Information Notice 1998-27, Steam Generator Tube End Cracking
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
July 24, 1998
STEAM GENERATOR TUBE END CRACKING
Addressees
All holders of operating licenses for pressurized-water reactors (PWRs) except those who have
permanently ceased operation and have certified that fuel has been permanently removed from
the reactor vessel.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this Information notice to inform
licensees of Instances of steam generator tube-end cracking. It is expected that recipients will
review the information for applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response in required.
Description of Circumstances
Entergy, the licensee for Arkansas Nuclear One, Unit I (ANO-1), conducted leak testing on
each once-through steam generator (OTSG) during the spring 1998 refueling outage to identify
the source of primary-to-secondary leakage measured during the previous operating cycle. The
leak testing revealed a small leak In one tube of each OTSG. The leak in each tube was in
flaws In the tube hardroll just below the upper tubesheet seal weld. Subsequent eddy current
inspections Identified primary-water stress-corrosion cracking (PWSCC) in each of the two
tubes. The through-wall flaws In the tubes were oriented both axially and circumferentially in
the tubes. Examinations of all upper tubesheet seal weld areas In both OTSGs revealed
possible flaws in 1896 additional tubes. For these tubes, the licensee installed an additional
hard roll in the upper tubesheet, thus redefining the reactor coolant pressure boundary.
Eddy current Inspections completed at Davis-Besse Nuclear Power Station in April 1998 Identified five tubes with "tube end anomalies" believed to be related to the flaws identified at
ANO-1. Duke Power Co. recently completed an assessment to determine If its Babcock and
Wilcox-designed, operating units, Oconee Nuclear Station, Units I and 3, were affected by
tube-end cracking. The licensee concluded that 372 indications in the Unit I OTSGs and 61 Indications in the Unit 3 OTSGs exceeded the repair criteria in the Oconee Technical
Specifications. Tube-end cracking has also been reported for the steam generators at Prairie
Island Nuclear Generating Plant.
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IN 98-27 July24, 1998 Steam generator tubes are secured into tubesheets using both mechanical expansion and
welding. In general, each tube protrudes slightly beyond the tubesheet. A seal weld provides a
leak-tight configuration. The mechanical expansion and seal welding processes introduce high
residual stresses in the ends of each tube. In addition, the high temperatures associated with
welding alter the metallurgical characteristics of the tube material in the heat-affected zone.
Although the tubes In the ANO-1 OTSGs were heat treated prior to operation (which should
have improved the material's resistance to stress-corrosion cracking), the licensee's preliminary
conclusion is that the rolling and welding processes contributed to the development of PWSCC
in the tubes at the seal weld. The licensee Is currently conducting a root-cause evaluation to
determine any future corrective measures necessary to prevent recurrence or to mitigate the
consequences of this mode of degradation.
Discussion
One of the safety functions of steam generators Is to maintain a barrier against fission product
release. Tube-to-tubesheet seal welds in steam generators minimize the potential for
primary-to-secondary leakage under normal-operating and postulated-accident conditions. The
development of PWSCC tube cracking In the vicinity of these welds may degrade the leak-tight
barrier between the primary and secondary coolant systems. Although the primary-to- secondary leakage during normal operation from a single tube-end flaw is likely to be small
relative to the limit specified in plant technical specifications, the combined leakage from tube- end flaws could become significant if a large number of tubes have through-wall cracks.
PWR technical specifications require tubes containing defects with depths that exceed the
repair criteria (typically 40 percent of nominal wall thickness) to be removed from service. The
repair criteria often require licensees to consider the entire tube in determining whether tubes
are degraded or defective. Defects located in any area of a tube are evaluated against the
repair criteria unless the area Is specifically excluded from the surveillance requirements. Upon
confirming the presence of tube-end flaws at ANO-1, the licensee utilized an industry-approved
repair method to install an additional hard roll In the upper tubesheet, thus redefining the reactor
coolant pressure boundary. The tubes In most recirculating steam generator designs are fully
expanded within the tubesheet and do not offer the option of this type of repair. While fully- expanded designs inherently provide considerable resistance to bypass leakage, licensee
technical specifications may still require licensees to consider flaws near tube-end seal welds In
dispositioning tubes as acceptable for continued service.
Steam generator tube-end cracking may not be limited to OTSGs. All PWR steam generators
contain similar welds that may be equally susceptible to the development of PWSCC in the
heat-affected zone of the tube. Due to the location of flaws associated with tube seal welds, the use of traditional Inspection techniques and analysis procedures may not be entirely
effective in detecting and characterizing this mode of degradation. The licensee for the Oconee
units found It necessary to perform supplemental eddy current Inspections on mockup test
samples to adequately address tube-end cracking.
IN 98-27- July24, 1998 This information notice requires no specific action or written response. If you have any
questions about the information In this notice, please contact one of the technical contacts listed
below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts:
Phillip J. Rush, NRR
301-415-2790
E-mail: pjrl@nrc.gov
Eric J. Benner, NRR
301-415-1171 E-mail: ejbl
nrc.gov
Attachment: List of Recently Issued NRC Information Notices
Attachment
July 24, 1998 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
96-48, Sup. 1 Motor-Operated Valve
Performance Issues
7/24/98
All holders of operating licenses
for nuclear power reactors except
those who have permanently
ceased operation and have
certified that fuel has been
permanently removed from the
reactor vessel.
98-26
98-25
98-24
Settlement Monitoring and
Inspection of Plant Structures
affected by Degradation of Porous
Concrete Subfoundations
Loss of Inventory from Safety- Related, Closed-Loop Cooling
Water Systems
Stem Binding In Turbine
Govemor Valves In Reactor
Core Isolation Cooling (RCIC)
and Auxiliary Feedwater (AFW)
Systems
7/24/98
7/24/98
7/08/98
All holders of operating licenses
for nuclear power reactors; all
of or applicants for licenses to
operate Independent Spent Fuel
Storage Installations; and
designers and fabricators of
Independent Spent Fuel Storage
Installations.
All holders of OLs for nuclear
power reactors; except those
licensees who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel.
All holders of operating licenses
for nuclear power reactors except
those who have permanently
ceased operation and have
certified that fuel has been
permanently removed from the
reactor vessel
OL = Operating License
CP = Construction Permit
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IN 98-27 July24, 1998 This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts listed
below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
[Original signed by David B. matthews]
for Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts:
Phillip J. Rush, NRR
301-415-2790
E-mail: pjrlnrc.gov
Eric J. Benner, NRR
301-415-1171 E-mail: ejb1 @nrc.gov
Attachment: List of Recently Issued NRC Information Notices
DOCUMENT NAME: A:\\SG-INFO.WPD
Tech Editor reviewed and concurred on 6/29/98
- SEE PREVIOUS CONCURRENCE
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IN 98-27 July 25,
98 Page of 3 This information notice requires no specific action or written response. If
u have any
questions about the information in this notice, please contact one of the echnical contacts listed
below or the appropriate Office of Nuclear Reactor Regulation (NRR roject manager.
Jack W. Roe, A ng Director
Division of Re tor Program Management
Office of Nu
ar Reactor Regulation
Technical contacts:
Phillip J. Rush, NRR
301-415-2790
E-mail: pjrl @nrc.gov
Eric J. Benner, NRR
301-415-1171 E-mail: ejbl@nrc.g
Attachment: List of Recently Issued N
Information Notices
DOCUMENT NAME: A:
G-INFO.WPD
Tech Editor reviewed nd concurred on 6/29/98
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ONCURRENCE
To receive a copy of this documen Indicate In the box C=Copy wlo attachmenVenclosure E=Copy wit attachmentenclosure N No copy
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DATE
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DATE
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07/21/98
OFFICIAL RECORD COPY
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July
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of3 This information notice requires no specific action or written response.
you have any
questions about the information in this notice, please contact one of e technical contacts listed
below or the appropriate Office of Nuclear Reactor Regulation (N ) project manager.
Jack W. Ro , Acting Director
Division of eactor Program Management
Office of ucear Reactor Regulation
Technical contacts:
Phillip J. Rush, NRR
301-415-2790
E-mail: pjrl@nrc.gov
Eric J. Benner, N
301-415-1171 E-mail: ejbl@n .gov
Attachment: List of Recently lssu
NRC Information Notices
DOCUMENT NAM
- %SG-INFO.WPD
Tech Editor revie ed and concurred on 6/29/98
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