Abstract:
The invention is a process of repairing cracked or microstructurally damaged portions of irradiated materials, such as nuclear reactor pressure vessels and shrouds. A damaged portion of the irradiated substrate is first removed, such as by electrical discharge machining (EDM). After removing the damaged portion, the recast layer inherent in the EDM process is then removed. Once the repair area substrate material has been removed to a calculated depth, the created cavity is then filled without releasing transmutated elements within the irradiated material. A chamber may be placed on the irradiated material surrounding the repair area to create an isolated work space.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to repair method, and, more particularly, the present invention relates to weld repair of irradiated materials. 
         [0003]    2. Description of the Related Art 
         [0004]    While the disclosed invention can be used in a variety of industries, the environment of a nuclear utility will be discussed herein for exemplary purposes. Nuclear utilities have a need to verify the integrity of their aging components within nuclear reactors and other plant systems. The neutron fluence in the reactor vessel core produces accumulated damage, and helium (He) and hydrogen (H) gas, through transmutation reactions. Thus, the reactor vessel internals are at risk of radiation embrittlement (irradiation-assisted stress corrosion cracking, or IASCC). Additionally, the reactor vessel material and cladding can become embrittled. 
         [0005]    Current weld repair technologies and process sequencings are not appropriate for irradiated material because they require a high ratio of energy/mass deposited and impart too much energy to the substrate irradiated material under repair. Machining techniques that produce chips are a foreign material exclusion (FME) risk to the primary coolant system of a reactor. Additionally, previously wetted surfaces and/or cracked surfaces are at risk of boric acid contamination prior to weld repair. 
         [0006]    There exists a need for a base material repair technique which permits a repair to the reactor materials and produces a true metallurgical bond. This repair technique must be made with low energy techniques that limit the diffusion of transmutated He and H gas species which produce porosity and cracking in the substrate. Additionally the technique will limit the risk of cracking caused by residual stresses in embrittled low alloy steels. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention is a process of repairing cracked portions of irradiated materials or portions of materials that have accumulated neutron irradiation damage, such as nuclear reactor pressure vessels and shrouds. As an initial step, a damaged portion of the irradiated substrate is removed, such as by electrical discharge machining (EDM). EDM is a preferred method of removal because other machining techniques would create “chip” debris, which is undesirable because of the irradiated nature of the substrate and because it would create a risk of foreign objects near or within the pressure vessel that must then be accounted for. After removing the damaged portion, the recast layer inherent in the EDM process is then removed. Laser ablation is a preferred process for removing the recast layer. Alternate methods, such as chemical striking and mechanical abrasion, may also be available. 
         [0008]    Once the repair area substrate material has been removed to a calculated depth, the created cavity is then filled. This may be done by coaxial powder injection laser welding, which entails injecting powder into a laser to instantaneously fuse the powder and substrate. This process beneficially does not heat the substrate as other welding techniques do. Heating of the substrate is undesirable because such heating promotes diffusion of helium and hydrogen that is entrapped within the irradiated substrate, causing porosity and or cracking in the weld and weld heat affected zone. 
         [0009]    If there is moisture within the crack being repaired, a chamber may be placed about the repair area and placed under a vacuum to dry out the moisture prior to powder injection welding. If the crack being repaired is a through-crack, the chamber may be placed in a hyperbaric condition with a positive pressure of an inert gas to expel the moisture from the crack. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention is described with reference to the accompanying drawings, which illustrate exemplary embodiments and in which like reference characters reference like elements. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
           [0011]      FIG. 1  shows a flow chart for a repair process of the present invention. 
           [0012]      FIG. 2  shows a perspective view of an electrical discharge machining tool for use in the present invention. 
           [0013]      FIG. 3  shows the electrical discharge machining tool of  FIG. 2  in a use position according to the present invention. 
           [0014]      FIG. 4  shows a side view of the tool of  FIG. 2  in a first position relative the vessel wall during use. 
           [0015]      FIG. 5  shows a side view of the tool of  FIG. 2  in a second position relative the vessel wall during use. 
           [0016]      FIG. 6  shows an outline drawing of a coaxial powder injection laser welding tool with a chamber for use in the present invention. 
           [0017]      FIG. 7  shows a close-up image of the laser and powder injection portion of the tool of  FIG. 6 . 
           [0018]      FIG. 8  shows an alternative access trunk design in a non-flooded reactor cavity. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  shows a flow chart for a repair process of the present invention. The method begins at step  100  in which a damaged portion of an irradiated metal is identified. This identification can be performed by visual inspection, ultrasonic testing, or other non-destructive examination (NDE) methods. NDE provides detailed information about the flaw(s) present in the damaged material, including the depth and thickness thereof. Thus, through this identification the amount of material that must be removed from the damaged metal is determined. 
         [0020]    Once the portion of the irradiated metal to be repaired has been identified, the next step  110  is to remove the damaged portion of the irradiated metal. This should be done while minimizing strain and deformation of the remaining section of the irradiated metal so that any transmutated He or H therein will not be released. Electrical discharge machining (EDM) is a preferred method of excavating the damaged metal. EDM uses electrical discharges between an electrode and the workpiece to remove material from the workpiece. An electrical discharge or “spark” is created between the electrode and the workpiece, producing intense heat that vaporizes a portion of the workpiece. The spark is controlled and localized so that it only affects the excavated surface of the workpiece material and does not affect the material properties below the excavated surface. Demineralized water can be used to flush any generated swarf from the area under repair. 
         [0021]    The EDM process may utilize a sinker-type electrode, harp-type electrode, or scoop type-electrode configuration to produce the excavation repair cavity. The dielectric to be utilized in this submerged process is deionized or demineralized water. The power supply can be set as electrode positive or electrode negative dependent upon the electrode material type (graphite, Cu—W, Ag—W). The operational discharge is performed by high frequency pulsing of the discharge power supply. A gap sensing circuit is utilized to control a motorized electrode advance and retract function to maintain consistent discharge machining and permit flushing of the swarf debris. The electrodes are designed to provide for flushing channels and injection paths. 
         [0022]    One advantage of the EDM process is that it limits the strain and deformation to the substrate produced by traditional machining methods. Additionally, the debris created by EDM is in the form of a fine particulate that can be removed in-situ. Thus, the risk of introducing foreign material, which could damage the reactor and fuel, is minimal compared to traditional machining techniques. The EDM process also removes the boric acid contamination and surface oxides. 
         [0023]      FIG. 2  shows a perspective view of a preferred EDM tool  1  for use in the present invention, and  FIG. 3  shows the EDM tool  1  of  FIG. 2  in a use position to repair a vessel wall  80 . The tool  1  includes a base  10  upon which a rotary shaft  11  is mounted. Two electrode mount arms  12  are rotatably mounted on the shaft  11 . An electrode  13  is coupled to the mount arms  12 . A harp- or wire-type electrode  13  is shown in the illustrated embodiment. A rotary drive assembly  14  and electrical control connections  15  are provided to rotate the mount arms  12  and electrode  13  about the shaft  11 . Additional connections are provided on the base  10  to provide demineralized water (connection  16 ), EDM electrical current (connection  17 ), swarf suction (connection  18 ), and purge air (connection  19 ). A rigging eyebolt  20  and handling pole connection  21  are provided to facilitate placement of the tool  1  into the desired use position. A catch pan  22  is coupled to the base  10  to collect and retain swarf and the removed portion of the wall  80  during use. The base  10  further includes railing  23  on a lower surface thereof so that the tool  1  can be moved left, right, forward, and backward relative the wall  80  to be repaired. Mounting connections  24  may be included on the underside of the railing  23  for mounting the tool  1  to a stand (not shown) during use. 
         [0024]    In use, the tool  1  is placed in position relative the vessel wall  80  in a known manner. The tool  1  is positioned such that rotation of the curvature of the wire-type electrode  13  will remove the damaged portion of the wall  80 . The tool  1  and electrode  13  are powered, and the rotary drive assembly  14  is engaged to rotate the electrode  13  toward the wall  80 . The powered electrode  13  is rotated into engagement with the wall  80 , and the rotation is continued until the electrode  13  exits the wall  80 . In this manner, a volume of the vessel containing the damaged material is removed, creating a crater in the wall having an outer surface  81 . The wall  80  illustrated in the figures includes a layer of cladding thereon.  FIGS. 4 and 5  show side views of the tool  1  and vessel wall  80  at various positions during use. 
         [0025]    The EDM process inherently creates a recast layer, also called a white layer, on the workpiece where EDM has altered the metallurgical structure of the workpiece. This recast layer may include a fine layer of oxide. It may be desirable to remove this recast layer (including any oxide) to prepare the surface for welding by performing a cleaning step  120  after the EDM process. There are a variety of ways in which the recast layer can be removed. For example, chemical striking with a pickling solution or a mechanical abrasive (wet or dry) process can be used. Laser ablation is a preferred process for removing the recast layer. The surface exposed by the EDM process is irradiated with a laser beam and the absorbed laser energy vaporizes the recast layer material, exposing the underlying substrate metal material which is readily weldable. 
         [0026]    The laser ablation process is typically performed in a pulsed mode at a high energy density (J/cm 2 ) and power density (Watt/cm 2 ) such that vaporization of material may occur. This typically requires a smaller focused spot size in comparison to laser welding. The ablated material must be evacuated from the area. The process may be performed in an environment of inert gas (such as argon or helium) or a vacuum. Some level of ablation can occur during the welding process if the beam is rastered or bifurcated ahead of the weld pool. 
         [0027]    Typically the ablation process is performed prior to welding. Test assemblies are used to verify parameters and procedures to ensure adequate ablation and coverage prior to executing the laser ablation process on the workpiece. 
         [0028]    After the damaged portion of the metal has been removed and the surface has been prepared for welding, the next step  130  is to replace the removed volume. While it is preferable to replace the same volume of material that was removed, such exacting precision is not necessary and is not contemplated by “replacing” as used herein. 
         [0029]    One preferred method of replacing the removed volume is powder injection laser welding, which entails injecting powder around/in a focused laser spot to instantaneously fuse the powder and substrate. This may preferably include injecting the powder coaxially to the laser. This process has a low energy to mass deposition ratio that beneficially does not heat the substrate as other welding techniques do. As used herein, an energy to mass powder flow rate (J/g) of less than 10 KJ/g mass is considered a low energy deposition rate, with a powder deposition efficiency ranging from 40 to 65%. Heating of the substrate is undesirable because such heating promotes diffusion of the He and H that is entrapped within the irradiated substrate, causing porosity and/or cracking in the welds. The powder material is provided in a cone-shape and positioned on the surface and heated with the laser to instantaneously fuse the powder and surface material. This process is continued until the removed volume has been filled as desired. If the cleaning step includes laser ablation, the same laser may be used to perform the coaxial powder injection laser welding. 
         [0030]      FIG. 6  shows a coaxial powder injection laser welding tool  5  for use in the present invention, and  FIG. 7  shows a close-up image of the laser and powder injection portion of the tool  5  of  FIG. 6 . The tool  3  includes robotics  51 ,  52 ,  53  that allow the head  55  to be translated and rotated about three orthogonal axes. The tool  3  includes a robotic manipulator  51  with pitch, yaw, and roll at the end effector or head  55 . Preferably, as shown in the illustrated example, the manipulator  51  is a six-axis robot. The tool  3  may include a powder feed line  52  and an optical fiber  53  for the laser. The head  55  includes a laser  56  in a central location and directed at a focus point  59 . A cone of shielding gas  57  surrounds the laser  56 . A powder supply  58  feeds powder to the focus point  59 , where the laser  56  fuses the powder to the surface  81  under repair. A collimating lens package (not shown) may also be provided. 
         [0031]    An example laser system is a neodymium-doped yttrium aluminum garnet (Nd:YAG) 1064 nm wavelength system with fiber optic delivery and transmissive optics. However, other is laser wavelengths (i.e. 800 to 940 nm) that work with fiber optics could also be used Example deposition rates are on the order of 0.4 to 0.6 lb/hr (3 g/min to 4.5 g/min), and track widths on the order of 0.6 to 3 mm are typical. A preferred standoff distance for the powder nozzle is 5 to 15 mm. 
         [0032]    If any moisture is present within the void, it should be removed prior to replacing the removed material. A preferred method of removing such moisture is to provide a chamber  70  around the void to isolate it, and using a vacuum to pull moisture out of the void. Tooling and materials to replace the removed material can then be inserted into the dry chamber and used to replace the removed material. The chamber may be put in place at the beginning of the repair process. 
         [0033]      FIG. 8  shows an alternative access trunk design  75  for use in the present invention in a non-flooded reactor cavity in lieu of the chamber  70 . In the illustrated embodiment, the trunk  75  extends from atop the reactor vessel  82  and extends down to the location of the surface to be repaired. The trunk  75  may include a mounting plate  76  to which tooling, such as the coaxial powder injection laser welding tool  5 , can be attached. Additional equipment such as pumps (not shown) may also be included to remove reactor vessel water from the trunk  75 . The trunk  75  preferably includes a seal  77 , such as a bellows, to form a foreign matter exclusion seal to prevent debris from entering the reactor vessel  82 . 
         [0034]    In some instances, the damaged material may contain cracks that extend completely through the material to be repaired. In these instances, it will be necessary to prevent moisture from entering into the area being repaired. In this instance a chamber is positioned about one side of the void, and a positive pressure is used to push the moisture out of the chamber and void. The chamber is placed in a hyperbaric condition with a positive pressure of an inert gas to expel the moisture from the crack. The replacement tooling and materials can then be inserted and/or manipulated in the dry chamber and used to replace the removed material. 
         [0035]    Another method of repairing a damaged portion of the workpiece is friction stir welding (FSW). FSW is a joining process in which the metals being joined are not melted. Rather, the bodies to be joined are placed adjacent each other and held together with some amount of force. A rotating tool is used to generate heat along the junction between two facing surfaces, causing a plasticized zone to form in the materials around the tool. The softened metal can then be joined using mechanical pressure. The plasticized zone creates a metallurgical bond under the rotating tool. FSW can be used directly on the damaged portion of the workpiece with no added metal, after removing at least a part of the damaged portion without added metal, or by removing the damaged portion and preparing a replacement metal portion which fits the removed volume, the replacement metal being welded in place by FSW. 
         [0036]    The process flow described herein provides for a technological means to produce weld repairs on ferritic, austenitic, and nickel based alloys that have neutron irradiation damage and embrittlement. The method improves the quality and volumetric acceptability of the weld repair by removing the defects when possible, eliminating the high irradiation damaged surface, eliminating boric acid contamination, eliminating moisture, and utilizing a low energy/mass deposited process which limits the evolution of porosity and cracking from transmutated gas species or elemental segregation. Coaxial powder injection provides for a low energy/mass deposit, limiting substrate heating. The process flow demonstrates a sequence of technological steps for in-situ reactor vessel and reactor vessel internals repair. 
         [0037]    While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Furthermore, while certain advantages of the invention have been described herein, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.