor structure, placed above the core and usually called "upper internals". The upper internals constitute a rigid structure comprising a guide tube for each cluster, each guide tube being in alignment with one of sleeves. It is periodically necessary to replace some of the fuel assemblies or to change their location in the core. For that purpose, the control clusters (that designation including possible spectrum shift clusters) are lowered into the core by actuation of their drive shafts. The vessel cover is removed for providing access to the upper internals. Each drive shaft is separated from the corresponding cluster using an axially introduced remote control tool. Then the upper internals can be raised along with the drive shafts and provide access to the fuel assemblies. DESCRIPTION OF THE PRIOR ART Up to now, the drive shafts were arranged for bearing on stop means formed in the bottom portion of the upper internals for being retained by the upper internals during removal of the latter. That approach relies for proper operation on satisfactory alignment of the clusters with respect to the drive shafts. However, it will not be possible to achieve sufficient accuracy of alignment in the reactors presently under development. There is a considerable risk that, in such reactors, the expected misalignments of the control clusters with respect to the cluster guides in the internals may cause jamming of the drive shafts on the upper "pommels" of the clusters provided for connection with the drive shaft gripping means. SUMMARY OF THE INVENTION It is an object of the invention to provide a retaining device for locking the drive shaft of a control cluster which is improved with respect to prior art devices having the same purpose. It is a more specific object to provide a device which makes it possible to achieve safe locking of the drive shafts on the higher part of the upper internals when the cover of the reactor should be removed after the temperature which prevails in the reactor has dropped from the value in operation (typically about 320.degree. C.) to a value lower than the boiling point of water (typicaly lower than 90.degree. C.). For that purpose, the invention provides a device for retaining a linearly movable drive shaft comprising: a base member fixedly secured to the upper portion of the upper internals and coaxial with a guide tube; a plurality of grippers spaced apart and distributed about the axis of said base member and mounted for pivotal movement about a horizontal axis between a position of engagement with a portion, formed with circumferential grooves, of said drive shaft contained in the guide tube, and a released position free from engagement with said drive shaft; slide means axially movable with respect to said base member between a higher position and a lower position, said slide means being operatively connected to said grippers for causing engagement of said grippers into a groove of said drive shaft when in said upper position; resilient means arranged to exert a force on said grippers tending to engage said grippers into said shaft; said slide means being dimensioned for being held by the corresponding sleeve at a position sufficiently remote from its upper position for maintaining the grippers in a released position whatever the temperature prevailing in the reactor and consequently whatever the variations of relative position of the sleeve and the upper internals due to differential thermal expension. In a preferred embodiment, the resilient means comprise three springs compressed between the base member and the slide means, distributed about the axis and circumferentially located between said grippers. Each gripper may be in the form of a bell crank lever, having one arm formed with at least one latching tooth and another arm having a terminal ball emprisoned in the slide means. According to another aspect of the invention, a method for retaining a vertically linearly movable shaft includes the steps of engaging the grippers of the device on the drive shaft following removal of the cover; lowering a remotely controlled tool on the drive shaft for locking the shaft in a predetermined position in which the shaft is entirely within the internals; forcing down the slide means with the tool for releasing the grippers at the same time as the drive shaft is seized by the tool; raising the drive shaft to the appropriate locking level; and releasing the slide means for allowing the grippers to engage into the drive shaft before the drive shaft is released by the tool.