Information Notice 2001-16, Recent Foreign and Domestic Experience with Degradation of Steam Generator Tube and Internals
| ML013030601 | |
| Person / Time | |
|---|---|
| Issue date: | 10/31/2001 |
| From: | Imbro E Operational Experience and Non-Power Reactors Branch |
| To: | |
| Petrone C | |
| References | |
| IN-01-016 | |
| Download: ML013030601 (7) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 October 31, 2001 NRC INFORMATION NOTICE 2001-16: RECENT FOREIGN AND DOMESTIC EXPERIENCE
WITH DEGRADATION OF STEAM GENERATOR
TUBES AND INTERNALS
Addressees
All holders of operating licenses for pressurized-water reactors (PWRs), except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees about findings from recent inspections of steam generator tubes and secondary- side internal components and structures. 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 is required.
Background
The NRC reported the degradation of steam generator tubes in several generic
communications including Information Notice (IN) 96-38, Results of Steam Generator Tube
Examinations; IN 97-26, Degradation in Small-Radius U-Bend Regions of Steam Generator
Tubes; IN 97-49, B&W Once-Through Steam Generator Tube Inspection Findings; IN 97-88, Experiences During Recent Steam Generator Inspections; and Regulatory Issue Summary
2000-22, Issues Stemming From NRC Staff Review of Recent Difficulties Experienced in
Maintaining Steam Generator Tube Integrity. In addition, the NRC reported the degradation of
steam generator secondary side structures in IN 96-09, Damage in Foreign Steam Generator
Internals; IN 96-09 Supplement 1, Damage in Foreign Steam Generator Internals; and
Generic Letter (GL) 97-06, Degradation of Steam Generator Internals. This generic
communication reports additional experience with the degradation of steam generator tubes
and internals.
Description of Circumstances
Foreign Sludge Lancing Experience
In 1998, a foreign reactor was shut down for a refueling outage. At the time of the shutdown, there was no evidence of primary-to-secondary leakage. During the outage, sludge lancing
was performed followed by a bobbin coil probe inspection of 100% of the tubes in all four
steam generators. The tube inspections revealed only minor wall thinning. However, during
plant startup following the outage, a very small primary-to-secondary leak was observed, and
the reactor was shut down to investigate its source.
Subsequent inspections identified several degraded steam generator tubes in the second and
third rows of the steam generator tube lane. The degradation consisted of localized loss of the
outer surface of the tubes just above the top of the tubesheet. Extensive wall loss in one of
these tubes resulted in a pinhole-sized perforation of the tube wall. Although the eddy current
examination performed during the refueling outage identified wall thinning in these tubes, the
technique was apparently not capable of identifying the very deep, localized degradation found
in the leaking tube.
An evaluation determined that the sludge-lancing technique damaged the steam generator
tubes. In 1997, a previously used sludge lancing technique was modified to improve tubesheet
cleaning and allow longer intervals between lancing operations. The modifications consisted of
enlarging the spray nozzles, increasing the water pressure of one of the 90- nozzles, and
adding an 8 minute stationary lancing step with the lancing mechanism at the lowest position.
Previously, the spray nozzles had been moved continuously up and down. Following the tube
leakage event, the facility performed mockup tests using the modified sludge-lancing technique.
The testing revealed that no damage to tubes occurred when the water jet angle was exactly
90-; however, it was reported that damage similar to that described above was observed on
tubes if the water jet angle deviated from 90-. The extent of the damage increased with the
duration of the stationary lancing, the size of the spray nozzle, the water pressure, and the
temperature of the water. The testing caused localized wall loss primarily in tubes in the
second and third tube rows. No damage was observed in the sixth and higher rows. The
mockup test results supported the facilitys root cause evaluation.
Degradation of the Calvert Cliffs Unit 2 Tube Support
During the performance of a steam generator secondary-side visual inspection in 1999, Baltimore Gas and Electric Company (BGE) identified degradation at the periphery of the
eggcrate tube supports in both steam generators at Calvert Cliffs Nuclear Power Plant Unit 2.
In the #21 steam generator, BGE found minor degradation of the eggcrate supports on the hot- leg side at the sixth, seventh, and eighth support elevations. In the #22 steam generator, BGE
found more extensive degradation of the eggcrate supports on the hot-leg side at the seventh
and eighth support elevations, as well as on the cold-leg side at the sixth support elevation. On
the basis of the location and nature of the degradation, BGE concluded that it was caused by
erosion-corrosion, similar to, but much less extensive than, that observed at San Onofre Unit 3.
(The San Onofre experience is discussed in GL 97-06).
BGE performed an upper bundle flush and sludge lancing of the steam generators during the
1999 inspection outage and adjusted chemistry levels to improve resistance to erosion- corrosion over the following operating cycle. BGE had performed similar secondary-side
inspections at Calvert Cliffs Unit 1 in 1996 and 1998 and found no eggcrate degradation. Possible Degradation in Thermally Treated Alloy 600 Tubes
The steam generators at Turkey Point Units 3 and 4 were replaced in 1982 and 1983, respectively, with steam generators of an improved design. The tubes of the replacement
steam generators were made of a more corrosion-resistant material, thermally treated Alloy
600, and were hydraulically expanded (and therefore, subjected to less stress). The quatrefoil
tube supports were also more resistant to corrosion, being made of stainless steel.
During a steam generator tube examination in the spring of 2000, the licensee for Turkey Point
Unit 3 detected 69 tubes which required plugging. Of the 69 plugged tubes, 41 had volumetric
pit-like indications, 15 had inside-diameter-initiated circumferential indications, eight had
outside-diameter-initiated circumferential indications, and five had wear indications. Most of
these indications were in the hot-leg hydraulic-expansion transition region at the top of the tube
sheet. The volumetric and circumferential indications were detected with rotating probes. This
was the first time rotating probes were extensively used at Turkey Point Unit 3.
As a result of these findings, the licensee reviewed historical data and industry experience to
assess the root causes of the tube degradation. Because of the lack of prior rotating probe
inspection data for Turkey Point Unit 3 and the limited number of defects identified by the
industry in thermally treated Alloy 600 tubes, the results were inconclusive for the
circumferential and volumetric indications.
In a subsequent outage at Turkey Point Unit 4 in the fall of 2000, the licensee detected seven
tubes with possible corrosion degradation and plugged these tubes immediately since a
qualified depth-sizing technique was not available. Based on the eddy current and ultrasonic
examination results in this inspection, the licensee reanalyzed the previous Unit 3 data. The
licensees judgement is that the circumferential and volumetric indications at Unit 3 were false
positive and caused by manufacturing anomalies or deposits at the top of tube sheet or by the
inspection techniques associated with the rotating probe.
Discussion
Regardless of steam generator design or materials, it is important to effectively monitor the
tubes and their support structures to ensure tube structural and leakage integrity are
maintained. The operating experience provided above illustrates several important aspects of
ensuring steam generator tube integrity.
The sludge lancing experience with the foreign steam generators illustrates the importance of
carefully monitoring the tubes after secondary-side activities. Inspections performed
subsequent to chemical and/or mechanical cleaning of the steam generators should be
comprehensive to ensure that degradation induced (or exacerbated) by secondary-side
activities is detected in a timely fashion to prevent a loss of tube integrity (structural and/or
leakage integrity). While the applicability of the foreign sludge lancing experience to domestic
facilities may be limited, the experience illustrates the importance of properly qualifying a
technique and then verifying that the technique performs as expected. The degradation of the Calvert Cliffs Unit 2 eggcrate supports illustrates the importance of
monitoring secondary side structures/components which may impact tube integrity. If support
structures such as eggcrate supports are permitted to excessively degrade, it may result in tube
damage through the loss of support to the tube (i.e., tube vibration) and/or through mechanical
damage by the introduction of loose material into the steam generator. In the case of Calvert
Cliffs, the degradation was not that severe, and the licensee was monitoring the support
locations since an analysis of their plant indicated it is one of the most susceptible plants to
eggcrate tube support degradation based on feedwater iron transport rates. This analysis is
documented in the licensees response to GL 97-06. With respect to the long-term integrity of
these steams generators, the licensee plans to install replacement steam generators at Calvert
Cliffs Unit 1 in spring 2002 and at Calvert Cliffs Unit 2 in spring 2003. The replacement steam
generators have stainless steel tube supports which are more resistant to erosion-corrosion.
The experience at Turkey Point illustrates the importance of performing comprehensive
inspections of steam generator tubes throughout the lifetime of a steam generator regardless
of the tube material. The thermally treated Alloy 600 steam generator tubes at Turkey Point
are less susceptible to corrosion than mill-annealed Alloy 600 tubes. Nonetheless, the tubes
are susceptible to degradation. In the case of Turkey Point Unit 3, the licensee postulated the
circumferential and volumetric eddy current signals could be attributable to manufacturing
anomalies similar to that observed from pulled tubes removed from Surry and other locations, and for several of the indications detected at Turkey Point Unit 4 where the licensee postulated
that possible corrosion degradation was occurring. Without comprehensive inspections early
in the life of a steam generator or without metallurgical examination of pulled tubes, evaluations to determine the cause of new indications are difficult to perform and are subject
to significant judgment. Since the likelihood of steam generator tube corrosion increases as
the steam generators age, it is important that special inspection processes and root cause
evaluations be comprehensive and conducted in accordance with Appendix B to 10 CFR Part
50. 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.
/RA/
Eugene V. Imbro, Acting Chief
Operational Experience
and Non-Power Reactors Branch
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: Z. Bart Fu, NRR Charles Petrone, NRR
301-415-2467 301-415-1027 E-mail: zbf@nrc.gov E-mail: cdp@nrc.gov
Attachment: List of Recently Issued Information Notices 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.
/RA/
Eugene V. Imbro, Acting Chief
Operational Experience
and Non-Power Reactors Branch
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: Z. Bart Fu, NRR Charles Petrone, NRR
301-415-2467 301-415-1027 E-mail: zbf@nrc.gov E-mail: cdp@nrc.gov
Attachment: List of Recently Issued Information Notices
Distribution:
IN Reading File
PUBLIC
Accession No.: ML013030601 Template No.: NRR-052 OFFICE EMCB:DE Tech Ed REXB:DRIP EMCB:DE EMCB:DE REXB
NAME ZBFu* PKleene* CPetrone* TSullivan* BBateman* JTappert*
DATE 10/29/01 09/28/01 10/29/01 10/29/01 10/30/01 10/31/01 C:REXB
GImbro
10 /31/00
OFFICIAL RECORD COPY
Attachment LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
______________________________________________________________________________________
2001-15 Non-Conservating Errors in 10/29/01 All holders of operating licenses
Minimum Critical Power Ratio or construction permits for boiling
Limits water reactors (BWRs)
2001-14 Problems with Incorrectly- 10/03/01 All holders of operating licenses
Installed Swing-Check Valves or construction permits for nuclear
power reactors except those who
have ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel
2001-13 Inadequate Standby Liquid 08/10/01 All holders of operating licenses
Control System Relief Valve for boiling water reactors
Margin
2001-12 Hydrogen Fire at Nuclear 8/08/01 All holders of operating licenses
(ERRATA) Power Stations or construction permits for nuclear
power reactors except those who
have ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel
2001-12 Hydrogen Fire at Nuclear 7/13/01 All holders of operating licenses
Power Stations or construction permits for nuclear
power reactors except those who
have ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel
2001-11 Thefts of Portable Gauges 07/13/01 All portable gauge licensees
2001-10 Failure of Central Sprinkler 06/28/01 All holders of licenses for nuclear
Company Model GB Series power, research, and test reactors
Fire Sprinkler Heads and fuel cycle facilities
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit