Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Vessel monitoring or inspection
Reexamination Certificate
2000-02-22
2001-12-18
Swiatek, Robert P. (Department: 3643)
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
Vessel monitoring or inspection
C073S622000, C228S104000
Reexamination Certificate
active
06332011
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to examination of nuclear reactors, and more particularly, to the examination of a top weld of a core shroud of a boiling water nuclear reactor.
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, sometimes referred to as a grid is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. The core shroud is a reactor coolant flow partition and structural support for the core components. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. A removable shroud head is coupled to a shroud head flange at the top of the shroud.
The shroud, due to its large size, is formed by welding a plurality of stainless steel cylindrical sections together. Specifically, respective ends of adjacent shroud sections are joined with a circumferential weld. During operation of the reactor, the circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the shroud. In particular, lateral seismic/dynamic loading could cause relative displacements at cracked weld locations, which could produce large core flow leakage and misalignment of the core that could prevent control rod insertion and a safe shutdown.
Known methods of inspecting the circumferential shroud welds for IGSCC and IASCC utilize ultrasonic probes positioned on the shroud outer surface at the weld joint. A series of scans are performed while projecting the ultrasonic beam through the weld from the outer side of the shroud to the inner side of the shroud. Some methods position the probe on the inner surface of the shroud and project the ultrasonic beam from the inner surface of the shroud to the outer surface of the shroud. The weld between the shroud head flange and the upper shroud section, sometimes referred to as an H
1
weld, is very difficult to access for inspection because of the plurality of shroud head locking lugs located around the outer surface of the shroud head flange which limits access to the weld from the outer surface of the shroud. Typically, less than 80% of the weld area can be examined. Additionally, because the shroud head flange extends radially inward, a probe cannot easily be placed against the weld between the flange and the upper shroud section on the inner surface of the shroud. Placing probes below the weld under the flange ledge and performing scans of the weld and upper heat affected zone by directing the ultasonic sound beam through the weld from the lower side has produced unreliable detection readings.
It would be desirable to provide a method of inspecting the H
1
weld between the shroud head flange and the upper shroud section that is reliable and that reliably examines greater than 80% of the weld circumference.
BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment, a method of inspecting an H
1
weld between a shroud head flange and an upper shroud section, and an upper heat affected zone of the H
1
weld includes the steps of positioning a phased array ultrasonic probe on a top surface of the shroud head flange, emitting an ultrasonic sound beam from the ultrasonic probe, electronically steering the ultrasonic sound beam to scan the weld joining the shroud head flange and the upper shroud section with the beam moving from an outer surface of the shroud to an inner surface of the shroud, and acquiring scan data over a length of the scan. The ultrasonic probe is then incrementally moved circumferentially along the top surface of the shroud head flange and the weld is again ultrasonically scanned. The ultrasonic probe is continuously moved circumferentially along the top surface of the shroud head flange in increments of between about 0.05 inch to about 1.0 inch with the H
1
weld ultrasonically scanned after each incremental move.
Initially, the ultrasonic beam is focused so that the focal point of the beam aligns with an upper fusion line of the weld and the outer surface of the shroud head flange. The beam is then repeatedly refocused so that the beam focal point moves along the upper fusion line of the weld from the outer surface of the shroud head flange to the inner surface of the shroud head flange in discrete increments. In one embodiment the beam focal point moves in increments of about 0.01 inch to about 0.5 inch.
After the ultrasonic probe has scanned the weld at the initial position on the shroud head flange, the ultrasonic probe is incrementally moved circumferentially along the top surface of the shroud head flange. At each predetermined incremental move of the probe the width of the weld is scanned by focusing the beam and moving the focal point incrementally along the fusion line as described above. Scans are performed at each incremental distance the probe is moved until the probe has traversed the complete circumference of the circumferential weld, or any desired portion of the circumference of the weld.
The above described method provides for reliable examination of greater than 80% of the H
1
weld circumference because the ultrasonic probe placement and movement are not restricted by the shroud head locking lugs that are located on the outer surface of the shroud head flange. The method provides an examination of the heat affected zone of the weld extending from the upper fusion line of the weld to about 0.5 inch above the upper fusion line. Further, the method provides for detection, length and through-wall sizing of surface-connected planar flaws within the weld metal, heat affected zone, and adjacent base metal material. The planar flaws resulting from IGSCC and IASCC. Also, the above described method can be used for the detection and sizing of cracking associated with attachment welds of the shroud head locking lugs.
REFERENCES:
patent: 3988922 (1976-11-01), Clark et al.
patent: 4302286 (1981-11-01), Lefebvre et al.
patent: 5009105 (1991-04-01), Richardson et al.
patent: 5784425 (1998-07-01), Morlan
Armstrong Teasdale LLP
General Electric Company
Swiatek Robert P.
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