Patent Number: 059057718
Section: summary

FIELD OF THE INVENTION This invention relates to maintenance and repair of nuclear reactors. In particular, the invention relates to the repair of the fuel core shroud of a boiling water reactor. BACKGROUND OF THE INVENTION A conventional boiling water reactor is shown in FIG. 1. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14. The feedwater flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and a core shroud 18. The core shroud 18 is a stainless steel cylinder which surrounds the nuclear fuel core 20, which is made up of a plurality of fuel bundle assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 1). Each array of fuel bundle assemblies is supported at the top by a top guide 24 and at the bottom by a core plate 26. The water flows through downcomer annulus 16 to the core lower plenum 25. The water subsequently enters the fuel assemblies 22, wherein a boiling boundary layer is established. A mixture of water and steam enters the core upper plenum under the shroud head 28. The steam-water mixture flows through standpipes 30 and enters steam separators 32. The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 34 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 36 (only one of which is shown) via recirculation water inlets 38. The jet pump assemblies are circumferentially distributed around the core shroud 18. Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC. Stress corrosion cracking has been found in the shroud seam welds or heat affected zones of the core shroud 18. This diminishes the structural integrity of the shroud, which vertically and horizontally supports the core plate 26, the top guide 24 and the shroud head 28. Thus, there is a need for a method and an apparatus for repairing a shroud which has cracks in or near the shroud seam welds. SUMMARY OF THE INVENTION The present invention is an apparatus for repairing a shroud in which one or more shroud seam welds have experienced SCC. The repair involves the attachment of a splice bracket to the shroud so that the bracket bridges the cracked weld seam and is applicable to both vertical and horizontal weld seams. The splice bracket is held by a plurality of shear pins. The bracket is intended to structurally replace the shroud seam weld which is cracked. Multiple splice brackets can be placed along the length of a crack. Each cracked shroud seam weld can be repaired independently. The shroud repair splice brackets in accordance with the invention are designed to withstand the thermal and structural loads and radiological conditions which the shroud is subjected to. Further, the shroud repair brackets of the present invention are designed and installed such that maintenance and inspection operations, such as removal of jet pump inlet mixers and RPV belt-line inspection, can be performed without removal of the repair brackets. In the case of a cracked shroud girth seam weld, the splice bracket is designed to support adjoining parts of the shroud above and below the cracked weld area, replacing the structural function of the girth weld. The splice bracket is fastened to the shroud above and below the cracked shroud girth seam weld in a manner which will prevent relative movement across the cracked shroud girth seam weld. Similarly, in the case of a cracked vertical shroud seam weld, the splice bracket is designed to support adjoining parts of the shroud on opposing sides of the cracked weld area, replacing the structural function of the vertical weld. In both cases, the splice plate serves to limit the amount of fluid which flows through a crack from the relatively higher-pressure interior of the shroud to the relatively lower-pressure exterior of the shroud, i.e., the downcomer annulus. In accordance with the preferred embodiments of the invention, the splice bracket has a plurality of circular holes for receiving a corresponding plurality of tapered fastener assemblies. A corresponding plurality of circular holes are machined in the shroud wall at positions which will be aligned with the holes in the splice bracket when the latter is correctly positioned adjacent the shroud. To facilitate machining alignments the holes in the shroud are preferably mutually parallel, as are the holes in the splice bracket. A respective tapered fastener assembly is installed in each set of aligned holes. The design provides for blind installation, i.e., with access from only one side. For example, some splice brackets can be installed on the outside of the shroud in areas where the jet pump assemblies pose no obstruction, while other splice brackets are installed on the inside of the shroud where access to the shroud exterior is blocked by the jet pump assemblies. The fastener assemblies are manipulated remotely to fasten the splice bracket to the shroud. In accordance with one preferred embodiment of the invention, each tapered fastener assembly comprises a threaded and tapered shear pin, a split (i.e., slotted) sleeve with a tapered bore and a threaded nut. The shear pin has threads on one end and a precise conical taper on the shank. When fully installed, the shear pin is encased by the sleeve. The split sleeve has a longitudinal slot which allows the sleeve to be flexed radially outward into a configuration having an expanded diameter. In the unflexed state, the split sleeve has a precise internal taper which matches the external conical taper of the pin; an external surface having a diameter which is smaller than the inside diameter of the holes in the shroud and in the splice bracket (for assembly clearance); and a raised annular flange to act as an axial position stop. The annular flange is sized to just pass through the ho/les in the bracket and shroud when the sleeve is unflexed. A shear bolt is used instead of a nut to minimize protrusion of the installed assembly when available space is limited. The nut is tightened to pull the pin enough to expand the sleeve by an amount sufficient that the annular flange will not pass through the holes. Then the pin is additionally torqued or tensioned to produce the desired preload, during which the sleeve expands further to tightly fit the hole, thereby making a rigid shear fastener joint. Thereafter the nut is locked against rotation relative to the pin. In accordance with another preferred embodiment of the invention, each tapered fastener assembly comprises a threaded and tapered shank, a split sleeve with a tapered bore, and a threaded shear bolt. The shank has a central threaded bore for receiving the threaded shank of the shear bolt and a precise conical taper on its outer circumferential surface. When fully installed, the tapered shank is encased by the sleeve. The sleeve has a longitudinal slot which allows the sleeve to be flexed radially outward into a configuration having an expanded diameter. In the unflexed state, the split sleeve has a precise internal taper which matches the external conical taper of the shank; an external surface having a radius of curvature which is smaller than the radius of curvature of the holes in the shroud and in the splice bracket; and a raised annular flange to act as an axial position stop. The annular flange is sized to just pass through the holes in the bracket and shroud when the sleeve is unflexed. The shear bolt is tightened to pull the shank enough to expand the sleeve by an amount sufficient that the annular flange will not pass through the holes. Then the shank is additionally torqued or tensioned to produce the desired preload, during which the sleeve expands further. To eliminate the need for a lock weld, a lock washer can be provided between the tapered shank and the shear bolt to prevent detorquing of the bolt, e.g., due to vibrations during reactor operation. A first preferred embodiment of the lock washer comprises a spring having a locking tang at one end and a tooth at the other end. The tang fits in a circular axial hole in the head of the shear bolt. The lock washer tooth meshes with one of a series of ratchet teeth machined into the opposing end face of the tapered shank. The ratchet teeth are slanted to allow relative rotation of the bolt and the shank in the tightening direction, while providing a positive lock against relative rotation in the detorquing or loosening direction. In a second preferred embodiment of the lock washer, the tooth and tang project from the same end of the spring. In either embodiment, the projecting tang may be checked for tightness after installation to ensure the fastener has been properly locked. For the second embodiment only, the tang has a recess which can be grasped by a tool. The grasping tool can be manipulated remotely to pull the tooth out of engagement with the ratchet teeth on the tapered shank, thereby unlocking the shear bolt and allowing it to be detorqued without damaging the lock washer. All steps in the installation of the shroud splice bracket assemblies in accordance with the invention are performed remotely with access from only one side of the shroud. In particular, the tapered fastener assemblies in accordance with the invention can be installed from outside or inside of the shroud. Prior to insertion, an unflexed split sleeve is slided onto a tapered fastener element having threads and then a threaded fastener element is threadably coupled to the threads of the tapered fastener element for a number of turns sufficient to hold the unflexed sleeve in place on the tapered fastener elements This yields a minimum flange diameter which is less than the diameter of the holes in the bracket and shroud wall, allowing the sleeve to pass through the holes. The assembly is then pushed through the aligned holes in the bracket and shroud wall. Once the raised flange of the sleeve clears the inner edge of the hole in the shroud wall, the threaded fastener element is tightened to pull the tapered fastener element back until the assembly is seated, i.e., the annular flange on the sleeve latches behind the shroud wall. During this operation, the sleeve is restrained axially initially by a thrust plate on the tool, reacting between the threaded fastener element and the sleeve, and then after some expansion, by the raised flange bearing against the inner circumferential surface of the shroud wall. Higher axial load is then applied by torquing or tensioning. This provides a tight fit which may also apply a contact pressure between the tapered fastener element, split sleeve, bracket and shroud. The magnitude of this contact pressure can be controlled by varying the torque or tension applied to the tapered fastener element, by varying the taper angle and by varying the surface conditions. Friction resulting from the contact pressure provides additional strength to the joint. This shroud repair design is advantageous because it allows fast installation using the minimum number of fasteners to achieve maximum joint strength. All holes in the shroud are circular cylindrical so that machining the shroud holes is simplified. The splice brackets can be installed entirely from one side of the shroud. When the splice brackets are installed from the outside of the shroud, it is unnecessary to remove the top guide or the fuel bundle assemblies inside the shroud. Conversely, when the splice brackets are installed from the inside of the shroud, it is unnecessary to remove the jet pump assemblies outside the shroud. The number of splice brackets needed to accomplish the repair is reduced due to the high load capacity of the fastener elements and the splice brackets. Also the brackets in accordance with the invention occupy little space in the reactor, which minimizes the impact on other activities inside the reactor. The bracket size and number of fastening assemblies per bracket can be selected based on the space available and the design loads. The loads on the shroud include: (1) the pressure differential between the inside and outside of the shroud; (2) seismic loading; and (3) the deadweight of the shroud head, steam separators etc.