ML13308C340

SG Unit 3 Telephone Call

ML13308C340
Person / Time
Site:	Palo Verde
Issue date:	11/04/2013
From:	Office of Nuclear Reactor Regulation
To:
Rankin J
References
TAC MF2982
Download: ML13308C340 (13)

Text

Page 1 Palo Verde U3R17 Steam Generators - FOSAR During 3R17 outage inspection of high pressure feedwater heater 5A, several metallic foreign objects were discovered on the feedwater (tube side) inlet of the heat exchanger. As a result, it was decided to perform Foreign Object Search and Retrieval (FOSAR) in the steam generators. NRC residents were briefed on the discovery, material, and locations of the foreign materials in the 5A feedwater heater and steam generators. Subsequent to this NRR has requested additional follow-up discussions. This document provides an APS response to the NRR request.

1. Discuss whether the steam generator includes any type of loose part trapping system and the results of any inspections of those locations.

APS Response: The PVNGS Replacement Steam Generators do not contain a loose parts trapping system.

2. Discuss the reason that prompted the secondary side visual inspections including any corrective actions to ensure no future loose parts will continue to enter the steam generator.

APS Response:

Since no primary or secondary inspections were planned for U3R17, an Operational Assessment/Degradation Assessment review was performed prior to the start of the refueling outage. With respect to the potential for foreign object-induced tube damage, the review stipulated that should potential foreign object intrusion from the secondary plant be identified, FOSAR of the steam generators should be considered.

During a routine outage inspection of high pressure feedwater heater 5A, several metallic foreign objects were discovered on the feedwater (tube side) inlet of the heat exchanger. As a result, it was decided to perform Foreign Object Search and Retrieval (FOSAR) in the steam generators.

With respect to corrective actions to ensure no future loose parts will continue to enter the steam generators, migration of foreign objects to the steam generators from the secondary plant is recognized as a generic industry challenge. This problem has resulted in numerous NRC publications (e.g. Generic Letter 85-02, NUREG 0844, several Information Notices) and industry operating experience reports. Complete prevention of all foreign objects from entering the steam generators from the feedwater trains is not feasible short of major design changes to install feedwater system strainers immediately upstream of the steam generators. As such, the nuclear industry has adopted a programmatic approach to minimizing the likelihood of foreign objects being introduced into the steam generators and precluding the potential for such objects to challenge the structural tube integrity requirements delineated in the EPRI Integrity Assessment Guidelines.

Page 2 Recognizing the challenge in maintaining the secondary side of steam generators completely free of foreign objects, the industry through EPRI has published several guidelines directed in whole or in part at programmatic actions and programs to help ensure tube integrity requirements are maintained and the potential for foreign object induced tube leaks minimized. The most recent publication Steam Generator Management Program: Foreign Object Prioritization Strategy for Triangular Pitch Steam Generators 1020989 dated July 2010 provides guidance for managing the presence of foreign objects in the steam generators such that those actions necessary to preclude a challenge to tube integrity are identified and performed.

Also guidance is provided to reasonably prioritize actions that need to be performed immediately to preclude significant tube damage in the next operating cycle. These guidelines are based on extensive industry operation experience and bounding wear calculations. Although only guidelines, use of these guidelines achieve the objective of minimizing the likelihood of tube leaks and preclude the likelihood of a condition that challenges tube integrity requirements.

By implementing a strict Foreign Material Exclusion (FME) program, Palo Verde has been successful in minimizing number of occurrences in which foreign objects have been introduced to the steam generators. Prior to this outage, there have been no identified metallic foreign objects likely to have been introduced to the Unit 3 steam generators from the secondary plant since the replacement steam generators were installed in 2007.

Specific actions to minimize the likelihood of flex gasket material being introduced to the steam generators are discussed below in the response to question 4.

3. Discuss the scope and results of the FOSAR inspection in both steam generators including a discussion of in-bundle inspections.

APS Response:

Scope of Inspections:

As described in the EPRI Steam Generator Integrity Assessment Guidelines and consistent with the design of the Palo Verde steam generators, foreign objects entering the steam generators from the feed train are most likely to locate on the tubesheet within the shell-to-tube bundle annulus region or the blowdown lane. Steam Generator tubes are typically susceptible to foreign object damage in areas of higher radial velocity which can produce sufficient contact force, between the object and tube, in the direction normal to the tube to produce relative motion and wear.

Accordingly, the inspection scope performed in both SG 31 and 32 during 3R17 included the following areas:

1. Top of Tubesheet (TOTS) annulus region on both the Hot and Cold Legs

• A general annulus inspection (area between the tubes and the wrapper) was performed

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• Approximately 5 tube rows in was viewed from the annulus (Since Palo Verde SGs have a tri-pitch tube bundle, inspections were performed from the 30 degree, 90 degree and 150 degree angles all of the way around the SG.

2. TOTS blowdown lane on both the Hot and Cold Legs from Hand Holes adjacent to the divider plate on both sides of the SG

• A general area inspection in the area between the tubes and the divider plate was performed

• Approximately 2 tube rows in (two passes were made to look down both angles of the tri-pitch oriented tube bundle)

3. In Bundle Inspection of tubes surrounding the location from which a thin metallic strip was identified after the part was removed (see discussion below in SG 31 Inspection Results). The in-bundle inspection was performed from the 90 degree angle and the 30 degree angle to assess the condition of the surrounding tubes and look for wear indications.

SG 31 Inspection Results:

As a result of the inspections performed in SG 31, 16 items were logged as potential foreign objects.

• 3 objects were removed. These included one piece of metallic material characteristic of flexitallic gasket material and two small pieces of tube scale/sludge rocks. After removal of the thin strip of gasket material, there appeared to be evidence of a wear mark on tube R167C84 (see attachment 4).

Re-inspection with in-bundle inspection equipment from 2 different angles confirmed a wear mark on this tube. The other tubes in this area were inspected with the in bundle inspection equipment, and one other tube showed a possible indication (R168C85).

• Of the 13 potential objects that were not retrieved from SG 31, 1 is characterized as a small sludge rock and 4 as small fragments of carbon graphite material. All of these were lodged firmly in between tubes and could not be retrieved. There was one very fine wire bristle that was identified in the annulus during the initial inspection, however, when retrieval efforts were attempted, the piece was no longer present. Inspections were re-performed in the entire annulus using both a cart from the wall of the wrapper and using a video probe, however, the piece was never found. It is suspected this very small object was unknowingly picked up by a magnet on the inspection equipment and removed from the steam generator.

• Seven locations were logged as unknown potential foreign objects. Three of these are characterized as extremely small parts that appear to be either small sludge rocks or a light reflection in bundle. The remaining four locations are potentially small objects that are located too far in-bundle to make definitive identification or characterization achievable.

Page 4 SG 32 Inspection Results:

As a result of the inspections performed in SG 32, 20 items were logged as potential foreign objects.

• Six (6) objects were removed, including one short piece of flex gasket material. The gasket material was lying free in the annulus and not lodged against any tubes. There was no evidence of indications on the tubes surrounding the object and it is concluded this piece of gasket material did not locate or orientate in a manner conducive to producing tube wear. Three small pieces of carbon graphite fragments, a small sludge rock that broke apart as it was being retrieved and a very small fine wire bristle were all removed from SG 32.

• Of the 14 potential foreign objects that were not retrieved from SG 32, 6 appear to be small carbon graphite fragments. One fragment was firmly lodged between tubes and could not be retrieved. The remaining fragments were not in a retrievable location or orientation. Additionally two small objects characterized as sludge rocks or scale were not retrievable.

• Six locations were logged as potentially unknown foreign objects. Two of these were later characterized following a subsequent inspection as likely being a reflection of water since after further dry out of the steam generator they were no longer observable. The remaining four are characterized as extremely small in size (0.05X0.05X0.05) and are suspected to be either reflections of light from a small amount of water on the tubesheet or small pieces of sludge.

From the FOSAR inspections performed in both steam generators, only the two pieces of flex gasket material (one from each steam generator) are considered to represent a potential challenge to tube integrity (see attachment 5). The remaining objects are judged as not likely to cause tube wear due to material composition, size and location.

Consistent with EPRI Steam Generator Integrity Assessment guidelines and Palo Verde steam generator program practice, the items that are irretrievable will be evaluated to determine the wear potential by formal thermal hydraulic (ATHOS),

vibration and wear calculations.

4. Discuss the source of the part. If the part is from a secondary side component, please discuss whether all pieces of the component have been retrieved. If it entered through the feedwater system, discuss the secondary side feed water flow pattern.

Page 5 APS Response:

The feedwater system is depicted in attachments 1 and 2. Feedwater to the steam generators is provided by two steam-driven feedwater pumps, which pump the feedwater through two trains of high pressure feedwater heaters and then into the two steam generators. As noted above, several foreign objects were discovered during planned maintenance of the 5A high pressure feedwater heater. The 5A heater is the first of three heaters in series on the A train, prior to entering the steam generators through the economizer and downcomer control valves.

The foreign object retrieved from steam generator 31 is a very thin strip of stainless steel approximately 2 1/2 in length, see attachment 5. The object is most likely a piece of a spiral-wound flex gasket. The exact source or component from which this short piece of gasket material originated from within the secondary plant has not been determined.

Dozens of similar gaskets located throughout the secondary plant in level control valves, pumps and flanged piping connections. Detailed reviews of historical and current work orders and gasket part histories are being performed in an attempt to identify a source or component(s). The most likely mechanism by which the gasket material can enter the flow stream is from radial buckling (see attachment 3). With this mechanism, it is possible for the gasket to degrade yet still perform its sealing function and not result in an external and observable leak of the process fluid.

Three similar short pieces of gasket material were retrieved from the 5A high pressure feedwater heater and one very short piece retrieved from steam generator

32. Whether these total five pieces of gasket material are from a single component or constitute the entire inventory of gasket material is unknown.

Palo Verde has been implementing a program, in response to several industry operating experience reports, to replace flex gaskets that are susceptible to radial buckling with gaskets that incorporate a steel inner ring that reduces the possibility of the gasket failing into the flow stream during installation or operation. Due to the large number of locations where these gaskets are installed throughout the secondary plant, wholesale replacement is not feasible. Industry experience indicates replacement during regular maintenance is the appropriate strategy to minimize recurrence.

Apparent Cause Condition Report 4466275 has been initiated to determine if any additional corrective actions are necessary or appropriate to further minimize the likelihood of recurrence of this condition.

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5. Discuss the planned scope for any non-visual inspections and tests (Eddy current, Ultrasonic, in-situ pressure testing)

APS Response:

Perform bobbin and plus point ECT examinations SG 1 in the area that surrounds the wear scar that was discovered in the area adjacent to the foreign object that was removed from the SG. Perform ECT exams on the tubes at four in-bundle locations where the FOSAR inspections were less definitive and prevented good characterization of the potential foreign objects size, material composition and orientation.

6. Discuss the results of the previous inspections for the affected tube. Discuss the extent to which a historical review was performed for the region where the wear/part was detected (visual and eddy current historical reviews)

APS Response: A look back (to the previous outage) was performed on all tubes bounding the area where the foreign object was found/removed and no prior ECT signals were detected.

7. If the scope of the eddy current inspections is limited to region where the wear scar/loose part was detected, please discuss the possibility that the loose part was present in another region of the tube bundle prior to ending up in the location that it was detected. The loose part could move as a result of a number of factors (e.g.,

temporarily restricted to a specific area, movement as a result of flow conditions, change in mass because the part broke apart, change in flow conditions from transients or start ups/shutdowns). Information notice 2010-05 discusses possible movement of a part in one steam generator.

APS Response:

The Palo Verde steam generators are equipped with axial economizers (preheaters) to enhance thermal efficiency. The foreign object is located on the cold leg side top of tube sheet within the economizer section near the outer periphery of the tube bundle. The design of the economizer section is such that incoming feedwater enters the cold leg side of the steam generator through the two economizer feedwater nozzles and into the economizer box. The flow is then evenly distributed circumferentially by the economizer box, directed downward through 70 evenly spaced holes towards the tubesheet and then radially inward through the tube bundle beneath a flow distribution plate.

Because the incoming feedwater flow is radially inward across the tubesheet, as would be expected the foreign object was oriented along a radial tube lane and wedged in place in a manner that allowed it to interact with this tube and produce tube wear. Because the incoming feedwater flow is radially inward; it is not feasible for this foreign object to have entered the tube bundle at another location, become wedged in such a manner as to produce tube wear, then back radially

Page 7 outward against flow, migrate circumferentially, re-enter the tube bundle, become wedged in its as-found location and produce the observed wear scar. The only reasonable migration path is radially inward.

8. Discuss the extent to which Palo Verdes eddy current scope reflects the limitations of FOSAR to detect loose parts (in light of operating experience which demonstrates the challenges in detecting loose parts, Information Notice 2004-10, 2004-17)

APS Response:

IN 2004-10: Loose Parts in Steam Generators There were several OEs listed in the IN describing utilities having FO wear and some with leaks. The IN discussed that it is important to limit, promptly detect and implement corrective actions upon identification of loose parts.

Actions will vary depending on nature and location of part. Retrieval is not always possible. An engineering evaluation should be performed to determine if it is okay to leave the part/tubes in-service. Based on the evaluation, tube should be plugged as needed.

Palo Verde actions have been consistent with the discussion section of IN 2004-

10. An engineering evaluation will be performed to bound the parts left in service and ensure that tube wear will be below tube integrity limits. The only parts found during FOSAR that were left in service that do not appear to be graphite or sludge rocks/scale are too far in bundle to get any better visual inspection information, so the area surrounding the tube location where these potential foreign objects were identified will be tested during ECT inspection.

2004-17: Loose Part Detection and Computerized Eddy Current Data Analysis in Steam Generators This IN discusses the experience from Shearon Harris and points back to IN 2004-10 (discussed above). There are challenges with detecting loose parts.

Shearon Harris experienced a SG tube leak. This was a result of a missed ECT indication from the previous outage because the secondary analysis computer algorithm had an error which missed the initial indication on one tube. Also, there was a missed foreign object during the FOSAR inspection even though the part could be seen when the inspection tape was reviewed.

Palo Verde performed an extensive FOSAR and the FOSAR video was reviewed by inspection and utility personnel outside of containment after the inspection was initially performed.

The primary limitation of FOSAR is the ability to accurately locate and characterize foreign objects more than a few tubes in from the outer periphery.

For that reason the most comprehensive approach to detecting foreign objects is to perform a full scope primary eddy current and corresponding FOSAR. That

Page 8 comprehensive approach is performed at Palo Verde every other refueling outage in accordance with our Operational Assessment and Condition Monitoring evaluations. In accordance with the EPRI Steam Generator Integrity Assessment Guidelines, since there is visual confirmation of tube wear from a foreign object, tube integrity is being evaluated with a qualified technique (i.e. eddy current testing). In four locations located too far in-bundle to obtain characterization of the potential objects, ECT will be performed.

Cumming s, Justin M(Z09570)

Digitally signed by Cummings, Justin M(Z09570)

DN: cn=Cummings, Justin M(Z09570)

Reason: I am the author of this document Date: 2012.07.24 14:29:19

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Cummings, Justin M(Z09570)

Digitally signed by Cummings, Justin M(Z09570)

DN: cn=Cummings, Justin M(Z09570)

Reason: SIGNATURES N/A Date: 2012.07.24 14:29:41

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Huber, Donald A(Z51610)

Digitally signed by Huber, Donald A(Z51610)

DN: cn=Huber, Donald A(Z51610)

Reason: I have reviewed this document Date: 2012.07.24 15:57:14

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