UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

June 5, 2003

NRC INFORMATION NOTICE 2003-05: FAILURE TO DETECT FREESPAN CRACKS IN

PWR STEAM GENERATOR TUBES

Addressees

All holders of operating licenses or construction permits for pressurized-water reactors (PWRs).

Purpose

This information notice (IN) is being provided to inform licensees of a recent problem

experienced at Comanche Peak Unit 1 concerning the detection of freespan outside diameter

stress corrosion cracking (ODSCC) in steam generator (SG) tubes. This has led to tube

integrity performance criteria not being met as defined in Nuclear Energy Institute (NEI) 97-06,

“Steam Generator Program Guidelines.” The NRC anticipates that recipients will review the

information for applicability to their facilities and consider taking appropriate actions. However,

suggestions contained in this IN do not constitute NRC requirements; therefore, no specific

action or written response is required.

Description of Circumstances

Comanche Peak Unit 1 is a four-loop Westinghouse PWR with four Westinghouse Model D4

recirculating SGs (1, 2, 3, 4). Each SG contains 4578 mill- annealed Alloy 600 tubes, which are

nominally 0.750 inch in diameter and have a nominal wall thickness of 0.043 inch. The tubes

are supported by a number of carbon steel tube support plates with circular holes and by

V-shaped chrome-plated Alloy 600 anti-vibration bars (AVBs).

Comanche Peak Unit 1 was shut down approximately 1 week prior to its scheduled refueling

outage as a result of a primary-to-secondary leak. A 5- to 15-gallon-per-day (gpd) leak was first

observed in SG 2 on September 26, 2002. Over the next 2 days, the leakage spiked to higher

values several times. On September 28, 2002, after a leakage spike to 52 gpd, the licensee

elected to shut down the plant and to commence refueling (1RF09). In response to the leak, a

special inspection by the NRC staff was conducted. The results of the special inspection were

documented in an inspection report dated January 9, 2003, ?Comanche Peak Steam Electric

Station - Special Team Inspection Report 50-445/02-09” (ADAMS Accession No.

ML030090566).

After shutting down the plant, the licensee began inspecting the SG tubes with eddy current

testing techniques. A bobbin coil and a rotating probe were used during these inspections. 

The rotating probe was equipped with various types of coils including a +PointTM coil. The

bobbin coil was used to inspect the full length of each tube while the rotating probe was used to

inspect selected regions of the tube (e.g., the top of tubesheet region) and to confirm and/or

characterize indications initially detected by the bobbin coil probe.

ML031550258
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The licensee determined that the leak was from an axially oriented flaw in the tube located at

row 41, column 71 (R41C71) in SG 2. The flaw, located in the U-bend region, was estimated

from the +PointTM coil to be approximately 0.9 inches in length, with a depth of approximately

90 percent over most of the indicated length. The licensee’s structural assessment of the flaw

indicated that the leaking tube did not meet the applicable structural and accident leakage

performance criteria in NEI 97-06. These performance criteria were developed consistent with

the plant design and licensing basis and include the three-times-normal-operating-pressure

criterion against burst (3800 pounds per square inch (psi)), the 1.4 times main steam line break

(MSLB) criterion against burst (3584 psi), and a 1- gallon-per-minute (gpm) MSLB- induced leak

rate criterion. The licensee estimated the burst pressure of R41C71 to be 2727 psi at the

location of the flaw based on analysis of the flaw profile as determined by the +PointTM coil. In

situ pressure testing of this tube was terminated at a test pressure of 2100 psi when leakage

exceeded the test system capacity of 2.5 gpm.

Review of the bobbin data for this tube (R41C71) from the previous inspection in 2001 (1RF08)

revealed that a clearly detectable indication was present at the location of the leak. This bobbin

indication did not meet the reporting criteria in the 1RF08 eddy current data analysis guidelines

and was not reported by either the primary or the secondary analyst in 2001. These reporting

criteria required a freespan bobbin indication in the absence of a dent or ding signal to be

reported if the phase angle response of the indication was less than the phase angle response

corresponding to a 0 percent through-wall flaw. Since a dent or ding signal can rotate a flaw

signal out of the normal phase angle window, the applicable reporting criteria for bobbin

indications in the presence of a detectable dent or ding signal were less restrictive (i.e., were

increased). If a dent or ding signal had been reported at this location in 1RFO8, the bobbin

indication in tube R41C71 would have been reportable. A reportable bobbin indication might

have triggered additional inspections with a rotating probe. However, no ding signal was

reported at the R41C71 location in 1RF08 by either the primary or the secondary analysts

during their review of the bobbin data since there was no clear evidence of a ding in the 1RF08

signal response. However, a large amount of horizontal noise attributable to probe wobble was

observed. This amount of horizontal noise could easily mask a 2 volt ding signal.

Based on these findings, the licensee revised its bobbin probe data analysis procedures for the

1RF09 inspection to increase the phase angle response reporting criteria for freespan

indications. The ensuing inspections identified about 20 freespan flaws. These included

freespan flaws associated with dents and dings and long freespan flaws not associated with

dents or dings. However, examination of the inspection results called into question the

reliability of the bobbin inspection. Of the 20 freespan flaws, only 5 had been detected during

both the primary analysis of the bobbin data, performed using automated (computerized) data

screening (ADS), and the secondary analysis of the bobbin data, performed by human analysts. 

The primary (ADS) analysis missed several of the bobbin indications called by the secondary

(human) analysis and vice versa. In general, the bobbin indications missed by the primary

(ADS) analysis exhibited bobbin amplitude responses less than the 0.2-volt ADS threshold. 

Furthermore, 8 of the 20 freespan flaws were not detected by either the primary or secondary

analysis of the bobbin data. These eight freespan flaws were found fortuitously rather than by

programmatic intent. They were found only because the licensee had performed a more

comprehensive +PointTM examination of the region to investigate an indication or dent located

elsewhere in the same region of tube where the flaw was eventually found.
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Accordingly, the licensee retrained the analysts and manually performed a third (tertiary)

independent analysis of the bobbin coil data, leading to the finding of additional freespan bobbin

indications. Several of these additional freespan bobbin indications were confirmed as flaws

during the +PointTM coil examination. Of these confirmed flaws, two had been detected during

the aforementioned primary and/or secondary analysis of the bobbin coil data. These bobbin

indications were not investigated with a +PointTM coil following the primary (ADS) and secondary

(manual) analysis since the bobbin signals at these locations were perceived to be similar to

those observed in 1999 (i.e., there was a perceived lack of change in the bobbin coil signal

indicating that the bobbin indication was not a result of a flaw, but rather it was within the

expected range of repeatability of the bobbin test). However, during the tertiary analysis, the

review of the prior inspection data for these indications revealed clear indications of signal

change, calling into question the effectiveness of the prior history reviews for bobbin indications.

To address this concern, the licensee prepared data analysis guidelines for the history reviews

and performed a new, supplemental history review of all bobbin indications. Two qualified data

analysts working as a team performed this supplemental review. They considered all data

extending back to the first inservice inspection, including data from the low-frequency absolute

channel. The analysts were also instructed to identify not only indications with changes

exceeding change criteria specified in the data analysis guidelines, but also indications with

changes which, in their experience and judgment, were beyond changes associated with

normal eddy current signal repeatability. This review led to the finding of three additional flaws.

Discussion

Early detection of stress corrosion cracks is key to ensuring that such cracks do not impair tube

integrity relative to the tube integrity performance criteria in NEI 97-06. It continues to be

standard industry practice to use bobbin probes to screen for indications potentially associated

with axially oriented stress corrosion cracks and, where such indications are found, to perform a

followup inspection with a rotating, surface-riding coil such as a pancake or +PointTM coil to

determine whether a crack is actually present. As evidenced by the recent experience at

Comanche Peak Unit 1, appropriate data analysis procedures, analyst training, and process

controls are critical to ensuring that all indications of actual stress corrosion cracking are being

identified during the bobbin coil data analysis and subsequently inspected with a +PointTM coil. 

The following are some of the lessons learned from the recent experience at Comanche Peak

Unit 1.

1. Care should be exercised when establishing reporting criteria for indications based on

phase angle response. Dings, dents, and other artifacts can rotate a flaw indication

outside the nominal range of phase angle response, even where the amplitude of such

artifacts is relatively small or less than the reporting value for such artifacts.

2. The presence of artifact signals which may potentially distort flaw indications can

themselves be masked by other artifacts such as probe wobble. Probe wobble signals

tend to be particularly large in the U-bend region of a tube. 

3. Depending on the value of the threshold criteria, indications with voltage responses less

than the ADS threshold criteria may sometimes be associated with flaws whose

maximum depths exceed the tube plugging limit (e.g., 40 percent through-wall). Thus,

data analysis procedures (including ADS threshold criteria) should be sufficiently robust 
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to reliably identify indications which may potentially exceed the plugging limit. For

example, the use of ADS at some plants is supplemented by an independent review of

the data by two teams of human analysts. 

4. A comparative review of indications called by the primary and secondary analysis teams

can provide insights on the effectiveness of the analysis effort. As an illustration, failure

of the primary or secondary analysis team to detect a high fraction of the indications

identified by the other team may be indicative of a need to evaluate the cause of the

discrepancies and whether corrective actions are needed with respect to the

examination technique, data analysis guidelines, and/or analyst training.

5. A robust approach is important for determining which bobbin indications exhibit change

over time in order to ensure all potential flaws are further evaluated (e.g., with a rotating

probe). A team could review the previous bobbin coil data for each indication identified

during an inspection or multiple independent reviews of the previous bobbin coil data

could be done. The analysts might be allowed to use their judgment and experience in

determining whether there has been a change in addition to determining whether

specific change criteria on phase angle and amplitude have been met. In addition,

previous inspection data could be reviewed as far back in time as possible since the

bobbin response for some of the flaws at Comanche Peak Unit 1 did not show a change

when compared only to the most recent previous inspection data.

6. The bobbin data from the low-frequency absolute data channel can sometimes be

helpful in detecting long freespan indications and for observing changes in these signals

over time.

7. The insertion of known flaw signals from a “Judas” (or “Cobra”) tube into the data

stream being reviewed by each data analyst can provide additional confidence in the

performance level of the analysts. This insertion could be done in such a manner that

the data analysts could not tell that the inserted flaw signal did not belong to the

population of actual flaws they were currently analyzing. At Comanche Peak Unit 1, the

Judas tube was a tube containing indications missed during the primary and secondary

analysis and found fortuitously during the subsequent +PointTM examination.

Related Generic Communications

The following documents describes other recent reactor operating experience with steam

generator tubes:

1. IN 2002-02 and IN 2002-02 supplement 1, “Recent Experience With Plugged Steam

Generator Tubes” dated January 8, 2002 and July, 17, 2002

2. IN 2002-21, “Axial Outside-Diameter Cracking Affecting Thermally Treated Alloy 600

Steam Generator Tubing” dated June 25, 2002

3. IN 2001-16, “Recent Foreign and Domestic Experience with Degradation of Steam

Generator Tubes and Internals,” dated October 31, 2001

4. NRC Generic Letter 97-05, “Steam Generator Tube Inspection Techniques,” dated

December 17, 1997
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This information notice does not require any 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 project manager in the NRC’s Office of Nuclear Reactor

Regulation (NRR).

/RA/

William D. Beckner, Program Director

Operating Reactor Improvements Program

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical Contacts: Emmett Murphy, NRR Matthew Yoder, NRR

(301) 415-2710 (301) 415-4017

E-mail: elm@nrc.gov E-mail: mgy@nrc.gov

Attachments: List of Recently Issued NRC Information Notices