Patent Publication Number: US-2020290145-A1

Title: Methods and apparatus for repairing a tubular structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims all benefit including priority to U.S. Provisional Patent Application No. 62/591,755, filed on Nov. 28, 2017, which is incorporated by reference. 
    
    
     FIELD 
     The present invention generally relates to component maintenance and in particular to repairing tubular structures. 
     BACKGROUND 
     Nuclear reactors contain numerous components that are prone to wear and damage over the life of a reactor, which can have implications to safety, reliability, efficiency and regulatory requirements. 
     Unfortunately, many components are difficult to repair, for example, reactor components are irradiated during their operational life, which tends to cause structures to become brittle. Moreover, many structures have very thin walls. As a result, conventional repair techniques are likely to cause further damage. For example, introduction of heat and stress into a brittle irradiated component may cause cracks to form or propagate. Likewise, heating of a thin structure may cause defects such as cracking due to thermal stress or deformation. 
     Tight clearances between parts further complicates repair. In particular, in many cases, it may be difficult to deliver equipment to the location that requires repair, which may preclude in-situ repairs. 
     Due to the difficulty of repair, damaged components often need to be replaced at significant cost. 
     SUMMARY 
     In some embodiments, aspects of the present disclosure can be used for repairing a tubular structure from the inside of the structure. In some embodiments, the repair can include using weld metal build-up from the inside diameter of the structure. 
     According to an aspect of the invention, a method of repairing a defect in a tubular structure is provided. The method may comprise depositing a protective weld layer over the defect by sequentially depositing weld droplets atop a weld pool on said structure, wherein said protective weld layer bonds to said structure surrounding said defect, wherein sequentially depositing weld droplets comprises selectively moving a wire electrode of a weld torch between a retracted position in which the weld droplet is formed, and an advanced position in which the weld droplet is deposited on the structure. 
     In an embodiment of the method of repairing a defect in a tubular structure, the weld droplet is dropped on the structure. 
     In an embodiment of the method of repairing a defect in a tubular structure, depositing the protective weld layer over the defect is by gas metal arc welding (GMAW) or Plasma Transferred Arc Welding (PTAW). In some embodiments, depositing the protective weld layer includes Gas Tungsten Arc Welding (GTAW). 
     In an embodiment of the method of repairing a defect in a tubular structure, the tubular structure is part of a nuclear reactor. 
     In an embodiment of the method of repairing a defect in a tubular structure, the method comprises inserting a repair apparatus into the tubular structure; and moving the repair apparatus to a location of the defect. 
     In an embodiment of the method of repairing a defect in a tubular structure, the method comprises depositing a reinforcing layer atop said protective weld layer by gas metal arc welding (GMAW) or Plasma Transferred Arc Welding (PTAW) or Gas Tungsten Arc Welding (GTAW). In some embodiments, the depositing forms a structurally sound reinforcing layer atop the protective weld layer to repair the defect. 
     In another aspect, a method of moving a repair apparatus through a tubular structure is provided. The method comprises: positioning a first brace at an first angle with respect to an interior surface of the tubular structure; securely engaging the tubular structure with a first brace; moving the repair apparatus relative to the securely engaged first brace by expanding or contracting at least one linear actuator to advance the repair apparatus in a direction along the longitudinal axis of the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, moving the repair apparatus relative to the securely engaged first brace comprises moving a second brace relative to the first brace and to a second angle with respect to the interior surface of the tubular structure by expanding or contracting the at least one linear actuator to advance the repair apparatus in the direction. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure comprises securely engaging the tubular structure with the second brace; disengaging the first brace from the tubular structure; and repositioning the first brace with respect to an interior surface of the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the first and second brace are each a brace ring having a cross section smaller than a cross section of the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, at least three linear actuators are configured to move said first and second braces relative to one another for positioning a plane of each brace at a desired spatial relationship relative to each other. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the method comprises positioning the first and second brace transverse to the longitudinal axis of the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the method comprises rotating a base plate of the repair apparatus to position a weld torch connected to the base plate into circumferential alignment with a weld location. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the method comprises depositing a weld droplet to the weld location. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, securely engaging said tubular structure with the first brace or second brace comprises expanding the first or second brace into contact with the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the method comprises extending a locking actuator into contact with the tubular structure and locking said actuator to fix the longitudinal position of the repair apparatus with respect to the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the method comprises rolling the repair apparatus on rollers rotatably connected to a support structure of said repair apparatus when the repair apparatus is advanced in the direction along the longitudinal axis of the tubular structure. In an embodiment, the rollers are biased into engagement with the tubular structure. 
     In an embodiment of the method of moving a repair apparatus through the tubular structure, the method comprises inserting the repair apparatus into the tubular structure, wherein the tubular structure is part of a nuclear reactor. 
     In another aspect, an apparatus for repairing a defect in a tubular structure is provided. The apparatus comprises: a body for insertion in a tubular structure of said tubular structure; an end effector mounted to said body, said end effector having a weld torch operable to deposit weld material on said tubular structure by forming molten weld droplets and depositing said weld droplets onto said tubular structure; a drive unit comprising: a first brace for selectively securely engaging the tubular structure to anchor the apparatus; at least one linear actuator for moving said apparatus relative to said first brace in a direction along the longitudinal axis of the tubular structure; and a rotational actuator coupled to said end effector for rotating said weld torch. 
     In an embodiment of the apparatus, the drive unit comprises a second brace for selectively securely engaging said tubular structure, the at least one linear actuator configured to move said first and second braces relative to one another in the longitudinal direction to move said apparatus in the direction. In an embodiment, the rings have a cross section smaller than a cross section of the tubular structure. 
     In an embodiment of the apparatus, the drive unit comprises at least three linear actuators configured to move said first and second braces relative to one another for positioning a plane of each brace at a desired spatial relationship relative to each other. 
     In an embodiment of the apparatus, the apparatus comprises a wire feed unit configured to selectively advance and retract a wire electrode of said weld torch. In an embodiment, the weld torch is configured to deposit said weld droplets onto said tubular structure by causing said weld droplets to fall by retracting said wire electrode away from said tubular structure. In another embodiment, the weld torch is configured to advance toward said tubular structure to deposit weld droplets on said tubular structure. 
     In an embodiment of the apparatus, the apparatus comprises a plurality of rollers structures connected to the end effector or the body, the roller structures comprising: a support structure; rollers rotatably connected to the support structure; and a plurality of biasing members each configured to bias at least one of the rollers into contact with the tubular structure. 
     In an embodiment of the apparatus, the apparatus comprises position sensors operable to report signals indicative of longitudinal and rotational position of the end effector and weld torch, and of the radial distance between weld torch and tubular structure. 
     In an embodiment of the apparatus, the apparatus comprises an extendable locking actuator for selectively contacting said tubular structure and anchor said locking actuator to fix the position of said end effector with respect to the tubular structure. 
     In an embodiment of the apparatus, the tubular structure is part of a nuclear reactor. In some embodiments, the methods and apparatuses can be used for repairing an irradiated tubular structure. 
     In some embodiments, the apparatuses and methods can be used to repair non-nuclear-related tubular structures. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the figures, which depict example embodiments: 
         FIG. 1  is an isometric view of a portion of a nuclear reactor; 
         FIG. 2  is a partial cutaway view of a repair apparatus positioned within a calandria relief duct of the reactor of  FIG. 1 ; 
         FIG. 3  is an isometric view of an end effector of the repair apparatus of  FIG. 2  depicting a base plate, including position sensors, of the repair apparatus; 
         FIG. 4  is a schematic view of stages of a weld deposition process; 
         FIG. 5  is an overhead view of a drive unit of the repair apparatus of  FIG. 2 ; 
         FIG. 6  is a perspective view of a portion of the drive unit of  FIG. 5 ; 
         FIG. 7  depicts a weld layer deposited on a calandria relief duct of the reactor of  FIG. 1 ; 
         FIG. 8  is a flow chart depicting a process for repairing a defect; and 
         FIG. 9  is a flow chart depicting a process for examining a weld. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a portion of a nuclear reactor  100 . As depicted, reactor  100  is a CANDU™ reactor. Reactor  100  has a plurality of calandria relief ducts  102  and vent lines  103 . Embodiments disclosed herein may be suitable for effecting repairs to calandria relief ducts  102 , and detailed examples will be provided with reference to calandria relief ducts  102 . However, it should be understood that apparatus and methods disclosed herein may also be suitable for repairing other components and structures of a nuclear reactor  100 , or components and structures not associated with a nuclear reactor or the nuclear industry (e.g. piping in a petrochemical plant). Moreover, although the apparatus and methods disclosed herein are described with reference to a CANDU™ reactor, they may be suitable for repairing reactors of other types. 
     As depicted, calandria relief duct  102  has a defect, namely a crack  104 . Crack  104  may form and propagate, due, for example to stress such as stress due to thermal cycling. Corrosion effects may worsen or accelerate the development of crack  104 . As will be apparent, crack  104  defines a region of weakness in the wall of calandria relief duct  102 , and also presents a risk of leakage. 
     Calandria relief duct  102  may be formed of stainless steel and may have walls approximately 0.375 inches thick. However, other components may be formed of other materials, such as plain (carbon) steel, aluminum or alloys thereof. Calandria relief duct  102  may be brittle as a result of irradiation during operation of reactor  100 . Likewise, calandria relief duct  102  may have been subjected to radiation hardening. 
     Conventional repair techniques may be unsuitable for repairing calandria tube duct  102 . For example, many such techniques introduce substantial quantities of heat into the work piece upon which they are formed. For example, welding using conventional Gas Metal Arc Welding (GMAW) may introduce sufficient heat to induce cracking or deformation due to thermal stress. Apparatus and methods disclosed herein are suitable for effecting a structurally sound mitigation of defect  104  by welding, while limiting heat input into the work piece (e.g. the calandria relief duct). More specifically, any type of welding technique may be suitable for use in connection the apparatus and methods of the present invention. 
     In some situations, direct physical access to a tubular structure may be limited or unavailable. For example, if the tubular structure traverses behind a wall or between other structures, it may be inaccessible from the outside. In other scenarios, the tubular structure may be in an area in which physical exposure should be minimized. 
     Structural repair of calandria relief duct  102  may rely on access to the duct interior proximate defect  104 . The duct interior may be accessed through seal disks  106 , which may be removed or broken. As used herein, the terms “proximal” and “distal” may be used to refer to positions along the length of a calandria relief duct  102 . Specifically, “proximal” refers to locations close to seal disk  106  and “distal” refers to locations close to where calandria relief duct  102  meets calandria body  108  of reactor  100 . 
       FIG. 2  depicts a repair apparatus  200  positioned within calandria relief duct  102  according to an embodiment. The wall of calandria relief duct  102  is partially cut away to show details of repair apparatus  200 . 
     As illustrated in  FIG. 2 , repair apparatus  200  has a head section  202  and a tail section  204 . Apparatus  200  may be inserted in calandria relief duct  102  with head section  202  at the distal end. An end effector  206  is mounted as part of head section  202 . Tail section  204  extends behind head section  202  and may protrude from calandria relief duct  102 . 
     Tail section  204  may comprise a plurality of segments  208  pivotably mounted to one another. Segments  208  may define an internal channel (not shown). One or more electrical power, control or data lines may be run through the channel for providing power, control and communication for end effector  206 . In addition, consumables such as weld wire and gas may be routed through the channel 
     Head section  202  and tail section  204  provide a base for transporting the element to effect welding and repair of the calandria relief duct  102 . However, head section  202  and tail section  204  may be varied or substituted in embodiments. For example, tail section  204  may be omitted when head section  202  is provided with the elements to effect welding, e.g. an electrical power source, control lines and/or wireless communication devices, weld wire, and/or an inert gas supply. 
     A plurality of roller structures  210  may be mounted around head section  202  and tail section  204  of apparatus  200 . Roller structures  210  are equipped with wheels for supporting apparatus  200  against calandria relief duct  102  such that apparatus  200  can be rolled and follow the path of the duct. Roller structures  210  also radially centre apparatus within calandria relief duct  102 . In an example, a plurality of roller structures  210  each comprising rollers  211  rotatably connected to a support structure. The support structure  212  may be rotatably connected to head section  202  and/or tail section  204  of apparatus  200 . The plurality of roller structures may be connected to end effector  206  and/or the body of apparatus  200 , e.g. tail section  204 . The plurality of roller structures are provided such that apparatus  200  may move within in interior of calandria relief duct  102  and minimize friction between apparatus  200  and an interior surface of caladria relief duct  102 . A plurality of biasing members  213  may also form part of the support structure, the biasing members configured to bias at least one of the rollers into contact with an interior surface of calandria relief duct  102 . The biasing members may be a spring, piston, or any types of mechanical actuator suitable to bias rollers radially outward from repair apparatus  200  and into contact with calandria relief duct  102 . 
     Head section  202  may have a drive unit  214  for providing linear motion of apparatus  200  along the length of calandria relief duct  102 . Drive unit  214  may include one or more braces  216 . As depicted, drive unit  214  includes two brace rings, i.e. a first brace ring  216 - 1 , and a second brace ring  216 - 2 . While the braces, specifically brace rings  216 - 1  and  216 - 2 , are illustrated as having a circular cross section, the shape of the braces may be varied to suit the environment in which apparatus  200  is used, e.g. the braces may be brace rings having a rectangular shape for insertion in a rectangular tubular structure. Each brace may have a cross section smaller than the cross section of calandria relief duct  102  and can be selectively radially extended to securely engage the tubular structure. In an example, each brace may be expanded to anchor against calandria relief duct  102  or retracted away from the duct  102 . 
     Each brace, e.g. brace rings  216 - 1 ,  216 - 2 , is longitudinally movable relative to one another, and/or the tail section  203 , by one or more linear actuators  218 . In the depicted embodiment, the linear actuators  218  are ball screws driven by electric stepper motors. However, in other embodiments, other types of linear actuators may be used. In another embodiment, linear actuators  218  can be used to define and control the spatial relationship of the planes of each brace disc relative to each other. In an example, at least three linear actuators are used to move adjoining brace rings in relation to each other. The three or more linear actuators may be spaced along the peripheral internal surfaces of the brace rings. Three or more linear actuators allow each of the brace discs to be moved in three dimensions relative to the adjoining disc, e.g. x, y, and z direction shown in  FIG. 5 . One or two linear actuators may be used to move adjoining brace rings relative to each other; however, one actuator or two actuators do not define a plane and cannot move a brace disc in three dimensions. Due to end-effector weight variations, the natural load posed on the apparatus due to gravity relative to deployed position, mechanical lost motion, and the tolerances of the linear actuators and coupling devices, an apparatus  200  with one or two linear actuators may not be positioned as accurately or effectively within the tubular structure, in comparison to embodiments with three or more linear actuators. Improper placement may impact the anchoring and bracing stability of the apparatus when used for welding, or high reactive force processes such as grinding or mechanical drilling. 
       FIG. 3  depicts end effector  206  in greater detail. End effector  206  has a base plate  230 . Base plate  230  is rotatably mounted to head section  202 , for rotation about a longitudinal axis of the head section  202 . Linear locking actuators  239  are also provided as part of end effector  206 , which may be extended in order to create a platform that is fixed in position and is configured to support the dynamic loads of welding or the reactionary load of tasks such as grinding. End effector  206  further includes at least one weld torch  232 . In some embodiments, the end effector  206  includes a wire feed unit for feeding a wire electrode  236  through the weld torch for melting and deposition onto the damaged part. End effector  206  further includes a plurality of position sensors  234 . Position sensors  234  may be linear variable displacement transformers (LVDTs) and may be operable to report signals indicative of longitudinal and rotational (circumferential) position, and of the radial distance between weld torch  232  and calandria relief duct  102 . 
     In some embodiments, unit  237  can comprise a wire or other component for positioning the tether to allow for torch rotation. 
     One or more cameras  238  are also mounted to the end effector for remote observation of weld deposition by weld torch  232 . Cameras  238  may be fixed relative to plate  230 , such that they rotate along with plate  230  and weld torch  232  and maintain a constant position relative to weld torch  232 . Cameras  238  may be connected with a remote display device for observation of welds by an operator. Cameras  238  may be equipped with filters, for example, for protection of operator vision or camera sensors from the weld arc. 
     Weld torch  232  may be designed to deposit weld material onto a workpiece such as calandria relief duct  102  while limiting the amount of heat introduced into the workpiece. In some embodiments, this may be achieved by modulating voltage and current provided to the wire electrode  236 , which may be combined with extension and retraction of the electrode. The deposit weld material may vary depending on the material of the tubular structure being repaired as well as the process conditions the weld material will be exposed to. Typical weld materials for a calandria relief duct may include 304, 304L, 316, or 316L stainless steel.  FIG. 4  depicts an example weld deposition process representative of embodiments. As shown in frame (i) and (ii), a consumable wire electrode  236  is extended toward calandria relief duct  102 . As the electrode  236  contacts calandria relief duct  102 , current to the electrode is increased and an arc initiates. The electrode is then withdrawn out of contact with calandria relief duct  102  and current to the electrode causes formation of a droplet  240  of weld material, as shown in frames (iii)-(iv). At frames (v)-(vi), the electrode  236  is extended into contact with calandria relief duct  102  and then withdrawn, leaving the droplet in a weld pool  242  on the surface (frame (vii)). The droplet detachment and the arc short circuit may occur almost without current as the electrode movement may remove the need for a short circuit to occur because of the electrode wire oscillation. The electrode  236  oscillation detaches the droplet of weld material on the surface of duct  102  as the electrode moves backwards and retracts from the surface. As the electrode  236  is withdrawn, current is increased and a new weld droplet  240  is formed (shown in frame (viii). The placement of weld droplets may be repeated to form a weld (e.g. weld shown in  FIG. 7 ) to remediate a defect in duct  102 . This weld technique may be referred to as cold metal transfer (CMT). In an example, CMT may have a feedrate capability of 40 inch/minute. 
     Overhead welding may also be achieved according to the embodiment described above with reference to  FIG. 4 . As described in relation to frames (v)-(vi), the electrode is extended into contact with calandria relief duct  102  and then withdrawn. In overhead welding, extension of electrode  236  into contact with calandria relief duct  102  will need to occur quickly to deliver droplet  240  from electrode  236  to form weld pool  242  to avoid premature solidification of weld material which may create conical peaks of weld material upon solidification. 
     Variations of the weld deposition process shown in  FIG. 4  are possible. In an embodiment, instead of the electrode  236  extending into contact with relief duct  102  (as shown in frame (v)) to deposit the weld material directly on the surface of duct  102 , the electrode droplet  240  may be allowed to fall to the surface of duct  102 . Because the droplet falls to the duct, an electrical short circuit is not created between wire electrode  236  and duct  102 , which minimizes the heat stress imparted to the duct. In another embodiment, after the droplet forms on electrode  236 , the electrode  236  may be drawn away from the surface of duct  102  to cause the droplet  240  to fall to the surface of calandria relief duct  102 . The motion of retracting the electrode  236  away from the duct  102  pulls droplet  240  off electrode  236  and causes it to fall to duct  102 . In another embodiment, electrode  236  is not retracted from the duct  102  after droplet  240  is formed, and the droplet  240  is deposited on calandria relief duct  102  by contact between the droplet  240  and duct  102 . 
     It has been determined that in embodiments, deposition of a weld using the technique described above has been found to structurally mitigate defect  104 , while limiting heat introduced to calandria relief duct  102  to acceptable levels. That is, it has been found that heat introduced does not create unacceptably high risk of causing further damage to relatively thin-walled structures such as calandria relief duct  102  that have been subjected to radiation hardening. 
     In comparison to other welding techniques, CMT may provide droplets of weld material to the weld pool at a cooler temperature that minimizes heat stresses imparted to the metal substrate (e.g. a calandria relief duct). However, other types of weld techniques may be used as part of the apparatus and method described herein. For example, in certain circumstances GMAW may be an desirable welding technique of which there are many variants such as: Gas Metal Arc Welding GMAW; Globular GMAW; Short-circuiting/Short-arc GMAW (Drip Transfer); Spray &amp; Spray Pulsed GMAW. 
     GMAW is a welding process in which an electric arc forms between a consumable wire electrode and the work piece metal(s), which heats the work piece metal(s), causing them to melt and join. A constant voltage, direct current power source is most commonly used with GMAW. However, pulsing current GMAW, such as Spray &amp; Spray Pulsed GMAW, may also be used to melt the filler wire and allow a small molten droplet to fall with each pulse. Pulses allow the average current to be lower, decreasing the overall heat input and thereby decreasing the size of the weld pool and heat-affected zone while making it possible to weld thin work pieces. The pulses of current may provide a stable arc with no spatter, since short-circuiting may not take place. 
     In another example, welding techniques other than GMAW may be used. For example, thermal spray processes such as Plasma Transferred Arc Welding (PTAW) may also be used as a welding technique in which at least one weld torch  232  may create a high-energy arc between an anode and cathode on torch  232 , and calandria relief duct  102 . Inert gas, e.g. argon, may be supplied to the arc to create a plasma column then a powder metal consumable is introduced into the plasma column, using the plasma as both a heat source to melt the powder, and a propelling agent to spray (atomized) and coat the substrate creating metallurgical bond(cladding) to the substrate. 
     In some embodiments, Gas Tungsten Arc Welding (GTAW) may be used. 
       FIGS. 5-6  depict drive unit  214  in greater detail. Linear actuators  218  are powered and controlled by lines routed through tail section  204  ( FIG. 2 ). Extension of linear actuators  218  pushes brace rings  216 - 1 ,  216 - 2  farther apart. Conversely, retraction of linear actuators  218  draws brace rings  216 - 1 ,  216 - 2  closer together. Linear actuators  218  are mounted to brace rings  216  at hinged connections  220 , such that brace rings  216  are free to pivot relative to linear actuators  218 . 
     Linear actuators  218  are operable to precisely control the positions of bracing rings  216  relative to one another. As noted, in the depicted embodiment, linear actuators  218  are driven by stepper motors, such that extension occurs in discrete increments. In an embodiment, three linear actuators are arranged to connect to points 120 degrees apart on brace rings  216 - 1  and/or  216 - 2 . One end of each linear actuator may be connected to a bracing rings and supported on bearings and the other end may be connected to an adjacent bracing rings and supported by a sleeve on a nut with swivel/rotary pins. 
     Drive unit  214  is operable to move repair apparatus  200  by sequentially extending one of brace rings  216  to anchor against calandria relief duct  102 , then moving the other brace rings  216  relative to the anchored brace ring  216 , anchoring the moved ring in its new location, and moving the other ring. For example, in the depicted embodiment, apparatus  200  may be moved in the distal direction by expanding the proximal brace ring  216 - 2  and retracting distal brace ring  216 - 1 ; extending linear actuators  218 ; expanding the distal brace ring  216 - 1  to anchor against calandria relief duct  102  and retracting proximal brace ring  216 - 2 ; and retracting linear actuators  218 . The brace rings  216  also support the repair apparatus  200  during welding. Brace rings  216  may have inflatable seals (e.g. pneumatically inflatable seals) which may be expanded to extend into contact with the tubular structure, i.e. calandria relief duct  102 , to anchor repair apparatus  200 . Brace rings  216  may extend transverse to longitudinal axis of tubular structure such that it contacts the cross sectional circumference of the tubular structure to maximize the surface contact area between brace rings  216  and the tubular structure. The position of head section  202  (and in particular, end effector  206 ), may be tracked, e.g., based on an encoder signal received from the stepper motors. 
     In an embodiment, a first brace (e.g. brace ring  216 - 2 ) may be positioned at a first angle with respect to an interior surface of a tubular structure (e.g. calandria relief duct  102 ). The first angle may be generally transverse to the longitudinal axis of the tubular structure, e.g. as shown in  FIG. 5  brace rings  216 - 1  and  216 - 2  are generally transverse to longitudinal axis of calandria relief duct  102 , or at an angle with respect to the plane transverse to the longitudinal axis of the tubular structure. The tubular structure may then be securely engaged by the first brace. For example, the first brace may extend into contact with an inner surface of the tubular structure, or the first brace may be expanded (e.g. by pneumatics) to sealingly engage the inner surface of the tubular structure, such that the first brace securely engages the tubular structure to provide an anchor against which the apparatus can be moved in relation to. At least one linear actuator may then be expanded or contracted to move the repair apparatus  200  relative to the first brace and advance apparatus  200  in a direction (e.g. the proximate or distill directions) within the tubular structure. Expansion or contraction of the at least one linear actuator may advance the repair apparatus depending on the configuration of the linear actuators. For example, a linear actuator on the distil side of the first brace that is expanded may push the repair apparatus in the distil direction; whereas, a linear actuator on the proximal side of the first brace that is contracted may pull the repair apparatus in the distil direction. As the at least one linear actuator expands or contacts, the repair apparatus advances generally along the longitudinal axis of the tubular structure. The repair apparatus may move in straight sections of the tubular structure as well as movement through tubular elbows (e.g. 45 and 90 degree, short and long radius elbows). A body of the repair apparatus, e.g. tail section  204 , may be connected to the first brace, for example by tether  224 , and advanced as the at least one linear actuator is actuated. The body of the repair apparatus may comprise roller structures  210  to reduce friction between the body and the surface of the tubular structure as repair apparatus  200  advances through the tubular structure. 
     In an embodiment, moving the repair apparatus relative to the securely engaged first brace includes moving a second brace relative to the first brace. The second brace may be positioned at a second angle with respect to the interior surface of the tubular structure by expanding or contracting at least one linear actuator to advance the repair apparatus. The second angle may be generally transverse to the longitudinal axis of the tubular structure or at an angle with respect to the plane transverse to the longitudinal axis of the tubular structure. As noted above, in an embodiment, at least three linear actuators may be configured to move the first and second braces relative to one another for positioning a plane of each brace at a desired spatial relationship relative to each other. 
     Continuing the above example, as shown in  FIG. 5 , a first brace, e.g. brace ring  216 - 2 , may be anchored to the tubular structure and linear actuators  218  may move the second brace relative to anchored first brace to advance repair apparatus  200  within the tubular structure. Tether  224  may be connected to the second brace, e.g. brace ring  216 - 1 , and advance within the tubular structure as linear actuators  218  expand and push against the anchored first brace. The second brace, e.g. brace ring  216 - 1 , may be securely engaged to the inner surface of the tubular structure. The first brace may then be disengaged from the tubular structure and repositioned with respect to the tubular structure and the second brace to engage the tubular structure once more to move the repair apparatus within the tubular structure as described above. When repair apparatus  200  has reached a defect location, the first and second braces may each securely engage the tubular structure to fix the position of the repair apparatus  200  with respect to the tubular structure. A locking actuator  239  may also extend into contact with the tubular structure to fix the longitudinal position of the repair apparatus with respect to the tubular structure. The weld torch may then be positioned by rotating a base plate of the repair apparatus  200  to position the weld torch into circumferential alignment with a weld location. Welding may then commence according to a desired welding technique, e.g. CMT welding. 
     To position brace rings  216  at a desired angle with respect to the tubular structure, or position weld torch and/or plate  230  at a desired angle with respect to the tubular structure during welding (e.g. perpendicular to the tangent of duct  102 ), position sensors  234  may provide input to controllers for the stepper motor controllers 
     In an example, each brace ring  216  may have a plurality of pairs of linear displacement devices. For example, each brace ring  216  may have a six linear displacement measurement devices. The devices may be arranged in pairs, with one of each pair located on opposite sides of the plate  230  or brace ring  216 . During (axial) motion the angular position of each brace disc or plate  230  may be monitored to insure that all three pairs of linear displacement sensors remain at equally spaced distances to the sensor on the opposite side of the disk. When the sensor pairs are equally displaced the disk has reached a position by which it is perpendicular to the inside surface of the duct. Once the brace rings  216 , and consequently the end-effector are in the desired position, linear locking actuators  239  may be extended in order to create a platform that is fixed in position and is capable of supporting the dynamic loads of welding or the reactionary load of tasks such as grinding. 
     Movement of apparatus  200  may create tension in apparatus  200  while drive unit  214  advances head section  202 . In particular, tail section  204  and rollers structures  210  drag against calandria relief duct  102  and therefore resist movement due to operation of drive unit  214 . Tension in tether  224  may aid in achieving accurate position control of head section  202  and end effector  206 . 
       FIG. 6  is an isometric view of drive unit  214  and brace ring  216 - 2 . As depicted, drive unit  214  has an electric motor  260 , which may be a stepper motor. Motor  260  drives linear actuators  218  by way of a first chain drive  262 . Specifically, motor  260  turns a ball screw, causing extension of linear actuators  218  in a first direction of rotation, or retraction in a second direction of rotation. 
     Motor  260  further drives one or more rotational actuators  264  by way of a second chain drive  266 . Rotational actuators  264  are coupled to end effector  206  for rotating the end effector to position weld head  232  circumferentially. 
     In combination with rotational actuators  264  and linear actuators  218  of drive unit  214 , weld torch  232  is operable to deposit a layer of weld material onto a work piece, e.g. calandria relief duct  102  by forming droplets of weld material and causing the droplets to fall onto the work piece. 
       FIG. 7  depicts an example weld  300  formed on calandria relief duct  102  using apparatus  200 . The location of defect  104  is indicated in broken line. Weld  300  is deposited as a series of droplets, as described above with reference to  FIG. 4 . During deposition of weld  300 , drive unit  214  is operated by a controller to define the longitudinal and radial extent of weld  300 . That is, plate  230  of end effector  206  is rotated to deposit a circumferentially-extending bead of weld. End effector  206  is extended or retracted by linear actuators  218  to deposit longitudinally-extending weld. 
     As depicted, weld  300  covers an area of calandria relief duct  102  surrounding defect  104  and structurally bridges the defect. That is; weld  300  structurally joins non-defective metal on both sides of defect  104 . 
     In some embodiments, it may be desired to deposit additional weld material atop weld  300  for further structural reinforcement. Conveniently, the structurally-sound weld  300  may be capable of withstanding further heat input without further damage to the work piece. In other words, weld  300  may effectively act as a heat sink for further repair procedures. Thus, in some embodiments, additional weld material may be deposited by conventional welding techniques such as gas metal arc welding (GMAW). Weld  300  may therefore be referred to as a protective weld layer. Subsequent layers may be referred to as reinforcing layers. 
     End effector  206  also allows for preliminary examination of weld  300 . Specifically, cameras  238  ( FIG. 3 ) are oriented and focused to capture images of electrode  236  during and subsequent to depositing of weld  300 . Optionally, cameras  238  may be equipped with optical filters (not shown), for example, to protect the eyes of a viewer or the camera sensor, or to aid in the assessment of weld integrity. 
       FIG. 8  is a flow chart depicting of an example process  1000  for repairing a defect. At block  1010 , end effector  206  is deployed to the location of defect  104  using drive unit  214 . Repair apparatus  200  is inserted in calandria relief duct  102  with head section  202 , including end effector  206 , at the distal end. Drive unit  214  moves repair apparatus  200  towards the defect location using sequential extension and anchoring of each of brace ring  216 - 1  and  216 - 2  and movement of the other brace ring relative to the anchored brace ring. Position of repair apparatus  200 , end effector  206  is tracked by position sensors  234  and reported to a controller. In an example, drive unit moves repair apparatus  200  to a location slightly distal of defect  104 . In other words, drive unit  214  may advance repair apparatus  200  until it slightly overshoots the location of defect  104 , preferably by less than a stroke length of linear actuators  218 . 
     At block  1020 , end effector  206  is retracted to a weld start location, indicated as point A in  FIG. 7 . Specifically, linear actuators  218  of drive unit  214  retract end effector  206  to align weld torch  232  to a weld start location. In some embodiments, the weld start location may be at an end point of the defect or at a specified location relative to an end point of the defect. Preferably, end effector  206  is retracted by less than a stroke length of linear actuators  218 . Thus, retraction may be performed by anchoring brace ring  216 - 2  against calandria tube duct  102  and operating linear actuator  218  to draw brace ring  216 - 1  toward brace ring  216 - 2 . Linear locking actuators  239  may also be extended into contact with an interior surface of calandria relief duct  102  to fix the position of end effector  206  with respect to the defect. 
     At block  1030 , weld torch  232  is positioned in relation to the defect location. Base plate  230  of the end effector  206  is rotated around a longitudinal axis of the head section  202  until weld head  232  is circumferentially aligned at a weld start location. Alignment may be determined based on signals received from one or more position sensors  234 . Alignment may be perpendicular to the tangent of tubular structure surface at the weld location or at an angle with respect to the surface of the weld location. 
     At  1040 , a protective weld layer is deposited. Weld head  232  may deposit weld material onto a workpiece by cold metal transfer as described above with reference to  FIG. 4 . That is, weld droplets are sequentially formed and released to drop onto the workpiece, namely, calandria relief duct  102 . As noted, the CMT welding technique limits the heat introduced into the workpiece, which may help maintain the integrity of the material. For example, this technique may reduce the likelihood of crack formation or propagation, especially in irradiated or thin components. In other embodiments, other types of welding techniques may be utilized to deposit weld material to a workpiece , e.g. PTAW. In some embodiments, weld torch  232  is moved in a path to deposit weld material over the entirety of the defect and a portion of the surrounding area of the workpiece. For example, weld material may be distributed circumferentially by rotating base plate  230  about a longitudinal axis of the head section  202  while weld torch  232  deposits weld material. Weld material may be distributed longitudinally by movement (e.g. retraction) of linear actuators  218  to move end effector  206  and weld torch. For example, at the end of a circumferentially-extending weld bead, drive unit  214  may move end effector towards the proximal end of the workpiece, such that an adjacent bead may be deposited. Depositing of a protective weld layer may continue in this manner until the weld reaches a weld end location, indicated as point B in  FIG. 7 . As depicted, the completed weld layer covers the defect and overlaps to the surrounding metal by a defined margin. Thus, the protective weld layer may structurally bridge the defect. In other words, the weld may extend between regions of sound metal surrounding the defect. 
     At  1050 , a reinforcement weld layer may be deposited on top of the protective weld layer, for example, where multiple weld layers are desirable for increased structural integrity. The reinforcement weld layer can be deposited by repair apparatus  200  using a cold metal transfer process as described herein to further minimize introduction of heat into the workpiece. In some embodiments, the reinforcement weld layer may instead be deposited using a different, e.g. conventional, welding process such as gas metal arc welding (GMAW). The protective weld layer can form a barrier between the workpiece and subsequent engagement with a weld head, electrode, weld, weld pool, or other component involved in a welding process, such that the protective layer absorbs heat rather than or in addition to the workpiece. This protective weld layer can therefore allow repair apparatus  200  or a cold metal transfer process described herein to be used with other welding apparatuses or welding processes to repair irradiated metal, for example. 
     One or more reinforcement weld layers can be sequentially deposited onto a previously deposited weld layer. For example, multiple reinforcement weld layers may be employed to effect a repair to a workpiece. In some embodiments, each reinforcement weld layer may be deposited according to one or more different patterns designed to improve structural integrity or effect a repair. 
     In another example, a through wall breach may have occurred and the primary requirement will be to stop the through wall leak path. Rebuilding the wall where a through wall breach exists may be performed by apparatus  200  tack welding a base plate or plug in place, and then welding an initial build-up layer over the around and over the plate or plug. Additional, reinforcement weld layers may continue to be added to build up the weld as described above. 
     At  1060 , the weld is examined, for example, according to one or more processes according to various regulatory standards. The protective weld layer and/or one or more reinforcement weld layers 
     At  1070 , the defect is monitored after completion of the repair to verify that further growth of defect  104  does not occur. 
       FIG. 11  is a flow chart depicting an example process for examining a weld at block  1020 . The depicted process of examining the weld may confirm structural integrity of the weld or of surrounding areas of the workpiece, and suitability of the repaired component for being returned to service in the reactor. Successive examination can be performed according to a schedule, for example, at every outage (e.g., when the workpiece is not in use), until the defect is removed. 
     At  1110 , visual inspection is performed on the weld, including one or more protective and/or reinforcement weld layers, and surrounding areas of the workpiece. For example, surface examination can be performed using artificial lighting. The weld crown can be examined with no surface preparation. Visual inspection may be performed using cameras  238  during or immediately following depositing of the weld. 
     At  1120 , an eddy current examination is performed on the weld, including one or more protective and/or reinforcement weld layers, and surrounding areas of the workpiece, for example, to detect flaws in the material. The weld crown can be examined with no surface preparation. 
     At  1130 , weld build-up thickness, including one or more protective and/or reinforcement weld layers, is measured by ultrasonic methods. 
     At  1140 , ultrasonic volume inspection is performed on the weld, including one or more protective and/or reinforcement weld layers, and surrounding areas of the workpiece. For example, ultrasonic angle beam examination can be used to validate the integrity of the weld deposit. The weld crown can be examined with no surface preparation in some embodiments. 
     Although the above examples are described with reference to repair of a calandria relief duct, apparatus and methods disclosed herein may be applicable to performing repairs on other reactor components. In particular, such methods and apparatus may be useful for repairing irradiated and thin-walled components. 
     Similarly, repair apparatus  200  may be useful for deploying end effector  206  within tubular structures other than calandria relief ducts. 
     Although the embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 
     As can be understood, the detailed embodiments described above and illustrated are intended to be examples only. The invention is defined by the appended claims. 
     The claims are not intended to include, and should not be interpreted to include, means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.