Abstract:
A method and apparatus for physically restraining an inlet mixer of a boiling water reactor (BWR) jet pump assembly. An inlet mixer clamp assembly is attached to the top of a transition piece of the BWR jet pump assembly, to provides a downward clamping force on the top of the inlet mixer. The clamping force restrains the inlet mixer from experiencing vibration and mitigates leakage between the inlet mixer and the transition piece sealing surfaces of the jet pump assembly. The clamping force also restrains the inlet mixer from undesirable rotation, especially during operation of the jet pump assembly. The inlet mixer clamp assembly may be used as an alternative to conventional jet pump beam assemblies. Alternatively, the inlet mixer clamp assembly may be used as a redundant physical restraint in addition to the conventional jet pump beam assemblies.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Example embodiments relate generally to nuclear reactors, and more particularly to a method and apparatus for a Boiling Water Reactor (BWR) inlet mixer clamp assembly that physically restrains and applies a downward force on an inlet mixer of a boiling water reactor (BWR) jet pump assembly. The inlet mixer main clamp assembly may be used as an alternative to conventional jet pump beam assemblies. Alternatively, the inlet mixer clamp assembly may be used as a redundant physical restraint in addition to conventional jet pump beam assemblies. 
     2. Related Art 
     A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends (for example by a bottom head and a removable top head). A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus between the cylindrical reactor pressure vessel and the cylindrically shaped shroud. 
     In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within jet pump restrainer brackets by a gravity actuated wedge. 
     Conventionally, the inlet mixer is held in place by jet pump beam assemblies at the top of the BWR jet pump assembly. The jet pump beam assemblies press down and provide a clamping force on the inlet mixer to mitigate movement, rotation and vibration of the inlet mixer and reduce potential leakage between the inlet mixer and the transition piece sealing surfaces of a jet pump assembly. Additionally, the jet pump beam assemblies mitigate the potential that the inlet mixer may be ejected from the diffusers of the jet pump assembly during operation of the jet pump assembly. Conventionally, jet pump beam assemblies are highly stressed, they are limited in their clamping force, and they are susceptible to stress corrosion cracking. Field failures of jet pump assemblies have previously caused significant plant downtime and expensive repair. 
     SUMMARY OF INVENTION 
     Example embodiments provide a method and an apparatus for providing vertical restraint of an inlet mixer of a BWR jet pump assembly to reduce leakage and vibration of the inlet mixer. Example embodiments provide a clamping force, preferably at locations off of the centerline of the transition seat, to resist inlet mixer rotation. The clamping force of example embodiments may exceed the force that may otherwise be applied by conventional jet pump beam assemblies to ensure that the inlet mixer stays seated against the transition piece. The clamping force of example embodiments may eliminate a single point of failure for stabilizing the inlet mixer, as the clamping force may be a redundant downward force on the inlet mixer that may be applied in addition to conventional jet pump beam assemblies. Alternatively, the clamping force of example embodiments may be used in lieu of conventional jet pump beam assemblies. 
    
    
     
       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 a perspective view of a conventional boiling water nuclear reactor (BWR) jet pump assembly; 
         FIG. 2  is a detailed view of the top of an inlet mixer, with conventional jet pump beam assemblies in place; 
         FIG. 3  is a detailed view of a BWR inlet mixer clamp assembly, in accordance with example embodiments; 
         FIG. 4  is a detailed bottom view of a BWR inlet mixer clamp assembly, in accordance with example embodiments; 
         FIG. 5  is a detailed view of a BWR inlet mixer clamp assembly in use in conjunction with conventional jet pump beam assemblies, in accordance with example embodiments; and 
         FIG. 6  is a detailed view of a BWR inlet mixer clamp assembly in use (without conventional jet pump beam assemblies), in accordance with example embodiments. 
     
    
    
     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. 
       FIG. 1  is a perspective view of a conventional boiling water nuclear reactor (BWR) jet pump assembly. The jet pump assembly includes a riser pipe  1  that brings water into inlet mixers  2 . The inlet mixers discharge water into diffusers  4 . The jet pump beam bolts  6  of the conventional jet pump beam assemblies (seen in better detail, in  FIG. 2 ) can be seen toward the top of the inlet mixer  2 . 
       FIG. 2  is a detailed view of the top of an inlet mixer  2 , with conventional jet pump beam assemblies  8  in place. The jet pump beam assemblies  8  are attached underneath a bridge  10   a  portion of transition piece  10  of a conventional jet pump assembly. The transition piece  10  is welded to the top of the riser pipe  1  (see  FIG. 1 ) to ensure stability of the transition piece  10 . The jet pump beam assemblies provide a downward clamping force on the top of inlet mixer  2 , to resist the vertical hydraulic load the inlet mixers see during operation and to reduce vibration of the inlet mixer. 
       FIG. 3  is a detailed view of a BWR inlet mixer clamp assembly  20 , in accordance with example embodiments. The inlet mixer clamp assembly  20  may include an inlet mixer clamp plate  22 . The inlet mixer clamp assembly may include inlet mixer jacking bolts  24  that provide a downward clamping force that is applied to the top of the inlet mixer  2 . The jacking bolts  24  may be threaded bolts that mate with eyelets of the clamp plate  22  as well as threaded connections that may be made in the top surface of the existing inlet mixer  2  (shown in detail in  FIGS. 5 and 6 ). The jacking bolts may alternatively penetrate clamp plate  22  and contact and/or press on the top surface of the existing inlet mixer  2  without actually penetrating the top surface of the inlet mixer. The inlet jacking bolts  24  may be lined up in a row along the centerline of the clamp plate  22 , as shown in  FIG. 3 . Alternatively, jacking bolts  24  may be provided on other convenient locations on the clamp plate  22 . The number of jacking bolts  24  may vary, although four jacking bolts  24  are shown in example embodiments. 
     The clamp plate  22  may include beam bolt holes  30 , especially in the case where the clamp assembly  20  is to be used in conjunction with the conventional beam assemblies  8 . The beam bolt holes  30  allow for clearance of the beams bolts  6  (as shown in  FIG. 5 ). 
     The clamp plate  22  may include mounting blocks  28 , that may be “C”-shaped components affixed to a bottom surface of the clamp plate  22 . Mounting blocks  28  may be securely attached to clamp plate  22  via mounting block bolts  26 . Alternative to mounting block bolts  26 , other reasonable means of securing mounting blocks  28  to clamp plate  22  may be used, including welding. 
     All clamp assembly components may be made by materials that are known to be acceptable for a nuclear environment. For instance, stainless steel (304, 316, XM-19, or equivalent) or nickel based alloys (Iconel, X-750, X-718, or equivalent) may be used. 
       FIG. 4  is a detailed bottom view of a BWR inlet mixer clamp assembly  20 , in accordance with example embodiments. Note that mounting blocks  28 , formed in a “C”-shape, include mounting block apertures  28   a  that cradle the transition piece bridge  10   a  (shown in  FIG. 2 ). The mounting blocks  28  grip the transition piece  10  to provide an anchor for the clamp plate  22 , thereby allowing jacking bolts  24  to then press forcefully down on the top of inlet mixer  2 . 
       FIG. 5  is a detailed view of a BWR inlet mixer clamp assembly  20  in use in conjunction with conventional jet pump beam assemblies  8 , in accordance with example embodiments. Note that jacking bolts  24  may mate with threaded connections that are drilled into portions of the thick upper surface  2   c  of the inlet mixer  2 . Mounting blocks  28  are shown cradling the bridge  10   a  (see  FIG. 2 ) of the transition piece  10 , to stabilize the clamp plate  22 . Existing beam bolts  6  of the conventional jet pump beam assemblies  8  are shown protruding through the beam bolt holes  30 , as the beam bolt holes  30  may be sized to allow the beam bolts  6  to clear the clamp plate  22  when the clamp assembly  20  is attached to the inlet mixer  2 . In this example embodiment, conventional jet pump beam assemblies  8  are working in unison with the clamp assembly  20  to provide redundant downward force on the top of the inlet mixer. 
     The clamp plate  22  is sized to fit between the lifting eyelets  12  of the inlet mixers  2 , but may alternatively extend beyond the lifting eyelets. The clamp plate has also been sized to fit over and cover the upper surfaces of opposing portions of transition piece  10  (note that only one side of transition piece  10  is shown in  FIG. 5 , as the opposing side of transition piece  10  is obscured from the drawing and located on the other side of the inlet mixer  2  as shown in  FIG. 2 ). Lobes  22   b  are sized to extend from the clamping plate  22  at locations directly above the transition piece bridges  10   a  (bridges  10   a  may be seen in  FIG. 2 ). The lobes  22   a  are located at this position of the clamping plate  22  to support mounting blocks  28  underneath the clamp plate  22  and allow the mounting blocks  28  to cradle the transition piece bridges  10   a.    
     By positioning the jacking bolts  24  at locations on either side of “centerline”  22   b  of plate  22 , the clamp assembly  20  restrains the inlet mixer  2  from rotation during jet pump assembly operation. Note that the “centerline”  22   b  divides the clamp plate  22  approximately in half, and runs through the center of each lobe  22   a  (and, centerline  22   b  also runs directly through the middle and above both transition piece bridges  10   a ). For the jacking bolts  24  to be effective in mitigating rotation of the inlet mixer  2 , it is preferable to have at least one jacking bolt  24  on either side of centerline  22   b . Furthermore, the farther the jacking bolts  24  are located away from centerline  22   b , the more effective the clamp assembly  20  will be at mitigating clockwise/counterclockwise rotation of the inlet mixers  2 . For this reason, lobes  22   c  may be used to extend the location of jacking bolts  24  away from centerline  22   b  to provide added leverage to allow clamp assembly  20  to mitigate inlet mixer  2  rotation that may occur especially during jet pump assembly operation. In one embodiment, the jacking bolts  24  may be located on the clamp plate  22  in a line that runs approximately perpendicular to the centerline  22   b . Alternatively, if jet pump beam bolts  6  are to be removed from operation upon the installation of clamp assembly, jacking bolts  24  may be located where jet beam bolts  6  would otherwise penetrate plate  22  (i.e., jacking bolts  24  may be located where beam bolt holes  30  may otherwise be located). Alternatively, only one jacking bolt  24  may be located on clamp plate  22  at a location that intersects centerline  22   b.    
       FIG. 6  is a detailed view of a BWR inlet mixer clamp assembly  20  in use (without conventional jet pump beam assemblies  8 ), in accordance with example embodiments. Note the vacant space  8   a  that is left under the clamp plate  22 , where the conventional jet pump beam assemblies  8  would otherwise be located. Also note that the beam bolt holes  30  do not have beam bolts  6  of the jet pump beam assemblies  8  penetrating through the holes  30 . Therefore, the beam bolt holes  30  are optional in this configuration. 
     The configuration of  FIG. 6  allows the clamp assembly  20  to be used in lieu of the conventional jet pump beam assemblies  8 , such that the jet pump beam assemblies  8  may be removed prior to installation of the clamp assembly  20 . 
     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.