Abstract:
A support clamp assembly for mechanically securing a thermal sleeve to an elbow conduit in a jet pump of a nuclear reactor vessel, the support clamp assembly including: a tension shaft having a first end extendable through an opening in a sidewall of the elbow conduit and an opposite end with a head; a cruciform assembly having a base with an opening to receive the tension shaft and to abut the head of the shaft, wherein the cruciform assembly seats in the thermal sleeve; a boss slidable over the first end of the tension shaft and having a curved surface seating an outside surface of the elbow conduit, and a coupling device engaging the first end of the tension shaft and abutting the boss, wherein the coupling device places the tension shaft under tension to secure the cruciform assembly to the thermal sleeve and the boss to the elbow conduit.

Description:
BACKGROUND OF INVENTION 
       [0001]    The present invention relates to an apparatus and method for supporting or repairing a welded joint between a thermal sleeve and an elbow of a jet pump in a water recirculation system of a boiling water nuclear reactor (BWR). 
         [0002]    In a boiling water nuclear reactor, an annular space is defined between the core shroud and the reactor pressure vessel wall. Jet pumps are located in the annular space for recirculating water in the reactor. Each jet pump typically includes a riser conduit, connected to an elbow conduit in the annular space. The elbow conduit is welded to a thermal sleeve which penetrates the reactor pressure vessel wall and supplies water for recirculation through the reactor via a jet pump. The weld between the thermal sleeve and the elbow conduit is typically a full penetration butt weld. The weld joint lies within the confined space of the annulus. Access to the weld joint is highly restricted. Moreover, the weld joint is subjected to the reactor environment and is subject to inter-granular stress corrosion cracking. 
         [0003]    The jet pump assemblies in the reactor vessel can flood the core to about two-thirds the core height in the event of a Loss of Coolant Accident (LOCA). If the integrity of the jet pump recirculation piping becomes compromised, such as due to a piping break or separation of the riser pipe elbow conduit from the inlet nozzle thermal sleeve, the jet pump system may lose its ability to flood the core during a LOCA. 
         [0004]    Over time, cracks may occur in the weld joint between the thermal sleeve and the elbow conduit. Propagation of cracks in the weld joint can compromise the integrity of the weld joint between the elbow and thermal sleeve. Some leakage flow through a cracked welded joint may be tolerable because water leakage does not degrade the ability of the jet pumps to flood the core. However, propagation of cracks in the weld joint may lead to separation of the thermal sleeve and elbow conduit to the jet pump. Upon separation of the sleeve and elbow, the jet pump recirculation system may be severely degraded due to reduction in coolant flow. 
         [0005]    Replacement or repair of the weld joint between the thermal sleeve and the elbow conduit might be accomplished by breaching the piping system external to the reactor in a drywell area of the containment system. This approach would necessitate off-loading the fuel from the reactor and draining the reactor vessel water level down to a level below the recirculation inlet nozzles. The associated thermal sleeve and piping safe-end could then be replaced to restore structural integrity of the recirculation piping system. Such an approach would be costly. Accordingly, there is a long felt need for a method and device to provide weld replacement or repair the weld joint between the thermal sleeve and the elbow. 
       SUMMARY OF INVENTION 
       [0006]    A support clamp assembly is disclosed for mechanically securing a thermal sleeve to an elbow conduit in a jet pump of a nuclear reactor vessel, the support clamp assembly including: a tension shaft having a first end extendable through an opening in a sidewall of the elbow conduit and an opposite end with a head; a cruciform assembly having a base with an opening to receive the tension shaft and to abut the head of the shaft, wherein the cruciform assembly seats in the thermal sleeve; a boss slidable over the first end of the tension shaft and having a curved surface seating an outside surface of the elbow conduit, and a coupling device engaging the first end of the tension shaft and abutting the boss, wherein the coupling device places the tension shaft under tension to secure the cruciform assembly to the thermal sleeve and the boss to the elbow conduit. 
         [0007]    A support clamp assembly is disclosed for mechanically securing a thermal sleeve to an elbow conduit in a jet pump of a nuclear reactor vessel, the support clamp assembly comprising: a tension shaft having a threaded end extendable through an opening in a sidewall of the elbow conduit and a head at an opposite end of the shaft; a cruciform assembly having a base with an opening to receive the tension shaft, wherein the base includes an end surface shaped to receive the head of the shaft, wherein the cruciform assembly seats in the thermal sleeve; a boss having an opening through which extends the threaded end of the tension shaft, the boss includes a first end with a curved surface to seat on an outside surface of the elbow conduit and an opposite end with a recess to receive a nut, and the nut screws onto the threaded end of the tension shaft, wherein the nut places the tension shaft under tension to secure the cruciform assembly to the thermal sleeve and the boss to the elbow conduit. 
         [0008]    A method is disclosed to assemble a support clamp assembly in a jet pump of a nuclear reactor vessel, the method comprising: attaching a cable to a first end of a shaft and lowering the bolt and cable into an elbow conduit and thermal sleeve of the jet pump; guiding at least one thermal sleeve anchor device down the cable to slide the anchor device on the shaft, wherein the anchor device abuts against a second end of the shaft; seating the anchor device against an inside surface of the thermal sleeve; pulling the cable through the hole in the elbow; extending the cable and shaft from the anchor device, through an aperture in the elbow conduit; attaching a securing device to the first end of the shaft extending through the aperture in the elbow conduit, wherein the securing device is against an outside surface of the elbow conduit, and applying tension to the shaft which biases the thermal sleeve to the elbow conduit. 
     
    
     
       SUMMARY OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a portion of a boiling water reactor with the vessel wall and shroud (core wall) a cut away to show in the gap therebetween exemplary jet pumps connected to a thermal sleeve at the vessel wall. 
           [0010]      FIG. 2  is an enlarged cross-sectional view of the riser pipe and elbow conduit welded to a thermal sleeve in the vessel wall, and showing a clamp assembly supporting the weld joint between the elbow conduit and thermal sleeve. 
           [0011]      FIGS. 3 and 4  are perspective front and rear views, respectively, of a cruciform assembly for the clamp assembly. 
           [0012]      FIG. 5  is perspective view of a primary cruciform of the cruciform assembly. 
           [0013]      FIG. 6  is perspective view of a secondary cruciform of the cruciform assembly. 
           [0014]      FIG. 7  is a perspective view of a draw bolt for the cruciform assembly, where one end of the bolt is supported by the cruciform assembly. 
           [0015]      FIGS. 8 and 9  are perspective front and rear views, respectively, of an elbow boss for securing an opposite end of the draw bolt to the elbow conduit. 
           [0016]      FIGS. 10 and 11  are perspective front-side and rear-side views, respectively, of a nut secured against the elbow boss. 
           [0017]      FIGS. 12 and 13  are perspective front-side and rear-side views, respectively, of a latch spring for the elbow boss of the cruciform assembly. 
           [0018]      FIGS. 14 and 15  are cross-sectional view of the elbow conduit and thermal sleeve, and show the assembly of the cruciform assembly. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0019]    An apparatus and method has been developed to support or repair a weld in an inlet nozzle thermal sleeve to riser pipe elbow conduit for a recirculation jet pump system in a boiling water nuclear reactor. A mechanical support clamp assembly has been developed to provide structural support to the welds that join the thermal sleeve to the elbow conduit leading to the riser pipe in a jet pump. The support clamp assembly maintains the spatial relationship of the jet pump riser pipe and recirculation inlet nozzle thermal sleeve. The support clamp assembly supports the welds by adsorbing some of the forces on the joints between the jet pump riser pipe and recirculation inlet nozzle thermal sleeve, and by ensuring that the elbow conduit of the riser pipe and thermal sleeve do not separate in the unlikely event of a weld failure. By maintaining the spatial relationship with the support clamp assembly, the positional relationship between the recirculation inlet nozzle thermal sleeve and the riser pipe elbow conduit is preserved during normal operation and in the unlikely event of a Loss of Coolant Accident (LOCA). 
         [0020]      FIG. 1  shows a reactor pressure vessel  10  having a reactor pressure vessel wall  12  and an inner core shroud  14  separated by a generally annular space  16 . Several jet pumps, one being generally designated  18 , are disposed in the annular space and in an annular array around the core shroud  14 . Each jet pump  18  typically comprises an inlet riser pipe  20 , a pair of inlet elbows  22  adjacent the upper end of the inlet riser, a pair of nozzles  23 , a pair of mixing assemblies  24 , and a pair of diffusers  26 . A riser pipe transition  28  splits the flow and redirects the flow through the elbows  22  and nozzles  23  and down to the mixing section. The riser pipe transition is at the top of the jet pump  18 . The riser pipe transition  28  is supported by braces  25  attached to the vessel wall  12 . In addition, braces and restrainers maintain the jet pump  18  in a fixed position in the annular space  16  between the core shroud  14  and pressure vessel wall  12 . 
         [0021]    The jet pump receives external water under pressure through a thermal sleeve  32  that penetrates and is seated in the pressure vessel wall  12 . The thermal sleeve is welded to an inlet elbow conduit  34  (see  FIG. 2 ). The opposite end of the inlet elbow  34  is secured to the lower end of the inlet riser  20 . Water enters the thermal sleeve  32  and flows through the elbow  34 , upwardly in the inlet riser  20 , through the inlet elbows  22  into the jet pump nozzles  23  for flow in a downward direction through the mixing assembly  24 , the diffusers  26  and into a plenum  30  for upward flow through the reactor core. The jet pump nozzles  23  induce water to flow from the annular space  16  to the mixing assemblies  24  where the water from the annular space  16  mixes with the water passing through the jet pump nozzles. 
         [0022]      FIG. 2  is a cross-sectional view of the riser pipe  20  and elbow conduit  34  that is welded (see RS-1 weld joint  36 ) to the thermal sleeve  32 , interfaces with the wall  37  of the reactor vessel at pads  42 . The thermal sleeve  32  includes a generally converging conical segment  38  seated in a nozzle  40  which is an opening in the wall  37 . 
         [0023]    The conical segment  38  of the thermal sleeve may be fabricated from inconel. The inside surface of the segment  38  includes a conical section that has a large diameter inlet facing an inlet tube  46  of the thermal sleeve and a narrow diameter towards an inner end  48  of the segment  38 . The inner end  48  of thermal sleeve conical segment  38  is welded  50 , e.g., a bimetallic weld designated a RS-1A weld, to a cylindrical coupling  52  fabricated from stainless steel. The opposite end of the cylindrical coupling  52  is welded  36 , e.g., weld RS-1, to the inlet elbow  34 . 
         [0024]    A support clamp assembly  60  mechanically couples the thermal sleeve  32  to the inlet elbow conduit  34 . The support clamp includes a cruciform assembly  62 , a draw bolt  64 , a boss  66 , a latch spring seated in the boss, and a nut  68 . The cruciform assembly  62  seats on the conical inside surface of the conical segment  38  of the thermal sleeve  32 . The cruciform assembly anchors the support clamp assembly to the thermal sleeve. A draw bolt extends through the cruciform and has a bolt head that seats on the cruciform. 
         [0025]    The opposite end of the bolt  64  extends through a wall of the elbow conduit and engages the boss  66  seated on the wall of the elbow conduit. The threaded nut  68  screws on the threaded end of the draw bolt. The nut abuts against the boss. The nut is turned on to the threaded end of bolt to place the bolt shaft under tension. The tension on the bolt applies a force to the cruciform assembly and the thermal sleeve  32  in a direction of the bolt shaft and the elbow conduit  34 . Similarly, the tension on the bolt pulls the boss and elbow conduit towards the thermal sleeve. Due to the tension of the bolt shaft, the support clamp assembly  60  secures the thermal sleeve  32  to the elbow conduit  34 . 
         [0026]      FIGS. 3 to 6  show the cruciform assembly which includes a primary cruciform  70  and a secondary cruciform  72 . The primary and secondary cruciform components may be fabricated from inconel, which is preferably the same material as the conical segment  38  of the thermal sleeve. By forming the cruciform assembly and thermal sleeve conical segment of the same material, they have similar thermal expansion characteristics. The primary and secondary cruciforms when assembled form a brace having outer surfaces  74  that abut the inner surface of the conical segment  38 . The outer surfaces  74  of the cruciform assembly seat against the inner surface of the conical segment to brace the clamp assembly against the thermal sleeve. 
         [0027]    The cruciforms have an open construction to allow substantially uninterrupted flow of coolant water through the thermal sleeve. Ribs  76  in the cruciforms  70 ,  72  span between the outer surfaces  74  and their center bases  78 ,  82 . The ribs are aligned with the flow direction through the thermal sleeve such that the ribs are narrow in the flow direction. Water flows across the ribs without much resistance or disturbance to the flow of the water. The primary cruciform  70  includes a center cylindrical base  78  having a slot  80  to receive the base  82  of the secondary cruciform and a spherical concave seat  84  to receive the head of the draw bolt. 
         [0028]    The seat  84  on the base  78  of the primary cruciform  70  allows the bolt head to pivot a few degrees and thereby adjust if the cruciforms are not entirely perpendicular to the bolt shaft. A spherical ledge surface  88  of the draw bolt when mated with the spherical seat  84  of the primary cruciform allows limited articulation of the draw bolt relative to the cruciform assembly, thus precluding the imposition of any bending loads on the draw bolt. 
         [0029]    The slot  80  in the base  78  of the primary cruciform receives and supports the secondary cruciform and allows the secondary cruciform to shift slightly angularly when the outer surfaces  74  of the cruciforms seat against the inside walls of the conical segment of the thermal sleeve. 
         [0030]      FIG. 7  is a perspective view of the draw bolt  64 . The bolt head  86  includes a convex spherical surface on a lower ledge  88  that seats in the concave spherical seat of the base of the primary cruciform. The bolt shaft  90  has a length sufficient to extend from the boss, through the elbow conduit and to the cruciform assembly in the thermal sleeve. The proximal end  92  of the bolt is threaded. Upon installation, the proximal end extends through apertures in the elbow conduit and boss, and engages the nut on the exposed side of the boss. The draw bolt may be formed from austenitic stainless steel type XM-19. 
         [0031]      FIGS. 8 and 9  are perspective views of the front and rear, respectively, of the elbow boss  66 . The boss may be fabricated from austenitic stainless steel, type  316 , and machined to conform to an outside surface of the elbow conduit. The elbow boss  66  is held in position relative to the elbow by a sleeve  94  extending through the boss. The sleeve  94  is inserted into a cylindrical opening extending through the elbow. A short end stub  95  of the sleeve protrudes from a surface  96  of the boss which abuts the elbow conduit. The abutting surface  96  of the boss is curved to match the outer surface of the elbow conduit in contact with the boss. The stub  95  protrusion of the sleeve slides into an aperture in the elbow conduit. The stub aligns the boss to the elbow conduit and keys the elbow boss to the elbow. The opening in the short radius turn of the elbow conduit is a circular hole that may be machined by electric discharge machining (EDM). The stub of the sleeve and the curved surface  96  of the boss forms a seal for the opening of the elbow conduit. The sleeve also provides a conduit through which extends the draw bolt as the bolt protrudes out of the elbow. 
         [0032]    A spherical annular seat  98  is recessed in the circular end of the boss which is opposite to the surface  96  for the elbow conduit. The seat  98  mates with a corresponding spherical surface  108  of the nut  68 . A machined cavity  100  in the boss is adjacent the seat  98  and receives a latch spring  102  ( FIGS. 12 and 13 ). 
         [0033]      FIGS. 10 and 11  are perspective views of the nut  68  that screws on the threaded end of the draw bolt. The nut includes an internal cylindrical aperture with threaded sidewalls and outside sidewalls extended to engage a wrench or other device to tighten the nut on the threaded end of the bolt. A flange  104  of the nut includes an outer circular edge having teeth to engage a pawl  106  ( FIG. 13 ) of the latch spring to prevent unintended rotational movement of the nut when seated in the seat  98  of the boss. The engagement of the pawl and teeth prevent loosening of the mechanical joint, when subjected to flow induced vibration. The spherical annular bottom surface  108  of the nut seats on the seat  98  of the boss to allow limited articulation of the nut and draw bolt relative to the boss. 
         [0034]      FIGS. 12 and 13  are perspective views of the sides of the latch spring  102 . The latch spring is generally shaped as a hairpin with a pair of extended legs  110  and a connecting corner end  112  having a rim engaging an overhanging lip in the back corner of the cavity  100  ( FIG. 9 ) of the boss, which is generally shaped to receive the latch spring. The latch spring is held in the cavity  100  by overhangs in the cavity that extend partially over the corner end  112  and legs  110 . To insert the latch spring into the cavity, the legs are pinched together as the spring is slid into the cavity. The latch spring provides a bias on the prawl  106  that engages the teeth  104  of the nut and thereby prevents loosening rotation of the nut. 
         [0035]      FIG. 14  is a cross sectional view of the riser pipe  20 , elbow conduit  22  and thermal sleeve  32 , with a cable  112  connecting the unassembled components of the support clamp assembly. The draw bolt  64  and cruciform assembly are assembled inside the jet pump riser pipe  20  and nozzle thermal sleeve  32 . A cable  112  is a guide wire to position the components of the cruciform assembly into the thermal sleeve and elbow conduit and slide together the components of the assembly. To provide an opening to the elbow and thermal sleeve, the jet pump inlet mixer assemblies  24  ( FIG. 1 ) are removed to expose two side circular openings  114  ( FIG. 1 ) of the riser pipe transition  28 . The cable, bolt and components of the cruciform assembly are inserted through one of the circular openings  114  and down the riser pipe. 
         [0036]    At the refuel floor of the core above the jet pump assemblies, the flexible stainless steel cable  112  is attached to the threaded end of the draw bolt  64 . The draw bolt is lowered by the cable into one of the circular openings  114  in the riser pipe transition and down the riser pipe. The cable is used to position the draw bolt  64  upstream  116  of the converging conical section of the thermal sleeve segment  32 . The cable extends from the bolt, through an opening in the riser pipe transition and has a terminating cable end at the refuel floor of the containment building. 
         [0037]    The opening in the base of the primary cruciform  70  is first slid over the terminating end of the cable and then the opening in the base of the secondary cruciform  72  is slid over the end of the cable. The cruciforms slide along the cable into the openings  114  of the transition  28 . The cable guides the primary cruciform and subsequently the secondary cruciform through the riser pipe, elbow conduit and upstream of the converging conical section of the thermal sleeve. 
         [0038]    The primary cruciform slides along the cable and onto the end of the draw bolt. Subsequently, the cable is used to manipulate the second cruciform to slide on the end of the draw bolt and to slide the base of the secondary cruciform into the slot in the base of the primary cruciform. The bolt and cruciforms are assembled upstream of the converging conical section of the thermal sleeve. 
         [0039]    After the bolt and primary and secondary cruciforms have been advanced to upstream  116  of the thermal sleeve, the cable  112  is slackened and lays on the inside surface of the elbow conduit and near an opening  118  in the elbow. A hook  120  is extended from the refuel floor and into the annular space  16  between the shroud and the vessel wall. The hook extends through the opening  118  in the elbow and grasps the cable  112 . The free end of the cable is pulled through the riser, out the opening  118 , up through the annular space  16  and to the refuel floor. The cable is used to pull the threaded end of the draw bolt  64  out through the opening  120  in the elbow conduit. As the bolt is pulled through the elbow opening, the primary and secondary cruciforms seat on the converging conical section  32  of the thermal sleeve. 
         [0040]      FIG. 15  is a cross sectional view of the riser pipe  20 , elbow conduit  22  and thermal sleeve  32 , with the cable  112  outside of the riser and guiding the boss  64  and nut  68  to the threaded end of the draw bolt  64 . After the hook  120  ( FIG. 14 ) pulls the cable out through the opening  118  in the elbow, the cable pulls the treaded end of the draw bolt through the opening  118  in the elbow. At the refuel floor, the cable is threaded through the elbow boss  66  and nut  68  and they slide down the cable to the end of the bolt. The elbow boss slides over the threaded end of the bolt and seats on an outside surface of the elbow surrounding the opening  118  in the elbow. The nut  68  is guided by the cable to the threaded end of the bolt. The bolt is twisted on the threaded end of the draw bolt such that the proper mechanical torque preload is applied to the nut and bolt. Finally, the stainless steel cable  112  is removed from the draw bolt and extracted from the annular space between the vessel wall and core shroud. 
         [0041]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.