Information Notice 2010-05, Management of Steam Generator Loose Parts and Automated Eddy Current Data Analysis

Management of Steam Generator Loose Parts and Automated Eddy Current Data Analysis
	ML093640691
Person / Time
Issue date:	02/03/2010
From:	Mcginty T; Division of Policy and Rulemaking
To:	;
	Beaulieu, D P, NRR/DPR, 415-3243
References
	IN-10-005
	Download: ML093640691 (5)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001 February 3, 2010

NRC INFORMATION NOTICE 2010-05: MANAGEMENT OF STEAM GENERATOR LOOSE

PARTS AND AUTOMATED EDDY CURRENT

DATA ANALYSIS

ADDRESSEES

All holders of an operating license or construction permit for a nuclear power pressurized-water

reactor issued under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic

Licensing of Production and Utilization Facilities, except those who have permanently ceased

operations 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 (IN) to inform

addressees of recent operating experience with (1) loose parts (foreign objects) in steam

generators and (2) the use of automatic steam generator eddy current data analysis systems.

The NRC expects recipients to review the information for applicability to their facilities and to

consider actions, as appropriate, to avoid similar problems. However, suggestions contained in

this IN are not NRC requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

At the Braidwood Station, Unit 1, in 2009, the licensee, Exelon Generation Company, LLC,

inspected steam generator tubes using eddy current techniques. As is common practice, the

licensee used two independent teams (i.e., primary and secondary teams) to evaluate the data.

Each of the two teams used an automated data screening system to evaluate the bobbin coil

eddy current data. Human analysts reviewed the results of each of the automated data

screening systems to accept, reject, or modify the classification of the signals identified through

the automatic data analysis.

During the 2009 inspections, one of the automated data analysis systems identified a distorted

signal from the bobbin coil eddy current data slightly above both the expansion transition and

the tubesheet on the hot-leg side of the steam generator. The human analyst accepted this

signal for further investigation. To resolve the nature of this indication, the licensee used a

rotating eddy current probe to inspect the location with the distortion. Based on the result of this

subsequent examination, the licensee concluded that mechanical wear between the tube and a

foreign object caused the indication. The depth of the wear indication was estimated from the

rotating probe as 73 percent through the tube wall. Because of its size, the indication was in

situ pressure tested to confirm that it did not significantly compromise the integrity of the tube.

The licensee did not observe any leakage during the in situ pressure test and confirmed the

tube had adequate integrity. Following the in situ pressure test, the licensee stabilized and

plugged the tube. The plant technical specifications require the licensee to plug any tubes with

flaws equal to or exceeding 40 percent of the wall thickness.

This tube had been inspected during prior outages. Upon review of the historical eddy current

data, the licensee concluded that an indication had existed at this location since 2006. The

indication in 2006 was smaller than that observed in 2009, whereas the indication in 2007 was

similar in size to the indication observed in 2009. As a result, the licensee concluded that the

tube should have been plugged in 2007.

As was the case for the 2009 steam generator tube inspections, two independent automated

data analysis systems were employed during the 2006 and 2007 inspections at Braidwood

Station, Unit 1. During all three inspections, the primary automated data analysis system

identified a distorted signal at the location where the flaw was observed in 2009. Because the

human analyst rejected these signals in both 2006 and 2007, no further investigation into the

nature of the signal was performed. The secondary automated data analysis system did not

identify this location as having a distorted signal in the 2006, 2007, or 2009 inspections, although a signal attributed to a potential loose part was initially identified in 2006 and was

subsequently rejected by the human analyst during that inspection.

During the 2009 outage, no foreign object was found near the tube with the 73 percent through- wall wear indication. However, the affected tube was located near a cluster of tubes that the

licensee had plugged in 2003 because of a foreign object that was identified but could not be

removed. During the 2007 outage, the licensee could not locate this foreign object and now

postulates that it moved from its original location and caused the 73 percent through-wall wear

indication. The licensee also postulates that the foreign object may have broken into smaller

pieces that were removed by the blowdown system or during the removal of sludge from the top

of the tubesheet (i.e., sludge lancing).

The licensee assessed the cause of this event and determined that it was a historic human

performance issue related to the amount of technical rigor applied during the review of the

distorted eddy current data that the automated data analysis system identified during the 2006 and 2007 inspections. A contributing cause was that one of the automated data analysis

systems did not identify the distorted indication.

The licensee took the following corrective actions:

It revised the guidelines for the eddy current data analysis to emphasize the requirement

to manually review available frequencies associated with distorted top of the tubesheet

indications before determining whether an indication requires additional testing and/or

analysis.

It incorporated the lessons learned from this issue in its site-specific performance

demonstration training and testing program to ensure that all data analysts and

computer screening systems can properly identify the signal as requiring additional eddy

current inspection.

It reevaluated the logic parameters in the automated data analysis system that the

secondary data analysis team used. * It determined and implemented changes to ensure that foreign object wear indications

are correctly identified at the top of the tubesheet region.

The licensee also assessed the eddy current method that it had chosen to size the wear

indication identified in 2009 (i.e., the rotating eddy current probe). Different sizing methods exist

for differently shaped wear scars (e.g., football-shaped and tapered-hole wear scars) and for

volumetric indications, like wear, in the free span. For at least one of these techniques

(e.g., sizing with a pancake coil), the resultant size would have been much smaller than

73 percent, thereby resulting in a flaw that would not require in situ pressure testing. The

licensee used a sizing method that conservatively estimated the flaw size and verified that the

integrity of the tube was not compromised through in situ pressure testing.

BACKGROUND

Related Generic Communications

Previous related generic communications include the following:

NRC IN 2004-10, Loose Parts in Steam Generators, dated May 4, 2004 (Agencywide

Documents Access and Management System (ADAMS) Accession No. ML041170480)

NRC IN 2004-17, Loose Part Detection and Computerized Eddy Current Data Analysis

in Steam Generators, dated August 25, 2004 (ADAMS Accession No. ML042180094)

NRC IN 2003-05, Failure To Detect Freespan Cracks in PWR [Pressurized-Water

Reactor] Steam Generator Tubes, dated June 5, 2003 (ADAMS Accession

No. ML031550258)

DISCUSSION

In addition to reinforcing the information in NRC IN 2004-10 and NRC IN 2004-17, the recent

operating experience at Braidwood, Unit 1 illustrates several important points relative to the

management and detection of loose parts and the use of automatic data analysis systems. The

loose part that may have caused the wear scar identified in 2009 was first identified in adjacent

tubes during a refueling outage in 2003. The licensee stabilized and plugged the tubes

surrounding the original location of the loose part because it was unable to remove the loose

part; however, the loose part eventually migrated from its original location.

Monitoring the location of loose parts that are left in the steam generator may help licensees

detect tubes potentially affected by loose parts. For example, if a loose part is left in the steam

generator, the licensee can perform secondary side visual inspections during subsequent

outages to verify that the loose part has not moved. If the loose part has moved, additional

secondary side visual inspections could be performed to locate the loose part, and primary side

inspections could be performed on active tubes surrounding the original location of the loose

part to determine whether the loose part has affected these tubes. If visual examination of the

loose part's original location is not possible, primary side inspections of the active tubes

surrounding the original location could determine whether the loose part has moved and whether additional tubes have been damaged. This process may lead to the more timely

detection of wear caused by loose parts.

At Braidwood, Unit 1 only one of the automated data analysis systems identified the distorted

signal attributed to the wear scar estimated as being 73 percent through the tube wall. The

qualification of an automated data analysis system is important for ensuring that all relevant flaw

signals are identified. Ensuring that an automated data analysis tool can detect the various

types of flaws that may occur along the entire length of the tube is important for ensuring tube

integrity, and plant technical specifications require licensees to perform inspections with the

objective of detecting flaws of any type that may satisfy the applicable tube repair criteria.

Most plants with thermally treated Alloy 690 tubing, such as Braidwood, Unit 1 have very little

degradation, and the forms of degradation that are observed at these plants tend to be easily

detectable. Because many of the signals identified during the automated data analysis

screening may not be flaws, it is important that eddy current data analysts do not inadvertently

dismiss relevant signals. The experience at Braidwood, Unit 1 highlights the importance of

ensuring that human data analysts effectively review eddy current data. A licensee can monitor

the performance of a human analyst by inserting a known flaw signal from a Judas (or

Cobra) tube into the data stream that is being reviewed, as discussed in NRC IN 2003-05.

Choosing the appropriate method to size an indication is important for verifying tube integrity. In

instances where multiple techniques exist for sizing a flaw, it is important to select the most

appropriate technique. If the licensee cannot determine an appropriate technique, a

conservative approach should be used to ensure that tube integrity is being maintained as

required by the plant technical specifications. At Braidwood, Unit 1 the licensee used a sizing

method that conservatively estimated the flaw size and in situ pressure tested the flaw to verify

that tube integrity was maintained.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below or to the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

/RA/

Timothy J. McGinty, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contact:

Kenneth J. Karwoski, NRR

301-415-2752 E-mail: kenneth.karwoski@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. whether additional tubes have been damaged. This process may lead to the more timely