ML20205D982

Safety Evaluation Supporting Amend 98 to License NPF-73

ML20205D982
Person / Time
Site:	Beaver Valley
Issue date:	03/26/1999
From:	NRC (Affiliation Not Assigned)
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ML20205D973	List: ML20205D969 ML20205D982
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NUDOCS 9904050021
Download: ML20205D982 (10)

Text

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UNITED STATES j:

g NUCLEAR REGULATORY COMMISSION f

WASHINGTON, D.C. 20555 4 001

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 98 TO FACILITY OPERATING LICE'NSE NO. NPF-73 DUQUESNE !IGHT COMPANY

_QHIO EDISON COMPANY THE CLEVEL^.ND ELECTRIC ILLUMINATING COMPANY THE TOLEDO EDISON COMPANY BEAVER VALLEY POWER STATION. UNIT 2 DOCKET NO. 50-412

1.0 INTRODUCTION

By letter dated March 10,1997, as supplemented by letters dated July 28,1997, September 17,1997, April 30,1998, January 29,1999, and February 26,1999, Duquesne 3

Light Company (the licensee) submitted a proposed license amendment which includes an i

amendment to the Technical Specifications (TSs) and new license conditions for Beaver Valley

- Power Station, Units 1 and 2 (BVPS-1 and BVPS-2), The proposed changes would allow sleeving of steam generator tubes at Beaver Valley Power Station with sleeves designed by the vendor, ABB Combustion Engineering (CE). Additionally, the proposed amendment would establish a repairlimit of 32-percent for the pressure boundary of any CE sleeve and delete a reference to a previously used sleeving technique referenced in the TSs. The licensee has

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also committed to perform inservice inspections of installed CE welded sleeves, complete a j

post weld heat treatment (PWHT) of CE sleeve welds, and visually inspect sleeve welds after

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installation. These commitments are included in license conditions described in Section 5 of this safety evaluation (SE). The following safety evaluation considers the proposed changes to the license for Beaver Valley Unit 2. The staff previously reviewed and approved similar changes for BVPS-1 as documented in Amendment No. 208 to Facility Operatin0 License No.

DPR-66 dated November 25,1997. The July 28,1997, September 17,1997, April 30,1998, January 29,1999, and February 26,1999, letters did not change the no significant hazards consideration or expand the amendment beyond the scope of the initial notice.

'Three types of ABB/CE leak tight sleeves are proposed for use at Beaver Valley. Two of the three designs are termed Full Depth Tubesheet (FDTS) sleeves. One FDTS sleeve is welded to the tube near both the upper and lower ends of the sleeve. The second type is wetoed to the tube near the upper end and a hard roll expansion in the lower tubesheet area secures the tube within the steam ge_nerator tubesheet. The third sleeve design spans degraded areas of the steam ' generator tube at tube suppori plates or in a free span section of the tube. This Tube Support sleeve is welded at both ends.

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- The revised TS would reference the current generic topical report for CE welded sleeves,

.CEN-629-P, Revision 02, " Repair of Westinghouse Series 44 and 51 Steam Generator Tubes Using Leak Tight Sleeves," and CEN-629-P Addendum 1, of the same title, both dated January 1997. Because the bulk of the technical and regulatory issues for the present request are identical to those reviewed in the previous SEs for CE sleeves, this SE discusses only those issues warranting revision, amplification or inclusion based on recent experience.

Details of prior suff evaluations of CE sleeves may also be found in the SEs for Waterford Steam Electric Station, Unit 3, Docket No. 50-362, dated December 14,1995; Byron Nuclear Power Station, Units 1 and 2, and Braidwood Nuclear Power Station, Units 1 and 2, Docket Nos. 50-454, 50-455, 50-456, and 50-457, dated April 12,1996; Zion Nuclear Power Station, Units 1 and 2, Docket Nos. 50-295 and 50-304, dated October 29,1996; and Kewaunee Nuclear Power Plant, Docket No. 50-305. The staff's conclusions in these evaluations apply

' to the proposed BVPS-2 license amendment unless superseded by an evaluation discussed herein.

2.0

SUMMARY

OF PREVIOUS REVIEWS Previous staff evaluations of CE sleeves addressed the technical adequacy of the sleeves in the four principal areas of pressure retaining component design: structural requirements, r'.sterial of construction, welding,' and non-destructive examination. The staff found the analyses and tests that were submitted to address these areas of component design to be l

acceptable.

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The function of sleeves is to restore the structural and leakage integrity of the tube pressure boundary. Consequently, structural analyses were performed for a variety of loadings

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including deWgn pressure, operating transients, and other parameters selected to envelope loads imposed during normal operating, upset, and accident conditions. Stress analyses of j

sleeved tube assemblies were performed in accordance with the requirements of the ASME

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Boiler and Pressure Vessel Code (ASME Code), Section Ill. These analyses, along with the t

results of qualification testing and previous plant operating experience were cited to demonstrate that the sleeved tube assembly is capable of restoring steam generator tube i

integrity.'

The material of construction of the sleeves is nickel Alloy 690, a Code-approved material (ASME SB-163), covered by ASME Code Case N-20. The staff has found that the use of Alloy 690 thermally treated (TT) sleeves is an improvement over the Alloy 600 material used in the original stearn generator tubing. Corrosion tests conducted under Electric Power Research Institute (EPRI) sponsorship confirm test results regarding the improved corrosion resistance of Alloy 890 TT over that of Alloy 600. The NRC staff has concluded as a result of these laboratory corrosion tests that Alloy 690 is acceptable as meeting the guidelines in Regulatory Guide (RG) 1.85, " Materials Code Case Acceptability ASME Section lll Division 1," Revision 24, dated July 1986. The NRC staff has approved use of Alloy 690 TT tubing in replacement steam generators as well as sleeving applications.

The welding process employed to join the sleeve to the parent tube is automatic autogenous

' gas-tungsten arc welding (GTAW). The application of this process to the CE sleeve design was qualified and demonstrated during laboratory tests employing full scale sleeve / tube i

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e 3-mock-ups. Qualification of the welding procedures and welding equipment operators was performed in accordance with the requirements of the ASME Code,Section IX (Welding).

The staff considers sleeves to be a long-term repair but not a repair with unlimited service life.

The installation of CE Leak Tight Sleeves involves welding the sleeve to the tube which potentially cieates new locations susceptible to stress corrosion cracking. The time for the initiation of service induced degradation in sleeve / tube assemblies is not quantified. Operating experience with tubes fabricated from Alloy 600 indicates that initiation times can vary significantly depending on residual stresses, variability in material properties, and the chemical environment adjacent to the tube material. Consequently, although vendors traditionally conduct accelerated corrosion tests of sleeve / tube assemblies for the purpose of making service life predictions, the staff finds this method too unreliable for deterministic predictions.

However, the staff does consider the corrosion test results a good relative indicater of potential performance.

Considering the unreliability of sleeve life predictions, the staff has typically required licensees to inspect a sample of sleeves at cach outage. Periodic inservice inspections will provide assurance that any service !nduced degradation in sleeves is detected and addressed appropriately. Inservice inspection requirements applicable to the licensee's proposed amendment request are discussed further in Section 3.3 of this SE.

3.0 DISCUSSION Recent experience with the installation and inspection of steam generator tube sleeves has highlighted several areas that were either not relevant to previous sleeving amendments or addressed in detailin the staff's evaluations of the sleeving amendments previously referenced. These issues include: (1) the preparation of tube surfaces prior to sleeve welding, (2) the adequacy oiinspection techniques used to accept sleeve welds, (3) inservice inspection requirements for sleeved tubes, and (4) the PWHT of sleeve welds. In addition to each of these topics, the licensee's proposal to establish a plugging limit for sleeve degradation is discussed in the following sections.

3.1 Weld Preparation In order to form an adequate sleeve to tube weld, the surface of the metal (s) to be welded must be cleaned prior to performing any weld. For sleeve installation, the inner diameter of the parent tube must be cleaned of service induced oxides at the desired weld location. The presence of impurities during the welding process can lead to unacceptable weld indications.

For the CE sleeving process, the tube inner surface is cleaned using motorized wire brushes.

Based on recent findings during the inspection of sleeves at other plants, the industry has identified the cleaning process as an essential step in the sleeva installation process. This conclusion is based on an ABB/CE root cause assessment which concluded that changes in the cleaning process, implernented in prior sleeve installations, resulted in the formation of weld zone indications. In order to minimize the potential for these indications, the vendor enhanced the tube cleaning process to ensure the optimum removal of service induced oxides prior to welding. In order to verify the removal of oxides, the vendor recommends a 100-percent visual examination of the cleaned area. At such time that process controlis demonstrated to ensure cleaning efficiency, a sampling program may be used. The licensee

. for BVPS-2 will confirm the effectiveness of the cleaning step in the sleeve installation by a visual examination of the tube inside diameter surface, as recommended by CE. If the visual inspections reveal that the cleaning process is effectively removing all tube oxide deposits, the licensee may relax the requirement to visually inspect tubes after cleaning.

3.2 Weld Acceptance Inspections To verify the acceptability of sleeve welds, the licensec will complete an initial examination of sleeved steam generator tubes prior to retuming a repaired tube to service. The acceptance examination includes an inspection using visual, ultrasonic, and eddy current techniques. The analysis of inspection data from three diverse methods of inspection improves the ability to

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detect fabrication induced defects. In addition, eddy current data are also used as a baseline j

for comparison with data obtained in future required periodic inspections.

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Past field experience has demonstrated that previous initial acceptance examinations based

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on visual and ultrasonic inspection techniques may not be sufficient to identify all fabrication induced defects. As a result, the weld acceptance nondestructive evaluation (NDE) was modified. The modifications include inspection using an ultrasonic tester with an enhanced digitized amplitude system. The enhanced ultrasonic inspection technique provides a greater

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number of signal properties to more fully characterize a weld. The modified procedure was

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extensively tested on laboratory produced welds containing a variety of inclusion and lack of fusion defects. Samples were destructively examined and the metallurgical sections compared with the inspection results. Comparison of results demonstrated the revised procedure was highly reliable, and that no significant defects should remain undetected when using the enhanced procedure.

In addition to ultrasonic testing, the sleeve weld acceptance criteria require a 100-percent eddy current inspection using a plus point probe. Field experience led CE to discover that weld suckback and circumferentially oriented oxide inclusions wou'd not be detected by' ultrasonic inspection techniques. CE has shown that the plus point probe reliably detects the varicus fabrication induced weld defects including blowholes, weld suckback and circumferentially oriented oxide inclusions. The vendor has also shown that eddy current methods can locate the position of a fabrication defect with respect to the weld centerline which is considered the pressure boundary. Thus, for an FDTS sleeve upper weld, indications located above the weld centerline that are acceptable with other inspection techniques may be left in service.

However, any indication found below the weld centerline requires the tube to be plugged. For the lower welds on Tube Support sleeves, this criterion is appropriately modified so that indien is below the weld centerline may be left in service.

During a recent installation of welded sleeves at Kewaunee Nuclear Power Plant, the licensee visually identified weld zone indications that were not identified with either eddy current or ultrasonic inspection techniques. This finding may indicate that all three inspection methods are needed to ensure acceptable sleeve welding. The licensee for BVPS-2 nas indicated its intent to perform a 100-percent visual examination of all sleeve welds for the acceptrnce examinations. This commitment will be included in the license condition included in Section 5.2 of this SE.

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n L,i t 3.3 Inservice inspection Requirements The licensee has proposed to complete a sample inspection of 20-percent of installed sleeves at each refueling outage. if degradation or defects are identified during the inspection of the sleeves, the scope of the inservice examination may be expanded to include additional inspections of repaired tubes. This proposalis consistent with current industry guidance for steam generator sleeve examinations. EPRI recommends a 20-percent sample inspection for sleeves. The licensee's commitment to inspect installed sleeves in each refueling outage is included in a license condition described in Section 5.1 of this SE.

~ 3.4 Post-Weld Heat Treatment One primary element in the development of service induced stress corrosion cracking in steam generator tubing is the residual stresses in the material. In unrepaired tubing, these stresses could originate from the original fabrication of the tubing or from deformation of the tube material at low temperatures (e.g., tubesheet expansion). The installation of sleeve welds will also introduce residual stresses from the welding process in both the sleeve and the tube.

These stresses have the potential for increasing the susceptibility of the welded materials to stress corrosion cracking. A PWHT can reduce these stresses, and thus potentially increase

' the time for the initiation of cracking within a welded joint. For the installation of CE welded sleeves at BVPS-2, the licensee has committed to perform a PWHT of the sleeve welds. This commitment is included in a license condit% discussed in Section 5.3 of this SE.

3.5 Sleeve Plugging Limits The sleeve minimum acceptable wall thickness is determined using the criteria of RG 1.121,

" Bases for Plugging Degraded PWR Steam Generator Tubes," and ASME Code Section 111 allowable stress values and pressure stress equations. According to RG 1.121 criteria, an I

allowance for NDE uncertainty and postulated operational growth of tube wall degradation within the sleeve must be accounted for when using NDE to determine sleeve plugging limits.

Therefore, a conservative tube wall combined allowance for postulated degradation growth and eddy current uncertainty of 20-percent through-wall per cycle was assumed for the purpose of determining the sleeve plugging limit. The sleeve structurallimit, which was calculated based on the most limiting of normal, upset, or faulted conditions for 7/8-inch outside diameter steam generator tubes in Westinghnuse Model 44 and 51 steam generators, wtw determined to be 52-percent of the sleeve nominal wall thickness based on ASME Code minimum material properties in accordance with staff positions. Removal of sleeved tubes from service when degradation in the sleeve pressure boundary reaches a depth of 32-percent provides assurance that the minimum acceptable wall thickness will not be violated during the next cycle of operation. This requirement is incorporated into TS 4.4.5.4.a.6.b.

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4.0 PROPOSED TECHNICAL SPECIFICATION CHANGES In order to incorporate the proposed changes to permit sleeving of the BVPS-2 steam generators using CE welded sleeves and to eliminate a reference to a sleeving methodology i

no longer used, the licensee has proposed the following changes to the TSs.

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1. Proposed Changas to TS 4.4.5.4.a.6.b, " Plugging or Repair Limit" The definition of Repair Limit is modified to specify that the repair limit for ABB/CE tungsten

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inert gas (TIG) welded sleeves is 32-percent of the nominal sleeve wall thickness.

2. Proposed Changes to TS 4.4.5.4.a.9.a, " Tube Repair" A reference to allow the installation of Babcock and Wilcox kinetic welded sleeves is deleted and replaced by a reference to the ABB/CE topical reports, CEN-629-P, Revision 02, and CEN-629-P Addendum 1, for the TIG welded sleeves.

3. Proposed Changes to B 4.12, " Bases" The Bases Section is modified to indicate that the technical bases for sleeving are described in the CE topical report referenced in Surveillance Requirement 4.4.5.4.a.9.

5.0 LICENSEE'S COMMn MENT IN SUPPORT OF AMENDMENT REQUEST As part of the proposed amendment request, the licensee has committed to perform an inservice inspection of CE welded sleeves in accordance with a license condition which establishes the sampling and expansion scope for examining CE welded sleeves. The licensee has committed to complete a visualinspection of sleeve welds in addition to eddy current and ultrasonic test eaminations at the time of sleeve installation. The licensee also committed to perform a PWHT of sleeve welds. These commitments are reflected in a new license condition that is being proposed for inclusion in Appendix D to BVPS-2, Facility Operating License No. NPF-73.

5.1 Inservice inspection of CE Welded Sleeves The following are imposed to require an inspection of sleeves during each inservice inspection.

1. The initialinspection sample at each scheduled inservice inspection shall be a rninimum of 20% of each repair method / design installed.

2. The repaired tubes selected as the second sample (if required by Table 1) during each scheduled inservice inspection shall include tubes containing similar repair design (s).

3. If the insetyice inspection of a steam generator conducted in accordance with Table 1 requires a sample inspection whose results fall in Category C-3, the inspection frequency shall be at least once per 20 months. This inspection frequency shall apply until a subsequent inspection demonstrates that a thirr; sample inspection is not required.

4.

Additional, unscheduled inservice inspections shall be performed on each steam generator in accordance with the first sample inspection specified in Table 1 during the shutdown subsequent to any of the following conditions:

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. a. Primary-to-secondary tube leaks (not including leaks originating from tube-to-tube sheet welds)in excess of the limits of TS 3.4.6.2,

b. A seismic occurrence greater than the Operating Basis Earthquake,

c. A loss-of-coolant accident requiring actuation of the engineered safeguards, or

d. A main steam line or feedwater line break.

5.

The steam generator shall be determined operable after completing the corresponding actions (plug or repair all tubes exceeding the plugging or repair limit) required by TS 3/4.4.5 (Tables 4.4-1 & 4.4-2) and Table 1 con *ained herein.

TABLE 1: STEAM GENERATOR REPAIRED TUBE INSPECTION (4)(5) 1ST SAMPLE INSPECTION (1)(2) 2ND SAMPLE INSPECTION (2)(3)

SAMPLE SIZE RESULT ACTION REQUIRED RESULT ACTION REQUIRED A minimum of C-1 None N.A N.A 20% of repaired tubes C-2 Plug defective repaired C-1 NONE tubes and inspect 100%

j of the same repair design

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in this S.G.

C-2 Plug defective repaired tubes C-3 Perform action for C-3 result of first sample C-3 Inspect all repaired tubes All other S.G.s NONE i

of the same repair design are C-1 in this S.G., plug defective tubes and inspect 20% of the same Somo S.G.s C-2 Perform action for C-2 repair design in each but no result of first sample other S.G.

additional S.G.s are C-3 Additional S.G.

Inspect all repaired is C-3 tubes of the same repair design in each S.G. and plug defective tubes.

Notification to NRC pursuant to 10 CFR 50.72(b)(2).

8-(1)

The initial inspection sample at each scheduled inservice inspection shall be a minimum of 20% of each repair method / design installed.

The inspection of repaired tubes may be per' ormed on tubes from 1 to 3 steam generators (2) f based on outage plans.

(3)

Each repair method / design is considered a separate population for determination of scope expansion.

(4)

The categories above (C-1, C-2 and C-3) are those described in TS 3.4.5 (5)

Repaired tube examinations shall be comprised of a volumetric examination of 100% of the repair pressure boundary.

5.2 Visual Acceptance Examination of Sleeve Welds The following is imposed to support the licensee's commitment to visually examine sleeve welds to identify weld zone indications that may not be detected using eddy current or ultrasonic inspection techniques:

Welds associated with steam generator tube sleeves shall be subjected to a visual examination (VT-1) at the time of installation in addition to eddy current (ET) and ultrasonic (UT) test techniques. Discrepancies between the visual, eddy current and ultrasonic examination results shall be reconciled.

5.3 PWHT of CE Sleeve Welds The following is imposed to support the licensee's commitment to perform a PWHT of ABB-CE TIG welded sleeve welds.

The installation process for ABB-CE TIG welded sleeves per CEN-629-P, Revision 2, will include a heat treatment of sleeve welds.

6.0 STAFF EVALUATION The staff concludes that the proposed sleeving repairs as described in the topical report,

" Combustion Engineering Leak Tight Sleeves," (CEN-629-P, Revision 2) can be accomplished to produce sleeved tubes of acceptable metal!urgical properties, corrosion resistance, and structural and leakage integrity. As indicated previously, recent experience has indicated the need to evaluate additional areas that were not applicable to previous CE sleeving amendments or were not addressed in detailin previously approved CE sleeving amendments. The staff evaluation of these issues is discussed below.

The licensee's commitment to visually inspect sleeve welds after installation addresses potential concems resulting from experience at another plant where indications were identified only by visual

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inspection methods. At this timo, industry experience with these indications is limited. The I

licensee has proposed a reasonable approach to identify indications that could potentially affect the structural and leakage integrity of CE welded sleeves.

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. The Beaver Valley Steam Generator Tube Surveillance Requirements currently require an examination of at least three percent of the inservice ; team generator tubes. As discussed in Section 5.1 of this SE, the licensee has committed to inspect a 20-percent sample of sleeves at each refueling outage. Since the proposed scope for the inservice inspection of sleeves increases the probability that any sleeve / tube degradation is identified during the course of inspections, the staff concludes that the licensee's proposal for examining the sleeves at each inservice inspection is acceptable.

A PWHT of sleeve welds may reduce the residual stresses in the vicinity of the sleeve / tube weld, and thus decrease the susceptibility of the assembly to stress corrosion cracking. Given that stress corrosion cracking is a significant degradation mechanism affecting steam generator tubing, the licensee's intent to complete a PWHT of sleeve welds in accordance with the CE topical report should extend the usefullife of sleeved tubes.

The structural analysis of sleeve / tube assemblies in CEN-629-P, Revision 02, assumes a material ultimate tensile strength of 90 ksi for Alloy 690 sleeves. According to the topical report, this value was determined based on actual test data. The sleeve design description in the report specifies that the Alloy 690 tubing "is procured to the requirements of the ASME Boiler and Pressure Vessel Code, Section ll SB-163, Code Case N-20." In addition, Section 2 of the topical report states that the sleeve dimensions, materials, and joints were designed to the applicable ASME Boiler and Pressure Vessel Code. The staff notes that ASME Code Case N-20-3 states,

" nickel-chromium-iron Alloys 600 and 690...may be used in the construction of Class 1 components in accordance with Section ill, Division 1., provided the tensile, yield strength, and design stress intensity values as listed in Tables 1,2, and 4, respectively are used." Table 1 in Code Case N-20-3 lists the ultimate arength of Alloy 600 and 690 as 80 ksi. Therefore, the staff has concluded that the use of 90 ksiin the calculations to evaluate sleeve margins of safety is inconsistent with the design requirements stated in CEN-629-P, Revision 2. However, the staff notes that the use of a 90 ksi tensile strength does not affect the conclusion that the sleeve design will provide adequate structural and leakage integrity for repaired steam generator tubes. In addition, the proposed sleeve repair limit is also unaffected by the use of an incorrect value for material tensile strength.

7.0

SUMMARY

The licensee has proposed changes to amend the license for BVPS-2 to permit the installation of CE Leak Tight Sleeves per topical report CEN-629-P, Revision 2. The changes include a licensee commitment to perform visual acceptance examinations of sleeve welds, perform a PWHT of CE sleeve welds during the installation process, and corhplete additional examinations of sleeve / tube assemblies during inservice inspections. The NRC staff has concluded, based on previous evaluations of CE sleeve repairs and the discussion included in Section 6 of this SE, that the ame.. ament to the BVPS-2 TSs to allow the installation of CE sleeves is acceptable. Although some of the calculations in the referenced topical report are in error due to the use of invalid material properties, the staff notes that the overall results obtained from the analysis are correct.

Therefore, the conclusion of acceptability of the proposed changes remains unchanged.

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8.0 STATE CONSULTATION

in accordance with the Commission's regulations, the Pennsylvania State official was notified of the proposed issuance of the amendment. The State official had no comments.

9.0 ENVIRONMENTAL CONSIDERATION

The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the cmendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released j

offsite, and that there is no significant increase in individual or cumulative occupational radiation.

1 exposure.. The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (62 FR 19829). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set

- forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.

10.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, that:. (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and secuiity or to the health and safety of the public.

Principal Contributor: P. Rush Date:

Fbrch 26, 1999 j

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