Information Notice No. 85-65: Crack Growth in Steam Generator Girth Welds
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
July 31, 1985
Information Notice No. 85-65: CRACK GROWTH IN STEAM GENERATOR GIRTH
All nuclear power pressurized water reactor (PWR) facilities holding an
operating license (OL) or a construction permit (CP).
This information notice is provided to alert recipients of a potentially
significant problem pertaining to the growth in indications in steam
generator circumferential welds. Ultrasonic examination had determined
previously that the welds were acceptable. It is suggested that recipients
review the information for applicability to their facilities and consider
actions, if appropriate, to preclude a similar problem occurring at their
facilities. However, suggestions contained in this information notice do not
constitute NRC requirements; therefore, no specific action or written
response is required.
The NRC is continuing to evaluate pertinent information. An additional
notification will be made if specific actions are determined to be required.
Description of Circumstances:
In 1982 Indian Point Station Unit 3 had a leak at weld No. 6 on one of their
steam generators (see Information Notice 82-37). Weld No. 6 is a full-
penetration circumferential weld located in the transition zone between the
tube bundle and steam dryer areas, below the feedwater nozzles, and subject
to thermal cycling. The crack was started by corrosion and operating
temperature fluctuations caused it to grow through the wall because of
low-cycle fatigue. The repair method reduced the defects to an acceptable
level. Ultrasonic examinations have been performed during outages since 1982
and in the summer of 1985. Previously known indications that appear to have
grown in size are being evaluated.
In 1983 Surry Power Station Unit 2 performed ultrasonic examinations of the
No. 6 welds. The original construction weld at Unit 2 is 6 inches above the
weld that attached the lower portion of all three replacement steam
generators in 1980 (see attached sketch). The examination showed widespread
indications of discontinuities on the inside surface of this weld in the "A"
steam generator. None of the indications seemed large enough to be rejected
and it was
July 31, 1985
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decided that they were surface blemishes of reflections from weld geometry.
In March 1985, an ultrasonic reexamination was performed on the original
construction weld at Surry and larger, but acceptable, discontinuities were
found in the same locations. The inside surface of the weld in generator A
was visually examined, but no defects were seen. However, when magnetic
particle testing was performed at the request of the NRC, closely spaced
linear cracks were found over a large portion of the circumference. The
appearance of these cracks was similar to those at Indian Point. The safety
significance is that substantial loss of secondary coolant could occur
without warning if cracking degradation continued undetected.
The cracks in generator A were in a narrow band at the upper edge of the
weld and covered almost the entire inside diameter. The cracks were as deep
as 1/2 inch and were covered by the surface oxide, which obscured detection
by visual examination. Generators B and C had numerous, smaller,
circumferential cracks in the same location. To complicate matters, there
were 10 unacceptable subsurface indications in generator B, based on the
requirements of ASME Section XI, IWB-3511. After a fracture and fatigue
evaluation, these subsurface indications were accepted by ASME IWB-3600. The
surface cracks in all three generators were removed by grinding; repair
welding was not necessary.
Weld No. 6 was made on-site and had high residual stresses as a result of
the low preheat and postweld heat treatment temperatures. The steel in the
vicinity of the weld pitted when the secondary water contained high oxygen
concentrations (higher than 25 ppb) and contaminants such as chlorides and
copper ions. In addition to internal pressure, this portion of the steam
generator has a change in cross-section and undergoes thermal cycling. Heat
treatment of the nearby replacement weld in 1980 reduced the residual
stresses, but could not undo any existing damage to the original
construction weld. The cracks ran from pit to pit and grew to an
unacceptable size in less than one inspection period.
At the next outage, the No. 6 welds in all three steam generators at Surry
Unit 2 will be partially examined by magnetic particle testing. The
subsurface indications in generator B also will be examined by ultrasonic
methods. Slow growing corrosion cracks are irregular in length and depth.
When the cracks are located in the fusion line between the weld and the base
metal, evaluation is very difficult by ultrasonic methods alone. Magnetic
particle testing is more sensitive than visual examination and supplements
ultrasonic examinations where there is a possibility of surface defects.
ASME Section XI specifies the maximum allowable planar indications and the
methods of examination, but these methods may not be sufficient to identify
indications and defects in all cases. Additional surface preparation,
calibration notches, personnel training, and smaller ultrasonic probes may
result in a better understanding of the ultrasonic indications.
July 31, 1985
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No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: P. Cortland, IE
1. Sketch of Steam Generator
2. List of Recently Issued IE Information Notices
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