Patent Publication Number: US-2005135537-A1

Title: Pressure vessel

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to pressure vessels and more particularly to pressure vessels that are susceptible to stress corrosion cracking (“SCC”), such as those employed in commercial nuclear reactor plants.  
      Commercial nuclear reactor plants generate high temperature, high pressure water for ultimately driving steam turbines to generate electricity. For example, pressurized water plants are designed to operate at temperatures of approximately 600° F.-650° F. or more and at pressures of approximately 2250 psi or more. As these plants have aged after decades of substantially continuous operation, highly stressed regions of pressure vessels exposed to these high temperature, high pressure corrosive environments have proven to be susceptible to SCC. In particular, the welds of some penetration nozzles in the hemispherical pressure vessel heads (sometimes known as “J-groove” welds) and the associated heat affected zones in the penetration nozzles have started to develop cracks.  
      The nuclear power industry periodically inspects the pressure vessel heads in pressurized water nuclear reactor plants to detect cracking using ultrasonic or eddy current techniques. However, because of the structure and geometry of pressure vessel heads and their welds, small cracks may not be detected before they grow across the welds or through the penetration nozzle walls. Once a crack develops across the entire weld or through the heat affected zone in the penetration nozzle, the high temperature water will start to leak through the crack. Thus, the industry visually inspects the outer surfaces of the pressure vessel heads for evidence of leaking (such as residual water stains or boric acid crystals) in the course of refueling outages. However, because the typically tight fit of the penetration nozzles in the pressure vessel heads may delay leakage, a relatively long time may pass between the initiation of cracking and the visual detection of leakage. During this time, the cracking may continue to develop and existing cracks may continue to grow to the point where the pressure vessel heads can not be economically repaired but must be replaced.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a pressure vessel design that permits cracks associated with penetration nozzle welds to be readily verified by visual inspection techniques. It is a further object to provide a design that permits leaks to be detected before cracks can substantially grow. As used herein, the word “penetration” means a hole extending through a pressure vessel wall and the expression “penetration nozzle” means a pipe or tube segment that extends through a “penetration” and is fixed to the pressure vessel wall.  
      With these objects in view, the present invention resides in a pressure vessel having a leak path in its pressure-retaining boundary extending from its penetration nozzle welds to its outer surface where a leak may be readily detected. A pressure vessel embodying the present invention has an inner surface (which may be exposed to a corrosive environment) and an outer surface with a plurality of penetrations defined by penetration walls extending between the inner surface and the outer surface. A plurality of penetration nozzles extend in the penetrations. Each penetration nozzle is sealed with the pressure vessel head by a circumferential structural weld at the inner surface of the pressure vessel. Each penetration wall and adjacent penetration nozzle defines a passageway extending therebetween from the circumferential structural weld outwardly to the outer surface of the pressure vessel. Each passageway includes a groove in the penetration wall or in the penetration nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention as set forth in the claims will become more apparent from the following detailed description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein:  
       FIG. 1  is a schematic representation of a pressure vessel having penetration nozzles extending from a hemispherical upper head;  
       FIG. 2  is a schematic representation of a penetration nozzle welded to the pressure vessel head of  FIG. 1  in accordance with prior art designs;  
       FIG. 3  is a top view of the penetration nozzle of  FIG. 2  taken along line  3 - 3 ;  
       FIG. 4  is a schematic representation of a first embodiment of the present invention depicting a penetration nozzle welded to the pressure vessel head of  FIG. 1 ;  
       FIG. 5  is a top view of the penetration nozzle of  FIG. 4  taken along line  5 - 5 ;  
       FIG. 6  is a schematic representation of a second embodiment of the present invention depicting a penetration nozzle welded to the pressure vessel head of  FIG. 1 ;  
       FIG. 7  is a top view of the penetration nozzle of  FIG. 6  taken along line  7 - 7 ; and  
       FIG. 8  is a schematic representation of a third embodiment of the present invention depicting a penetration nozzle welded to the sidewall of the pressure vessel of  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring now to the drawings in detail and in particular to  FIG. 1  there is shown a pressure vessel  10  containing fuel assemblies  12  for transferring heat to recirculating water in a pressurized water nuclear reactor plant. The pressure vessel  10  has a hemispherical upper head  14  attached to a vessel body  16  by a plurality of nuts  18  threaded onto studs  20 . The pressure vessel  10  also has a welded lower head  22 . The upper head  14  has a plurality of penetration nozzles  24  extending from its outer surface  26 . The penetration nozzles  24  in the upper head  14  of a pressure vessel containing fuel assemblies  12  are designed to support control rod drive assemblies (not shown) or in-core instrumentation (not shown). The lower head  22  of a pressure vessel  10  containing fuel assemblies  12  may also have penetration nozzles (not shown) for supporting additional instrumentation. The pressure vessel  10  is also representative of other pressure vessels that contain high temperature water and/or steam in pressurized water nuclear power plants such as pressurizers and steam generators and of other pressure vessels used in other plants such as boiling water nuclear reactor plants, petrochemical plants and chemical plants wherein the penetration nozzles can be employed for the same or other purposes such as supporting thermocouples and the like.  
       FIGS. 2 and 3  illustrate a known pressure vessel head design wherein a plurality of penetration nozzles, represented by penetration nozzle  24 , extend through a plurality of penetrations, represented by penetration  28 , in the upper head  14 . The upper head  14  has an inner surface  30  that is exposed to high temperature, high pressure water and radiation during operation. Each penetration nozzle  24  and penetration  28  extends generally vertically along an axis  32 . Each penetration  28  is generally defined by a penetration wall  34  extending between the inner surface  30  and the outer surface  26 . Each penetration nozzle  24  is typically fixed in the penetration  28  by a specific interference (or a so-called “shrink fit”) with tie penetration wall  34  and sealed with the upper head  14  by a circumferential structural weld  36  at the inner surface  30  of the upper head  14 . The seal is designed to be water-tight under all conditions. The structural weld  36  is designed to support the penetration nozzle  24  against all of the mechanical and hydraulic forces to which it may be exposed while in service. Typically, for a pressure vessel in a commercial nuclear reactor plant, the weld design takes no credit for the interference fit. The interference fit is intended to fix the penetration nozzle  28  in position prior to welding and then to minimize the effect of any externally applied loads on the weld. As is illustrated, each penetration  28  may have an enlarged countersunk portion  38  at the inner surface  28  of the upper head  14 . In other designs, there may be no countersunk portions or there may be additional countersunk portions at the outer surface  26  of the pressure vessel  10 .  
      As discussed above, these welds  36  and their heat affected zones  37  are subject to SSC after years of exposure to high temperature, corrosive environments. Once a crack grows across a weld  36  or through the wall of the penetration nozzle  24 , the water in the pressure vessel  10  will tend to leak through the crack. However, in the prior art penetration nozzle designs illustrated by  FIGS. 2 and 3 , the tight interference fit may restrict the passage of leaking water for many years while the cracking becomes more extensive.  
      As is illustrated by  FIGS. 4-8 , the present invention provides a leak path extending between a penetration nozzle  24  and a penetration  28  from the weld  36  to the outer surface  26  of the pressure vessel  10  so that a leak can be readily detected in the course of a visual inspections during following outages. In pressure vessels embodying the present invention, each penetration wall  34  and adjacent penetration nozzle  24  defines a passageway extending from the weld  36  to the outer surface  26 , which passageway includes at least one groove cut into the penetration wall  34  or in the penetration nozzle  24  in the interference fit region. Most preferably, the grooves are cut into the penetration walls  34 . The grooves may be cut by electrical discharge machining or other suitable means.  
       FIGS. 4 and 5  illustrate a first embodiment of the present invention, which includes a leak path or passageway  40  extending axially from its inner end at the circumferential structural weld  36  to its outer end at the outer surface  26 . As is best seen in  FIG. 4 , each passageway  40  may include a countersunk portion  38  at its inner end. Each passageway  40  generally includes one or more (depicted in  FIG. 5  as three) grooves  42  machined into pressure vessel head  14 , which extend axially from the countersunk portion  38  to the outer surface  26 . In one design having three grooves  42 , the grooves may be approximately a fourth of an inch wide and a sixteenth of an inch deep. Preferably, the grooves  42  will have generous radii  44 . In other embodiments, the grooves  42  may be machined in the penetration nozzles  24  (not shown) rather than the pressure vessels  10 . The designs may be varied to suit the capabilities of the manufacturers.  
       FIGS. 6 and 7  illustrate a second embodiment of the present invention, which provides a passageway  50  including a groove  52  spirally extending around the axis  32  from a counterbore  38  at its inner end to an optional countersunk portion  54  at its outer end at the outer surface  26 .  
       FIG. 8  illustrates a third embodiment of the present invention, wherein a penetration nozzle  60  is welded to the inner surface  62  of a vertical sidewall  16  of the pressure vessel  10 . As is illustrated, an axially extending groove  64  is machined into the penetration nozzle  24 , which extends from the weld to the outer surface  66  of the pressure vessel  10 . Preferably, the groove  64  is positioned at the bottom of the penetration nozzle  24 .  
      While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.