Abstract:
An apparatus for reinforcing weakened portions of the top guide assembly in a boiling water reactor. The repair apparatus includes a cruciform lattice segment which reinforces the damaged or weakened region of the top guide beam lattice. This cruciform lattice segment is held in place atop the existing top guide with specially designed straps. The cruciform lattice segment and associated straps are arranged so that a beam segment of the cruciform lattice segment bridges the weakened region in the cracked top guide. Thus, the bridging beam segment transmits loads across the weakened region of the top guide. The straps are designed to avoid interference with removal and installation of the fuel assemblies and the control rod blade which is inserted between the fuel assemblies.

Description:
FIELD OF THE INVENTION 
     This invention relates to the repair of boiling water reactor components which have been damaged or weakened by stress corrosion cracking. In particular, the invention relates to the repair of cracked beans in the top guide of a boiling water reactor. 
     BACKGROUND OF THE INVENTION 
     A conventional boiling water reactor (BWR) comprises a reactor pressure vessel (RPV) filled with cooling water, a core of nuclear fuel submerged in that water and a stainless steel cylinder, called the core shroud, which surrounds the nuclear fuel core and is concentric with the RPV walls The core shroud comprises a shroud flange for supporting the shroud heads, a circular cylindrical upper shroud wall having a top guide welded to the shroud flange, an annular top guide support ring welded to the bottom rim of the upper shroud wall, a circular cylindrical middle shroud wall comprising multiple sections welded in a stack, with a top rim of the middle shroud wall being welded to the top guide support ring, and an annular core plate support ring welded to the bottom rim of the middle shroud wall and to the top rim of a lower shroud wall. The entire shroud is supported by a shroud support, which is welded to the bottom rim of the lower shroud wall, and by an annular shroud support plate, which is welded at its inner diameter to the shroud support and at its outer diameter to the RPV wall. Reactor water flows down the annular space between the RPV wall and the shroud, around the lower rim of the shroud and up through the fuel core located within and surrounded by the cylindrical shroud. 
     The fuel core consists of a multiplicity of upright and parallel fuel bundle assemblies arranged in 2×2 arrays, each assembly consisting of an array of fuel rods inside a Zircaloy fuel channel. The assemblies of each array are separated by a cruciform gap which allows vertical travel of a cruciform control rod blade in between the fuel channels. Each control rod blade contains neutron-absorbing material. The power level is maintained or adjusted by positioning control rods up and down within the core while the fuel bundle assemblies are held stationary. Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate. In particular, the top guide provides lateral support to the upper end of the fuel assemblies, neutron monitoring instrument assemblies and installed neutron sources, and maintains the correct fuel channel spacing to permit control rod insertion. The top guide is designed so that during periodic refueling operations, the fuel bundle assemblies can be lifted out of and lowered into the core without removing the top guide. 
     One type of top guide installed in certain types of BWRs has a fabricated design comprising a lattice of interlocking upper and lower beams held together by a large circular ring. The circular ring of the top guide sits on the top guide support ring of the shroud, and is provided with radially inwardly directed flanges that capture the distal ends of the beams. The beams and support ring are typically made of Type 304 stainless steel with high carbon content. The composition of standard Type 304 stainless steel is 18.0-20.0 wt. % Cr, 8.0-10.5 wt. % Ni, 2.0 wt. % Mn, 1.0 wt. % Si, 0.08 wt. % C, 0.045 wt. % P and 0.03 wt. % S. 
     The foregoing top guide design contains many creviced welded and unwelded connections, which are used to attach the lattice beam supports to the inner surface of the support ring and for rigid span support of the top guide structure over the core. The Type 304 stainless steel with high carbon content which is typically used in early BWR plants, in conjunction with the many creviced regions, results in the top guide being susceptible to intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC). Sustained exposure to conditions conducive to IGSCC and IASCC will eventually require repairs or complete top guide replacement. 
     SUMMARY OF THE INVENTION 
     The present invention is a device employed to repair damaged top guides having cracks in the beam lattice region. The invention permits use of a limited number of local repairs to the top guide without welding. Therefore, employment of the invention avoids the need for complete top guide replacement. Thus, this invention is expected to benefit the end user by facilitating in-reactor repair with reduced costs and reduced downtime. 
     The repair in accordance with the invention entails the installation of a cruciform lattice segment which reinforces the damaged or weakened region of the beam lattice. This cruciform lattice segment is held in place atop the existing top guide with specially designed straps. The cruciform lattice segment and associated straps are arranged so that a beam segment of the cruciform lattice segment bridges the weakened region in the cracked top guide. Thus, the bridging beam segment transmits loads across the weakened region of the top guide. The straps are designed to avoid interference with removal and installation of the fuel assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of portions of two intersecting beans of a typical top guide. 
     FIG. 2 is an isometric view of a peripheral portion of the typical top guide shown in FIG.  1 . 
     FIG. 3 is an isometric view of a top guide repaired in accordance with a preferred embodiment of the invention. 
     FIG. 4 is a partially exploded isometric view of portions of two intersecting beams of a top guide being reinforced with repair hardware in accordance with a preferred embodiment of the invention. 
     FIG. 5A is a sectional view showing the top guide repair hardware installed on a top guide, the section being taken along line  5 A— 5 A indicated in FIG.  4 . 
     FIG. 5B is a sectional view showing the top guide repair hardware of FIG. 5A in an unfastened condition during an intermediate stage in the repair hardware installation procedure. 
     FIGS. 6 and 7 are sectional views showing the top guide repair hardware, the sections being respectively taken along lines  6 — 6  and  7 — 7  indicated in FIG.  5 A. 
     FIG. 8 is an exploded isometric view of a cruciform reinforcement beam and modified top guide prior to coupling in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a typical top guide comprises a lattice of interlocking upper and lower beams  12  and  14 , respectively. The lower edge of each upper beam  12  has a plurality of laterally spaced, vertically upwardly extending slots  16  for receiving a portion of a respective one of a plurality of mutually parallel lower beams extending generally perpendicular to the upper beams. Conversely, the upper edge of each lower beam  14  has a plurality of laterally spaced, vertically downwardly extending slots  18  for receiving a portion of a respective one of a plurality of mutually parallel upper beams extending generally perpendicular to the lower beams. The slots  16  and  18  enable the upper and lower beams  12  and  14  to be interlocked in “egg-crate” fashion. The beams are held at their ends to maintain the “egg-crate geometry so that the beams can provide lateral support to the 2×2 arrays of fuel bundle assemblies  60  separated by spacers  62 , shown in FIG.  3 . 
     The opposing ends of each beam of the typical top guide are supported by a circular support ring  20 , shown in FIG.  2 . The support ring  20  comprises a rim  22  and a rim top  24  which are fastened together by a multiplicity of bolts  26  circumferentially distributed along the ring  20 . The rim  22  comprises a circular cylindrical wall  22   a  and a radially inwardly directed annular flange  22   b  integrally joined to a bottom end of the rim wall  22   a . The rim top  24  also forms a radially inwardly directed annular flange extending generally parallel to flange  22   b . The radius of the radially inner periphery of flange  22   b  is generally equal to the radius of the radially inner periphery of rim top  24 . The rim  22  and rim top  24  form a channel for receiving the ends of the lattice beams. 
     As seen in FIG. 2, the end of each beam is coupled to a bracket  28  by means of a plurality of pins  30 . The bracket  28  has a U-shaped cross section with a channel for receiving the end of the beam. The sidewalls  28   a  and  28   b  extend in parallel from opposite ends of a base  28   c  of bracket  28 . (In the alternative, two separate plates not connected by a base could be used instead of bracket  28 .) Bracket  28  may be a welded or cast structure. The sidewalls  28   a  and  28   b  each have a plurality of holes which extend in the thickness direction for receiving pins  30  which fasten the bracket  28  and the beam end together. In addition, each sidewall has a bore extending vertically downward from an upper end face for receiving a pin  32  which fastens the bracket  28  and the rim top  24  together. For the sake of clarity, only the portion of pin  32  which passes through the rip top is indicated by dashed lines, although it should be understood that the pin extends into the bracket. Each sidewall also has a bore extending vertically upward from a lower end face for receiving a pin (not shown) which fastens the bracket  28  and the rim flange  22   b  together. The pins which fasten bracket  28  to ring  20 , in conjunction with the pins that fasten the bracket to the beam end, hold the beans and ring together, thus forming a top guide assembly which can be installed and removed as a single unit using a crane or other lifting equipment. The width of the bracket channel is slightly greater than the thickness of the beam end, so that the parallel sidewalls of bracket  28  block displacement of the beam end in the circumferential directions and oppose rotation of the beam end about a vertical axis. 
     During reactor operation, the reactor components are exposed to conditions which render welded joints and heat affected zones thereof susceptible to stress corrosion cracking. In particular, the crevice welds and heat affected zones at the intersections of the upper and lower beams of the top guide assembly are susceptible to stress corrosion cracking. The cracks which result can weaken the top guide and impair its ability to function as a lateral support. To avoid the cost of replacing the entire top guide when crack indications are discovered in certain regions of the bean lattice, it is desirable to provide a local repair to reinforce the cracked region. 
     Hardware for carrying out such a local repair of the top guide assembly in accordance with a preferred embodiment of the invention is depicted in FIG.  3 . This repair hardware is installed while the reactor is shut-down. The repair hardware shown in FIG. 3 comprises a cruciform lattice segment  34  having two parallel beam segments  36  and  38  which intersect a beam segment  40  at right angles, the beam segments  36  and  38  each being joined to the beam segment  40  either integrally or by welding. However, it should be understood that the term “cruciform lattice segment” as used herein includes any reinforcement structure having at least one beam segment intersecting at least one other beam segment at right angles. At a minimum, the cruciform lattice segment of the invention has only two beam segments which intersect at right angles. At a maximum, the cruciform lattice segment of the invention may have a first plurality of parallel beam segments intersecting a second plurality of parallel beam segments at right angles, thereby forming a reinforcement lattice. The cruciform lattice segment sits on top of the beam lattice of the top guide assembly, each beam segment of the cruciform lattice segment being aligned with a corresponding beam of the top guide lattice. The cruciform lattice segment is placed such that a respective beam segment of the cruciform lattice segment overlies each weakened region of the top guide lattice which requires reinforcement. Thus, for those embodiments of the cruciform lattice segment which have parallel beam segments, the parallel beam segments must have the same spacing as that of the top guide lattice beams to which the reinforcement beam segments are attached. 
     Regardless of the number of beam segments, the cruciform lattice segment is attached to the weakened beam lattice of the top guide assembly by a multiplicity of spaced straps  42 . In the fully installed state, each strap  42  rigidly restrains the reinforcement beam segment and the underlying lattice beam against relative displacement in the vertical and lateral directions. As shown in FIG. 3, the cruciform lattice segment and associated straps are arranged so that the beam segments of the cruciform lattice segment bridge the weakened regions in the cracked top guide. Thus, the bridging beam segments transmit loads across the weakened regions of the top guide. 
     The structure of the strap  42  in accordance with the preferred embodiment of the invention will be described with reference to FIGS. 4-7. As best seen in FIG. 5B, each strap  42  comprises a pair of strap members  44  and  46  having a truncated U-shaped profile. Strap member  44  comprises a longitudinal member  44   a , an upper transverse member  44   b  extending generally perpendicular to longitudinal member  44   a  and a pair of lower transverse members  44   c  extending generally perpendicular to longitudinal member  44   a  and parallel with each other. As best seen in FIG. 7, lower transverse members  44   c  form a clevis, each transverse member  44   c  having a hole for receiving the opposing ends of a clevis pin  48 . Strap member  46  comprises a longitudinal member  46   a , an upper transverse member  46   b  extending generally perpendicular to longitudinal member  46   a  and a lower transverse member  46   c  extending generally perpendicular to longitudinal member  46   a  and parallel with transverse member  46   b . Lower transverse member  46   c  is a plate-shaped projection having a hole for receiving an intermediate portion of the clevis pin  48 . The ends of the clevis pin are secured to the lower transverse members  44   c  of strap member  44 . Clevis pin  48  is not secured to lower transverse member  46   c , which allows strap member  46  to swing relative to strap member  44  about the clevis pin axis. As seen in FIG. 5B, the end face of transverse member  46   c  of strap member  46  has a chamfered surface  50  which contacts a corresponding chamfered surface  52  at the end of a slot formed in strap member  44  to retain the strap member  46  at the free swing-out position shown in FIG.  5 B. Placement of strap member  46  in the free swing-out position shown in FIG. 5B creates a gap between the upper transverse members  44   b  and  44   c  which is wide enough to allow passage there-through of the stacked cruciform lattice beam segment  36  and top guide lattice beam  12  during installation of the straps. 
     As seen in FIG. 5A, the distance separating the, transverse members  46   b  and  46   c  is greater than the overall height of the stacked cruciform lattice beam segment  36  and top guide lattice beam  12 . Therefore, the stacked beam segment and beam can fit between the transverse members  46   b  and  46   c . Furthermore, the distance separating the transverse members  44   b  and  44   c  is greater than the overall height of the stacked cruciform lattice beam segment  36  and top guide lattice beam  12  by an amount sufficient to allow the upper transverse member  46   b  to enter the gap between the upper transverse member  44   b  and the cruciform lattice beam segment  36  when strap member  46  is swung from the free swing-out position shown in FIG. 5B to the closed position shown in FIG.  5 A. 
     In the closed position, the strap members  44  and  46  are fastened together using a hex screw  54  as shown in FIG.  5 A. The upper transverse members  44   b  and  46   b  have respective threaded bores  56  (see FIG. 4) and  58  (see FIG.  5 B). The threaded bores  56  and  58  are located such that they are coaxial when the strap member  46  is swung into the closed position. In the coaxial state, the threaded bores  56  and  58  can engage the threads on the hex screw screwed therein. As shown in FIG. 5B, the hex screw  54  is preinstalled by threadably coupling the distal portion of the screw shaft in the threaded bore  56  formed in upper transverse member  44   b . As shown in FIG. 5A, the shaft of hex screw  54  has a length such that the distal end of the shaft will bear against the top surface of the cruciform lattice beam segment  36  when the hex screw is further threadbly coupled in the threaded bore  58  formed in upper transverse member  46   b  and then fully torqued to urge the bottom surface of the top guide lattice beam  36  against the upper surfaces of the lower transverse members  44   c  and  46   c . The hex screw  54  is torqued to produce a desired preload in the strap. 
     In accordance with the preferred embodiments of the invention, the cruciform lattice segment  34 , the straps  42  and the hex screws  54  are made of XM-19 stainless steel, and the clevis pin  48  is made of Type 304 stainless steel. As previously noted, the beams of the top guide lattice are also made of Type 304 stainless steel. XM-19 stainless steel has higher strength than Type 304 stainless steel. 
     During repair hardware installation, the reactor is shutdown and the temperature of both the top guide beams and the repair hardware are relatively low. After the repair has been completed and the reactor has been restarted, the temperature of the top guide lattice beams and the repair hardware rises. Because the straps and the top guide lattice beams are made of different materials having different coefficients of thermal expansion, the strap and the beam undergo differential thermal expansion, thereby applying a thermal load to the strap in addition to the preload. For example, Type 304 stainless steel has a coefficient of thermal expansion, α 304SS =9.4244×10 −6  inch/inch/° F. In contrast, XM-19 stainless steel has a coefficient of thermal expansion α XM-19 =8.9464×10 −6  inch/inch/° F. The thermal load arising from differential thermal expansion of these materials helps to secure the repair hardware in place. 
     A further refinement in accordance with the present invention is shown in FIG.  8 . In order to resist displacement of the cruciform lattice segment relative to the top guide beam lattice in directions parallel to the upper and lowers top guide beams, a plurality of spaced grooves  64  can be formed on the upper edges of the upper and lower beams  12  and  14  in the reinforcement region. Grooves  64  can be formed by electrodischarge machining in a conventional manner. Correspondingly, the lower edges of the cruciform lattice beam segments are provided with projections  66  which are configured and spaced to form-fit inside the grooves  64 . The interlocking relationship of grooves  64  and projections  66  prevents shear displacement of the cruciform lattice segment  34  relative to the top guide beams  12  and  14 . 
     The preferred embodiments of the top guide repair apparatus in accordance with the invention have been disclosed for the purpose of illustration Variations and modifications of the disclosed structure which fall within the concept of this invention will be readily apparent to persons skilled in the art. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.