Patent Number: 043303706
Section: summary

The present invention relates generally to nuclear reactors of the type including a reactor vessel having a vessel cover which utilizes at least one rotatable plug for performing certain position related functions within the vessel. The present invention relates more particularly to a combination seal and bearing arrangement for use with this nuclear reactor and specifically for use with its vessel cover. An example of the nuclear reactor just recited, at least with respect to its rotating plugs and internal components may be seen in co-pending United States patent application, Ser. No. 13,420, filed Feb. 21, 1979 and entitled A CABLE HANDLING SYSTEM FOR USE IN A NUCLEAR REACTOR, said application having been assigned to the Assignee of the present application. As seen there, the cover of the reactor vessel includes a horizontally extending deck containing a plug assembly made up of three horizontally extending, circular plugs, an outermost plug which is the largest of the three, an intermediate plug which is eccentrically located entirely within the outermost plug and an innermost plug which is eccentrically located entirely within the intermediate plug. These plugs serve to perform certain position related functions within the reactor vessel, specifically to facilitate refueling of the reactor core by rotating the plugs relative to one another and to the stationary deck. Each of the rotating plugs just described and its surrounding plug or the stationary deck in the case of the outer plug define an annular opening or clearance gap therebetween. This gap should contain some sort of bearing system to facilitate rotation of the plug and some sort of sealing system for containment of the reactor core coolant and cover gas within the reactor vessel while preventing an influx of oxygen, water vapor or other ambient materials into the reactor vessel. Heretofore, this has been typically accomplished by means of an arrangement typically including what is commonly referred to as a dipseal, a separate annular backup seal and a system of bearings. In this arrangement, the dipseal is comprised of means located on opposite sides of the annular clearance gap for containing a pool of liquid metal sealant, usually liquid sodium where the latter is used as the reactor coolant. The system of bearings is located externally of the liquid sealant and the backup seal is disposed between the two. This backup seal is typically constructed of an inflatable and deflatable elastomer which is maintained in its inflated state during normal operation of the reactor for providing a tight seal in order to prevent reactor cover gas pressure surges from forcing the liquid sodium sealant up into the bearing system. When the cover plugs are rotated the backup seal is partially deflated, sufficient to allow rotation of the plug but nevertheless remaining in sealing engagement. There are a number of drawbacks associated with the arrangement just described. First, with respect to the dipseal, while the use of sodium as a sealant has the advantage of being compatible with sodium in the reactor vessel, it causes oxidation of the surrounding components and particularly the bearing system which is very difficult to clean. Other compatible liquid metal sealants which have been suggested include mercury and an alloy of tin and bismuth. However, this latter alloy also results in a severe oxidation problem and the mercury results in toxicity problems. As will be seen hereinafter, in accordance with one preferred aspect of the present invention, a non-metallic liquid sealant is used in place of the liquid metal sealant and does not provide an oxidation or toxicity problem. While this non-metallic sealant is not compatible with the reactor coolant, it is reliably isolated from the latter, as will also be seen hereinafter. A second drawback associated with the prior art arrangement described above resides in the utilization of its backup, elastomeric seal between the pool of liquid metal sealant and the bearing system. More specifically, during normal operation of the sealant over extended periods of time, it is necessary to periodically replace the elastomeric seal due to wear. Because of its positional relationship to the bearing system, it is necessary to disassemble the latter in order to make this change which may be complicated and, in any event, time consuming. However, since this seal at least in part serves to protect the bearing system from the liquid metal sealant, it must be positioned inwardly of the latter. On the other hand, the liquid sealant utilized by the present invention is one which is compatible with its associated bearing system and, in fact, serves as a lubricant for the latter. Hence, the bearing system is positioned in direct contact with the latter, thereby eliminating the need for an inflatable elastomeric seal therebetween. However, an inflatable and deflatable elastomeric seal is nevertheless used, but it is located externally of the bearing system and, hence, can be easily replaced without disassembling the latter. Moreover, since the elastomeric seal does not serve to protect the bearing system from internal cover gases or from the liquid sealant itself, it does not have to remain in engagement at all during rotation of its associated plug and will therefore last longer than the previous elastomeric seals that remained in engagement with their plugs during rotation. In view of the foregoing, one object of the present invention is to provide a combination arrangement for providing a seal and bearing system within the annular clearance around a rotating plug in a nuclear reactor of the type described and particularly to an arrangement which is uncomplicated in design, relatively economical to provide and, most important, reliable in use. Another object of the present invention is to provide a combination seal and bearing arrangement which in a preferred embodiment utilizes a non-metallic sealant for sealing the previously mentioned clearance gap and, in any event, a sealant which is compatible with the bearing system used, thereby eliminating the need for an inflatable elastomeric seal therebetween. Still another object of the present invention is to minimize the possibility of any of the sealant reaching the liquid metal coolant within the reactor vessel in the unlikely event that a seismic occurrence might cause the liquid sealant to splash. Yet another object of the present invention is to select a liquid sealant which also serves as a lubricant for its bearing system. Still another object of the present invention is to provide the combination sealing and lubricating fluid just mentioned and to periodically replace it with clean fluid without disassembling any of the components making up the overall seal and bearing arrangement or any other components of the reactor. Still another object of the present invention is to provide a combination seal and bearing arrangement which includes an inflatable elastomeric seal, but one which is easily accessible from the ambient surroundings for replacement and one which does not need to be placed in engagement during rotation of its associated plug, thereby increasing its useful life. As will be discussed in more detail hereinafter, the overall combination seal and bearing arrangement meeting the foregoing objectives is one which includes stationary means fixedly mounted around the outside of the annular clearance gap which extends around the particular plug to be sealed and supported for rotation. This combination arrangement also includes rotating means fixedly mounted to the plug itself for rotation therewith and extending around the inside of the annular clearance gap. This rotating means and the stationary means just mentioned together define a circumferential leakage path therebetween, which path extends outwardly from the annular clearance gap to the ambient surroundings. In accordance with the present invention, the overall arrangement disclosed herein is provided with bearing means located between and engaging both the stationary and rotating mens inside the annulus path just described and sealing and lubricating fluid, specifically, a liquid, filling an entire circumferential section of the leakage path between the bearing means and annular clearance gap. This fluid is provided for sealing the leakage path while, at the same time, lubricating the bearing means. In accordance with another aspect of the present invention, the combination seal and bearing arangement constructed in accordance with a preferred embodiment includes means for easily providing and replacing the sealing and lubricating fluid without disassembling the arrangement and means for minimizing the possibility that this fluid will reach the liquid metal coolant within the reactor in the unlikely event that a seismic occurrence might cause the fluid to splash. Still another aspect of the present invention resides in the utilization of an inflatable and deflatable seal means which does not have to remain in a seal engaged position during rotation of the plugs and which is located externally of the arrangement's bearing means for easy replacement without the necessity of disassembling the latter.