Metal working – Method of mechanical manufacture – Heat exchanger or boiler making
Reexamination Certificate
1999-08-10
2001-01-30
Cuda, I. (Department: 3726)
Metal working
Method of mechanical manufacture
Heat exchanger or boiler making
C029S890054, C029S402140
Reexamination Certificate
active
06178635
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to boiling water reactors and, more particularly, to a method and apparatus for repairing welds in a boiling water reactor shroud.
2. Discussion of the Related Art
Boiling water reactor shrouds are disposed concentrically within reactor vessels to divide the flow of coolant water through the reactor vessels and to structurally support and align the fuel assemblies, steam separator assemblies and control rod guide tubes. The shrouds are generally cylindrical and are typically formed of multiple arcuate steel plates joined by welds along their abutting vertical and horizontal edges. After periods of use, cracking of the shroud within heat affected zones of the welds tends to occur as a result of corrosion, radiation and stress. When excessive cracking of the welds occurs, the shroud must either be replaced or repaired.
U.S. Pat. No. 5,402,570 to Weems et al, the disclosure of which is incorporated herein by reference, describes a method of repairing boiling water reactor core shrouds having horizontal cracks in heat affected zones of welds by securing plural tie rods in vertical orientation about the periphery of the cracked shroud to axially compress the shroud and thereby urge the opposing surfaces of the horizontal cracks toward one another. Lateral or radial spacers or bumpers may also be installed between the inner surface of the reactor vessel and the outer surface of the shroud and/or tie rods to resist horizontal seismic loads and to hold the shroud in place relative to the vessel wall in the event of cracking along any horizontal weld during a seismic event.
While the tie rod repair described in U.S. Pat. No. 5,402,570 provides an acceptable method for mitigating the consequences of cracking along horizontal welds, there exists a need for a vertical weld repair which can be used alone or in combination with tie rods to mitigate the consequences of cracking along vertical welds in a boiling water reactor core shroud.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a method and apparatus for repairing cracked vertical welds in a boiling water reactor core shroud.
The present invention is generally characterized in a method of repairing a weld in a boiling water reactor shroud using a clamp with a pair of pins connected by a structural member such as a plate. The method includes the steps of forming a pair of openings in the shroud on opposite sides of the weld, inserting a pin into each opening in the shroud, and moving at least one of the pins toward the weld from a rest position to an engaged position by rotating the pin about an axis of rotation offset from a central longitudinal axis of the pin so that the pins are positioned to transmit loads from the shroud to the structural member. The pins preferably protrude from disks that are rotatably mounted on the structural member and include circumferentially spaced recesses for receiving set screws to lock the pins in the engaged position. The pins can also be threaded into the disks and provided with inboard flanges so that the flanges can be tightened against the inner surface of the shroud by turning the pins. Alternatively, the flanges can be mounted on bolts extending through the pins. The openings formed in the shroud can be circular or of decreasing diameter in the direction of the weld. Seals can be positioned between the weld repair clamp and the inner surface of the shroud to prevent leakage from the openings.
Some of the advantages of the vertical weld repair according to the present invention are that the repair can be performed without unloading the reactor fuel, that the repair can be performed in combination with other types of repairs such as those involving installation of tie rods, that the repair can be designed to meet all operating and design basis loads for a reactor, that the repair eliminates the need to inspect the repaired weld thereby minimizing in-service inspection of the shroud, that the repair can mitigate the effects of cracking along welds without preloading, that the repair can accommodate cold feedwater injection, that the repair provides installation clearance take-up while holding opposed surfaces of a crack together for normal operation, that the repair can accommodate cracking well away from the weld as needed, that stresses on the shroud are minimized, that interference with reactor internals such as the top guide and core plate is minimized, that welds behind core spray piping and other structures in the reactor vessel can be repaired, that the repair will not loosen in service or create any shroud leak paths, and that the repair can be performed using no loose pieces in a cost effective and timely manner.
Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference numerals. Parts identified by a reference numeral followed by one or more primes are similar to parts identified by the same reference numeral without primes unless indicated otherwise.
REFERENCES:
patent: 5392322 (1995-02-01), Whiting et al.
patent: 5402570 (1995-04-01), Weems et al.
patent: 5502754 (1996-03-01), Erbes
patent: 5521951 (1996-05-01), Charnley et al.
patent: 5577082 (1996-11-01), Weems et al.
patent: 5600689 (1997-02-01), Weems et al.
patent: 5600690 (1997-02-01), Weems et al.
patent: 5623526 (1997-04-01), Wivagg
patent: 5675619 (1997-10-01), Erbes et al.
patent: 5729581 (1998-03-01), Loock et al.
patent: 5737379 (1998-04-01), Erbes
patent: 5781603 (1998-07-01), Wivagg
patent: 5793828 (1998-08-01), Wivagg et al.
patent: 5803686 (1998-09-01), Erbes et al.
patent: 5839192 (1998-11-01), Weems et al.
patent: 5905771 (1999-05-01), Erbes et al.
patent: 5964029 (1999-10-01), Weems et al.
McCurdy, Jr. Harold William
Sylvester William E.
Weems Sterling J.
Cuda I.
MPR Associates, Inc.
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