Information Notice 1997-26, Degradation in Small-Radius U-Bend Regions of Steam Generator Tubes

Degradation in Small-Radius U-Bend Regions of Steam Generator Tubes

ML031060007
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	05/19/1997
From:	Slosson M Office of Nuclear Reactor Regulation
To:
References
IN-97-026, NUDOCS 9705140336
Download: ML031060007 (10)

1/4 UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 May 19, 1997 NRC INFORMATION NOTICE 97-26: DEGRADATION IN SMALL-RADIUS U-BEND

REGIONS OF STEAM GENERATOR TUBES.

Addressees

All holders of operating licensees or construction permits for pressurized-water reactors

(PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to

disseminate information about recent degradation affecting small-radius (rows I and 2)

U-bend regions of tubes in recirculating steam generators (SGs), in order to alert utilities to

potential problems in ensuring the integrity of the small-radius U-bends, and to provide

information about action taken by certain licensees to ensure adequate integrity. It is

expected that recipients will review the information for applicability to their facilities and

consider this information, as appropriate, in their SG inspection programs. However, suggestions contained in this information notice are not NRC requirements; therefore, no

specific action or written response is required.

DescriDtion of Circumstances

Licensees that use Westinghouse-designed recirculating SGs have for many years identified

indications in the U-bend regions of tubes with small radii. During the late 1970s and early

1980s, many units, as a preventative measure, plugged small-radius U-bend tubes to avoid

potential forced outages due to leakage. However, some licensees subsequently unplugged

these tubes and performed in situ stress relief to reduce the susceptibility for degradation.

Also SG designs evolved over time and a number of different material conditions are

represented in currently operating PWRs. These include mill-annealed alloy 600, mill- annealed alloy 600 in situ stress relieved, thermally treated alloy 600, and thermally treated

alloy 690. The following discussion of experience at four plants represents recent operating

experience regarding U-bend degradation that involved various tube material conditions.

During a 1996 inspection, Commonwealth Edison Company (ComEd) identified a total of

64 axially oriented and 2 circumferentially oriented indications in the U-bends of the row 1 SG

tubes at Zion Unit 2. ComEd characterized the indications as primary water stress-corrosion

cracking. The tubes at Zion Unit 2 were fabricated with mill-annealed alloy 600 material, and

the U-bends had not been heat treated. As a result of the inspection findings, ComEd

preventively plugged all the row I tubes at Zion Unit 2. - _ lI C ;C

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IN 97-26 May 19, 1997 During a 1996 inspection, the Tennessee Valley Authority (TVA) identified axial indications in

17 small-radius U-bends of SG tubes at Sequoyah Unit 2 and characterized the degradation

as primary water stress-corrosion cracking. The tubes were fabricated with mill-annealed

alloy 600 material and the U-bends werein situ heat treated during the cycle 6 outage in

1994. TVA plugged the 17 row I tubes that contained the U-bend indications.

During 1992 and 1995 inspections, Pacific Gas & Electric Company (PG&E) identified

circumferential indications having relatively small arc angles in the small-radius U-bends

of SG tubes at Diablo Canyon Unit 1. The tubes were fabricated with mill-annealed alloy

600 material and the small-radius U-bends were in situ heat treated after the second

refueling outage in 1988. PG&E plugged the degraded tubes.

During a 1996 inspection, ComEd identified a single axial indication in the U-bend of one of

the SG tubes at Braidwood Unit 2. The Braidwood Unit 2 tubes were fabricated with

thermally treated alloy 600 tubes and the U-bends in the first seven rows received additional

thermal stress relief after bending during the manufacturing process. ComEd plugged the

degraded tuve.

A small number of axial indications originating on the outside diameter of the tubes have

been reported in the small-radius U-bend regions of the SGs at Palo Verde 1, 2, and 3 and

St. Lucie 1. These SGs were designed by Combustion Engineering.

Discussion

U-bend degradation has occurred in mill-annealed alloy 600 tubes irrespective of whether

they have been heat treated. Tubes with thermally treated alloy 600 material are less

susceptible to degradation than mill-annealed alloy 600 tubes. However, thermally treated

alloy 600 tubes have also begun to experience U-bend degradation. None of the degraded

thermally treated alloy 600 tubes have been removed from SGs for confirmation of the

degradation mechanism. Reports of U-bend degradation have been based on eddy current

inspection results. The susceptibility to cracking in small-radius U-bends and the findings of

recent field inspections have emphasized the importance of inspection of this area of SGs

with techniques capable of accurately detecting U-bend degradation.

U-bend degradation can potentially impair tube integrity if not effectively managed. Concerns

in this regard stem from limitations of eddy current testing to detect and size U-bend cracks, the potential for some U-bend cracks to have relatively long lengths, and the potential for

high crack growth rates for some of these cracks. The industry standard bobbin coil has

proven unreliable for detecting U-bend cracks and, in addition, is not qualified for this

application under the Electric Power Research Institute (EPRI) technique qualification

protocol. The industry has developed special probes for these inspections. The industry has

qualified a rotating pancake coil and a Plus Point coil for detecting indications in small-radius

U-bends, in accordance with enhanced qualification criteria developed by EPRI.

- 1 IN 97-26 May 19, 1997 There continues to be an absence of pulled tube information to confirm that the detection

threshold for these cracks is better than 40 or 50-percent through wall. In addition, available

inspection techniques are not capable of reliably sizing crack depths and, for this reason, it

has been industry's practice to "plug on detection" U-bend indications that are found.

Information available on crack growth rates bting experienced in the field is very limited by

virtue of the inability to perform reliable crack depth measurements and the resulting need to

"plug on detection." However, U-bend cracks have led to leakage as early as the first cycle

of operation and, thus, crack growth rates may potentially be high for some cracks. Given

the relatively high detection thresholds, the relatively long operating cycles, and the

potentially high growth rates, the depth of cracks may be in excess of 50-percent through

wall when they are first detected.

In view of these concems, effective management of the degradation of SG tubes is

important to ensure that adequate tube integrity is being maintained in accordance with

10 CFR Part 50, Appendices A and B. One such approach being Implemented by a number

of licensees involves the use of tube integrity assessments to ensure that inspection

sensitivity to U-bend cracks and the frequency and scope of inspection are sufficient to

ensure that U-bend flaws are being detected and removed from service before tube integrity

is impaired.

For example, ComEd performed in situ pressure tests at Zion Unit 2 on four tubes having

the longest axial U-bend indications and on two tubes with circumferential U-bend indica- tions using pressure loading consistent with the margins recommended in Regulatory Guide

(RG) 1.121, "Bases for Plugging Degraded PWJR Steam Generator Tubes." The two tubes

having circumferential indications satisfied RG 1.121 margins without leaking. Three of the

four tubes having axial indications leaked at a pressure of main steamline break conditions

but did not burst under a pressure loading of three-times-normal operating pressure.

Because of the limitations of the test equipment, the pressure in the fourth tube did

not reach the three-times-normal operating pressure criterion of RG 1.121. For this tube, ComEd performed analyses to show that the tube would not burst under a pressure loading

of three-times-normal operating pressure. These analyses are based on eddy current

test measurements. Since these measurements may have large uncertainties, ComEd

conservatively assumed that the cracks were 100-percent through wall. On the basis

of the leakage measurements at main steamline break pressures, CoinEd was able to

demonstrate that accident leakage would satisfy the requirements of 10 CFR Part 100.

For U-bend indications at Sequoyah Unit 2, TVA did not perform in situ pressure testing;

instead, it performed bounding analyses to show that the three tubes having the largest

U-bend cracking satisfied RG 1.121 criteria. However, it should be noted that in situ pressure

testing provides more definitive assurance of structural and leakage integrity than analyses.

For axial indications in the small-radius U-bend regions of the SGs at Palo Verde 1, 2, and 3 and St. Lucie 1, the licensees plugged the tubes.

IN 97-26 May 19, 1997 As shown by the examples discussed above, the integrity of the small-radius U-bend regions

can be more fully ensured by efforts that include performing inspections of rows 1 and 2 U-bends using qualified eddy current techniques; performing in situ pressure testing, as

necessary, to assess the condition of defective tubes; taking appropriate corrective actions, including plugging defective tubes; and assessing the appropriate operating intervals until the

next SG tube inspection.

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 contacts list below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Marylee M. Slosson, Acting Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

John C. Tsao, NRR

(301) 415-2702 E-mail: jct@nrc.gov

Eric J. Benner, NRR

(301) 415-1171 E-mail: ejbl @nrc.gov

Attachment: List of Recently Issued1RCInformatiog Notices

Attachment

IN 26 May 19, 1997 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

87-10, Potential for Water 05/15/97 All holders of OLs or CPs

Sup. 1 Hammer During Restart of for boiling-water reactors

Residual Heat Removal

Pumps

97-25 Dynamic Range Uncertain- 05/09/97 All holders of OLs or CPs

ties in the Reactor Vessel for Westinghouse pressurized- Level Instrumentation water reactors

97-24 Failure of Packing Nuts 05/08/97 All U.S. Nuclear Regulatory

on One-inch Uranium Commission licensees and

Hexafluoride Cylinder certificatees authorized

Valves to handle uranium hexa- fluoride in 30- and 48-inch

diameter cylinders

97-23 Evaluation and Reporting 05/07/97 All fuel cycle conversion, of Fires and Unplanned enrichment, and fabrication

Chemical Reactor Events facilities

at Fuel Cycle Facilities

97-22 Failure of Welded-Steel 04/25/97 All holders of OLs or

Moment-Resisting Frames CPs for nuclear power

During the Northridge reactors

Earthquake

97-21 Availability of Alternate 04/18/97 All holders of OLs

AC Power Source Designed for nuclear power

for Station Blackout Event reactors

97-20 Identification of 04/17/97 All holders of OLs

Certain Uranium for nuclear power

Hexafluoride Cylinders

that do not comply

with ANSI N14.1 Fabrication

Standards

OL = Operating License

CP = Construction Permit

_J

IN 97-26 May 19, 1997 As shown by the examples discussed above, the integrity of the small-radius U-bend regions

can be more fully ensured by efforts that include performing inspections of rows 1 and 2 U-bends using qualified eddy current techniques; performing in situ pressure testing, as

necessary, to assess the condition of defective tubes; taking appropriate corrective actions, including plugging defective tubes; and assessing the appropriate operating intervals until the

next SG tube inspection.

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 contacts list below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

original signed by S.H. Weiss for

Marylee M. Slosson, Acting Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

John C. Tsao, NRR

(301) 415-2702 E-mail: jct@nrc.gov

Eric J. Benner, NRR

(301) 415-1171 E-mail: ejbl@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

Tech Editor has reviewed and concurred on 04/07/97

• SEE PREVIOUS CONCURRENCES

DOCUMENT NAME: G:\EJB1\UBEND.IN

OFC Contacts BC/EMCB BC/PECB:DRPM ADIDRPM

NAME JTsao* . JStrosnider* AChaffee* MS

l EBenner S /

DATE 04/08/97 04/16/97 05/08/97 05'397

04/08/97

[OFFICIAL RECORD COPY

IN 97-XX

May XX, 1997 As shown by the examples discussed above, the integrity of the small-radius U-bend regions

can be more fully ensured by efforts that include performing inspections of rows I and 2 U-

bends using qualified eddy current techniques; performing in situ pressure testing, as

necessary, to assess the condition of defective tubes; taking appropriate corrective actions, including plugging defective tubes; and assessing the appropriate operating intervals until the

next SG tube inspection.

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 list below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Marylee M. Slosson, Acting Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

John C. Tsao, NRR

(301) 415-2702 E-mail: jct~nrc.gov

Eric J. Benner

(301) 415-1171 E-mail: ejblnrc.gov

Attachment: List of Recently Issued NRC Information Notices

• SEE PREVIOUS CONCURRENCES

DOCUMENT NAME: G:XEJBIUBEND.IN

OFC Contacts BC/EMCB BCIPECB:DRPM D/DRPM

NAME JTsao* JStrosnider AChaffee MSlosson

EBenner __ __ _ _ _ _

DATE 04/08197 04/16/97 T/r/97 I,197

04/08/97

[OFFICIAL REC.RD COPY

IN 97-XX

May XX, 1997 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 list below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Marylee M. Slosson, Acting Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

John C. Tsao, NRR

(301) 415-2702 E-mail: jct@nrc.gov

Eric J. Benner

(301) 415-1171 E-mail: ejbl@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

• SEE PREVIOUS CONCURRENCES

DOCUMENT NAME: G:NEJBIUBEND.IN

OFC Contacts BC/EMCB BC/PECB:DRPM D/DRPM

NAME JTsao* JStrosnider* AChaffee tuY MSlosson

EBenner*

DATE 04/08197 04/16/97 QJi97 I /97

04,08/97 ________ __COPY

[FFICIALRECORD COPY

. I

IN 97-XX

May XX, 1997 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 list below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Marylee M. Slosson, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

John C. Tsao, NRR

(301) 415-2702 E-mail: jct@nrc.gov

Eric J. Benner

(301) 415-1171 E-mail: ejblnrc.gov

Attachment: List of Recently Issued NRC Information Notices

• SEE PREVIOUS CONCURRENCES

DOCUMENT NAME: G:\EJB1UBEND.IN

OFC Contacts BCIEMCB BCIPECB:DRPM DIDRPM

NAME JTsao* JStrosnider* AChaffee MSlosson

EBenner* l

DATE 04/08/97 04116/97 1/97 I /97

04/08/97 I

L[ICi-IAL KhtCUKU OUIYX

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IN 97-xx

May XX, 1997 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 list below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Thomas T. Martin, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

John C. Tsao, NRR Eric J. Benner

(301) 415-2702 (301) 415-1171 E-mail: jct@nrc.gov E-mail: ejbl@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:XEJB1%UBEND.IN

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