Patent Publication Number: US-2007121776-A1

Title: System and method for multiple usage tooling for pressurized water reactor

Description:
BACKGROUND OF INVENTION  
      1. Field of the Invention  
      This invention relates generally to tools for inspecting, repairing and mitigating stress corrosion cracking on pressure water reactor vessels.  
      2. Description of Related Art  
      A reactor pressure vessel (RPV) of a pressurized water reactor (PWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head.  
      At various times during the operational life of a nuclear reactor, there is a need to remove the core and internals from the reactor vessel via the top head. Such instances include refueling, inspecting, annealing, repairing and mitigation of stress corrosion cracking (SCC).  
      SCC is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds which are exposed to high temperature water. The reactor components may be subject to a variety of stresses. These stresses may be associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other stress sources, such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can influence the susceptibility of metal in a component to SCC.  
      Reactor components in contact with reactor coolant may occasionally be replaced as a result of failure due to SCC. Replacing the internal components typically may require removing the core internals from the reactor vessel. For example, in the event a safe end and interconnecting coolant pipes require replacement, the reactor must be shut down for maintenance and drained to an elevation below that of the nozzle safe end. The safe end and/or interconnecting coolant pipes are then removed and a replacement safe end and/or interconnecting coolant pipes are welded to the RPV nozzle. Replacing a safe end and/or interconnecting coolant pipes is typically time consuming and costly since such replacement generally requires a lengthy reactor outage.  
      During reactor operations, however, circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the reactor components.  
      Known methods of inspecting the circumferential welds for IGSCC and IASCC have utilized ultrasonic probes positioned on the outer surface at the weld joint. A series of scans are performed while projecting the ultrasonic beam through the weld from the outer side of the component to the inner side of the component. Other methods rely on positioning an ultrasonic or eddy current probes on the inner surface of the component and projecting the ultrasonic beam from the inner surface of the component toward the outer surface of the component. In any event, most methods for inspection require temporarily shut down of the reactor vessel.  
      Further, in order to apply corrosion resistance cladding (CRC) to reactor components, the reactor must be kept dry during welding process. In this event, it is required to drain a refueling pool in the reactor to keep the welding area dry. However, the refueling pool may be difficult to drain because the high dose reactor components are stored inside the pool with the reactor vessel open to the pool at the same time.  
      Accordingly, a need exists for reliable and relatively easy temporarily shielding and access to the internals of the reactor vessel, allow the reactor vessel to be drained of water, and provide a safe work place for personnel.  
     SUMMARY OF THE INVENTION  
      Exemplary embodiments of the present invention relate to a system for shielding high radiation dose from inside reactor vessel wall and attached components. The system may include a radiation shield positioned within the reactor vessel, and a coffer dam. The radiation shield reduces the radiation dose from irradiated vessels. The coffer dam allows draining of the vessel and keeps the refueling pool filled with water.  
      Another exemplary embodiment provides the coffer dam with a working deck, and a coffer dam support for supporting the working deck. The working deck may include a rotatable access lid. The rotatable access lid may include a plurality of openings to access the interior of the reactor vessel.  
      Exemplary embodiments of the present invention provide a method of preparation of reactor vessel for services. The method may include removing core barrels in the reactor vessel, installing a radiation shield in the reactor vessel, installing a coffer dam, and draining the reactor vessel.  
      Exemplary embodiments of the present invention related to a system for inspecting, repairing and mitigating stress corrosion cracking on a pressure water reactor vessel. The reactor vessel includes inlet nozzles, outlet nozzles, and bottom mounted instrumentation (BMI) nozzles. The system may include a radiation shield positioned within the reactor vessel, a coffer dam, a tooling delivery robot lowered into the reactor vessel, and a tool cradle for holding the tools.  
      Exemplary embodiments of the present invention relate to a method for inspecting, repairing and mitigating stress corrosion cracking on a pressure water reactor vessel. The method may include removing core barrels in the reactor vessel, installing a radiation shield in the reactor vessel, installing a coffer dam, draining the reactor vessel, lowering a tooling delivery robot into the reactor vessel, attaching the tooling delivery robot at a surface of the reactor vessel, lowering a tool cradle which hold tools into the reactor vessel, and attaching the tool cradle at the surface of the reactor vessel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Exemplary embodiments of the present invention will become more apparent by describing, in detail, exemplary embodiments thereof with reference to the attached drawings, wherein like procedures are represented by like reference numerals, which are given by way of illustration only and thus do not limit the present invention.  
       FIG. 1  is a schematic view of a reactor pressure vessel in accordance with an exemplary embodiment of the present invention.  
       FIG. 2  is a schematic view of a reactor pressure vessel with core barrels removed in accordance with an exemplary embodiment of the present invention.  
       FIG. 3A  is a schematic view of a radiation shielding in accordance with an exemplary embodiment of the present invention.  
       FIG. 3B  is a schematic view of a radiation shielding installed in the reactor vessel in accordance with an exemplary embodiment of the present invention.  
       FIG. 4  is a schematic view of a coffer dam installed on the reactor pressure vessel in accordance with an exemplary embodiment of the present invention.  
       FIG. 5  is a schematic view of the working deck in accordance with an exemplary embodiment of the present invention.  
       FIG. 6  is a schematic view of a reactor pressure vessel and a filter in accordance with an exemplary embodiment of the present invention.  
       FIG. 7  is a schematic view of a bottom vessel wall with internal components in accordance with an exemplary embodiment of the present invention.  
       FIG. 8  is a flowchart illustrating the installation of tooling delivery robots in accordance with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      It should be noted that these Figures are intended to illustrate the general characteristics of method and apparatus of exemplary embodiments of the present invention, for the purpose of the description of such exemplary embodiments herein. These drawings are not, however, to scale and may not precisely reflect the characteristics of any given embodiment, and should not be interpreted as defining or limiting the range of values or properties of exemplary embodiments within the scope of this invention. Like numerals are used for liked and corresponding parts of the various drawings.  
       FIG. 1  is a schematic view of a reactor pressure vessel in accordance with an exemplary embodiment of the present invention. In particular,  FIG. 1  illustrates a perspective view of the containment building  10  with a cutaway of a containment wall  11  to show a reactor vessel  12 , refueling pool  16   a  and reactor pool  16   b  therein. The reactor vessel  12  is an elongated, generally cylindrically shaped member. The reactor vessel  12  has the usual hemispherical bottom and a plurality of inlet and outlet primary system water nozzles. As an exemplary embodiment, the inlet nozzle may introduce coolant pumps into reactor vessel to cool the reactor&#39;s core which creates heat, and the outlet nozzle discharges heated pressurized-water in a primary coolant loop to carry the heat to a steam generator  20 . The steam generator  20  vaporizes the water in a secondary loop to drive the turbine (not shown), which then ultimately produces electricity.  
       FIG. 2  is a schematic view of a reactor pressure vessel with core barrels removed in accordance with an exemplary embodiment of the present invention. As shown in  FIG. 2 , a closure head of the vessel reactor  12  and fuel (not shown) are removed. Further, radioactive lower and upper internals have been removed and stored. The reactor vessel  12  may include inlet nozzles  13  for inlet of coolant and outlet nozzles  14  for outlet of hot-pressurized water to produce energy for steam generators  20  (as shown in  FIG. 1 ). The reactor vessel  12  further includes bottom mounted instrumentation nozzles  15  (BMI). The BMI nozzles  15  are penetrated tube attached to the bottom head of the vessel  12 . The BMI nozzles  15  may be welded (via a J-groove weld, for example) to the vessel  12 .  
      Before performing an inspection, repair and/or mitigation for stress corrosion cracking (SCC) in the containment building  10 , precautions must be taken to prevent radiation emitted by the stored internals from being introduced to humans. In this regard, temporary radiation shielding  30  (shown in  FIG. 3A and 3B ) of the stored internal should be employed, and coffer dams  40  (shown in  FIG. 4 ) assembled on the reactor vessel  12  to seal the interior of vessel from water in refueling pool  16   a  and reactor pool  16   b .  
       FIG. 3A  is a schematic view of a radiation shield  30  in accordance with an exemplary embodiment of the present invention; and  FIG. 3B  is a schematic view of a radiation shield  30  installed in the reactor pressure vessel in accordance with an exemplary embodiment of the present invention. One of the purposes served by the radiation shield  30  is to minimize radiation dose from the irradiated vessel  12 .  
      The radiation shield  30  is generally cylindrical which conforms to shape of the reactor vessel  12 . In other words, the circumference of the radiation shield  30  should closely resemble the inner circumference of the reactor vessel  12 . The radiation shield  30  includes a plurality of openings  31  near the top end to correspond with nozzles  13  in the reactor pressure vessel  12 . It should be appreciated that the openings  31  may be varied depending on the number and size of corresponding nozzles found in the pressure vessel  12 . The radiation shield  30  may include notches  33  on an exterior surface of the shield  30 . The notches  33  provide support for the radiation shield  30  when positioned on internal brackets (not shown) in the vessel  12 . The notches  33  may also act as a locating means for locating the position of the radiation shield  30  while positioned within the reactor vessel  12 . The radiation shield  30  may be made from steel. However, one skilled in the art would appreciate that other metals, such as, but not limited to, stainless steel, may be employed.  
      It should be appreciated that the design of the radiation shield  30  may be varied depending on, for example, but not limited to, shape of the reactor vessel, radiation measurement, and thermal data.  
      Once the radiation shield  30  is installed in the reactor vessel  12 , a coffer dam may be installed on the reactor vessel to provide temporary shielding, and designed to allow draining of the vessel and keep the refueling pool and reactor pool filled with water. The structure of the coffer dam will now be described with reference to  FIG. 4 , wherein the coffer dam is generally referred to by reference number  40 .  
       FIG. 4  illustrates a schematic view of a coffer dam  40  installed on the reactor pressure vessel  12  in accordance with an exemplary embodiment of the present invention.  
      The coffer dam  40  is generally cylindrical and includes a plurality of segments  40   a ,  40   b ,  40   c  . The plurality of segments  40   a ,  40   b ,  40   c  may be sealed via sealing means (not shown) positioned between fit up flanges or edges  41   a ,  41   b ,  41   c , of adjacent segments  40   a ,  40   b ,  40   c . Fasteners (not shown) may be included for attaching together the edges  41   a ,  41   b ,  41   c  of adjacent segments  40   a ,  40   b ,  40   c . Further, sealing means (not shown) may be positioned between the bottom flange  41   a  of the completed coffer dam  40  and the reactor vessel upper flange  22 ; and fasteners (not shown) may be used for attaching the bottom flange  41   a  to the reactor vessel upper flange  22 .  
      Each segment  40   a ,  40   b ,  40   c  is an elongated vertical cylindrical section, (e.g., each segment is an equal longitudinal curved section of the total cylindrical coffer dam). If four segments are used, each segment may be curved 90 degrees; if three segments, 120 degrees; and so on. In an alternate embodiment, each segment  40   a ,  40   b ,  40   c  may be a horizontal cylindrical section (e.g., each segment is a cross sectional portion of the cylinder).  
      Further, if desired, the coffer dam  40  can be made of a combination of vertical and horizontal sections connected together.  
      In any case, the size of the segments of the coffer dam  40  is selected so as to fit through an equipment hatch (not shown) of the containment building  10  and yet still correspond to the size of the reactor vessel  12 . In other words, the circumference of the segment should closely resemble the inner circumference of the reactor vessel  12 . The choice size and quantity of the segments of the coffer dam  40  can also be varied to satisfy other manufacturing, transport and plant specific conditions.  
      Each segment  40   a ,  40   b ,  40   c  includes vertical and horizontal fit up flanges or edges  41   a ,  41   b ,  42   c . The adjacent edges  41   a ,  41   b ,  42   c  are mated and connected by fasteners, such as bolt and nut combinations, for example. The lowermost set of horizontal fit up flanges or edges  41   a  may form the bottom flange of the completed coffer dam  40 , whereas the uppermost set of horizontal fit up flanges or edges  41   c  may form the upper flange of the coffer dam  40 .  
      Each of the segments  40   a ,  40   b ,  40   c  may be pre-fabricated to contain the sealing means described below. Alternatively, all or some of the sealing means could be installed when the segments  40   a ,  40   b ,  40   c  are being assembled on the operating floor. In an example embodiment, the sealing means may be a thermal insulator gasket-type seal in combination with metallic and non-metallic  0 -rings. The sealing means helps resolve a significant feasibility issue by allowing a plurality of segments  40   a  ,  40   b ,  40   c  to be passed through the hatch and to form the complete coffer dam  40 . In order to prevent any leakage through the joint with sealing features, the space formed between two sealing device can be pressurized to prevent and/or reduce any coolant leak into the dry reactor vessel.  
      Once the segments  40   a ,  40   b ,  40   c  are connected together to form the completed coffer dam  40 , the coffer dam  40  is moved and attached to the reactor vessel flange  22 .  
      The bottom flange  41   a  of the coffer dam  40  may be connected to the reactor vessel flange  22  via fasteners (not shown), for example, but not limited to, a threaded bolt arrangement. More particularly, the bottom flange  32  of the coffer dam  40  may have a plurality of holes to allow the completed coffer dam  40  to be bolted to the threaded holes formed in the reactor vessel  12  for receiving the closure head. This bolt down arrangement prevents a catastrophic seal failure because the flanges  41   a - 41   c  are in intimate contact.  
      Accordingly, with the coffer dam  40  installed on the reactor vessel  12 , a temporary shield to reduce and/or prevent radiation emitting from the stored internals is provided and/or to allow draining of the reactor vessel  12  and keep the refueling pool filled with water.  
      It should be appreciated that the segments  40   a ,  40   b ,  40   c  can be pre-fabricated in a factory or each segment may be brought into the containment building  10  and preferably assembled by humans in a low radiation area of the operating floor.  
      On top of the coffer dam  40  is a working deck  50 , in which the coffer dam  40  is mounted on a supporter  49  as shown in  FIG. 4 . The working deck  50  may include a plurality of access holes  51 ,  53  (shown in  FIG. 5 ) for accessing the interior of the reactor vessel  12  by a user. For example, the access holes  51 ,  53  may be used to inspect/repair any nozzles in the bottom head of the vessel and/or nozzles in the side surface of the vessel. The access holes  51 ,  53  may provide ease in maintenance, repair, inspection and mantling/dismantling of parts from inside of the reactor vessel  12 .  
      Referring further to  FIG. 5 , the working deck  50  includes two large access openings  51 ,  53  for large tools to enter the vessel  12 . The large access openings  51 ,  53  accommodate large plugs  52 ,  54 , respectively, which may be removable from the access openings. The size of plug  52  may be ½ of the size of the inside diameter of reactor vessel  12 . As a result, this would allow instruments to reach the center of the reactor and/or near the edge of the reactor. The size of plug  54  may be determined by a particular tool size as it is designed for tooling access from above the working deck  50  into the reactor vessel  12 . However, it should be appreciated that the size of the plugs  52 ,  54  and the openings  51 ,  53  may vary depending on the operation required. The large plug  52  may further include a small opening  55  for smaller tool access. The small opening  55  may include a removable small plug  56  to be inserted therein. The removable small plug  56  may be, for example 8 inch to 16 inch in diameter. However, it should be appreciated that other diameter size may be employed depending on the size of the tools employed. Similarly, the small plug  54  may include a small opening  57  for tool access. The small opening  57  may include a removable small plug  58  to be inserted therein.  
      The working deck  50  rotates (e.g., 360 degrees) with respect to the coffer dam  40 . Further, it should be appreciated that the large plug  52  and the small plug  54  rotate within their respective openings. As a result, accessing of parts in the reactor vessel  12  may be easily handled and manipulated.  
      Referring back to  FIG. 4 , the working deck  50  is introduced through the equipment hatch (not shown) in the containment building  10 . The working deck  50  may be secured to the coffer dam support  49  by fasteners and with rotating mechanism (not shown).  
      Further, the working deck  50  may rotate 360 degrees to operate in tandem with a transport robot which will be described later. The rotation of the working deck  50  provides an ease in lowering and retrieving an application robot that is configured for inspection, repair, welding and/or machine operation.  
      Referring to  FIG. 6 , the coffer dam  40  according to an exemplary embodiment of the present invention may include a filter  59  for effective control of airborne particles in the reactor vessel  12 . The filter  59  is positioned on the coffer dam support  49  near the access opening. It should be appreciated that the filter  59  may also be located on the operating floor. Flexible ventilation duct may be connected from the access opening to an inlet port of filter  59 .  
      Further, in order to keep the vessel drain without airborne issues, the filter  59  may maintain a negative pressure inside the vessel  12  in operation. The negative pressure inside the vessel  12  may prevent airborne spreading to the operating floor and minimize contamination.  
      As an example embodiment, the filter  59  may be a high efficiency particulate air (HEPA) filter.  
      Once the temporary radiation shield  30  and the coffer dam  40  are installed in the reactor vessel  12  and the HEPA filter  59  operating, the vessel is drained of all fluid. The reactor vessel  12  may be drained by lowering a pump (not shown) into the vessel via the access openings  55 ,  57  in the working deck  50 , as shown in  FIG. 5 . The drainage operation is continued until the vessel is dry.  
      Once the vessel is completely dry, protectors  17  (shown in  FIG. 7 ) are installed on all bottom mounted instrumentation (BMI) nozzles  15 . The protectors  17  are used to protect the BMI  15  surface from damage by the mounted tooling on top of BMI  15 . Further, the design of the protectors  17  may be employed to engage with the tools for inspecting, repairing, and/or mitigating SCC.  
      Once the protectors  17  are installed on the BMI  15 , the operation proceeds to preparation of cleaning the internals of the reactor vessel  12  as described below.  
       FIG. 8  is a flowchart illustrating the installation of tooling delivery robots in accordance with an exemplary embodiment of the present invention.  
      To perform the operation, a tooling delivery robot is lowered into the reactor vessel (S 100 ), and installed at a surface of the vessel wall (or vessel bracket) (S 200 ). A tool cradle which holds the tools for operating is then lowered into the vessel (S 300 ), and affixed at the surface of the vessel wall (S 400 ). Thereafter, the delivery robot moves toward the tool cradle and engages a cleaning tool from the tool cradle (S 500 ). The delivery robot then moves away from the tool cradle along with the tool, and commences the operation (S 600 ).  
      Referring back to  FIG. 7 , the tooling delivery robot  70  and tooling cradle  75  in the reactor vessel  12  are shown in accordance with an exemplary embodiment of the present invention.  
      The tooling delivery robot  70  is lowered into the reactor vessel  12  to be installed. The tooling delivery robot  70  may be lowered using, for example, but not limited to, jig hoist, ropes and/or poles. The lowering of the tooling delivery robot  70  may also work in tandem with the rotating working deck  50  to position the delivery robot  70  in the appropriate position. In other words, the tooling delivery robot  70  may be lowered via openings  51 ,  53  in the working deck  50 , wherein the working deck  50  rotates so as to provide ease in positioning the tooling delivery robot  70  in the reactor vessel  12  for installation.  
      The tooling delivery robot  70  may be typically comprised of two segment arms  70 A,  70 B. Segment arm  70 A is interposed between a connection means  71  and an end of segment arm  70 B. Segment arm  70 B is interposed between an end of segment arm  70 A and a tool connector  72 . One end of each segment arms  70 A and  70 B are rotatable at a connection joint  74 . The arms  70 A,  70 B may rotate 360 degrees. Further, the tooling delivery robot  70  may provide the necessary translation movement to cover the entire bottom area of the reactor vessel  12 .  
      It should be appreciated that more than two segments may be employed to make up the tooling delivery robot depending on the angles and positions required for the robot arm.  
      Once the tooling delivery robot  70  is lowered, the connection means  71  is mounted to a small platform  77  attached at the surface of the vessel wall. The connection means  71  may be fastened to the platform  77  using, for example, but not limited to, nuts and bolts.  
      The tool cradle  75  is then lowered into the reactor vessel  12 . The tool cradle  75  may include tools, such as, a weld held tool  81  for repairing SCC and a surface improvement tool  82  for cleaning SCC, for example. However, it should be appreciated that other tools may be included in the tool cradle depending on the operation desired.  
      Once the tooling cradle  75  is positioned and installed in the vessel  12 , the tooling delivery robot  70  moves to engage with a tool in the tool cradle  75 . As an exemplary embodiment, if the operation is to mitigate the SCC in the BMI  15 , the tool delivery robot  70  engages with the tool weld head  81  and performs the repairing operation on the BMI  15 .  
      It should be appreciated that other tools may be employed besides the tool weld head to perform other operations, such as, inspecting, cleaning, repairing and/or machining.  
      As shown in  FIG. 7 , two tooling delivery robots  70  and two tool cradles  75  are provided in the reactor vessel  12 . The two tooling delivery robots  70  should provide sufficient movement and coverage in the reactor vessel  12  to cover the entire BMI  15  locations. In other words, the tooling delivery robots  70  operate to guide the tools  80  for inspection, repair, welding and/or machine tooling for all of the BMIs  15 .  
      Further, the tooling delivery robots  70  may provide the ability to perform different IGSCC mitigation functions simultaneously. For example, a first tooling delivery robot may perform at least one of inspecting, welding and machining operation simultaneously in different BMI to create a parallel path work flow, while simultaneously, a second tooling delivery robot  70  may retrieve the tools when the first tooling robot delivery have completed its tasks.  
      The exemplary embodiments of the present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the exemplary embodiments of the present invention, 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.