Abstract:
A method of repairing a substrate includes the step of excavating a well in the substrate with an electrical discharge machining operation. A wire is then delivered to the well. Simultaneously, laser energy is routed to the well such that the laser energy intersects with the wire to produce a reconstruction weld in the well. A tool delivery system with an electrical discharge machining tool head and a reconstruction welding tool head is used to perform the excavation and welding operations.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates generally to the repair of thick-walled components susceptible to corrosion, such as reactor pressure vessel control rod drive mechanisms in pressurized water reactor nuclear power plants. More particularly, the present invention relates to a technique for repairing such thick-walled components through precision excavation and welding. 
     BACKGROUND OF THE INVENTION 
     A number of technologies have been developed to repair corroded or damaged thin-walled, small diameter tubes used in applications such as heat exchangers or material transport systems. U.S. Pat. Nos. 5,430,270; 5,514,849; 5,430,270; 5,656,185; 5,573,683; and 5,653,897 disclose technologies of this type. Each of these patents is owned by the assignee of the present invention and is incorporated by reference herein. FIG. 1 illustrates an apparatus described in several of the foregoing patents. In particular, the figure illustrates a rotating apparatus  20  used to repair damaged tubes. A rotating welding head  22  is fixedly positioned at the end of a rotating sleeve  24 . 
     A rotating drive mechanism  25  rotates the sleeve  24 , thus the rotating sleeve  24  and the rotating welding head  22  synchronously rotate. The rotating drive mechanism  25  simultaneously rotates a filler assembly  26  that includes a filler metal receptacle  28  and a filler metal delivery system  30 . The filler metal receptacle  28  holds the filler metal to be welded. Generally, the filler metal receptacle  28  will be in the form of a reel of filler metal wire. The filler metal delivery system  30  receives the filler metal and delivers it to a filler passage within the rotating sleeve  24 . Since the rotating sleeve  24  and the filler assembly  26  rotate synchronously, the filler metal does not become tangled. 
     The filler metal delivery system  30  is powered through filler assembly slip rings  32 . The speed of the wire feed motor can be varied to permit different wire feed speeds, providing control of clad thickness and to allow adjustment for variations in laser output levels, travel speed, rotational pitch, and other factors. 
     The rotating apparatus  20  also includes a gas coupler  36  that is connected to a gas supply  38 . The rotating sleeve  24  includes a rotating fiber optic cable  40 . A laser  44  supplies energy to a fixed fiber optic cable  43 . The laser energy is transferred from the fixed fiber optic cable  43  to the rotating fiber optic cable  40  through an optical coupler  42 . 
     The rotating apparatus  20  is moved along its longitudinal axis by an axial drive system  50  mounted on shaft  51 . Guide rollers  49  may be used to guide the rotating sleeve  24  into position. A computer controller  53  is used to control the operation of the rotating apparatus drive mechanism  25 , the axial drive system  50 , and the filler metal delivery system  30 . In particular, the computer controller  53  is used to set the speed of the rotating apparatus drive mechanism  25 , the position for the axial drive system  50 , and the filler delivery rate for the filler metal delivery system  30 . 
     The operation of the rotating apparatus  20  is more fully appreciated with reference to FIG. 2, which provides an enlarged cross-sectional view of the rotating welding head  22 . The rotating welding head  22  includes a body  80 , which defines a filler passage  86 . The filler passage  86 , also called the wire conduit runs the length of the rotating sleeve  24 . Filler  88  is forced from the filler metal delivery system  30  through the filler passage  86  to a body aperture  94 . The laser energy is delivered through the body aperture  94  and welds the filler  88 . Gas conduit  89  delivers a shielding gas to the welding head  22 . Preferably, the gas conduit  89  terminates in distribution channels that distribute the gas to the aperture  94  at a number of locations. 
     FIG. 2 also depicts the rotating fiber optic cable  40  positioned within the body  80  of the rotating welding head  22 . The rotating fiber optic cable  40  runs the length of the rotating sleeve  24  and is affixed thereto. 
     The rotating fiber optic cable  40  terminates at a laser energy directional modification assembly  92 . Preferably, the assembly  92  is implemented as an optical assembly. FIG. 3 discloses an assembly  92  that includes an input lens assembly  96 , a wedge prism  97 , and an output lens assembly  98 . The wedge prism  97  serves to change the direction of the laser energy. Preferably, the laser energy is directed toward the receiving surface  99  at a non-orthogonal angle θ. When the laser energy is impinged upon a surface to be welded at an angle, of say 45°, as shown in FIG. 3, then reflective laser energy does not disrupt the incoming laser energy. 
     The device of FIGS. 1-3 has been used for clad weld repair of thin-walled (e.g., 0.05 inches thick) heat exchanger tubes. The device can also be used for fusing defects by melting and re-solidifying the metal of a thin-walled heat exchanger tube. 
     Most corrosion in pressurized water reactors has been associated with thin-walled heat exchanger tubes. However, there have been recent reports of water stress corrosion cracking in reactor pressure vessel control rod drive mechanisms. FIG. 4 illustrates a prior art reactor vessel dome  110  with a set of control rod drive mechanism (CRDM) nozzles  112 . A prior art repair system is positioned underneath the reactor vessel dome  110 . The prior art repair system includes a tool delivery system  114 , which supports a tool arm  116  that has a tool head  118  positioned at its end. The tool delivery system  114  executes radial motion as shown with line  120 , rotational motion as shown with arc  122 , and lift motion as shown with line  124 . These motions are used to deliver the tool head  118  to different locations in a CRDM nozzle  112  so that repairs can be effectuated. 
     A variety of tool heads  118  are used to effectuate repairs. A detection probe that uses eddy current techniques may be used to identify flaws in the CRDM nozzle  112 . Similarly, a detection probe that uses ultrasonic testing may be used to identify flaws in the CRDM nozzle  112 . A detection probe to execute dye penetrant examinations may also be used. Such a probe is used to verify information found from other detection techniques and to examine completed weld repairs. 
     An excavation tool may also be used as a tool head  118 . Prior art excavation tools generally rely upon milling, grinding, or cutting tools. Such tools typically require large motor power that is difficult to deliver to remote locations, such as CRDM nozzles. Another class of prior art excavation tools relies upon a welding mechanism to melt damaged surface areas. The problem with this approach is that it is rather difficult to handle the molten metal that is removed from the damaged surface areas. Both of the foregoing excavation techniques also share the shortcoming that they are imprecise and therefore result in relatively large and unnecessary excavations that must be reconstructed. 
     A cavity repair weld head may be used for reconstruction operations. Such a weld head is used to fill the excavated area with a filler material, such as weld beads. Alternately, an arc welding cavity repair weld head may be used. For example, a gas-tungsten arc welding tool may be used. 
     A boring tool head may also be used as a tool head  118 . A boring tool is used to bore the weld buildup after a weld repair. This allows the nozzle  112  to be returned to original design specifications. 
     As indicated above, one problem with prior art excavation tools is that they are imprecise and therefore produce relatively large excavations. Consequently, relatively voluminous reconstruction operations must be performed. This can result in high residual stresses and welding distortion, which may promote future cracks. Another problem arises when welding excavation operations produce a molten metal byproduct that is difficult to dispose. Finally, prior art techniques require a relatively large number of tool heads. It would be desirable to reduce the number of tool heads required to effectuate a repair. 
     In view of the foregoing, it would be highly desirable to provide an improved technique for repairing thick-walled components susceptible to corrosion, such as reactor pressure vessel control rod drive mechanisms in pressurized water reactor nuclear power plants. Such a technique should provide precision excavations to reduce the amount of reconstruction required. Further, such a technique should provide precision reconstruction welding operations to reduce residual stresses and welding distortion. Ideally, the technique would not produce a molten byproduct and would reduce the number of tool heads required to effectuate a repair. 
     SUMMARY OF THE INVENTION 
     The invention includes a method of repairing a substrate by excavating a well in the substrate with an electrical discharge machining operation. A wire is then delivered to the well. Simultaneously, laser energy is routed to the well such that the laser energy intersects with the wire to produce a reconstruction weld in the well. A tool delivery system with an electrical discharge machining tool head and a reconstruction welding tool head is used to perform the excavation and welding operations. 
     The technique of the invention achieves a precision excavation. Thus, when repairing a CRDM nozzle, less radioactive material needs to be disposed. Further, the radioactive material is in the form of dust, not a molten metal, so it is easier to handle. In addition, the precision excavation reduces the welding volume and the amount of filler material required for a repair. The precision welding eliminates the need for an additional step involving a boring tool head. The invention reduces the residual stresses and welding distortion resulting from a weld repair. The technique also provides corrosion protection to prevent future degradation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a prior art rotating welding apparatus. 
     FIG. 2 illustrates a weld head that may be used with the apparatus of FIG.  1 . 
     FIG. 3 illustrates a laser energy directional modification assembly that may be used with the apparatus of FIG.  1 . 
     FIG. 4 illustrates a reactor vessel dome and a prior art tool delivery system used to repair it. 
     FIG. 5 illustrates a reactor vessel dome and a tool delivery system in accordance with the invention that is used to repair it. 
     FIG. 6 illustrates an electrical discharge machining tool head used in accordance with an embodiment of the invention. 
     FIG. 7 illustrates a laser reconstruction welding tool head used in accordance with an embodiment of the invention. 
    
    
     Like reference numerals refer to corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5 illustrates a reactor vessel dome  130  and its accompanying CRDM nozzles  132 . The CRDM nozzles  132  are an example of a thick-walled component. As used herein, the term “thick-walled” means a substrate that is not practically amenable to repair by remelting operations. For example, heat exchanger tubes are thin-walled devices that are typically 0.050 inches thick. Thus, it is practical to repair such tubes by melting and restoring 80% of the tube wall thickness. On the other hand, thick-walled devices, such as reactor pressure vessel walls are ½ inch or thicker. Thus, it is impractical to remelt a wall of this type. In other words, due to the considerably thicker material, it is not practical for a laser welding method to melt a majority of the wall thickness and then provide filler material to provide an effective repair. 
     FIG. 5 illustrates a tool delivery system  134 . In accordance with the invention, the tool delivery system  134  operates to deliver an electrical discharge machining (EDM) tool head and a laser reconstruction welding tool head. Preferably, prior art tool heads are also used in connection with the tool delivery system  134 . For example, an eddy current detection probe tool head, an ultrasonic detection probe tool head, and a dye penetrant tool head may also be used with the tool delivery system  134 . The tool delivery system  134  is disclosed by way of example. A variety of tool delivery systems may be used with the prior art tool heads and the tool heads of the present invention. 
     The tool delivery system  134 , by way of example, includes a primary positioning mechanism  136  to execute lift and rotational motion. The primary positioning mechanism  136  is positioned on a belted track  138 , which is used to provide radial motion. The tool delivery system  134  includes a horizontal support arm  140 . In one embodiment of the invention, a rotating apparatus  142  is positioned on the horizontal support arm  140 . 
     The rotating apparatus  142  includes a housing  144 , which encloses a gas coupler, gas supply, optical coupler, laser, and computer controller. The rotating apparatus  142  also includes a shaft  146  and a local axial lift mechanism  148 . The rotating apparatus  142  also includes a rotating sleeve or tool arm  150 . A rotating drive mechanism  151  and a filler metal delivery system  152  are also associated with the rotating apparatus  142 . As discussed below, the rotating sleeve or tool arm  150  may be operated without rotation. In sum, the rotating apparatus  142  is consistent with the device described in reference to FIG.  1 . 
     FIG. 6 illustrates a thick-walled substrate  160 , which may be, for example, a CRDM nozzle. The figure also illustrates an electrical discharge machining (EDM) tool head  162  constructed in accordance with an embodiment of the invention. The EDM tool head  162  is positioned on top of a EDM tool arm  164 . The EDM tool arm  164  may be the previously discussed rotating sleeve  150  or a similar device. The EDM tool arm  164  preferably includes high voltage signal lines  166 , an electrode position control signal line  168 , and fluid lines  170 . These lines may be incorporated into the rotating sleeve  150  or a similar device. 
     The EDM tool head  162  includes an electrode  172  positioned between two electrode positioning arms  174 . An electrical system and electrode position controller  176  receives signals from the high voltage signal lines  166  and the electrode position control signal line  168 . The signal from the electrode position control signal line  168  is used to adjust the position of the two electrode positioning arms  174 . The signals from the high voltage signal lines  166  are applied to the electrode  172 , as will be further discussed below. 
     The EDM tool head  162  also includes a fluid controller  178 , which injects fluid through a nozzle  180  and collects it at a drain  182 . Preferably, the EDM tool head has a top elastomer seal  184 , a bottom elastomer seal  186 , and an axial elastomer seal (not shown) to enclose a region of the substrate  160 . That is, the electrode  172  is enclosed between the electrical discharge machining tool head  162 , the substrate  160 , and the elastomer seal  184 ,  186 . Control operations for the tool head may be performed from a controller positioned in the housing  144  or at another location. 
     As indicated above, the signal from the electrode position control signal line  168  is used to adjust the position of the electrode. The electrode  172  is configured in the shape of the region of the substrate  160  that is to be removed. Prior art techniques are used to determine the location and shape of a region to be removed from a substrate. For example, an eddy current detection probe tool head, an ultrasonic detection probe tool head, or a dye penetrant tool head may be used. 
     The signals from the high voltage signal lines  166  are applied to the electrode  172  such that the electrode extracts a region of the substrate corresponding to the shape of the electrode. The extracted region is in the form of dust. That is, an electric arc or spark is created between the electrode  172  and the substrate  160 . The spark erodes the material in dust form, which is flushed away by the fluid moving from the nozzle  180  to the drain  182 . The electrode positioning arms  174  are rapidly moved toward the substrate  160 , until a spark occurs, and then they are moved away from the substrate. This process may be repeated thousands of times a second. Although the dust removed with each spark is extremely small, the repetitive action results in a well  190 . 
     The advantage of the EDM tool head  162  is that it provides a precision excavation of the degraded substrate area  160 . For example, excavations with a width of as little as ⅛ of an inch are practical using the EDM tool head. This precision excavation means that a minimal amount of material is removed. In the case of CRDM nozzles, the removed material is radioactive, thus it is important to minimize the amount of material that is removed. Another advantage of the EDM tool head  162  is that the material removed is in the form of dust, not a molten metal. It is relatively easy to process the dust with the fluid controller  178 . 
     The excavated area or well  190  is then reconstructed by filling the cavity with a corrosion resistant welding alloy. Preferably, the filler metal is Inconel Alloy  52  produced by the International Nickel Company. This metal has excellent compatibility with Alloy  600 , which is widely used in pressurized water reactors. 
     FIG. 7 illustrates a laser welding tool head  200 . The tool head  200  may be positioned at the top of the rotating apparatus  142  of FIG.  5 . The device preferably includes a laser energy directional modification assembly  92  for focusing laser energy  201  into the well  190 . A filler passage  86  with a filler wire  88  is preferably provided, which allows the filler wire  88  to intersect with the laser energy  201  to produce a reconstruction weld  202 . The term “reconstruction weld” is used to denote that the weld is performed in a well and results in a substrate substantially at its original thickness. This term stands in contrast to the term “clad weld”, which implies that the deposited metal is placed on an un-excavated substrate and results in an increase in the original thickness of the substrate. 
     Advantageously, the laser reconstruction welding approach of the invention allows a precision reconstruction weld that is as little as ⅛of an inch. Prior art electric arc welding approaches cannot achieve precision welds of this type. Instead, prior art bulk weldments create significantly higher residual stresses due to the shrinkage upon cooling of the larger volume of weld metal. Since stress is a major factor in stress-corrosion cracking, any method to reduce the residual stress level of a repair weld will assist in extending its service life. The precision welding approach of the invention also eliminates the need for a boring tool head to repair the welded region. 
     Cracking typically occurs along the axial length of CRDM nozzles. While the rotating apparatus  142  can be used to repair helical excavations, it can also be operated without rotation to repair axial excavations. 
     In sum, the invention provides precision repairs for thick-walled components susceptible to corrosion, such as a reactor pressure vessel control rod drive mechanism in a pressurized water reactor nuclear power plant. The invention combines a precision electrical discharge machining excavation technique with a precision laser reconstruction welding technique. The tool heads used to achieve these functions are relatively simple to construct. In the case of the laser reconstruction welding tool head, prior art devices may be used. In the case of the electrical discharge machining tool head, a tool head for use in difficult to access geometries, such as control rod drive mechanisms has been described. Advantageously, the EDM tool head does not have to process molten metal. Furthermore, it does not require large motor power, as in the case of a milling, grinding, or cutting tool head. 
     Since the technique of the invention achieves a precision excavation, less radioactive material needs to be disposed. Further, the precision excavation reduces the welding volume and the amount of filler material required for a repair. The invention reduces the residual stresses and welding distortion resulting from a weld repair. The technique also provides corrosion protection to prevent future degradation. 
     Since the invention entails precision excavation and welding, the original thickness of the substrate is maintained. In other words, the excavation and welding operations do not result in build-up of the substrate. Prior art techniques can be used to excavate, melt, or weld a surface. For example, prior art arc welding techniques may be used for excavation and welding. However, such techniques do not provide the precise wall thickness tolerance, as afforded by the present invention. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.