Abstract:
A clamp assembly connects a diffuser adapter or lower ring to a diffuser tail pipe of a jet pump diffuser in a boiling water nuclear reactor. The clamp assembly includes at least two clamp segments shaped generally corresponding to an exterior circumference of the diffuser, a swivel link affixed at each end of each of the clamp segments, and at least two connecting bands pivotably secured to the swivel links between the ends of the clamp segments. The clamp segments each includes a locking assembly engageable with the diffuser adapter or lower ring and the diffuser tail pipe. The clamp assembly structurally replaces/repairs the weld joining the diffuser adapter or lower ring and the diffuser tail pipe.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to repairing or replacing a bimetallic weld in a jet pump diffuser assembly and, more particularly, to a jet pump diffuser clamping assembly that structurally replaces/repairs the weld. 
   In a boiling water nuclear reactor, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core flow. The lower portion of the jet pump is a long conical tubular section. The function of the diffuser is to slow the high velocity flow stream and thus convert the dynamic head of the flow stream into static pressure. 
   The jet pump diffuser assemblies in boiling water reactors can be one of two differing design configurations. The first configuration incorporates an adapter component, which is comprised of an upper austenitic stainless steel ring and a lower inconel alloy 600 ring. These two rings were shop-welded, as this was a bimetallic weld (a weld of dissimilar metals) and more difficult to perform than a weld joining similar metals. This adapter component then facilitated the assembly of the diffuser in the reactor, as the lower section of the adapter was welded to the inconel alloy 600 shroud support plate. The diffuser tail pipe being constructed of austenitic stainless steel was then joined to the upper section of the adapter, which was constructed of austenitic stainless steel. As such, at the time of field construction of the reactor these two welds of similar metals were performed. 
   The second design configuration consists of a lower ring constructed of inconel alloy 600, shop-welded to the diffuser tail pipe. Since the diffuser tail pipe is fabricated from austenitic stainless steel, this was a bimetallic weld. This design approach results in the welding of the lower ring to the shroud support plate as the only weld required to be performed at the time of field construction of the reactor. 
   Regardless of diffuser assembly design configuration under consideration, a bimetallic weld is present in the diffuser assembly. In the event that the structural integrity of the bimetallic weld of the diffuser assembly should become degraded, a means of reinforcing or structurally replacing this weld is desired. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In an exemplary embodiment of the invention, a clamp assembly connects a diffuser adapter or lower ring to a diffuser tail pipe of a jet pump diffuser in a boiling water nuclear reactor. The clamp assembly includes at least two clamp segments shaped generally corresponding to an exterior circumference of the diffuser, a swivel link affixed at each end of each of the clamp segments, and at least two connecting bands pivotably secured to the swivel links between the ends of the clamp segments. The clamp segments each includes a locking assembly engageable with the diffuser adapter or lower ring and the diffuser tail pipe. 
   In another exemplary embodiment of the invention, a method of repairing a weld joining a diffuser adapter or lower ring to a diffuser tail pipe of a jet pump diffuser in a boiling water nuclear reactor includes the steps of forming a plurality of holes in the diffuser adapter or lower ring and the diffuser tail pipe; positioning two clamp segments each including a clamp body and upper and lower pin inserts on the diffuser such that the upper pin inserts engage the holes in the diffuser tail pipe and such that the lower pin inserts engage the holes in the diffuser adapter or lower ring; connecting a swivel link at each end of each of the clamp segments; securing two connecting bands to the swivel links between the ends of the clamp segments; and tightening the connection between the clamp segments and the swivel links to ensure proper fit-up and substantially equal distribution of loads. 
   In still another exemplary embodiment of the invention, the clamp assembly includes two clamp segments shaped generally corresponding to an exterior circumference of the diffuser. Each clamp segment includes a clamp body housing an upper pin insert and a lower pin insert. The upper and lower pin inserts have pins engageable with corresponding holes formed in the diffuser adapter or lower ring and the diffuser tail pipe. A swivel link is affixed at each end of each of the clamp segments, and at least two connecting bands are pivotably secured to the swivel links between the ends of the clamp segments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of the clamp assembly affixed to a jet pump diffuser; 
       FIG. 2  is an elevation view of the clamp assembly in  FIG. 1 ; 
       FIG. 3  is a plan view of the clamp assembly; 
       FIG. 4  is a half-section view showing the locking assembly of the clamp segments; 
       FIG. 5  shows the holes formed in the diffuser for engagement with the clamp assembly pin inserts; 
       FIG. 6  is an exploded view of the clamp assembly; and 
       FIG. 7  illustrates the clamp installation process. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The jet pump diffuser clamp assembly  10  is shown installed on a jet pump diffuser PD in  FIGS. 1-3 . Components of the clamp assembly  10  include clamp segments  12  shaped generally corresponding to an exterior circumference of the diffuser PD, swivel links  14  affixed at each end of the clamp segments  12 , and connecting bands  16  pivotably secured to the swivel links  14  between the ends of the clamp segments  12 . 
   As shown in  FIGS. 4 ,  6  and  7 , the clamp segments  12  are comprised of a clamp body  18  and a pair of pin inserts, including an upper pin insert  20  and a lower pin insert  22 . The clamp bodies  18  each include a pair of circumferential channels  24  on an inside surface thereof that are sized to receive the upper and lower pin inserts  20 ,  22 . The pin inserts  20 ,  22  include a plurality of pins  26  engageable with corresponding holes H formed in the diffuser adapter or lower ring DA and the diffuser tailpipe TP. Preferably, the pins  26  are conically shaped, and the holes H formed in the diffuser adapter or lower ring and the diffuser tailpipe are correspondingly tapered. 
   The preferably conical pins and mating tapered holes are sized such that the pins seat in the tapered holes (i.e. the pin inserts and clamp bodies do not bottom-out or touch the diffuser when the clamp assembly bolts are tightened). As such, leakage of coolant is minimized, since the conical pins theoretically seal the tapered holes. Although cylindrical pins could be used, and the invention is not meant to be limited to conical pins per se, if the pins were cylindrical and they fit into cylindrical holes, there would need to be a clearance between the pins and holes in order to facilitate assembly. The radial clearance would therefore provide a leakage path at every pin-hole interface. In an attempt to minimize leakage, the conical pins and tapered holes are provided. 
   With continued reference to  FIGS. 1 and 6 , each of the clamp segments  12  includes a bolt collar  28  at each end thereof including at least one aperture  30  for receiving a bolt  32  engageable with a corresponding one of the swivel links  14 . 
   Ends of the connecting bands  16  include openings  34  therein that are aligned between openings  36  in the swivel links  14 . A pin  38  is fit through the aligned openings to secure the connecting bands  16  within the swivel link. Each of the bolts  32  and the pins  38  preferably also include crimp collars  40  to ensure fastener retention. 
   Since the diffuser tailpipe TP and the diffuser adapter or lower ring DA are typically formed of different materials having different coefficients of thermal expansion, it is desirable to match the coefficients of thermal expansion between any interfacing components. As such, the upper pin insert  20 , which is engageable with the diffuser tailpipe TP, is formed of a first material having a coefficient of thermal expansion substantially matching that of the diffuser tailpipe TP. Correspondingly, the lower pin insert  22 , which is engageable with the diffuser adapter or lower ring DA, is formed of a second material having a coefficient of thermal expansion substantially matching that of the diffuser adapter or lower ring DA. In one exemplary arrangement, the first material of the upper pin insert is type 316 austenitic stainless steel, and the second material of the lower pin insert is inconel alloy 600. 
   In a similar context, since the clamp body  18  spans across both the diffuser tailpipe TP and the diffuser adapter or lower ring DA, it is desirable to engineer the thermal expansion of the clamp body to be substantially equivalent to the combined thermal expansion of the diffuser tailpipe TP and the diffuser adapter or lower ring DA at operating temperatures. As such, the clamp body  18  is formed of a material having a coefficient of thermal expansion that is intermediate between the coefficients of thermal expansion of austenitic stainless steel (upper section of the adapter and diffuser tail pipe) and inconel alloy 600 (lower section of the adapter or lower ring). In an exemplary embodiment of the invention, the material of the clamp body  18  is type XM-19 austenitic stainless steel (nitronic 50). 
   As a consequence, as the reactor heats up from ambient to operating temperature, the pins  26  expand in concert with the expanding holes H, and since the geometry of the pins  26  substantially matches the geometry of the holes H, leakage is minimized and thermal stresses are not imposed on clamp and diffuser components. Moreover, thermal expansion of the clamp body  18  is engineered to be equivalent to the combined thermal expansion of the jet pump tailpipe TP and adapter piece or lower ring. DA. As a result, thermal stresses at the bimetallic weld of the adapter are alleviated. 
   The holes H, which are conical in a preferred exemplary embodiment of the invention, may be formed via electric discharge machining (EDM) in the tailpipe TP and the adapter or lower ring DA components, as shown in  FIG. 5 . The holes H include a row of holes in the tailpipe TP and a row of holes directly below in the adapter or lower ring DA. The center lines of the holes H in each row of holes are preferably equidistant from each other and lie in a common plane. Mirrored holes are also provided in the opposite side of the tailpipe TP and adapter or lower ring DA as shown in  FIG. 6 . The relative distance of the holes above and below the bimetallic weld is also a function of the coefficients of thermal expansion of the various materials. In an exemplary embodiment, the holes H are approximately ¾ (three fourths) inch in diameter with a total taper angle of 10°. Twenty-eight holes are shown, fourteen of which are machined in one side of the diffuser, and the other fourteen of which are machined in the other side of the diffuser. The holes H in any given side of the jet pump are preferably machined simultaneously in order to preserve the hole spacing above and below the weld line. The distance of these holes above and below the weld and the material selection of the clamp body components minimizes the introduction of additional thermal stress in the weld. 
   The repair clamp installation process is shown in  FIG. 7 . Once the holes H are machined in the jet pump diffuser, clamp components are installed. In a preferred exemplary embodiment, pre-assembled subassemblies including the clamp segments  12 , swivel links  14 , bolts  32 , and bolt crimp collars  40 , are installed onto each side of the jet pump diffuser PD such that the pins of the upper and lower pin inserts  20 ,  22  interface with the holes H in the jet pump diffuser. Subsequently, the connecting bands  16  are installed and secured to the swivel links  14  at each end of the connecting bands by virtue of the pins  38 . The bolts  32  are then tightened sequentially to ensure proper fit-up and equal distribution of loads in the clamp components. Finally, the pin and bolt crimp collars  40  are crimped to ensure fastener retention. 
   The clamp assembly described herein serves to structurally repair/replace the weld connecting the jet pump adapter or lower ring to the jet pump tailpipe. Since the clamp assembly is designed to structurally replace the attachment bimetallic weld, it is not necessary that the existing weld be accessible for visual inspection after the repair clamp has been installed. The clamp assembly is remotely installable in the reactor. The clamp assembly is simplified in design and installation, resulting in a superior alternative for jet pump diffuser repair installations. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.