Abstract:
A method and apparatus for repairing and/or preventatively maintaining a jet pump restrainer assembly used in a Boiling Water Reactor (BWR), the repair including attaching a bearing plate or plates to a horizontal surface of a jet pump restrainer assembly bracket, the bearing plate or plates acting to supplement and/or replace an existing contact surface between a mixer wedge and the jet pump restrainer assembly bracket of a jet pump restrainer assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Example embodiments relate in general to a method and apparatus for the mechanical repair of a Boiling Water Reactor (BWR) jet pump restrainer assembly. Specifically, example embodiments relate to mechanical repair of a potentially damaged contact area on either an inlet mixer wedge or a restrainer bracket of a restrainer assembly used to horizontally support a jet pump assembly against riser piping to reduce vibration. 
     2. Related Art 
     BWRs are designed to generate steam in reactor pressure vessels (“RPVs”) by heating the water surrounding uranium-containing tubes of fuel assemblies located in the RPV core regions. The RPVs have recirculation loops designed to facilitate the circulation of water in the core regions. The recirculation loops generally include large centrifugal pumps that pump water out of the RPVs and return the water to the inlets of jet pump assemblies located in annular regions in the RPVs surrounding the core regions. The jet pump assemblies are designed to entrain the surrounding water in the annular regions and then discharge the water in a manner that induces a desired flow pattern in the core regions. 
     The jet pump assemblies are subject to vibrations caused by hydraulic forces due to the flow of water and/or by the rotation of the centrifugal pumps. Thus, in one BWR design, the jet pump assemblies are horizontally supported against vibration with a jet pump restrainer assembly including a bracket using a three point suspension system. A three point system generally includes a wedge movably mounted on a vertically oriented guide rod that is attached to a jet pump assembly and extends through the space between the bracket and the jet pump. The wedge, which may weigh about seven pounds, is designed to slide downwardly under the force of gravity into the space between the bracket and the jet pump assembly and thereby urge the jet pump against the adjustment screws. 
     It has been found that the mating (or seating) surfaces of the wedges and/or the brackets of some commercial BWRs have worn substantially after operation over long periods of time. In some cases, the softer interior metal underlying the hardened surfaces of the wedges have worn extensively. It is believed that the wearing is caused by a fretting type of action when the hydraulic forces and/or pump vibrations induce the wedges to chatter or to rise upwardly and then fall back against the bracket. In addition, it is believed that the jet pump assemblies may move away from the adjustment screws in extreme cases. 
     The jet pump restrainer assemblies may be repaired by replacing the worn wedges and/or brackets. However, the jet pump assemblies and/or brackets would need to be disassembled, machined and reassembled, and the old parts would need to be replaced. 
     SUMMARY OF INVENTION 
     Example embodiments provide a method of repairing the BWR jet pump restrainer assemblies without requiring the disassembly of the jet pump assemblies and/or the associated brackets. Example embodiments may also allow for the continued use of the worn wedges and/or brackets. 
     Example embodiments include a method of repairing a BWR jet pump restrainer assembly, in situ. The jet pump assembly extends vertically through a hole in a bracket attached to a riser pipe and is supported against horizontal movement by a plurality of screws extending from the bracket toward the jet pump assembly by a wedge extending into the hole. The wedge is moveably mounted on a vertically extending guide rod fastened to the jet pump assembly. The wedge has a vertically extending inner surface designed to contact the jet pump assembly and an outer surface inclined relative to the vertically extending inner surface designed to contact the bracket. The mating surfaces of the wedge and bracket are the surfaces that are susceptible to wear over time. Example embodiments provide for the placement of bearing plates above and/or below the bracket to provide an additional bearing surface or surfaces between the inlet mixer wedge and the bracket. Example embodiments allow for a bearing plate or plates to assist in supplementing, or in essence expanding the existing contact surface between the inlet mixer wedge and the restrainer bracket. Alternatively, example embodiments allow the existing inlet mixer wedge to be repositioned (the wedge may be partially withdrawn), allowing the bearing plate or plates to be shifted toward the wedge such that a new contact surface between the bearing plates and the wedge replaces the contact surface between the wedge and the existing bracket (i.e., following repair, the wedge and bracket no longer directly contacts each other). Additionally, example embodiments allow for a replacement and/or machining of either the wedge or the bracket, or both, in addition to the placement of a bearing plate or plates to provide additional support. Example embodiments also allow for a replacement wedge that is either smaller (i.e., narrower), larger (i.e., wider), or the same size as the original wedge, or a replacement wedge with a smaller or larger angle of inclination as the original wedge, thereby allowing the wedge to contact the bearing plate or plates while not necessarily contacting the bracket directly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of example embodiments will become more apparent by describing in detail example embodiments with reference to the attached drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
         FIG. 1  is schematic representation of a conventional BWR characterized by a reactor pressure vessel (“RPV”) having two recirculation loops; 
         FIG. 2  is a partial perspective schematic representation of a RPV taken along Line  2 - 2  of  FIG. 1 , which depicts a cut-away showing a conventional jet pump assembly arrangement; 
         FIG. 3  is a partial perspective elevation view of a jet pump assembly horizontally supported by a conventional jet pump restrainer assembly including a bracket, the view taken along Line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a simplified rendition of an example embodiment showing a side view of a jet pump restrainer assembly repair including bearing plates interfacing with a conventional bracket and mixer wedge, the view taken along Line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a simplified rendition of another example embodiment showing a side view of a jet pump restrainer assembly repair including bearing plates interfacing with a conventional bracket and mixer wedge, the view taken along Line  4 - 4  of  FIG. 3 ; 
         FIG. 6  is a perspective view of an example embodiment showing a jet pump restrainer assembly repair interfacing with a conventional bracket and mixing wedge, the view from a slight overhead angle; 
         FIG. 7  is a perspective view of an example embodiment showing a jet pump restrainer assembly repair interfacing with a conventional bracket and mixing wedge, the view from a slight underneath angle; 
         FIG. 8  is a detailed depiction of an example embodiment of a jet pump restrainer assembly repair showing top and bottom bearing plates, mounting bolts, optional locating bosses, and optional adjusting bolt collars; 
         FIG. 9  is an overhead view of an example embodiment of a jet pump restrainer assembly repair, shown without a top bearing plate in order to show optional locating bosses and optional adjusting bolt collars; 
         FIG. 10  is the overhead view of the example embodiment of  FIG. 9 , shown with a top bearing plate; 
         FIG. 11  is a perspective view of an example embodiment showing mounting bolts penetrating the bracket; and 
         FIG. 12  is a perspective view of an example embodiment showing mounting bolts penetrating the bracket, with a tighter bolt pattern than the one shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     Referring to  FIG. 1 , the drawing is a schematic representation illustrating a commercial boiling water nuclear reactor (“BWR”)  10  that generates steam in a reactor pressure vessel (“RPV”)  12 . Commercial BWRs are designed to drive turbines (not shown), which in turn generates electrical power. The RPV  12  has a main feedwater inlet nozzle  14  for receiving condensate from a condenser (not shown) and a main steam outlet nozzle  16  for providing generated steam to a turbine. The RPV  12  supports a core shroud  18  containing a plurality of fuel assemblies  20  that generate the steam in its core region and a steam separator/dryer assembly  22  located over the core shroud  18 . 
     The RPV  12  illustrated by  FIG. 1  has two recirculation loops  30  for facilitating the flow of water in its core region. Each recirculation loop  30  has a large centrifugal reactor coolant pump (“RCP”)  32  connected with a recirculation water outlet nozzle  33  of the RPV  12  by pump suction piping  34  for pumping water out of the RPV  12  and pump discharge piping  36  for pumping the water back into the RPV  12 . The pump discharge piping  36  generally includes a header  38  and parallel branch piping, which is illustrated by piping  40 . Each of the piping branches  40  is connected by a recirculation water inlet nozzle  42  to riser piping  44 , which extends to a pair of jet pump assemblies  46  operating in parallel (only one of which is illustrated by  FIG. 1 ). 
     As is best seen in  FIG. 2 , the riser piping  44  terminates at a manifold  48  sometimes referred to as a “ramshead”. Each jet pump assembly  46  of the pair generally includes an inlet  50  adjacent the manifold  48  that is open to an annular region defined by the wall of the RPV  12  and wall of the core shroud  18  for entraining the surrounding water in the annular region, a mixing section  52  and a diffuser section  54  supported on a crossplate  56 . A jet pump restrainer assembly  46 A is used to horizontally restrain jet pump assembly  46  to riser pipe  44 . The jet pump assembly  46  may have a boss  49  on its periphery surface as shown in  FIG. 3 . 
     As shown in  FIG. 3 , jet pump restrainer assembly  46 A includes bracket  70  (with horizontal edge  78  in between vertical side-wall  70 A), wedge  60 , adjustment screws  80 , horizontal plates  64 , vertical plates  65 , and welds  72 , described in detail, herein. Wedge  60  is movably mounted on a vertically extending guide rod  62  fastened to jet pump assembly  46 . Guide rod  62  may have threaded ends engaged with nuts  63  fastened to horizontal plates  64  extending between vertical plates  65  that extend from the mixing section  52  of the jet pump assembly  46 . The wedge  60  is designed to slide vertically through a hole  68  in a bracket  70 , which is attached to the riser piping  44  by welds  72  or other suitable means. As shown by  FIG. 3 , the upper end of the guide rod  62  is above the bracket  70  and the lower end of the guide rod  62  is below the bracket  70 . Also, the wedge  60  may move downwardly on the guide rod  62  under the force of gravity to a location where an inner surface of the wedge  60  contacts the jet pump assembly  46  (and preferably the boss  49 ) and an outer surface  76  of the wedge  60  that is inclined contacts an edge  78  of the bracket  70 . The weight of the wedge  60  provides a sufficient force urging the jet pump assembly  46  against two (or more) adjustment screws  80  for horizontally supporting the jet pump assembly  46  against hydraulic forces and vibrations. The adjustment screws  80  may be fixed in place by welds (not shown). Preferably, the outer surface  76  of the wedge  60  is inclined relative to the edge surface  78  of the bracket  70 . Advantageously, this three point suspension system can accommodate substantial thermal expansion differences. 
       FIG. 4  is a simplified rendition of a jet pump restrainer assembly  46 A including a wedge  60  located between the boss  49  of mixing section  52  and bracket  70 . Area  61  represents a damaged area between the contact surfaces of the wedge  60  and the bracket  70 . Damage may occur on the contact surfaces of either the wedge  60 , or the bracket  70 , or both, due to thermal expansion, fretting, or wear between the wedge  60  and bracket  70 , generally. Jet pump restrainer assembly repair  104  may include a top bearing plate  100 , or a bottom bearing plate  102 , or both, attached to bracket  70 . The plates  100 / 102  may be fashioned above and/or below bracket  70 , preferably on a horizontal surface of the bracket  70 , such that plates  100 / 102  may increase the effective contact area between bracket  70  and wedge  60 . This may be accomplished by using bearing plates  100 / 102  with a same angle of inclination as the existing wedge  60  and bracket  70 , and aligning the contact surface between wedge  60  and plates  100 / 102  and the contact surface between wedge  60  and bracket  70 , such that wedge  60  contacts both the plates  100 / 102  and bracket  70 . While a benefit of the embodiment is that it allows a jet pump restrainer assembly  46 A to be repaired in situ, without the disassembly or machining of jet pump restrainer assembly  46 A, this embodiment may still allow for the disassembly and/or machining of the wedge and/or bracket during the repair. Specifically, the wedge  60  and/or bracket  70  may be machined in place, or jet pump restrainer assembly  46 A may be disassembled allowing the machining and/or replacement of either the wedge  60  or the bracket  70  in addition to the fashioning of plates  100 / 102  on bracket  70 . Additionally, example embodiments may be used as a preventative measure prior to actual wear between wedge  60  and bracket  70 . 
       FIG. 5  is another example embodiment, similar to  FIG. 4 . However,  FIG. 5  depicts a replacement wedge  60 A with a different angle of inclination than the wedge  60  originally in use. Replacement wedge  60 A allows an upper bearing plate  110  and/or a lower bearing plate  112 , also with a different angle of inclination matching replacement wedge  60 A, to be shifted toward wedge  60 A such that bracket  70  does not contact replacement wedge  60 A (notice area  61  to indicate the difference in the angle of inclination between bracket  70  and wedge  60 A, shown as a smaller angle of inclination for exemplary purposes). In an alternative embodiment, the existing wedge  60  (shown in  FIG. 4 ) may be partially withdrawn from bracket  70  in order to allow room for bearing plates  110 / 112  to be shifted toward the location of wedge  60 , thereby allowing only the bearing plates  110 / 112  and not bracket  70  to contact the wedge. In this alternative embodiment, the contact surface of bearing plates  110 / 112  may provide for a same angle of inclination as bracket  70  and wedge  60 , ensuring that contact surfaces between plates  110 / 112  and wedge  60  match. Alternatively, a wedge with a larger angle of inclination may also be used. Whether the existing wedge  60  or a new wedge  60 A is to be used, plates  110 / 112  should be provided with a contact surface angled to allow plates  110 / 112  to flushly contact the wedge, ideally allowing both plates  110 / 112  to flushly mate with the wedge, although example embodiments may allow for just one of plates  110 / 112  to flushly mate with the wedge. 
       FIG. 6  is a perspective view, similar to  FIG. 3 , with the restrainer assembly repair  104  shown interfacing with wedge  60  and bracket  70 . Top bearing plate  100  and bottom bearing plate  102  may be held together by mounting bolts  120 . The positioning of the mounting bolts  120  may be in any location that ensures that bearing plates  100 / 102  are securely affixed to bracket  70 , to provide plates  100 / 102  with stable support to place a horizontal force on the outer surface  76  of wedge  60 . Additionally, adjusting bolt collars  122 , such as an eccentric cam, may be used in conjunction with mounting bolts  120  to allow for the fine positioning of plates  100 / 102  relative to bracket  70 . 
       FIG. 7  is a perspective view of  FIG. 6 , from a slightly underneath angle. Locating bosses  130  (shown in  FIGS. 8 and 9 ) may be provided to ensure the proper placement of plates  100 / 102  relative to brackets  70 . The locating bosses  130  (shown in  FIGS. 8 and 9 ) may include locating boss bolts  132  used to stabilize the bosses  130 . Alternatively, plates  100 / 102  may be machined to allow for locating bosses  130  to be an integral part of the plates themselves. 
       FIG. 8  is a detailed drawing showing an example embodiment of a restrainer assembly repair  104  including a top bearing plate  100  and a bottom bearing plate  102 , the bearing plates  100 / 102  held together and able to be secured to bracket  70  by mounting bolts  120 . Optional bolt collars  122 , such as an eccentric cam, may be used for fine positioning of the restrainer assembly repair  104  relative to bracket  70  and wedge  60 . Cut-out areas  65 A may be included on the top bearing plate  100 , to allow the top plate  100  to fit down over vertical plates  65  (vertical plates  65  are shown in at least  FIGS. 6 and 7 ). Optional locating bosses  130  may be included to allow the restrainer assembly repair  104  to be more easily positioned relative to the bracket  70 . Locating boss bolts  132  are used to secure the locating bosses  130  once they are positioned, allowing the restrainer assembly repair  104  to apply a horizontal force to wedge  60 . Alternatively, plates  100  or  102  may be machined such that locating bosses  130  are an integral part of the plates themselves. 
     While example embodiments shows two bearing plates (one to be positioned above bracket  70 , and the other to be positioned below bracket  70 ), four mounting bolts  120  (two to be located on either side of a bracket  70 ), four adjusting bolt collars  122  (designed to contact bracket  70  on the inner and outer surface of the bracket), and two sets of locating bosses  130  and locating boss bolts  132  (designed to contact bracket  70  on the inner surface of the bracket), it should be understood that example embodiments are not limited to this specific design. Specifically, restrainer assembly repair  104  may be provided with just one bearing plate (to be positioned either above or below bracket  70 ), a greater or lesser number of mounting bolts  120  to be located in any position that securely attaches bearing plates  100 / 102  to bracket  70 , the optional adjusting bolt collars  122  may be provided to interface with either the inner and/or outer surface of bracket  70  (any number of bolt collars  122  may be used; alternatively, no bolt collars  122  may be used), and the optional locating bosses  130  may be provided to interface with either the inner or outer surface of bracket  70  (any number of locating bosses  130  may be used; alternatively, no locating bosses  130  may be used). 
     Additionally, while example embodiment use mounting bolts  120  to hold plates  100 / 102  together and affix the restrainer bracket repair  104  to bracket  70 , any means may be used to fulfill this purpose. Specifically, clamps, welds, screws, nails, adhesive, or other means may be used to attach plates  100 / 102  to bracket  70 . While plates  100 / 102  are referred to as plural (specifically, two plates) throughout this document, it should be understood that, alternatively, only one bearing plate may be used instead. Furthermore, while example embodiments show mounting bolts  120  that preferably do not penetrate bracket  70 , it should be understood that mounting bolts, clamps, screws, nails, or other attachment means may alternatively penetrate bracket  70  as a way of attaching plates  100 / 102  to bracket  70 . 
       FIG. 9  is an overhead view of  FIGS. 6 and 7 , shown without top bearing plate  100 . Notice bolt collars  122  contacting the inner and outer surfaces of bracket  70 , while locating bosses  130  are contacting the inner surface of bracket  70 . Also notice that a main body of the lower bearing plate  102  spans across an entire length of a flexible area ( 70 B, signifying the gap between vertical side-walls  70 A) of the restrainer bracket. 
       FIG. 10  is the same view as shown in  FIG. 9 , but with the addition of top bearing plate  100 . Notice cut-out area  65 A which allows top bearing plate  100  to slide over vertical plates  65 . Also notice additional cut-out areas  55 B of top bearing plate  100  which allow vertical side-walls  70 A of the restrainer bracket  70  to fit within the additional cut-out areas  65 B. The additional cut-out areas  65 B allow a main body of the top bearing plate  100  to span across an entire length of the flexible area ( 70 B) of the restrainer bracket  70 . 
       FIG. 11  is an example embodiment showing mounting bolts  120  penetrating bracket  70 . 
       FIG. 11  is an example embodiment showing mounting bolts  120  penetrating bracket  70  using a tighter bolt pattern than  FIG. 11 . 
     Example embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the intended spirit and scope of example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.