Patent Number: 055219519
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, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. A core spray inlet 11 supplies water to a core spray sparger 15 via core spray line 13. The feedwater from feedwater sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18. Core shroud 18 is a stainless steel cylinder which surrounds the 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 19 and at the bottom by a core plate 21. The core top guide provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion. The water flows through downcomer annulus 16 to the core lower plenum 24. The water subsequently enters the fuel assemblies 22, wherein a boiling boundary layer is established. A mixture of water and steam enters core upper plenum 26 under shroud head 28. Core upper plenum 26 provides standoff between the steam-water mixture exiting core 20 and entering vertical standpipes 30, which are disposed atop shroud head 28 and in fluid communication with core upper plenum 26. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. The steam passes through steam dryers 34 and enters steam dome 36. The steam is withdrawn from the RPV via steam outlet 38. 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 43 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 42 (only one of which is shown) via recirculation water inlets 45. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The pressurized driving water is supplied to each jet pump nozzle 44 via an inlet riser 47, an elbow 48 and an inlet mixer 46 in flow sequence. A typical BWR has 16 to 24 inlet mixers. 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 girth seam welds or heat affected zones of the core shroud 18. This diminishes the structural integrity of the shroud, which vertically and horizontally supports core top guide 19 and 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 girth seam welds. SUMMARY OF THE INVENTION The present invention is a method and an apparatus for repairing a shroud in which one or more shroud girth seam welds have experienced SCC. The method involves the placement of a plurality of brackets around the outer circumference of the shroud at a plurality of azimuthal positions, held by shear pins positioned between jet pump assemblies. In the event of multiple cracked shroud girth seam welds, respective pluralities of brackets are installed at respective elevations. The brackets are intended to structurally replace the shroud girth seam welds which are cracked. The shroud repair brackets in accordance with the invention are designed to support the top guide, the fuel bundle assemblies and the shroud head. The brackets are further designed to withstand the thermal and radiological conditions which the shroud is subjected to. The shroud repair brackets are 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 welds during all normal and upset conditions. Further, the shroud repair brackets of the present invention are designed and installed such that removal of jet pump inlet mixers and RPV beltline inspection can be performed without removal of the repair brackets. Each bracket has a plurality of circular holes for receiving a corresponding one of a plurality of tapered pin assemblies. A corresponding plurality of circular holes are machined in the shroud wall at positions which will align with the holes in the bracket. Then the bracket is correctly positioned outside the shroud with the circular holes of the bracket and shroud in alignment. A corresponding tapered pin assembly is blindly installed in each set of aligned holes and then manipulated remotely to fasten the bracket to the shroud. In accordance with the preferred embodiment of the invention, each tapered pin assembly consists of three types of parts: a threaded tapered pin, a slotted sleeve with a tapered bore, and a threaded nut. The pin has threads and a socket on one end and a precise conical taper on the other end. When fully installed, the tapered pin 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 slotted sleeve has a precise internal taper which matches the external conical taper of the pin; an external surface having a radius of curvature which is smaller than the radius of curva-ture of the holes in the shroud and in the repair 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 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 tensioned to produce the desired preload, during which the sleeve expands further. Thereafter the nut is lock welded to the pin. All steps in the installation of the shroud repair bracket assemblies in accordance with the invention are performed remotely and outside the shroud. In particular, the tapered pin assemblies in accordance with the invention can be entirely installed from outside of the shroud. Prior to insertion, the unflexed sleeve is slided onto the tapered pin and then the nut is threaded onto the pin for a number of turns sufficient to hold the unflexed sleeve in place. 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 nut is tightened to pull the tapered pin 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 held in place initially by a thrust plate on the tool, reacting between the nut 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 with a tensioner. This applies a contact pressure between the pins, sleeve, bracket and shroud. The magnitude of this contact pressure can be controlled by varying the tension applied to the pin, by varying the taper angle and by varying the surface conditions. This shroud repair design is advantageous because it allows fast installation using the minimum number of fasteners. All holes in the shroud are circular cylindrical so that machining the shroud holes is simplified. Because the repair brackets can be installed entirely from the outside of the shroud, it is unnecessary to remove the top guide or the fuel bundle assemblies. The number of brackets needed to accomplish the repair is reduced due to the high load capacity of the shear pins and the splice plates. Also the brackets in accordance with the preferred embodiment of the invention occupy little space in the reactor, which minimizes the impact on other activities inside the reactor. The bracket size and number of pins per bracket can be selected based on the space available and the magnitude of the seismic loads anticipated.