Information Notice 1994-88, Inservice Inspection Deficiencies Result in Severely Degraded Steam Generator Tubes
| ML013100218 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear, Maine Yankee |
| Issue date: | 12/23/1994 |
| From: | Grimes B Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| +sunsimjr=200611, -RFPFR, FOIA/PA-2001-0256 IN-94-088 | |
| Download: ML013100218 (3) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
D.C.
20555 December 23,
1994 NRC INFORMATION NOTICE 94-88:
INSERVICE INSPECTION DEFICIENCIES RESULT IN
SEVERELY DEGRADED STEAM GENERATOR TUBES
Addressees
All holders of operating licenses or construction permits for pressurized- water reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert addressees
to recent findings from steam generator (SG) tube inspections and investigations at Maine Yankee Atomic
Power Station (Maine Yankee). A number of tubes were degraded to the point where they potentially no
longer retained adequate structural margins to sustain the full range of normal operating, transient, and
postulated accident conditions without rupture. This occurrence is the result of inservice inspection
deficiencies during past inspections. On December 7, 1992, NRC informed addressees of a similar problem
at Arkansas Nuclear One, Unit 2 (ANO 2), by issuing NRC IN 92-80, "Operation With Steam Generator
Tubes Seriously Degraded." 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.
Description of Circumstances
On July 15, 1994, Maine Yankee Atomic Power Company, the licensee for Maine Yankee, shut down the
plant when the measured primary-to-secondary leak rate approached 189 liters [50 gallons] per day. The
technical specification limit is 818 liters [216 gallons] per day. Beginning at a low rate before the last
refueling outage, this leakage had slowly increased during the operating cycle, which began in October
1993.
After shutting down the plant, the licensee tested for leaks and found four leaking tubes. The licensee
conducted an eddy current test (ECT) inspection of the leaking tubes using a motorized rotating pancake
coil (MRPC) probe and found circumferential cracks, initiating from the tube inner diameter surface, at the
hot leg expansion transition location, which is near the top of the tubesheet. One of these tubes contained a
circumferential indication with an average depth of 94% and extending 360 degrees around the tube
circumference. Inner diameter circumferential cracking at the tube expansion transition location had been
observed previously in other tubes at Maine Yankee during inspections dating back to 1990.
Reanalysis of previous inspection data for the leaking tubes, performed with the benefit of "hindsight,"
indicated that circumferential indications were present in these tubes since at least 1990 when MRPC
inspections were first performed at Maine Yankee. These indications had not been previously reported due
to the difficulty of discriminating the flaw indication from the interference signal associated with probe
"liftoff' effects caused by the transition geometry and denting. This problem was intensified by the fact that
cracks initiating from the inner diameter surface produce a signal with a phase angle rotation that is small
in comparison to the geometry- and denting induced interference signal such that the composite signal was
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interpreted as a normal, non-flawed indication. However, terrain plots of these signals, generated as part of
the data reanalysis, revealed the presence of the circumferential crack indications. These terrain plots had
not been generated as part of the original field analysis for these tubes.
The licensee performed a 100% MRPC inspection of all expansion transition locations on the hot leg side
of each steam generator. The licensee employed a 3-coil MRPC consisting of a 2.9-mm [0.115-inch]
diameter pancake coil which is sensitive to both axial and circumferential cracks, an "axial" coil which is
primarily sensitive to axial cracks, and a "circumferential" coil which is primarily sensitive to
circumferential cracks. The licensee reported that improved signal-to-noise performance was achieved
through the use of the larger 2.9-mm [0.115-inch] diameter pancake coil in lieu of the previously used
2.0-mm [0.080-inch] diameter coil together with the use of larger diameter copper cables ("low loss
cables") for the MRPC probe.
The licensee also employed improved data analysis procedures to reflect information learned from the
reanalysis of the previous inspection data for the leaking tubes and from recent experience at ANO-2. All
primary and secondary analysts were trained and tested on the 1992 and 19.93 data for the Maine Yankee
tubes that leaked in 1994. The primary and secondary analysts were required to generate a pancake coil
terrain plot for each tube at the expansion transition. Any circumferential indication from the terrain plot
was to be recorded as a possible crack. Level III resolution analysts reviewed all indications of possible
cracks using voltage ratios from the axial and circumferential coils to assist in interpretation of the pancake
coil signals. The voltage ratio criterion is based on the premise that pancake coil indications caused by
geometry or dents are likely to produce a response on both the axial and circumferential coils whereas
circumferential crack indications are likely to produce a significant response only on the circumferential
coil. The licensee considered a voltage ratio of 2.0 (circumferential coil response divided by the axial coil
response) to indicate a circumferential crack. The licensee stated that it would likely have found the cracks
in previous inspections and plugged the tubes if it had used these data analysis methods.
The licensee found indications of circumferential cracks in a total of 303 tubes, including the four leaking
tubes. All of these tubes were plugged and staked, irrespective of the measured depth of the indications.
Several of these indications were quite large, including 23 indications with average depths (over the
360-degree circumference) exceeding 79 percent of the tube wall thickness and 10 indications with average
depths exceeding 89 percent of the tube wall thickness. The licensee reports that 79 percent is the
allowable average crack depth, consistent with the most limiting burst pressure criterion from Regulatory
Guide (RG) 1.121, "Bases for Plugging Degraded PWR Steam Generator Tubes," August 1976. Analyses
performed for a similar plant (ANO-2) indicate that a tube with an average crack depth exceeding 89 percent may not sustain a postulated main steam line break (MSLB) pressure of 17,340 kPa [2,500 psi].
Most of the indications found were determined to date back to at least 1990.
Discussion
The recent eddy current test results at Maine Yankee indicate that certain tubes may have degraded to the
point that they did not meet the structural margin criteria of Regulatory Guide 1.121 and that, in addition, some of these tubes may not have been capable of sustaining the differential pressures associated with a
postulated MSLB. That this was, in fact, the case is not conclusive, given the large degree of uncertainty
associated with eddy current depth measurements for cracks and the inability of eddy current testing to
resolve small ligaments of sound material between crack segments.
The licensee performed in-situ pressure tests of ten tubes at Maine Yankee containing some of the largest
indications to assess their actual burst integrity. The results of the in-situ pressure tests indicate that the
eddy current test measurements for at least some of the cracks were conservative. The licensee concluded
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on the basis of these tests that the majority of tubes with MRPC sized indications exceeding 79 percent
average through wall had structural margins consistent with the most limiting Regulatory Guide 1.121 criterion and that all tubes were capable of sustaining an MSLB without rupture. The staff is evaluating the
results of the in-situ pressure tests, but has not yet reached a conclusion regarding the validity of the tests to
simulate an actual pressure transient in the steam generators.
Inadequate eddy current test procedures from 1990 or earlier appear to be the primary reason the tubes at
Maine Yankee became severely degraded before their recent discovery. In INs 90-49, "Stress Corrosion
Cracking in PWR Steam Generator Tubes," of August 6, 1990, and 92-80, "Operation with Steam
Generator Tubes Seriously De-raded," of December 7. 1992, NRC stressed the importance of using
appropriate probes such as pancake type coils when inspecting locations that are potentially subject to
circumferential cracks. The Maine Yankee findings, however, indicate that use of pancake type coils is not
necessarily sufficient to ensure the timely detection of circumferential cracks.
This difficulty in obtaining accurate eddy current test results also demonstrates the importance of (1)
optimizing the test methods to minimize electrical noise and signal interference and to maximize flaw
sensitivity; (2) anticipating potential sources of interfering signals, such as from probe liftoff caused by
tube transition geometry and from dents and understanding their potential effect on flaw detection; (3)
developing test and analysis procedures that will allow the flaw signal to be discriminated from any
unavoidable signal noise or interference; and (4) being alert to plant unique circumstances (e.g., dents, copper deposits) which may necessitate special test procedures found not to be necessary at other similarly
designed steam generators or not included as part of a generic technique qualification. Recent information
from ANO-2, where circumferential cracks initiated from the outer diameter surface at the expansion
transition, indicates that procedures which rely on what nondestructive examination analysts refer to as
"good or expected phase angle correlation" between base test frequencies can lead to missed indications.
Data from pulled tube specimens are useful for developing and validating effective inspection methods for
each plant. Appropriate training and performance demonstration testing of the data analysts on the test and
analysis procedures are essential elements of an effective inspection program.
Circumferential cracks have been reported at the tube expansion transition locations of several Combustion
Engineering (CE) designed plants (including Maine Yankee and ANO-2) and at several Westinghouse
plants. Tubes in the CE steam generators and at most of the affected Westinghouse plants were explosively
expanded against the tubesheet (called an "explansion" process at CE units and a "WEXTEX" process at
Westinghouse units). Tubes in the remaining affected Westinghouse plants were roll expanded against the
tubesheet.
The staff is continuing to evaluate the generic implications of the Maine Yankee occurrence.
This information notice requires no specific action or written response. If you have any questions about the
information in this notice, please contact the technical contact listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/S/'D BY BDLIAW/FOR
Brian K. Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation
Technical contact:
Emmett L.
Murphy, NRR
(301)
504-2710
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