Patent Number: 047284790
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to fluid seals and more particularly to a fitting for a high pressure seal that must be periodically disassembled and replaced with a low pressure seal arrangement. The invention has particular utility in connection with a nuclear reactor wherein the high pressure seal between the thimble guide tube and the thimble at the seal table on the exterior of the well in which the reactor vessel is seated must be released and replaced with a low pressure seal during the refueling of the reactor. 2. Description of the Prior Art Refueling of pressurized water reactors is an established, routine operation carried out with a high degree of reliabilty. Refueling is normally performed once or twice a year depending upon load requirements. For the sake of economy, it is desirable that the refueling operation be accomplished as quickly as possible. In recent years a number of design innovations have considerably simplified refueling operations, reducing the number of operator actions required during refueling and, hence, considerably reducing the amount of time for a complete refueling operation from approximately four weeks, to less than seven days. The present invention is generally directed toward further simplifying fying the refueling operations, minimizing the downtime of the reactor during such refueling, and increasing the reliability of certain components which must be manipulated during refueling. In a typical pressurized water nuclear reactor arrangement, the reactor vessel is seated in a concrete well and contains the usual nuclear reactor core and instrumentation. Among the instrumentation are a plurality of stainless steel tubes, referred to as thimbles, which extend, during normal reactor operation, from the upper plate of the core downwardly to the bottom of the vessel where they pass through a penetration in the bottom of the vessel and terminate at a point exterior to the vessel well. The thimble is normally empty, however, approximately once a month a neutron flux detector is pushed through the thimble to the top of the core and then slowly retracted while neutron flux readings are taken with suitable flux mapping equipment to which the detector is connected outside the thimble. The thimble itself is housed in a larger stainless steel tube, referred to as a guide tube, which stainless steel tube, referred to as a guide tube, which is welded to the bottom of the vessel and which forms the vessel penetration through which the thimble passes. The interior of the guide tube is exposed to the reactor cooling water and hence the operating pressure of the system, whereas the inside of the thimble is essentialy dry and at atmospheric pressure. The guide tube extends on the exterior of the vessel from the vessel pentration through the concrete wall forming the well of the vessel to a stainless steel plate, known as the seal table. A termination stub of the guide tube penetrates the seal table and is welded thereto. The thimble passes entirely through the guide tube and extends beyond the seal table toward the flux mapping equipment. Because the space between the outer surface of the thimble and the inner surface of the guide tube is exposed to the operating pressure of the reactor, it is necessary during normal reactor operation to have a high pressure seal at the seal table at the point where the thimble exits the guide tube in order to prevent reactor coolant from being expelled from the guide tube. During a refueling operation, it is necessary to retract the thimble a number of feet in order to remove it from the volume of the nuclear core. To do this, the high pressure seal at the seal table between the guide tube stub and the thimble is disassembled so that the thimble can be retracted the necessary distance. Although the reactor is powered down during the refueling operation and the water pressure within the vessel is concomitantly reduced, it is still necessary to have a low pressure seal between the guide tube stub and the thimble because the seal table is approximately at or below the upper flange of the vessel and is thus below the water level of the refueling canal which is normally filled during the refueling operation. In the past it was necessary to temporarily lower the water level within the vessel to a point below the seal table during the time that the high pressure seal at the seal table was disassembled and a low pressure seal installed in its place. Thereafter, the vessel and refueling canal would be filled with water to provide sufficient shielding to keep the radiation within an acceptable level when fuel assemblies are removed from the vessel. The prior technique of physically installing a low pressure seal at the seal table after disassembling the high pressure seal, and then, after the refueling operation, subsequently reinstalling the high pressure seal, has several disadvantages. First of all, it is time consuming because the water level has to be reduced to a level below the seal table each time the seal between the guide tube stub and the thimble is replaced. Further, the action of changing from a high pressure seal to a low pressure seal, and vice versa, is time consuming in that, depending on the design of the reactor, there could be in excess of 50 thimbles, each of which must be retracted through a separate guide tube stub at the seal table. Another disadvantage is that the reliability of the seal at the seal table between the guide tube stub and thimble could be impaired in the course of changing from one to the other type of seal. The reason for this is that the high pressure seal is a compression type seal employing a compression ring, or ferrule, which is compressed via a lock nut to form a friction-tight, fluid seal at the exterior of the guide tube stub. When the high pressure seal is disassembled, the ferrule remains in place and is utilized as an anchor point for the compression nut in the formation of the low pressure seal. If during this conversion from a high pressure seal to a low pressure seal, the ferrule slips, the integrity of the low pressure seal may be impaired. Further, and more importantly, when the high pressure seal is reassembled the ferrule, once having slipped, would have to be removed and a portion of the guide tube stub may have to be cut off in order to provide a clean, unmarred surface for the compression fitting of the high pressure seal. Since the guide tube stub only projects beyond the seal table by several inches, the entire guide tube may eventually have to be replaced in order to provide a sufficiently long guide tube stub on which to form a high pressure compression seal. SUMMARY OF THE INVENTION It is an object of the present invention to provide a high pressure compression seal fitting which incorporates a built-in low pressure seal arrangement in order to eliminate the actions required to physically disassemble a high pressure seal and to install a low pressure seal in its place, and vice versa, and at the same time to provide such a fitting which is both simple in construction and highly reliable. The above and other objects are accomplished according to the invention by the provision of a high pressure seal fitting with a built-in low pressure seal arrangement for sealing a space defined by an inner diameter of a pipe and a rod slidably disposed within the pipe and extending beyond an end of the pipe, the fitting including: a fitting body provided with an axial passage for slidably accommodating the rod and having a first end region constructed for engaging the end of the pipe for forming a high pressure seal between the pipe and the fitting body, and a second end region opposite the first end region and including an outer circumferential protrusion having the shape of a ferrule for cooperating with a compression fitting and nut to form a releasable high pressure seal between the fitting body and the rod; and low pressure seal means disposed in the axial passage at the second end region of the fitting body for maintaining a low pressure seal between the fitting body and the rod when the releasable high pressure seal is disassembled. As used herein, rod is intended to include a hollow rod such as a pipe. Because the low pressure seal is built into the fitting body, when the high pressure compression seal between the fitting body and the rod is disassembled, the low pressure seal is automatically in place at the time that the high pressure compression seal is released. Thus, in the context of a seal between a guide tube stub and a thimble at a seal table of a nuclear reactor as discussed above, such a fitting body avoids the actions required in lowering the water level to effect the low pressure seal as was required in the past, and also avoids the reliability problems involved in using the same ferrule, which heretofore was a separate component, to effect multiple compression seals. Further, because the circumferential protrusion which forms the ferrule of the high pressure seal at the second end region of the fitting body is integral with the fitting body itself, the problem of the ferrule loosening at the guide tube stub is eliminated. Preferably, the fitting body is comprised of first and second components which are joined together, for example by a welded joint. The first component may comprise a socket which contains the first end region for engaging the end of the pipe, or guide tube stub, and for forming a high pressure seal therewith. The second component incorporates the second end region with the integral ferrule and low pressure seal means. Accordingly, in the event that the integral ferrule does become damaged, the second component of the fitting body can be removed, for example by breaking the weld with the first component, and a new second component installed in its place. The guide tube stub is thus preserved, avoiding the possibility of having to replace an entire guide tube as could happen with the use of the prior technique for sealing the guide tube stub to the thimble.