Patent Number: 047537698
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT In FIG. 1, part of a nuclear reactor is illustrated diagrammatically. The reactor has a pressure vessel 10, a cover 12 for closing the vessel and upper internals in vessel 10. The upper internals comprise a plate 14, guide tubes 16 extending down as far as to an upper core plate (not shown) and guide tubes 18 projecting above plate 14 and all ending substantially at the same level, below the cover. A sleeve 20 is located in alignment with each guide tube 18 and is arranged for guiding a corresponding drive shaft 22. That part of each sleeve 20 which is above the cover includes a linear motion mechanism which may be of any one of the well known types, for instance as described in U.S. Pat. No. 3,480,807 (Downs et al). Each drive shaft has a terminal coupling for disconnectable connection between the drive shaft 22 and the pommel 24 of a corresponding control cluster 26. The arrangement which has been described up to now is well known in the art and consequently does not require a detailed description. Each of the guide tubes 18 is provided with a retaining device (situated in the zone designated by a dash-dot circle in FIG. 1) which will now be described with reference to FIGS. 2 to 4. The retaining device comprises a base member 28 securely connected by suitable means, for instance bolts 30, to the upper part of each guide tube 18 which slidably receives a drive shaft 22. The base member is formed with three recesses 32 mutually spaced apart by 120.degree. about the axis of the drive shaft. A horizontal pin 36 is located across each recess and a gripper 34 is pivotally received on the pin for pivotal movement in a plane passing through the axis of the guide tube 18. Each gripper is formed as a bell crank lever. One of the arms of the lever has a latching lip engageable in any one of the peripheral grooves 38 formed in shaft 22. The other arm has a terminal ball 40. A slide member 42, consisting of a plurality of parts secured together, is mounted on base member 28 for vertical sliding movement thereon. Referring to FIG. 3, keying means 46 are provided for preventing rotation of the slide member about the shaft axis. The balls 40 of all three grippers are retained in an annular chamber 48 defined by the slide member. Three springs 44 compressed between the base member and the slide member exert on the slide an upwardly directed force which tends to urge the slide member to a higher position (FIG. 3). When the slide member is in that higher position, it retains the lips of the grippers 34 in engagement with a groove 38 of shaft 22. Referring again to FIG. 2, a tulip shaped end piece 50 is permanently connected to the lower end of sleeve 20 and has a heat protection function. A calibrated hole in the tulip is formed for circulating a predetermined flow rate within the gap between the drive shaft and the guide tube. Flow calibration is important when there is a large number of clusters, since the cumulated flows short-circuit the core and may affect the overall thermal balance. During operation of the reactor, sleeve 20 and its end piece 50, as well as all upper internals, are immersed in the high temperature high pressure coolant, which also contacts the vessel wall. End piece 50 is secured in a position so selected that, whatever the differential thermal expansion between the parts, the end piece forcibly maintains slide member 42 in a position in which the grippers 34 are out of contact with shaft 22. In FIG. 2, the two endmost positions that gripper 34 may assume when the cover is in position are indicated. The position farthest from the shaft is indicated in full line. The dash-dot line indicates the position closest to the shaft that may be taken by gripper 34 when the cover is in position. It will be appreciated that there still exists an angular clearance of about 20.degree. in the latter case, where the differential expansions are the most unfavorable. On the other hand, springs 44 always engage the grippers 34 into the confronting groove 38 when the slide member 42 is free to move upwardly. The device which has just been described may be operated as follows for reactor refueling. First, the reactor is shut down. Then, the coolant temperature steadily decreases. When the temperature has become lower than 90.degree. C., typically about 70.degree. C., the cover is removed. The control clusters are then in lower position in the reactor. When the cover is lifted, the end pieces release the slide members 42. The slide members move up under the action of the return springs 44 and they apply the grippers 34 on the drive shafts 22. Each drive shaft 22 in turn is then separated from the corresponding cluster. Typically, each drive shaft has a flexible finger grab for gripping a handling enlarged head of the cluster, such a construction being described, for instance, in French Pat. No. 2,537,764. Then separation may be carried out with a tool which is axially inserted into the drive shaft. After the cluster has been released, each drive shaft is locked in the upper internals in a predetermined position, so selected that the lower end of the drive shaft be within the upper internals and there is no danger that the drive shaft later jams against the corresponding enlarged head, when the internals are inserted again into the reactor vessel. Referring to FIG. 5, each drive shaft is unlocked using a tubular tool whose lower end is shaped to force down the drive member 42 for spreading apart the grippers 34 and releasing shaft 22. The driving rod of tool 52 makes it possible to lift the drive shaft up to the predetermined locking level while the drive member 34 remains forced down. Last, the slide member 42 is released by lifting the lower end of the tool while the tool rod remains stationary. Then, the return springs 44 move back the slide member to its upper position and engage the grippers 34 in a groove 38 of the drive shaft 22. Then the tool may be removed. The tool may be provided with a graduated scale for checking the level at which the drive shaft was locked. After all drive shafts have been brought to the preselected locking position, the upper internals are removed as a whole and reloading may be carried out. After reloading has been completed, the upper internals are reinserted back. Then the drive shafts are moved down into engagement with the enlarged ends of the clusters. That operation may be carried with a tool (not shown) which is also arranged for unlocking the locking device by forcing down the slide member. It will be appreciated that the device of the invention provides for safe locking of the drive shafts, whether or not they are attached to their clusters, at such a level that the handling steps and the abutment when the upper internals are inserted back are rendered easier. The time spent for connection, disconnection and associated check-up are reduced while safety is improved over that of the prior art devices.