Abstract:
A method for providing weld inlays and onlays to primary nozzles of a nuclear reactor comprising: providing a first welding device in a first primary nozzle of the nuclear reactor; providing a second welding device in a second primary nozzle of the nuclear reactor; providing a third welding device in a third primary nozzle of the nuclear reactor; and operating the first, second and third welding devices at the same time. Other methods are also provided.

Description:
[0001]    Priority to U.S. Provisional Patent Application Ser. No. 61/269,628 filed Jun. 26, 2009, is claimed, the entire disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to nuclear power plants, and more specifically to methods for repairing welds on primary nozzles of nuclear power plants. A nuclear power plant typically has a nuclear reactor and a reactor coolant system (RCS) for removing heat from the reactor and to generate power. The two most common types of reactors, boiling water reactors (BWRs) and pressurized water reactors (PWRs), are water-based. In a pressurized water reactor (PWR), pressurized, heated water from the reactor coolant system transfers heat to an electricity generator, which includes a secondary coolant stream boiling a coolant to power a turbine. The RCS section downstream of the electricity generators but upstream of the reactor is typically called the cold leg, and downstream of the reactor and upstream of the electricity generators is typically called the hot leg. 
         [0003]    PWRs typically have either three hot legs and three cold legs or, more commonly in the United States, four hot legs and four cold legs. A PWR reactor vessel thus typically will have six or eight primary nozzles connecting the hot and cold legs to the reactor vessel. Tubing of the hot or cold leg typically is welded to the nozzle at a primary nozzle weld. The reactor vessel is typically made from carbon steel and the hot or cold leg piping from stainless steel. In the past, alloy 600 was used as a weld material between the reactor vessel nozzle and the hot or cold leg piping, and was felt to be a good material for use in such a dissimilar metal weld. However, primary water stress corrosion cracking (PWSCC) has been found in many of such welds, and without any mitigation, regulatory agencies may require more frequent inspection of such welds than in the past. Such inspections are expensive and time consuming, as the reactor must be shut down. 
       SUMMARY OF THE INVENTION 
       [0004]    Several companies thus offer mitigation of PWSCC of large diameter alloy 600 welds. Westinghouse markets a mechanical stress improvement process, which has several disadvantages, for example spacing constraints. Westinghouse thus also has proposed welding on the inside of the primary nozzles in conjunction with its parent company Toshiba using underwater laser beam welding. 
         [0005]    Areva also has proposed a solution called the AEGIS inlay program that delivers robotic tooling to primary nozzles for welding operations. This program allows for welds on multiple nozzles simultaneously to minimize schedule impact, and remains in development. 
         [0006]    One object of the present invention is to provide a time-efficient method for permitting welding on the inside of primary nozzles to further minimize schedule impact. 
         [0007]    Another alternate or additional object of the present invention is to provide additional operations to the welding in an efficient manner. 
         [0008]    The present invention provides a method for providing welds to primary nozzles of a nuclear reactor comprising: 
         [0009]    providing a first welding device in a first primary nozzle of the nuclear reactor; 
         [0010]    providing a second welding device in a second primary nozzle of the nuclear reactor; 
         [0011]    providing a third welding device in a third primary nozzle of the nuclear reactor; and 
         [0012]    operating the first, second and third welding devices at the same time. 
         [0013]    The present invention also provides a method for providing welds to primary nozzles of a nuclear reactor comprising: 
         [0014]    providing a first welding device in a first primary nozzle of the nuclear reactor; 
         [0015]    providing a first pre-weld processing device in a second primary nozzle of the nuclear reactor; and 
         [0016]    operating the first welding device and the first pre-weld processing device at the same time. 
         [0017]    The present invention also provides a method for providing welds to primary nozzles of a nuclear reactor comprising: 
         [0018]    flapping a weld of a primary nozzle; and 
         [0019]    welding the flapped surface using a tool manipulator within the primary nozzle. 
         [0020]    The present invention also provides a method for providing welds to primary nozzles of a nuclear reactor comprising: 
         [0021]    providing a barrier layer at a primary nozzle using a tool manipulator within the primary nozzle; and 
         [0022]    providing a further weld over the barrier layer using the tool manipulator or a further tool manipulator. 
         [0023]    The present invention also provides a method for providing welds to primary nozzles of a nuclear reactor comprising: 
         [0024]    identifying a location of a weld of a primary nozzle; 
         [0025]    fixing a locator in the primary nozzle as a function of the weld location; 
         [0026]    placing a tool manipulator in the primary nozzle; and 
         [0027]    locating the tool manipulator using the locator, the tool manipulator providing a weld. 
         [0028]    The present invention also provides a method for providing welds to primary nozzles of a nuclear reactor comprising: 
         [0029]    providing a first working device in a first primary nozzle of the nuclear reactor; 
         [0030]    providing a second working device in a second primary nozzle of the nuclear reactor; 
         [0031]    providing a third working device in a third primary nozzle of the nuclear reactor; and 
         [0032]    operating the first, second and third working devices at the same time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    One preferred embodiment of the present invention will be described with respect to the drawing in which: 
           [0034]      FIG. 1  shows schematically in cross section the reactor area of a PWR nuclear reactor, as well as two of the primary nozzles; 
           [0035]      FIG. 2  shows a repair support structure for placement in the reactor area of a reactor with eight primary nozzles to aid in performing a primary nozzle welds; 
           [0036]      FIG. 3  shows placement of a turntable in the repair support structure; 
           [0037]      FIG. 4  shows placement of loading tubes in the primary nozzles using the turntable, the loading tubes having plugs at one end; 
           [0038]      FIG. 5  shows the slot of  FIG. 4 ; 
           [0039]      FIG. 6  shows placement of plugs using a common tool manipulator; 
           [0040]      FIG. 7  shows schematically a non-destructive examination (NDE) device on the common tool manipulator; 
           [0041]      FIG. 8  shows schematically a machining or grinding head for machining or grinding using the common tool manipulator; 
           [0042]      FIG. 9  shows a preparation robot for preparation of the weld; 
           [0043]      FIG. 10  shows schematically a gas-tungsten arc-weld device for arc-welding using the common tool manipulator to provide a weld inlay, and  FIG. 10B  shows a weld onlay; 
           [0044]      FIG. 11  shows schematically a common tool manipulator in four of the primary nozzles, with the preparation robot in a fifth primary nozzle; and 
           [0045]      FIGS. 12A , B, C, D, E, F, G and H show a preferred plan for performing a repair operation on eight primary nozzles using three common tool manipulators and one preparation robot at a same time, or four CTMs at a same time. Four CTMs and two preparation robots overall can be used. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0046]      FIG. 1  shows schematically in cross section a reactor vessel  100  of a PWR nuclear reactor, as well as two of the primary nozzles  10 ,  20 . The reactor vessel  100  is typically made of carbon steel, with a main section  105  and integral extending nozzle areas  110 ,  120  for the hot and cold legs all made of the same material. The vessel  100  may be a single cast piece. During construction of a nuclear power plant, tubes  210 ,  220 , made for example of stainless steel, are welded to the nozzle areas  110 ,  120 , respectively, with welds  310 ,  320 . These welds  310 ,  320  in the past have been made of alloy 600 or alloy 82/182, which was believed to be resistant to PWSCC. However, cracking and other defects have been found in alloy 600, alloy 182 or alloy 82 present in such welds, particularly at an interior surface  312  of such welds that presents to water or steam located in the primary nozzles. The present invention thus is directed to a method for providing a further weld over the weld at surface  312  to prevent PWSCC at the welds  310 ,  320 . The further weld can be placed directly over the older weld material, or can be a weld inlay, provided after machining or grinding away some of the old weld material. The present invention thus also advantageously provides for removal of cracks in the welds  310 ,  320  via machining or grinding, thus creating a new weld-appropriate surface for a weld inlay. A PWSCC resistant material such as alloy 52, 52M, 152, 152M, 52MS or 52MSS may be provided at surface  312  after pre-weld preparation so that no PWSCC susceptible materials, such as alloy 600 or alloy 82/182 presents to the steam or water in the primary nozzles. 
         [0047]      FIG. 2  shows a repair support structure  40  for placement in the reactor vessel  100  with, in this embodiment, eight primary nozzles. A repair support structure for six or four primary nozzles could also be provided. The reactor vessel, emptied of its fuel rods and internals, is drained and dried so that a dry environment for the primary nozzles  10 ,  20  in present. The repair support structure  40  then is placed over the reactor vessel so that flanges  44  are attached to the top of the opened reactor vessel  100 . The repair support structure  40 , preferably made of steel, provides a solid base  42 , and eight openings  46  that align with the primary nozzles  10 ,  20  of the reactor vessel  100 . The repair support structure  40  has the advantage that the cavity can remain flooded, so that the water still provides shielding. 
         [0048]      FIG. 3  shows a turntable  50  in the repair support structure  40 , the turntable  50  including a base  52  for attaching to floor  42 , a motor  54  to rotate a holding platform  56  and a linear actuator  58  slidable on platform  56  by a second motor  59 . Work devices to be inserted into primary nozzles  10 ,  20  and the other six primary nozzles thus can be lowered onto holding platform  56 , which may include rails or other devices to proper position the work devices. Linear actuator  59  can have pins or another type of connectors  57  for connecting the work devices to the actuator in a removable fashion. The linear actuator  58 , via motor  59 , then can push or slide the work devices into the primary nozzles  10 ,  20 , deposit the work device into one of the nozzles, and then return to remove the work devices at a later time. Since the platform  56  is fully rotatable via motor  54 , all eight nozzles can be accessed. 
         [0049]      FIG. 4  shows placement of a loading tube  60  in the primary nozzle  10  using the turntable  50 . Shown solely schematically, a hole or other connector on the loading tube can interact with connectors  57  to hold the loading tube on the linear actuator  58 . 
         [0050]    Each loading tube  60  can have a plurality of sliding feet  62  which can be actuated by hydraulic cylinders and can press out to lock the loading tube  60  into a fixed position with respect to the closest edge  311  on surface  312  of weld  310 , for example 2 inches. The loading tube  60  preferably is placed based on known information about the location of weld  310 , for example from plant design information or schematics, to be a certain distance, for example 2 inches from the expected closest edge of the weld. 
         [0051]    Loading tube  60  also has radially extending supports  64 , for example made of steel, with slot  66 . Once locked, various work devices can be provided that have necks which extend through slot  66  and lock the work device with respect to the loading tube  60  via the interaction of the necks with slot  66 . 
         [0052]      FIG. 5  shows for example one embodiment of a slot  66 , through which a neck  66 B can pass and then rotate and retract to lock a work device onto support  64 . 
         [0053]      FIG. 6  shows placement of plugs  70  using a common tool manipulator. Although not shown in  FIG. 4  for clarity, when first placed the loading tubes can have plugs  70  attached to the end, for example with a spring-loaded air-actuated ball detent controlled by the operator, placed in the primary nozzle. After placement of the loading tubes  60  with plugs  70 , a common tool manipulator  90  can be placed on turntable  50  and inserted into the loading tube  60  using the linear actuator  58 . Common tool manipulator  90  has an arm  92 , preferably with at least degrees of movement, the arm  92  capable of having different tools attached to its end for different operations, for example an attachment head for plug installation, a non-destructive examination head, a machining head, and a welding head. In  FIG. 7  arm  92  of common tool manipulator  90  has an attachment head  94 , for example by latching onto the plug  70  after the detent is released. Attachment head  94  can move plug  70  down tube  210  to seal tube  210 , the plug having a an expandable diameter, for example via a screw actuated expanding mandrel. Plug  70  can prevent materials from moving down the hot or cold legs. Once plug  70  is installed, the CTM  90  is removed via an attachment to the turntable and brought back up to the top of the reactor vessel so that the attachment head  94  can be removed and replaced by an NDE head. 
         [0054]    Once the plugs  70  have been placed, a non-destructive examination of the weld  310  can take place.  FIG. 7  shows schematically an NDE head  95  on the arm  92  of common tool manipulator  90 . Advantageously, NDE head  95  may be exactly the same device as used on so-called in-service inspection (ISI) devices, for example those used a TRANS-WORLD REACTOR VESSEL EXAMINATION SYSTEM from Areva. The NDE head  95  preferably has both an eddy current sensor and ultrasonic transducer. The ultrasonic transducer can detect the physical structure of any flaws. The eddy current sensor detects when materials change, so that a transition from for example stainless steel to alloy 600/82/182 can be detected. The NDE thus can provide details of the weld  310 , namely the physical location and extent of the weld and of any flaws. Since arm  92  can fully rotate 360 degrees within the tube, the circumferential, axial and radial extent of any flaws can be determined. For example, a circumferential reference point of zero degrees can be set at the top of nozzle area  110 , an axial reference point of zero can be set at an end  61  of the loading tube  60 , and a circumferential reference point can be set at an inner surface  161  of the nozzle area  110  at end  61 . A flaw  313  thus could extend from for example 15 degrees to 32 degrees, and have a maximum axial extent on one side of 0.17 inches and at another side of 0.28 inches, and have a maximum radial depth of 0.5 inches. To prepare the flaw for remediation before a corrective weld inlay, a pre-weld operation could occur in which a machining or grinding operation occurs from 12 to 35 degrees from 0.15 to 0.30 inches and with a constant radial depth of 0.6 inches. The entire flaw is thus removed. Alternatively, depending on the type of machining or grinding used or the extent of the flaws, it may be advantageous to machine or grind all 360 degrees. A software program, such as ACCUSONEX from Areva, can be used to provide a visual three-dimensional representation of the flaws, and accurately map the locations of the weld and any flaws. Should the NDE determine that the loading tube is located too far or too close to the weld  310 , a repositioning of the loading tube can occur. 
         [0055]      FIG. 8  shows schematically a machining or grinding tool  97  for machining or grinding using the common tool manipulator arm  92 . Tool  97  can machine or grind away any flaws, and also can be used to machine or grind away a small portion, for example 0.1 inch, of all of inner surface  312  of the weld  310 , for example using CNC control. A vacuum can be provided with the CTM  90  to permit vacuuming of the machined away or ground material. The CTM  90  then can be removed from tube  60  and brought to the top of reactor  100 . A laser profilometry head also can be provided at the CTM  90  at the same time as the machining or grinding tool  97  is attached, and is used to determine the shape of the nozzle, for example if it is not perfectly circular. The CNC control thus can be modified as the machining or grinding is occurring to ensure the proper machining or grinding depth. 
         [0056]      FIG. 9  shows a preparation robot  200  for preparation of the weld after machining or grinding. The robot may be for example one available from the STÄUBLI Corporation, and may be used for example to flap the weld  310  to compress the weld material, and also to clean the weld  310 , for example with a sponge or wipe, or perform a surface examination with a die penetrant. One advantage of the preparation robot is that certain tools used can be replaced in situ, i.e. carried and stored on the robot itself, without the robot needing to be removed from the primary nozzle. Thus during a surface exam, a sponge or wipe  205  can be used to perform a further pre-weld operation, and then a surface examination head  204  can be used after the sponge or wipe  205  without withdrawing the robot  200 . 
         [0057]    While the robot  200  is operating, the machining or grinding head  97  of CTM  90  can be removed manually and an arc-welding device installed on manipulator arm  92 .  FIG. 11  shows schematically an arc-weld device  99  installed on arm  92  for arc-welding using the common tool manipulator  90 , for example a gas-tungsten arc welding head. An inlay  410  can be laid over any PWSCC susceptible alloy, and can extend a distance X axially beyond the weld  310 , for example 0.25 inches. The weld inlay may be made of alloy 52MS for example, and have a thickness of at least 0.13 inches for example. After the welding of the inlay, a machining or grinding can occur. Any machined flaws can also be filled with weld material. Once the weld inlay  410  is placed, a final inspection can occur using preparation robot  200 . 
         [0058]      FIG. 10B  shows an alternate weld  412  to the weld inlay  410 , in which the weld  412  is placed over the weld  310  without machining or grinding, a so-called onlay. With both the inlay  420  and the alternate onlay weld  412 , a barrier layer  411  made of for example alloy 309 can be placed over any stainless steel material, and the alloy 52MS, for example, then placed over the alloy 309 barrier layer  411 . The barrier layer  411  is helpful since certain alloys such as 52MS may not weld well directly on stainless steel material with high sulfur content. Alternately the barrier layer  311  can be both alloy 309 over the stainless steel and carbon areas, and alloy 82 over the weld  310 . 
         [0059]    It should be noted that in some embodiments of the present invention, the machining or grinding step is not necessary, and the arc-weld device  99  can place the new weld material directly over weld  310  without machining or grinding, i.e. without performing a weld inlay operation. 
         [0060]      FIG. 11  shows schematically common tool manipulators in four of the primary nozzles, with the preparation robot in a fifth primary nozzle. Advantageously, three CTMs  90  can be welding, while a fourth can be machining. The preparation robot  200  can be flapping the weld of yet another primary nozzle  20 . In one preferred embodiment, only four total devices, three CTMs and one preparation robot are used. 
         [0061]      FIGS. 12A through 12H  shows a preferred plan for performing a repair operation on eight primary nozzles using three common tool manipulators and one preparation robot at a same time, with four CTMs  90  and two preparation robots  200  being available for placement. The four CTMs are identified as CTMA, CTMB, CTMC and CTMD, and the two preparation robots as STAUBLIA and STAUBLIB. A vacuum tool is also used, thus completing the first seven columns. The plan will be described with respect to the operations on the first hot leg primary nozzle, although as shown all eight nozzles are processed. 
         [0062]    As shown in the eighth column  8 , the loading tubes  60  with plugs  70  are placed during hours zero to seven of the first day of the repair procedure. 
         [0063]    As shown in the first column, the first CTMA then is used from hour seven to hour nineteen to install all of the plugs  70  in the four hot loop primary nozzles and four cold loop primary nozzles. 
         [0064]    As shown in the second column, the second CTMB has an NDE head installed and calibrated at hour eight, and from hour twelve to hour twenty is used for a non-destructive examination of the primary nozzle weld in the first hot loop. 
         [0065]    As shown in the third column, the third CTMC is then used to perform the NDE on the primary nozzle weld in the second hot loop from hour sixteen to the beginning of the second day. 
         [0066]    Once CTMB is removed at hour twenty from the first hot loop nozzle (column two), the fourth CTMD shown in column four, with a machining head  97 , is installed in the first hot loop nozzle and begins machining until hour ten of the third day. 
         [0067]    As shown in column seven, the first hot loop nozzle is then vacuumed at hours sixteen to twenty of the third day. As shown in column five, the first preparation robot then can abrade the first hot leg primary nozzle surface from hour 20 on day three to hour three on day four, while thereafter the second preparation robot, as shown in column six, can wipe the abraded surface from hours four to six on the fourth day. 
         [0068]    As shown in the first column, at hour 10 on the fourth day, the welding of a barrier layer of alloy 309 over any stainless steel material and alloy 82 over existing alloy 82/182 occurs. This barrier layer operation can proceed with CTMA until hour one on the fifth day. At this point the primary nozzle of the first hot leg primary has its barrier layers installed. 
         [0069]    As shown in  FIGS. 12B and 12C , third column, CTMC is then used to provide the weld inlay to the first hot leg from hour ten on day three to hour eleven on day four. As shown in  FIGS. 12D and 12E , second column, at hour eight on day  11 , the weld inlay in the first hot leg can be machined by CTMB until hour three on day thirteen. 
         [0070]    Vacuuming can occur again in the first hot leg on day thirteen from hour eleven to hour thirteen, as shown in  FIG. 12E . The first preparation robot then can abrade and FOSAR (foreign object search and retrieve) from hours five to ten on day fourteen. As shown in  FIG. 12F , the final post-weld examination can occur using the second preparation robot on day fifteen from hours twenty to twenty-two, at which point the first primary nozzle is fully remediated with its new weld. 
         [0071]      FIGS. 12F and 12G  show final steps for all eight nozzles. 
         [0072]    As shown for example at hour eight on day three, four nozzles can be occupied at once, by four CTMs. Alternately, four nozzles can be occupied by three CTM and one preparation robot, as shown for example at hour twenty-one on day three. Preferably, not more than half the nozzles are ever occupied, but at least half the nozzles are occupied by working devices during certain periods. This arrangement permits time-efficient use of the turntable, CTMs and preparation robots.