Abstract:
An improved permanent seal for a refueling canal of a nuclear power plant is disclosed and claimed. The seal includes a support structure and a membrane. The support structure includes a first annular plate with a number of ribs connected to and extending from a to surface of the support structure annular plate. The support structure is positioned atop the shield wall on the refueling canal floor, encircling and positioned near the annulus. The membrane includes a first end that is connected to the seal ledge and a second end that is connected to the refueling canal floor. The membrane has a stepped profile, with side walls extending substantially perpendicularly from a central annular plate to form a pocket configured to fit over the support structure. Loads imparted to the membrane are transferred through the support structure annular plate and ribs to the refueling canal floor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/772,171 filed on Mar. 4, 2013, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a mechanical seal, and, more particularly, the present invention relates to a seal for a refueling canal of a nuclear power plant. 
         [0004]    2. Description of the Related Art 
         [0005]    While the present invention may be used in a variety of industries, the environment of a pressurized water reactor (PWR) nuclear power plant will be discussed herein for illustrative purposes. A typical pressurized water reactor is a cylindrical steel vessel with a hemispherical head on each end of the cylinder. The reactor holds the fuel and system components used to generate electricity then the reactor is on-line. One of the hemispherical closure heads can be removed fur the purposes of refueling the reactor and performing maintenance within the reactor vessel. A peripheral flange is located near the joint between the reactor vessel and the removable closure head. This flange extends radially outward from the vessel centerline and is referred to herein as the “seal ledge.” 
         [0006]    The reactor vessel is positioned within a shield wall forming an annulus between the seal ledge and the shield wall. The top of the seal ledge and shield wall form the base of the refueling canal, which is used to transport spent fuel from the reactor to a storage facility. At the top of the shield wall and refuel canal floor juncture there is a piece of embedded steel to which a canal floor liner is attached. The embedded steel, as part of the shield wall, forms the outer boundary of the annulus. Some nuclear plants have a wide annulus that may be two to three feet wide, while others have a smaller annulus that is as little as two to four inches wide. 
         [0007]    Under normal operating conditions the annulus allows the reactor vessel to move in the vertical and horizontal directions, as well as allowing airflow around the reactor vessel. During refueling activities the annulus is sealed so the refueling canal can be flooded with water to reduce radiation levels. During reactor operation, however, the refueling canal is kept dry. 
         [0008]    If air cannot flow around the vessel when the reactor is operating, then damage could occur to the nuclear instrumentation used for monitoring the core, the shield wall, and the reactor vessel supports. If the annulus cannot be fully sealed during refueling, then the refueling canal would drain down and refueling water would contact the outside of the reactor vessel. Refueling water must be cleaned from the outside surface of the vessel because such water contains boric acid, which is corrosive to the vessel base material. This vessel cleaning is costly and incurs radiation dose exposure, both of which are preventable. 
         [0009]    One known attempt to seal the annulus to prevent leakage through the refueling canal uses a device that compresses an elastomer seal. These devices must be installed prior to refueling and then removed after refueling before the reactor operation cycle can begin. Additionally, these seals need to be inspected and replaced to ensure reliability. The inspections, installation, and removals are costly and incur plant personnel radiation exposure. 
         [0010]    Other knowm attempts to seal the annulus use permanent reactor cavity annulus seals. These devices can be broken into several categories; some bear structural members on the shield wall and the seal ledge, others bear the structural members on beams extending into the annulus, while still other devices cantilever structural members over the annulus. All of these devices provide a permanent membrane that has a welded connection to the seal ledge and the shield wall. 
         [0011]    One known attempt uses an annulus sealing device that is supported by beams that extend from the shield wall towards the reactor vessel, but do not contact the vessel. There is a U-shaped sealing feature that extends below the plane of the sealing flange. However, it is not possible to support this type of device in an annulus without support beams. 
         [0012]    Other known attempts use annulus sealing devices that bear structural members on the seal ledge and the shield wall However, these devices cannot be used in locations where it is not feasible to bear structural members on both the seal ledge and shield wall. 
         [0013]    One known attempt uses a device that cantilevers over the annulus with the support structure either anchored on the shield wall or the seal ledge. The support structure forms part of the sealing membrane, and the membrane&#39;s flexibility is gained from a C-shaped flexure. However, the C-shaped flexure creates a side pocket that will be difficult to decontaminate after the refueling canal has been drained. 
         [0014]    Thus, what is needed is a better way to seal the refueling canal. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention is an improved permanent seal for a refueling canal of a nuclear power plant. The seal includes a support structure and a membrane. The support structure includes a first annular plate with a number of ribs connected to and extending from a lower surface of the support structure annular plate. The support structure is positioned atop the shield wall on the refueling canal floor, encircling and positioned near the annulus. Some or all of the ribs are connected to the canal floor at their distal ends. The ribs can be positioned symmetrically about the support structure annular plate, at least in part. 
         [0016]    The membrane includes a first end that is connected to the seal ledge and a second end that is connected to the refueling canal floor. The membrane has a stepped profile, with side walls extending substantially perpendicularly from a central annular plate. The membrane is formed of a flexible material, such as stainless steel, to accommodate for thermal expansion and contraction of the membrane as well as the plant components to which the membrane is connected. The membrane side walls and annular plate form a pocket, which is configured to fit over the support structure. The membrane has a greater radial length than does the support structure, and thus completely overlies the support structure. Thus, only the membrane is in contact with the refueling liquid during the refueling process. 
         [0017]    In use, the support structure is assembled and affixed in the desired position. The membrane is assembled and affixed in the desired position with a lower surface of the membrane annular plate adjacent to an upper surface of the support structure annular plate. Loads imparted by the refueling water and equipment used during the refueling outage that are placed atop the membrane are imparted through the support structure annular plate and ribs to the refueling canal floor. 
         [0018]    The membrane annular plate may contain resealable port openings to allow for access to various plant components and instrumentation, and for ventilation flow. The support structure annular plate would then contain holes that are aligned with the port openings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention is described with reference to the accompanying drawings, which illustrate exemplary embodiments and in which like reference characters reference like elements. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
           [0020]      FIG. 1  shows a cross-sectional view through a narrow annulus permanent canal seal plate of the present invention. 
           [0021]      FIG. 2  shows a partial perspective view of the membrane component of the seal plate of  FIG. 1 . 
           [0022]      FIG. 3  shows a partial perspective view of the structural support component of the seal plate of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The present invention is a permanent seal for a refueling canal of a nuclear power plant; the present invention provides a liquid barrier over the annular space between a nuclear reactor vessel flange (seal ledge) and the surrounding annular ledge (shield wall).  FIG. 1  shows a narrow annulus permanent canal seal plate  10  of the present invention.  FIG. 1  shows the seal plate  10  in its use environment of a nuclear power plant and illustrates components thereof, including the seal ledge  1 , the refueling canal floor  2 , the shield wall  3 , and the annulus  4 . The seal plate  10  is an annulus sealing device that is comprised of two major subassemblies: a flexible membrane  110  and a support structure  120 . 
         [0024]    The flexible membrane  110  is a peripheral plate that is coupled to the seal ledge  1  and the refueling canal floor. As shown in  FIG. 2 , the membrane  110  is configured to cover the annulus, having an outer cylindrical wall  111  coupled to the refueling canal floor  1  and an inner cylindrical wall  112  coupled to the seal ledge  1 . One preferred method of coupling the membrane  110  to the plant components  1 ,  2  is by welding. Preferably, the entireties of the inner and outer circumferential walls  112 ,  111  of the membrane  110  are welded to the plant components  1 ,  2 . In this manner,water-tight integrity of the seal  10  is ensured. 
         [0025]    As further illustrated in  FIG. 2 , the membrane  110  further includes an annular plate  113  connecting the inner and outer diameter walls  112 ,  111 , which are angled relative the plate  113  to define a pocket  114  therewith. The annular top plate  113  includes an upper surface  113 A and a lower surface  113 B. 
         [0026]    Access port openings  115  are provided in the membrane  110  to allow for access to the reactor vessel nozzles and the nuclear instrumentation that surrounds the reactor vessel. Thus, the present invention does not impair the ability to access, inspect, repair, or replace components of the nuclear plant. During refueling activities, the port openings can be closed via covers  116  that are bolted thereto to provide a water tight seal that will withstand the flooded canal water load. The port covers  116  can be sealed with replaceable O-rings to ensure that they are water-tight. During reactor operation, the port covers  116  are removed and ventilated covers  117  can be installed to allow for HVAC airflow. While the size of the port openings  115  can vary, a diameter of approximately 19 to 20 inches is preferred. Likewise, the number of openings  115  can vary. In one preferred embodiment, the port openings  115  are symmetrically positioned is about the membrane  110  with approximately 15 to 25 inches between adjacent openings  115 . 
         [0027]    The membrane  110  preferably is formed of a flexible material, one example being stainless steel. This flexibility allows for the expansion and contraction the seal plate  10  and plant components will be subjected to due to the temperature fluctuations between the relatively cold environment during a refueling shutdown of the reactor and the relatively hot environment of the operating nuclear plant. 
         [0028]      FIG. 3  illustrates the support structure  120 , which is provided underneath the flexible membrane  110  within the membrane pocket  114 . The support structure  120  includes a horizontal annular plate  121  having an upper surface  121 A and a lower surface  121 B. A plurality of ribs  122  are coupled to and extend substantially perpendicularly from the lower surface  121 B. Proximal ends of the ribs  122  may be coupled to the lower surface  121 B in a variety of manners, such as by welding. In use, the support  120  is positioned atop the refueling canal floor  2  (see  FIG. 1 ). Distal ends of the ribs  122  are in contact with the canal floor  2 , with select ones of the ribs  122  being coupled, such as by welding, to the canal floor  2 . The ribs extend radially from an inner diameter side  123  of the support structure  120  to an outer diameter side  124  of the support structure  120 . Preferably, the ribs  122  are positioned radially with respect to the membrane annular plate  121  and/or center of the reactor vessel. Gaps between the ribs  122  allow for HVAC flow from the annulus  4  to the access port  115  locations. The number of ribs  122  may vary. Preferably, the ribs  122  are positioned symmetrically about the support plate  121 , such as at a spacing of one rib  122  per every 5° to 15°, with one rib  122  per every approximately 10° being more preferred. 
         [0029]    The support plate  122  defines a number of holes  125  therethrough. The holes  125  are positioned such that they are aligned with the access ports  115  of the membrane  110 . Portions of the ribs  122  underlying the holes  125  may be removed. Thus, the support  120  does not interfere with the access provided by the ports  125 . A back wall  126  may be provided to connect adjacent ribs  122  and provide support for the support structure  120  around the holes  125 . 
         [0030]    In use, the support structure  120  is assembled in known manner. This may include forming a number of arc sections of the support plate  121  that are eventually assembled, such as by welding, to form a complete 360° ring to fit completely around the annulus  4 . The ribs  122  are attached, such as by welding, to the lower surface  121 B of the support plate  121 . The arc sections are then positioned atop the refueling canal floor  2  and affixed thereto. Adjacent arc sections may or may not be coupled together. 
         [0031]    The flexible membrane  110  is likewise assembled in known manner. This may also include forming a number of arc sections that are eventually assembled, such as by welding, to form a complete 360° ring to fit completely over the annulus  4 . Each arc section may include a top plate  113  to which the inner and outer walls  112 ,  111  are coupled. Alternatively, the top plate  113  and walls  111 ,  112  may be a single integral piece, with the walls  111 , 112  being formed by plastically deforming end portions of the top plate  112 . Preferably, the walls  111 ,  112  are substantially perpendicular to the top plate  113 . 
         [0032]    Once formed, the membrane  110  is positioned such that its inner cylindrical wall  112  is atop the seal ledge  1  and its outer cylindrical wall  111  is atop the refueling canal floor. The membrane pocket  114  is positioned over the support structure  120  such that the lower surface  113 B of the membrane annular plate  113  rests atop the upper surface  121  A of the support structure annular plate  121 . The membrane  110  is positioned such that the port openings  115  overlie the support plate holes  125 . Any arc sections of the membrane are coupled together, such as by welding. The membrane  110  and the support structure  120  need not be coupled together. As shown, fur example, in  FIG. 1 , both the membrane  110  and the support structure  120  have radial lengths.  FIG. 1  shows a cross-sectional view through the seal  10  and plant components, the radial lengths of the membrane  110  and the support structure  120  being in the plane of the figure. The membrane  110  radial length is greater than the support structure  120  radial length, such that the membrane annular plate lower surface  113 B completely overlies the support structure annular plate upper surface  121 A and extends beyond radial ends thereof. Thus, the membrane outer diameter wall  111  is coupled to the canal floor  2  along a circumference that completely encircles the support structure  120 . 
         [0033]    Prior to a refueling of the reactor, the refueling canal is flooded with refueling water to a depth of 26 feet or more. The weight of this water exerts a force on the membrane  110 . Equipment used in the refueling process is also placed within the canal  2  atop the membrane  110 . The weight load of this water and equipment is transferred from the membrane  110 , to the support plate  121  and ribs  122  to the refueling canal floor  2 . Thus, the structure  120  supports the membrane  110 , helping to ensure its structural integrity. 
         [0034]    Stainless steel is a preferred material for both the flexible member  110  and the support structure  120 . In addition to having flexibility to withstand the compression and tension loads imparted by thermal expansion, stainless steel can also withstand the chemical conditions of the refueling processes. 
         [0035]    While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Furthermore, while certain advantages of the invention have been described herein, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.