Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Vessel monitoring or inspection
Utility Patent
1999-03-22
2001-01-02
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
Vessel monitoring or inspection
C376S258000, C376S260000
Utility Patent
active
06169776
ABSTRACT:
BACKGROUND OF THE INVENTION
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide.
Intergranular Stress Corrosion Cracking (IGSCC) is a known phenomenon occurring in reactor components, such as structural members, shrouds, piping, fasteners, and welds, exposed to stress in a corrosive environment. Typically, IGSCC initiates from a crevice at the base material along a weld in what is referred to as the heat affected zone. Reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses 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 IGSCC.
Over the life of the reactor, the shroud is often inspected to verify integrity. For example, the shroud welds must be periodically inspected for Intergranular Stress Corrosion Cracking (IGSCC). Based upon such inspections, the shroud may require either repair or replacement.
Known methods of inspecting shroud welds require an operator to stand on a refuel or auxiliary bridge positioned above the RPV and to manipulate inspection tooling within the RPV. Particularly, the operator couples the inspection tooling to a pole, inserts the pole and tooling into the RPV, and then positions the tooling adjacent to the weld to be examined. Because different inspection tooling is required to inspect different welds, i.e., vertical, attachment, and circumferential welds, the operator must periodically change the tooling coupled to the pole. In addition, due to the reactor internals configuration and the piping system obstructions, the operator typically must make extensive use of the bridge to inspect each shroud weld. This extensive bridge use substantially prevents other repair and inspection operations from being conducted simultaneously with the weld inspections.
It would be desirable to provide a shroud inspection apparatus particularly suitable for use in nuclear reactor applications which is easy to use and does not require support from the refuel or auxiliary bridge for performing shroud inspections. It would also be desirable to provide such an apparatus which minimizes the number of insertions, removals, and tooling change-out sequences to facilitate reducing reactor shut-down time. It would further be desirable to provide such an apparatus which navigates around anticipated shroud obstructions to facilitate reducing inspection time and operator activity.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by an inspection apparatus which, in one embodiment, inspects a shroud, specifically shroud welds, in a nuclear reactor. The inspection apparatus includes a drive system having four drive assemblies coupled to three support frame members. Each drive assembly includes dual cylindrically shaped drive wheels sized to rest on the shroud and move the inspection apparatus relative to the shroud. Each drive wheel pair independently moves between a position resting on the shroud top and a position rotated up and away from the shroud top. The drive assembly is maintained in proper radial position by inner and outer guide rollers. The inner guide rollers ride along the inner surface of the upper shroud circumference and the outer guide roller ride along the outer surface of the upper shroud circumference.
The inspection apparatus also includes a mast subassembly and a scanner subassembly. The mast subassembly includes a substantially elongate member and a roller foot coupled to the elongate member second end. The mast subassembly elongate member first end is coupled to the drive system support frame. The scanner subassembly has a substantially elongate horizontal frame, a scanner carriage having a turntable, and an ultrasonic scanner. The horizontal frame is movably coupled to the mast subassembly elongate member.
The vertical position of the scanner subassembly is changed by moving the horizontal frame between the ends of the mast subassembly. The horizontal position of the scanner is controlled by movement of the scanner carriage between the horizontal frame ends. The turntable rotatably couples the scanner to the scanner carriage. The scanner additionally has a fine vertical movement on the scanner carriage. The four degrees of motion allows the scanner subassembly to perform volumetric examinations of circumferential, vertical, and attachment shroud welds.
Additionally, the inspection apparatus includes an umbilical control system which uses gravity to feed an umbilical cable to a proper position along side the mast and couples the scanner subassembly and a data collection system. The umbilical control system maintains an appropriate amount of umbilical cable between the data collection system and the scanner subassembly.
To scan the shroud, and more specifically, the shroud welds, the inspection apparatus is lowered into the RPV until the drive system drive wheels rest on the shroud top. The mast subassembly is then positioned adjacent to the shroud so that the mast subassembly roller foot is tangent to the shroud. The drive assembly wheel and mast subassembly, including the roller foot, clamp the inspection apparatus to the shroud with the aid of a kicker clamp. The kicker clamp extends from the mast and engages the RPV wall which causes the bottom roller foot to engage the shroud and provides a fixed rigid structure from which to perform shroud inspections. This rigidity ensures no wandering of the inspection apparatus resulting from frictional forces during scanner subassembly motion to help reduce azimuthal positioning errors.
Once the inspection apparatus is clamped in position, the scanner subassembly scans the shroud welds. Particularly, in one embodiment, the scanner subassembly is moved adjacent to the mast subassembly first end with the scanner carriage adjacent the horizontal frame first end. The scanner carriage is then moved between the first and second ends of the horizontal frame as the scanner performs a complete scan for each weld. The horizontal frame is then moved toward the mast subassembly second end a distance equal to the height of the completed scan. The fine vertical movement can then be used to precisely control the vertical position of the scanner. This scan procedure is repeated until the horizontal frame is adjacent to the mast subassembly second end. The mast subassembly kicker clamp is then retracted away from the RPV wall and the drive system moves the inspection apparatus relative to the shroud a distance equal to the width of the horizontal frame. This scan procedure is repeated until the inspection apparatus encounters an obstruction at the shroud top or the entire shroud has been scanned.
If the drive system encounters an obstruction, the obstruction must be avoided or stepped around. In stepping around the obstruction, one of the drive assemblies currently retracted from the shroud is moved so that the drive wheels rest on the shroud top. The drive assembly encountering the obstruction is then moved so that the drive wheels are rotated inward away from the shroud. As a result, the inspection apparatus may be moved relative to the shroud.
Should the inspection apparatus encounter the obstruction with a second drive assembly, a drive assembly retracted from the shroud can be placed on the shroud top and the drive assembly currently encountering the obstruction is rotated inward away from the shroud. This stepping around process is repeated as each drive assembly encoun
Armstrong Teasdale LLP
General Electric Company
Jordan Charles T.
Mun K. Kevin
LandOfFree
Methods and apparatus for examining a nuclear reactor shroud does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods and apparatus for examining a nuclear reactor shroud, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods and apparatus for examining a nuclear reactor shroud will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2452079