ML20085N758

Submits Response to GL 95-03, Circumferential Cracking of SG Tubes

ML20085N758
Person / Time
Site:	Davis Besse
Issue date:	06/13/1995
From:	Selva J BABCOCK & WILCOX OPERATING PLANTS OWNERS GROUP
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20085N744	List: ML20085N740 ML20085N758
References
GL-95-03, GL-95-3, OG-95-1519, NUDOCS 9506300309
Download: ML20085N758 (12)

Text

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.",, OWNERS GROUP I j .d::"'-llllh.,

Duke Pnwer Company Oconee 1,2,3 '~~

Entergy operations, Jnc. ANO-1 Tole @ Edison Corrpeny Onvin Besse 1 Fborsda PowerCorporation CrystalRiver3 '

Tennessee Veney Authorty

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Bswearrow,movee Benebate1.2 Wcrking Together to Economically Provide Reliable and Safe Electrical Power Suite 525 e 1700 Rockville Pike e Rockville, MD 20852 e (301) 230-2iOO l

June 13,1995 i OG-95-1519 U.S. Nuclear Regulatory Commission l

Attn: Document Control Desk j

Washington, DC 20555-0001 '

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Subject:

B&W Owners Group Generic Response to GL 95-03 Gentlemen:

The B&W Owners Group (BWOG) has reviewed NRC . Generic Letter 95-03 regarding

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circumferemial cmcking 'of steam generator tubes and is submitting the attached generic response for all utilities with operating B&W Once-TLrough Steam Generators (OTSG). This generic BWOG response will be referenced and incorporated in each Utility's individual response to GL 95-03.

The circumferential cracking issue addressed.in GL 95-03 has some applicability to the B&W OTSG's. However, by virtue of the OTSG design and tubing material, the tubes have not been susceptible to many of the cracking mechanisms known to exist in other steam generator designs.

Preventive sleeving has minimized the occurrence of circumferential fatigue failures. Past tube inspections have concentrated on susceptible areas and circumferential cracking has not been identified as a significant degradation mechanism at B&W plants. The next inspection of OTSG's will continue to concentrate on these areas. As required by Generic Letter 95-03, the attached information provides the justification for continued operation of the B&W plants to their next scheduled steam generator inspection outage.

i Very truly yours, ed Y John A. Selva B&W Owners Group Steering Committee Chairman c: J.D.- Woodward (SGMP) i T.E.' Tipton (NEI)  !

9506300309 950613 C. Welty (EPRI) PDR ADOCK 05000346 p PDR

June 13, 1995 Generic Response to GL 95-03 for B&W OTSG Relative to circumferqntial Crackina of Steam Generator Tubes I Anolicability of B&W OTSGs to Circumferential Crackina Issue 1.0 Introduction NRC Generic Letter 95-03 "Circumferential Cracking of Steam Generator Tubes" dated April 28, 1995 addressed circumferential cracking of steam generator tubes. It notified all holders of PWR operating licenses of recent steam generator tube inspection findings at Maine Yankee and the safety significance of these findings. The letter also requested a review of recent operating experience at each plant with respect to the detection and sizing of circumferential indications, in addition to a safety assessment to justify continued operation until the next scheduled steam generator tube inspection.

This document is a generic safety assessment which applies to the licensees whose plants use once-through steam i generators (OTSGs).

2.0 OTSG Backaround Information .

The OTSG is a vertical once-through steam generator i containing typically 15,531 5/8-inch diameter Alloy 600 straight tubes, each over 56-feet long (see attached Figure l 1). The tubes are aligned and' supported by 15 carbon steel I tube support plates (tsp) which have broached holes to l provide three-point support and a space for' axial flow around each tube. During manufacture each tube was roller expanded 1-inch deep at each primary face of the 24-inch thick tubesheets for tube-to-tubesheet welding. Each fully assembled steam generator underwent a furnace stress relief heat treatment process at 1100 to 3150 degrees for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. I At full power, the primary coolant enters the upper head at about 604 degrees F and exits the lower head at about 554 degrees F, while approximately 450 degree F feedwater is preheated in an annulus around the tube bundle and then boils and superheats as it flows axially up around the tubes. Superheated steam leaves the OTSG at about 570 degrees F.

The first OTSG went into operation at Oconee 1 in 1974, and the 14 now in operation have accumulated EFPYs ranging from 9 to 16 years. All OTSGs started up using AVT water treatment and full-flow condensate polishing, and the feedwater chemistry has generally been maintained well below the maximum EPRI guideline values.

3.0 Susceptible Circumferential Crack Locations The areas of the OTSG tubes which may be susceptible to circumferential cracking are (1) those which may be subjected to high-cycle fatigue, and (2) those roll transitions which have residual tensile stresses which are primarily in an axial orientation. These criteria indicate four regions which may be susceptible. However, it should be noted that service-induced circumferential cracking has been observed only in region (a) below,

a. The uppermost span of tubes in the lane / wedge region,

b. Tube-to-tubesheet expansion transitions, including 189 total tubes which have not been stress-relieved and are therefore expected to be more susceptible to cracking,

c. Tube kinetic expansion transitions, which have not been stress relieved (TMI-1 UTS only), and

d. Tube / sleeve expansions at sleeve roll transitions.

Each of these regions is discussed separately.

3.1 The Uonermost Soan of Tubes in the Lane /wedae Reaion This region consists of tubes in the lane / wedge region at elevations between the upper tubesheet secondary face (UTSF) and the uppermost tube support plate (15th TSP). The inspection lane is an un-tubed channel which extends from the periphery to the center of the tube bundle along Row 76 of the tube layout. The lane / wedge region has been defined as a region of tubes which abuts the outer half of this lane and then flares out into a wedge shape near the periphery of the tube bundle, per the attached Figure 2.

The steam flow in the uppermost tube span is primarily across the tubes as it turns to exit the bundle. The open lane is a cooler area of the generator which in turn lets wet steam deposit contaminants higher in the bundle, contributing to local degradation. The lower flow resistance of the lane / wedge region results in higher crossflow velocities, and this high velocity steam may cause flow-induced vibration of these tubes. This vibration could result in high cycle fatigue failure (Reference NRC l Information Notice 91-43).

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3.2 Tube-to-Tubesheet Exnansion Transitions

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.f~ lDuring manufacture of.the OTSGs each tube was roller ,,  :

expanded 1-inch. deep:at the primary face of both tubesheets in preparation for-the tube-to-tubesheet welds. This" roller l expansion process produces a diameter _changeLin the tube _and~ -l leaves'the transition. area with residual stresses which are. (

primarily tensile on the OD surface. These cold-worked  !

areas are expected to be_more susceptible to cracking:than the balance of the tube-which has not been cold-worked. ..

-Later the entire assembled OTSG was subjectedito a furnace f.

stress relief process, which reduced the residual stresses

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in the transitions and thermally treated the tubes.

However, a few tubes in each plant were re-expanded during _;

the shop hydrostatic: tests as a temporary seal until the  !!

leaking tube could be rewelded. These deeper' expansions did .,

not have a post-roll' stress relief, which makes.them more l likely candidates for cracking. . .There are a total of 189 known non-stress relieved transitions existing in all the OTSGs according to manufacturing records, including:47 in l the UTS (hot 3eg upper tubesheet) and.142 in the LTS (cold leg lower tubesheet). Most of these expansions are in the ',{

Oconee-1 plant, including 29 located in the UTS. j Stress relief of Alloy 600 is neither required nor_ l prohibited by ASME Section III Code. However, this 12-hour j furnace stress relief at 1100 to 1150 F constitutes a  :

f thermal treatment which both reduces the residual stresses and enhances the resistance of; Alloy 600 to caustic  ;

corrosion. Therefore the roll transition joints which did'  !

not get the standard treatment are likely to be'the location of the first tube transition failures in the OTSGs.1 Unlike1  :

an RSG, the tunes are normally in compression during operation, which reduces the likelihood of circumferential- ,

cracking in the' transitions. ~

l- Three tubes in the Oconee-3 B-0TSG were explosively expanded

, full length in both upper and lower tubesheets. It could i not be determined from available records whether the expansions were performed before or after the: full bundle I stress relief. !I 3.3 Tube Kinetic Expansion Transitions (TMI-1 UTS'only)

, , During an extended cold shutdown period, the tubes of the i TMI-1 steam generators were found to have many ID-initiated IGA assisted cracks near the top of the upper tubesheet

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, (Reference NRC Information Notices 82-14 and 84-18). This  ;

was a singular event due to a reduced sulphur excursion  ;

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during the shutdown. Some tubes were plugged, and all tubes remaining in service were kinetically expanded part-length through the upper tubesheet to create a new load-carrying and essentially leak-tight joint. The lower transition of the new expanded length has not been subjected to a stress relief process. These kinetic expansion transitions are similar to the rolled tube transitions discussed above, and could be considered to be potential sites for circumferential cracking.

3.4 Tube / Sleeve Exoansions at Sleeve Roll Transitions Mechanical sleeves have been installed in all operating B&W plants in order to mitigate tube leaks due to high cycle fatigue and to repair tubes with other UTS or 15th TSP degradation. Sleeves are in service that are fabricated from both Alloy 600TT and Alloy 690TT material. The mechanical sleeve has roller expanded joints at each end to seal it into the parent tube. These tube expansion transitions have had no stress relief process. Thus, consistent with earlier discussions, this is a potential site for circumferential cracking. However, the orientation of the crack can not be reliably predicted because the resultant of the residual and operational stresses may vary.

Six tubes were sleeved at Oconee-1 in 1979 on a test basis with hydraulically expanded sleeves. Four of these had short sleeves at both UTSF and 15th TSP, and the other two were sleeved only at UTSF. The four sleeved tubes in the lane region have subsequently been plugged for reasons unrelated to the sleeves, but two of the double-sleeved tubes remain in service. These hydraulic expansions are also potential sites for circumferential cracking.

4.0 Scone of Past Tube Inspections Past tube inspections have concentrated upon the susceptible regions described above. The susceptible regions have been inspected as follows:

4.1 The upper span of any unsleeved tubes in the lane / wedge region has been examined by MRPC at the UTSF and 15th TSP.

4.2 Duke Power has inspected all the non-stress relieved UTS 1 roll transitions in the oconee-1 plant during the most l recent inspection outages using MRPC. I I

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4'.3 Kinetic transitions in the. lane / wedge region at TMI-l have I been frequently examined using an 8x1 array pancake coil probe.

l 4.4 All plants have examined a portion of their in-service tube l sleeves'using both standard and crosswound bobbin coils at l each scheduled inspection. In' addition, some sites have '

l recently used rotating plus point and rotating crosswound techniques for examination of sleeve rolled joints. During the Spring 1995 outage ANO-1 used the plus point probe to ,

inspect the rolled joints of approximately 222 Alloy 600 sleeves, which included the 50 sleeves which have the .

longest service life of any rolled sleeves in an operating .

B&W OTSG. Ten of these sleeves have now seen over 6 EFPY of operation. ,

5.0 Past Tube Inspection Results The results of past tube inspections related to circumferential cracking are listed for the four susceptible  :

locations described above:  !

e 5.1 Uppermost Span of Tubes in the Lane / wedge Region There have been 41 OTSG tube leaks which have been attributed to circumferential cracks caused by high cycle fatigue in the lane / wedge region. These cracks generally occur where the bending is restrained at the upper edge of the 15th TSP and at the secondary face of the upper tube 1 sheet, initiating in any local discontinuities. Tube failure due to high cycle fatigue usually progresses rapidly ,

after crack initiation, with primary to secondary leakage typically occurring within a period of hours.

The OTSG plant licensees have relied upon the installation f of preventative sleeves in the top span of these tubes to minimize tube failures. The 80-inch mechanical sleeve stiffens the tube and provides an inner pressure boundary.

All OTSG plants now have completed sleeving tubes in their defined lane / wedge regions. The annual rate of OTSG tube leaks decreased as the preventive-sleeving program neared -

completion, confirming *he effectiveness of this program.

Note that sleeving mitigates the consequences of fatigue failures, and it is not dependent upon inspection by non-destructive examination.

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5'. 2 Tube Roller Expansion Transitions None of the rolled tube transitions have leaked or shown service-induced eddy current indications to date, whether stress relieved or not.

5.3 Tube Kinetic Expansion Transitions There have been no confirmed service-induced tube cracks or '

leaks found in the TMI-1 kinetic expansions to date.

5.4 Tube / sleeve Expansions at Sleeve Roll Transitions To date, no sleeved tubes have been removed from service due to service-induced degradation in the sleeved area of the parent tube or sleeve.

II. Justification for Continued Oper3 tion The B&W plants by virtue of their design and tubing material have not been susceptible to many of the circumferential cracking mechanisms known to exist in other plants. The OTSG plants have not identified any service-induced circumferential cracking in the tubes or sleeves other than fatigue-induced cracks in the upper span of the lane / wedge region. Degradation in this region has been adequately addressed by preventative sleeving, as discussed in Section I. Inspections of the other susceptible regions have not identified circumferential cracks.

The inspection techniques used a't B&W plants are capable of  ;

reliable detection of circumferential cracking. These {

techniques have been shown to prevent uncontrolled tube i degradation. l If a tube should develop a circumferential crack and sever I at a rolled or kinetic expansion transition, the i consequences of this failure are minimized because the j severed ends of the tube would be captured within the tubesheet. In a similar manner, the sleeve prolongation beyond the rolled joint would keep the ends of the tube from separating if a tube should sever at a rolled sleeve j transition. The consequences of a circumferential crack  ;

which severs a tube are minimized by these design features.

Identified circumferential cracking has been successfully repaired at B&W plants. The inspection techniques which have been used will reliably detect the onset of e

circumferential cracking where it is known to occur. The potential for the rupture of a tube is very low, and the consequences of a severed tube are mitigated by the capture provisions of the design. Therefore, based upon the demonstrated success of preventative sleeving and the lack of observed failures in other areas believed to be most susceptible to circumferential cracking, there is no known risk incurred by continuing to operate the OTSG plants.

Continued operation of the OTSG plants until the next scheduled tube inspection is safe and is therefore justified.

III. Plans for the Next Inspection for Circumferential Crackina 1.0 Scone of the Insoections The specific locations considered to be most susceptible to circumferential cracking in B&W OTSGs will be inspected at the next scheduled tube inspection outage as follows:

1.1 The Uppermost Span of Tubes in the Lane / Wedge Region Tubes in the lane / wedge region have been preventively sleeved in the uppermost span to mitigate the consequences  !

of fatigue failures. A one-tube wide border of un-sleeved tubes in service around the lane / wedge region of sleeved tubes will be examined to confirm that the sleeved region is still adequately bounded. This examination will consist of inspection of the 15th TSP and UTSF areas of each tube.

Examination of sleeved tubes is addressed in item III.1.4 below.

If cracking is confirmed in the uppermost span of the border tubes surrounding the sleeved lane / wedge region, the inspection scope will be expanded until the area of cracking is demonstrated to be bounded.

1.2 Tube-to-Tubesheet Expansion Transitions Each of the rolled tube transitions which has not been stress relieved will be examined if it has not been sleeved or plugged . This includes transitions within both UTS and LTS. Note that these transitions in OTSG tubes are deep i

within the tubesheet where the signals are not distorted by dents or the face of the tubesheet.

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4 If cracking is-confirmed in the rolled expansion transitions which have not been stress relieved, sample expansion into the stress relieved transitions will be evaluated.

1.3 Tube Kinetic Expansion Transitions The examination of unsleeved tubes in the lane region at TMI-1 will include inspection of the kinetic expansion trans.itions for those tubes (approximately 280 tubes per OTSG). Indicfcions of circumferential cracking in the kinetic expansion transitions will be cause for additional examinations as necessary to bound that occurrence.

Leak testing by bubble test is scheduled for both TMI-1 OTSGs during the next refueling outage; such leak testing is capable of identifying upper tubesheet kinetic expansion transition crack leaks for follow-up eddy current examination and repairs as required. Note that previous leak tests since 1985 have consistently found that there-were no leaking kinetic expansion transitions.

1.4 Tube / sleeve Expansions at Sleeve Roll Transitions.

A 20 percent random sample of all sleeved tubes will be selected for examination. This examination will include tubesheet and freespan expansions in the pressure boundary portions of the sleeve and parent tube.

If cracking is found in the sleeved tubes, the sample will be expanded to 100% of the installed sleeves or until the expanded inspection scope demonstrates that the cracking is localized to a specific area or population of tubes.

2.0 Inspection Methods and Eauipment The technique to be used for the detection of circumferential cracking in OTSG tubes and sleeves will use a probe which has been qualified for detection of circumferential cracks per Appendix H of EPRI Report NP-6201 "PWR Steam Generator Examination Guidelines", or other site-specific qualification.

The analysts will be qualified to SNT-TC-1A Level II or higher, as well as to site-specific guidelines with respect t to detection of circumferential cracking.  ;

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The circumferential cracking issue is.applicablecto the B&W. .

OTSGs. By virtue of the design'and the tubing material, the j tubes have not been susceptible to many of the cracking j mechanisms known'to exist in other. steam generator designs. l' Preventive sleeving has' minimized the occurrence of fatigue'-

failures. Past tube inspections have concentrated upon  !

certain areas ~of the tubes which may be susceptible', and i circumferential cracking has.not been identified as a i significant degradation mechanism at B&W plants.-

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The inspection techniques which have been used are capable ,

of reliable detection of circumferential cracking. The i potential for the rupture of a tube remains acceptably low  :

and the. consequences of a rupture.are mitigated by the j design features. The next inspect' ions of OTSGs will- s continue to concentrate upon areas which may be susceptible. l to circumferential cracking. . :Thus,Las required by Generic Letter 95-03, it is concluded that continued operation of t the.OTSG plants until the next scheduled tube inspection ~1s .i justified. ,

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