Abstract:
A U-framed structure that spans a reactor vessel of a pressurized water reactor power plant and rides on wheels that fit on the rails of the nuclear plant&#39;s refueling machine. A curved monorail is supported on the underside of the U-frame structure and guides a trolley system which travels on the monorail. The trolley system supports a hoist which is used for lifting, positioning and lowering reactor service equipment on the floor of the power plant&#39;s refueling canal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Provisional Application Ser. No. 61/040,194, filed Mar. 28, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates in general to the servicing of pressurized water reactors and more particularly to equipment and a process for implementation of a mechanical stress improvement process for reactor vessel nozzle welds to reduce the susceptibility to primary water stress corrosion cracking. 
         [0004]    2. Description of Related Art 
         [0005]    The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core comprised of a plurality of nuclear fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer and reactor coolant pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and reactor coolant pumps to the reactor vessel independently. Each of the parts of primary side comprising a steam generator, a reactor coolant pump and a system of pipes which are connected to the vessel form a loop of the primary side. The piping leading from the reactor pressure vessel and to each steam generator is referred to as a hot leg, through which hot water flows from the reactor pressure vessel to the steam generator. After heat is extracted from the reactor primary coolant in the steam generator, the coolant water is returned to the reactor through the reactor coolant pumps and cold leg piping. Typically, there are two, three or four reactor cooling loops associated with a single reactor pressure vessel with each such cooling loop communicating with a steam generator through its hot leg and cold leg piping. 
         [0006]    Typically, the reactor systems are in service for extended periods ranging from one year up to 18 months between refueling outages. During those extended operating periods the reactor coolant system operate at between 547 (at the inlet nozzle to the reactor vessel) and 615 (at the outlet nozzle to the reactor vessel) degrees Fahrenheit (286° and 324° C.) almost on a continuous basis. After years of service at high temperatures and pressures, welds between the reactor pressure vessel nozzles and the coolant leg piping have begun to exhibit a susceptibility to primary water stress corrosion cracking. One method for mitigating the susceptibility of the welds to the primary water stress corrosion cracking is known as the Mechanical Stress Improvement Process (MSIP) described more fully in U.S. Pat. Nos. 4,683,014 and 4,612,071. When piping is welded together by means of a circumferential weld, significant residual tensile weld stresses can be produced in the weld metal and in the heat affected zone of the piping. These tensile stresses tend to enhance the possibility of stress corrosion cracking in the weld regions and result in potential cracks propagating in the weld metal and in the heat affected zone of such piping. The MSIP reduces the tensile residual weld stresses by imparting a compressive force to the sides adjacent to the weld using very large and extremely heavy clamps and presses. 
         [0007]    The MSIP equipment which includes the aforementioned clamps and presses must be transported into a plant&#39;s containment building in which the reactor coolant systems are located. The equipment is transported by a polar crane to a laydown area adjacent the reactor pressure vessel and then stored adjacent to the nozzle welds located in sandboxes in the refueling canal flooring through which the reactor pressure vessel nozzles pass. The installation of the MSIP equipment is restricted due to the limited space between the reactor pressure vessel and the refueling cavity walls and also due to the presence of electrical ports located between each of the sandboxes. These restrictions and a requirement to limit the use of the polar crane in the containment during outages, to accommodate other work being conducted during a plant outage, complicates the use of the MSIP process. 
         [0008]    Accordingly, an alternate means is desired for transporting the relatively heavy MSIP equipment, weighing up to 1,000 pounds or more, from a laydown area to an installation site adjacent the reactor vessel nozzle. 
         [0009]    Furthermore, such a transport means is desired that will support the MSIP equipment during its installation. 
         [0010]    Additionally, such a means is desired that will facilitate and expedite the application of the mechanical stress improvement process. 
       SUMMARY OF THE INVENTION 
       [0011]    This invention provides a mobile rigging structure (MRS) specifically suited for transporting the MSIP equipment from the laydown area to the sandbox installation site and facilitates and expedites the application of the MSIP. The MRS includes a frame structure with wheeled trucks attached to the bottom of the frame structure that is designed to ride on tracks on the floor of the operating deck that was originally designed for a refueling manipulator. A curved monorail is supported by and protrudes from the underside of the frame structure and extends partially around the reactor vessel when the frame structure is in position, so that the monorail extends over the center line of four of the eight sandboxes. A system consisting of seven trolleys is supported by and movable along the monorail in a direction at least partially around one half of the reactor vessel. The frame structure extends around the reactor vessel and in the preferred embodiment extends around 180 degrees so that the trolley system can move along the monorail over the center line of half of the sandboxes without further moving the frame structure. 
         [0012]    In one preferred embodiment, the frame structure is approximately U-shaped having a center portion and two peripheral, laterally extending arms that respectively extend from either side of the center section. The MRS is constructed in a plurality of frame sections that are separable so that they can easily be transported into the containment vessel. Desirably, the wheels of the trucks are attached to the laterally extending arms and the interface between frame sections includes inserts that fit between at least two of the frame structure sections and are sized to align the wheeled trucks with the tracks on the floor of an operating deck to accommodate different nuclear plant layouts. Preferably, the wheeled trucks include a lock that lock the frame structure in position on the track upon an operator&#39;s command, so the trolleys and hoists can be manipulated without movement of the frame structure. 
         [0013]    In one embodiment, the hoist comprises a plurality of slings supported by the trolley system with each sling having a hand chain fall hoist suspended therefrom. 
         [0014]    Thus, this invention enables two mobile rigging structures to be employed, one on either side of the reactor vessel, at the same time enabling the MSIP to be applied on at least two nozzles of the pressure vessel, simultaneously applying compressive stresses on adjacent welds of the two nozzles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0016]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention may be applied; 
           [0017]      FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internal components showing the nozzle arrangements to which the mechanical stress improvement process and this invention can be applied; 
           [0018]      FIG. 3  is a perspective view of portion of the interior of the containment of a pressurized water nuclear reactor power plant including a reactor pressure vessel located in a reactor cavity with an installed integrated head assembly extending upwardly into a refueling canal and four adjacent steam generators connected thereto; 
           [0019]      FIG. 4  is a plan view of the mobile rigging structure of this invention; 
           [0020]      FIG. 5  is a plan view of the unassembled components of the mobile rigging structure of this invention, which when assembled, assumes the form shown in  FIG. 4 ; 
           [0021]      FIG. 6  is a rear view of the mobile rigging structure of this invention; 
           [0022]      FIG. 7  is a side view of the mobile rigging structure of this invention; 
           [0023]      FIG. 8  is a plan view illustrating two mobile rigging structures of this invention situated on either side of a reactor vessel opposed to each other around the integrated head package of the reactor vessel and bordered on one side by the headstand area and on the other side by the equipment laydown area; 
           [0024]      FIG. 9  is an elevational view of the two mobile rigging structures adjacent the integrated head assembly; 
           [0025]      FIG. 10  is a plan view of a schematic of the floor of the refueling canal around the reactor cavity and shows the curved monorail of the rigging structure of this invention centered over the sand boxes; 
           [0026]      FIG. 11  is a vertical sectional view through a reactor vessel cavity; and 
           [0027]      FIG. 12  shows in the upper half, above the phantom line, a plan view taken from above the nozzles of the reactor cavity shown in  FIG. 11  and the lower half is a plan view taken below the nozzles. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Referring now to the drawings,  FIG. 1  shows a simplified pressurized water nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  10  having a closure head  12  enclosing a nuclear core  14 . A liquid reactor coolant, such as water, is pumped into the vessel  10  by pump  16  through the core  14  where heat energy is absorbed and is discharged to a heat exchanger  18 , typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump  16 , completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . 
         [0029]    An exemplary reactor design is shown in more detail in  FIG. 2 . In addition to a core  14  comprised of a plurality of parallel, vertical co-extending fuel assemblies  22 , for purposes of this description, the other vessel internal structures can be divided into the lower internals  24  and the upper internals  26 . In conventional designs, the lower internals function to support, align and guide the core, core components and instrumentation, as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assembly  22  (only two of which are shown for simplicity) and support and guide instrumentation and components such as control rods  28 . 
         [0030]    In the exemplary reactor shown in  FIG. 2 , coolant enters the vessel  10  through one or more inlet nozzles  30 , flows downward about a core barrel  32 , is turned 180 degrees in a lower plenum  34 , passes upwardly through a lower support plate  36  and lower core plate  37  upon which the fuel assemblies  22  are seated and through and about the assemblies. In some designs, the lower support plate  36  and the lower core plate  37  are combined into a single lower core support plate (at the same location as  36 ), which eliminates the separate lower core plate  37 . The coolant flow through the core and surrounding area  38  is typically large, in the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate  40 . Coolant exiting the core  14  flows along the underside of the upper core plate  40  and upwardly through a plurality of perforations  42 . The coolant then flows upward and radially to one or more outlet nozzles  44 . 
         [0031]    The upper internals  26  can be supported from the vessel and include an upper support assembly  46 . Loads are transmitted between the upper support plate  47  of the upper support assembly  46  and the upper core plate  40 , primarily by a plurality of support columns  48 . A support column is aligned above a selected fuel assembly  22  and perforation  42  in the upper core plate  40 . 
         [0032]    Rectilinearly moveable control rods  28 , typically including a drive shaft  50  and spider assembly  52  of neutron poison rods are guided through the upper internals  26  and into aligned fuel assembly  22  by control rod guide tubes  54 . The guide tubes are fixedly joined to the upper support assembly  46  and connected by a split pin to the top of the upper core plate  40 . The support columns  48  assist in retarding guide tube deformation under seismic and design basis accident conditions, which could detrimentally affect control rod insertion capability. 
         [0033]    Thus, it can readily be appreciated that the reactor vessel and its components are highly engineered to be rugged and continuously operated in a severe environment of high pressures and high temperatures. Thus, any potential for breaching the integrity of the system through cracks in the nozzles  30  and  44  should be timely addressed to avoid a need for unscheduled and costly outages. That is why it is important that the nozzles  30  and  44  be treated with the mechanical stress improvement process in order to prevent the initiation and propagation of cracks that could otherwise result in leaks in the primary system. 
         [0034]      FIG. 3  shows a perspective view of a portion of the interior of a containment of a pressurized water reactor nuclear power plant. The reactor pressure vessel  10  is shown located in a reactor cavity  56  that is a recess in the floor of a refueling canal  58  into which the integrated head assembly  60  of the reactor extends. A more detailed description of the integrated reactor head assembly can be found in U.S. Pat. No. 7,158,605. As previously mentioned, the reactor pressure vessel is connected to each of the steam generators  18  by a hot leg piping not shown which conveys the heated reactor coolant to the steam generators where heat is extracted to perform useful work, such as drive a generator for the production of electricity. The cooled reactor coolant is then returned through a cold leg to the reactor pressure vessel. Typically, in a conventional pressurized water reactor plant, there would be as many as four such steam generators, as shown, each one connected between a hot leg and cold leg to the reactor vessel. 
         [0035]      FIG. 11  provides a better view of an exemplary reactor cavity  56  and an appreciation of the difficulty in accessing the nozzle areas  30  and  44  to perform the mechanical stress improvement process.  FIGS. 11 and 12  provide views of a two-loop advanced pressurized water reactor pressure vessel having a single hot leg  44  and two cold legs  30  for each loop. The typical 4 loop PWR reactor vessel  10  is supported on two inlet and two outlet nozzle supports  66  upon which the cold leg nozzles  30  rest, with the vessel suspended in the reactor cavity  56 . The nozzle chamber  62 , also known as the sandbox, in which the vessel is supported, is square while the reactor cavity  56  is octagonal in plan view. The reactor cavity  56  is defined by a massive concrete structure  64  which forms a biological shield which protects personnel as well as adjacent structures and equipment, from the high neutron flux and radiation generated within the core  14  of the reactor. The floor  68  of the refueling canal is provided with sandboxes in which the coolant piping adjacent the reactor vessel nozzles  30  and  44  are buried and may be accessed. The sandboxes  70  and the refueling canal floor  68  can be observed from  FIG. 10 . The operating deck  96  also includes manipulator tracks or rails  72  which extend on either side of the pressure vessel  10  outward of the sandboxes  70  and were originally provided to guide a refueling manipulator that is designed to remove spent fuel from the core, reposition of the fuel assemblies within the core and insert new fuel assemblies at designed core locations to replace the spent fuel assemblies that had been removed. 
         [0036]    The mechanical stress improvement process equipment that is employed to reduce the residual tensile stresses in the high nuclear-chromium alloy nozzle safe-end welds are transported in the plant&#39;s containment building in which the reactor pressure vessel and steam generators are located by a polar crane that is supported from the containment, to a laydown area. The laydown area may be located at different locations depending on the containment configuration. One such area  74  is shown in  FIG. 3 . The mechanical stress improvement process equipment is assembled in the laydown area  74  and then is stored adjacent to the nozzle welds located in the sandboxes  70  in the refueling canal flooring  68 . As can be seen from  FIGS. 3 and 8 , the installation of the mechanical stress improvement process equipment is restricted due to a limited space between the reactor pressure vessel  10  and the refueling cavity walls and also due to the presence of electrical ports located between each of the sandboxes  70 . These restrictions and a requirement to limit the use of the polar crane during outages complicate the use of the mechanical stress improvement process equipment. 
         [0037]    This invention provides a mobile rigging structure for transporting the relatively heavy mechanical stress improvement process equipment, weighing up to 1,000 pounds or more, from the laydown area  74  to the installation site.  FIG. 3  generally illustrates two mobile rigging structure units  78  disposed on opposite sides and around the integrated head assembly  60  installed on the reactor pressure vessel  10 .  FIG. 4  illustrates a top view of the structure of one mobile rigging structure unit. Each such unit  78  includes a base member  80  with a lateral arm  82  extending from each end and supported by a brace  84  diagonally positioned and connected at opposite ends to the base member  80  and the arm  82 . A curved monorail  86  on which a trolley rides, is suspended from the underside of the base member  80  and the laterally extending arms  82 . The trolley is not shown in  FIG. 4  but will be described hereafter with regard to  FIG. 6 .  FIG. 4  shows the mobile rigging structure unit of this invention fully assembled while  FIG. 5  shows the separate components of the mobile rigging structure unit  78  in the form it is transported into the containment, before assembly. As shown in  FIG. 5 , the base unit  80  is moved separate from the laterally extending arms  82  and the monorail  86  is provided in two halves. In addition,  FIG. 5  shows the trucks  88  on which the laterally extending arms  82  ride over the tracks (i.e., rails)  72  on the floor of the operating deck  96 . 
         [0038]      FIG. 6  shows a rear view of the mobile rigging structure unit  78 , which as previously described is a generally U-frame structure  78  including a base or bridge member  80  which may be formed from one, two as illustrated, or more structural steel pieces and which extends between the two side arm members  82 . The U-frame structure  78  supports trucks  88  which extend under the arms  82  and ride in the tracks  72  on the floor of the operating deck  96 . As can be appreciated from  FIG. 7 , the trucks  88  extend below and are spaced from the laterally extending arms  82  by spacer members  90  which are inserted therebetween. The trucks ride on the rails  72  of the plant&#39;s refueling machine, which is described more fully in U.S. Pat. No. 4,832,902. Mobility is required so that the mechanical stress improvement process equipment can be positioned over the sandboxes after the mechanical stress improvement process equipment has been first lowered into the refueling cavity by the plant&#39;s polar crane. 
         [0039]    The U-frame structure  78  supports the monorail half  86  shown in  FIGS. 4 ,  5 ,  6 ,  8  and  9 . The monorail halves  86  are located on the underside of the U-frame structure  78  for supporting a trolley system  92  shown in  FIGS. 6 and 9 . The trolley system  92  travels on the monorail  86  and spans the center line of half of the sandboxes  70  in the floor of a refueling canal  68  around the reactor vessel  10 . The trolley system  92  supports slings and hand chain fall hoists  94  shown in  FIGS. 6 and 9 . The hand chain fall hoists  94  are suspended from the slings and raise and lower the mechanical stress improvement process equipment from and to the sandbox region. As illustrated in  FIG. 9 , in this preferred embodiment there are six hand chain fall hoists suspended from the slings.  FIG. 8  is a plan view illustrating two mobile rigging structure units of this invention positioned adjacent and on either side of the reactor vessel (with the integrated head assembly lift rig positioned on the adjacent reactor headstand  76 ).  FIG. 9  is an elevational view of the two mobile rigging structure units  78  adjacent and on either side of the integrated head assembly  60 . 
         [0040]    The mobile rigging structure units&#39; components shown in  FIG. 5  may be assembled in the containment building, including the trolley system  92  and slings and chain falls  94  and then lifted by the plant&#39;s polar crane onto the refueling machine rails  72 . In a practice of the present invention, the mechanical stress improvement process equipment is rigged and raised by a mobile rig structure unit  78  and the mobile rigging structure unit  78  is moved to position the equipment over the first sandbox and locked into position. The mechanical stress improvement process equipment is then lowered into the sandbox, installed adjacent to a weld and a compressive squeeze is completed. The mechanical stress improvement process equipment is then raised and moved to the next sandbox by way of a chain hoist  94  and the trolley system  92  located on the underside of the mobile rigging structure unit  78 . These activities are completed in all the sandboxes on each side of the reactor pressure vessel  10  by personnel located in the cavity. This eliminates the need to use the polar crane and assures a much safer operation. Advantageously, two mobile rigging structure units  78  may be used simultaneously as illustrated, one on each side of the reactor pressure vessel  10  which allows work to proceed on each side of the reactor pressure vessel without having to remove the reactor pressure vessel head  60 . In addition, the use of two mobile rigging structure units will assure that the work will be completed during aggressive critical path schedules. It should further be appreciated that the number of sandboxes around the reactor will vary depending upon the number of loops and type of reactor. However, the number of sandboxes does not change how the mobile rigging structure of this invention is used. Additionally, it should also be appreciated that while the preferred embodiment was described in an application to a pressurized water reactor pressure vessel the mobile rigging structure of this invention can also be applied to other nuclear power plant components and other types of reactor systems such as a boiling water reactor. 
         [0041]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.