ML20202E640

Safety Evaluation Supporting Amend 208 to License DPR-66

ML20202E640
Person / Time
Site:	Beaver Valley
Issue date:	11/25/1997
From:	NRC (Affiliation Not Assigned)
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ML20202E636	List: ML20202E631 ML20202E640
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NUDOCS 9712080099
Download: ML20202E640 (11)

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NUCLEAR REGULATORY COMMISSION waswiwovow, o.c. smess4em

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i SAFETY EVALUATION BY THE OFFICf 0F NUCLEAR REACTOR REGULATION i

RELATED TO 4NENDNENT No.208 TO FACILITY OPERATING t

LICENSE N0. DPR-66 DUQUESNE LIGHT CMPANY OHIO EDIS0N CMPANY PENNSYLVANIA POWER CMPANY 4

g gyER VALLEY POWER STATION. UNIT N0. 1 DOCKET N0. 50-33'4

1.0 INTRODUCTION

By letter dated March 10, 1997, as supplemented July 28 and September 17, changes to the-Beaver Valley Power Statio(n Unit Nos. I and 2 (8VPS-1 and1997, BVPS-2), Technical Specifications (TSs).

The requested changes would modify TS 3/4.4.5, " Steam Generators," and its associated Bases and would aad new license conditions to Appendices C (BVPS-1) and D (BVPS-2) to allow sleeving of steam generator tubes with sleeves designed by ABB Combustion Engineering

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(CE). The sleeves would be welded in place with the tungsten inert gas (TIG) process. The proposed amendment would establish a repair limit of 32 percent for the pressure boundary of any CE sleeve and would delete reference to the previously approved kinetic sleeving methodoloqy. The licensee also committed to perform inservice inspection of installed Cd welded sleeves, to complete a post weld heat treatment (PWHT) of CE sleeve welds, and to visually inspect sleeve welds after installation. These commitments would be reflected in a new license condition.. The following safety evaluation proposed changes for BVPS-1-only.

The staff's review of(SE) considers the the proposed changes to the BVPS-2 is still ongung and will be addressed at a later date.

The July 28 and September 17, 1997, letters provided clarifying information that did not change the initial proposed no significant hazards consideration determination or expand the amendment request beyed the scope of the

. April 23, 1997, Federal Reaister notice.

Three types of CE leak tight sleeves are propossd for use at 8VPS-1.

Two of the three designs are termed Full-Depth Tubesheet (FDTS) sleeves. One FDTS sleeve is welded to the tube near both the u)per and lower ends of-the sleeve.

.The second type is welded to the tube near tie upper end and a hard roll expansion in the lower tubesheet area secures the tube within the steam generator tubesheet.. The third sleeve design spans degraded areas of the steam generator tubs at tube support plates or in a free span section of the

. tube. This Tube Support sleeve is welded at both ends.

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l 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 Generatar Tubes Using Leak Tight Sleeves," and CIN-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 staff evaluations of CE sleeves may also be found in the SEs for Waterford Steam Electric Station, Unit 3, Docket No. 50-382, 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-1 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, material 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 acceptable.

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 including design pressure, operating transicnts, and other parameters selected to envelope loads imposed during normal operating, upset, and accident conditions.

Stress analyses of sleeved tube assemblies were performed in accordance with the requirements of the ASME Boiler and Pressure Vessel Code (ASME Code),Section III.

These analyses, along with the 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 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 im)rovement over the Alloy 600 material used in the original steam generator tu )ing. Corrosion tests conducted under Electric Power Research Institute (EPRI) sponsorship confirm test results regarding the improved corrosion resistance of Alloy 690 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 1.85, ' Materials Code Case Acceptability ASME Section III 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 GTAW (gas-tun? sten arc welding).

The application of this process to the CE sleeve design was qualified and demonstrated during laboratory tests employing full scale sleeve / tube 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 re) air but not a repair with unlimited service life. The installation of CE.eak Tight Sleeves involves welding the sleeve to the tube which potentially creates 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 indicator of potential performance.

Considering the unreliability of sleeve life predictions, the staff has typically required licensees to inspect a sample of sleeves at each outage.

Periodic inservice inspections will provide assurance that any service induced degradation in sleeves is detected and addressed appropriately.

Inservice inspection requirements applicable to the licensee's proposed amendment request are discussed furtier 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 amen hents or addressed in detail in the staff evaluations of the sleeving amendmats previously referenced. These issues include:

(1) the preparation of tube surfaces prior to sleeve welding, (2) the adequacy of inspection 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 liceasee's

)roposal to establish a plugging limit for sleeve degradation is discussed in tie following sections.

3.1 Weld Preparation in order to form an adequate sleeve to tube weld, prior to performing any weld, the surface of the metal (s) to be welded must be cleaned.

For sleeve installation, the inner diameter of the parent tube at the desired weld location must be cleaned of service induced oxides. The presence of impurities during the welding process can lead to the 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 sleeve installation process. This conclusion is based on a CE root cause assessment that concluded that changes in the cleaning process, implemented 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 ths optimum removal of service induced oxides prior to welding.

In order to verify the removal of oxides, a 100 percent visual examination of the cleaned area is performed. CE recommends the visual examination as an interim step until enough field experience is gained to consider adoption of a statistical sampling plan in the future. The licensee for BVPS-1 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 licensee will complete an initial examination of sleeved steam generator tubes prior to returning a repaired tube to service. The acceptance examination includes an inspection using visual, ultrasunic, and eddy current techniques.

The analysis of inspection data from three diverse methods of inspection improves the ability to detect fabrication induced defects, in addition, eddy current data are also used as a baseline for comparison with data obtained in future required periodic inspections.

Past field experience has demonstrated that previous initial acceptance examinations based 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 numbe; of signal properties to more fully characterize a weld. The modified procedure was 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 circumferential1y oriented oxide inclusions would not be detected by ultrasonic inspection techniques.

CE has shown the plus point probe reliably detects the various fabrication induced weld defects including blowholes, weld suckback, and

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' circumferential1y 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 a FDTS 51eeve upper weld, indications located above the weld centerline that are acceptable with other inspection techniques may be left in service.

Hownver, 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 indications 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 veld zone indications that nre 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-1 has indicated its intent to perform a 100 percent visual examination of all sleeve welds for the acceptance examinations. This commitment will be included in a license condition described in Section 5.2 of this SE.

3.3 Inservice inspection Requirements The licensee has proposed to complete sample inspection of 20 percent of installed sleeves at each refueling outage.

If degradation or detects 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 proposal is 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 arimary element in the development of service induced stress corrosion crac(ing 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 Lf 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-1, the licensee has committed to perform a PWHT of the sleeve welds.

This commitment is included in a license condition discussed in Section 5.3 of this safety evaluation.

! 3.5 Sleeve Plugging Limits The sleeve minimum acceptable wall thickness is determined usinr: the criteria ofRegulatoryGuide(RG)1.121,"PasesforPluggingDegradedPWASteam Generator Tubes" and ASME Code Section III allowable stress values and pressure stress equations. According to RG 1.121 criteria, an allowance for NDE uncertainty and postulated operazional growth of tube wall degradation within the sleeve must be accounted fcr when using NDE to de^trmine 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 structural limit, nich was calculated based on the most limiting of normal, upset, or faulted conditions for 7/8-inch outside diameter steam generator tubes in Westinghouse Model 44 and 51 steam generators, was 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.

4.0 PROPOSED TECHNICAL SPECIFICATION CHANGES In ordtr to incorporate the proposed changes to permit sleeving of the BVPS-1 steam geneiators using CE welded sleeves and to eliminate a reference to a sleeving methodology no lenger used, the licensee has proposed the following changes t) the TSs.

1. Proposed Changes 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 Combustion Engineering 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 installatien of Bat,tock and Wilcox kinetic welded sleeves is deleted and replaced by a reference to the ABB Ccmbustion Engineering topical reports, CEN-629-P, Revision 02, and CEN-624-P Addendum 1, for the TIG weldad sleeves.

3. Proposed Changes to Bases 3/4.4.5, " Steam Generators" The Bases Section is modified to indicate that the technical bases for sleeving are described in the CE topical report referenced in TS 4.4.5.4.a.9.

1 l 5.0 LICENSEE COMMITMENTS IN SUPPORT OF AMENDMENT RE00EST In support of the proposed amendment request, the licensee has comitted to perform an inservice inspection of CE welded sleeves which establishes the sampling and expansion scope for examining CE welded sleeves. The licensee has comitted to complete a visual inspection of sleeve welds in addition to eddy current and ultrasonic test examinations at the time of sleeve installation. The licensee also comitted to perform a PWHT of sleeve welds.

Details of these comitments are described below. These comitents are reflected in a new license condition that is being added to Appendix C to BVPS-1 Facility Operating License No. OPR-66 as proposed in the licensee's September 17, 1997, letter.

5.1 Inservice Inspection of CE Welded Sleeves The following license condition is imposed to require an inspection of sleeves during each inservice inspection.

J. The tubes selected as the second sanple (if required by Table 1) during each inservice inspection may be subjected to a partial tube inspection provided:

a. The tubes selected for the se and sample includes the tubes from those areas of the tube sheet array where tubes with imperfections were previously found, and,

b. The inspections in:1ude those portions of the tubes where imperfections were previously found.

2. If the inservice inspection of a steam generator conducted in accordance with Table 1 requires k 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 the inspection criteria cre satisfied; the interval may then be extended to a maximuin of 40 months.

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

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.

4. 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 Table 1.

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I TABLE 1: STEAM GENERATOR (5.G.) REPAIRED TUBE INSPECTION (3)

IST SAMPLE INSPECTION 2ND SAMPLE INSPECTION SAMPLE SIZE

! RESULT ACTION REQUIRED RESULT ACTION REQUIRED A minimum of C-1 None N.A N.A

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20% of It re> aired tu)es (1)(2)

C-2 Plug defective C-1 HONE t

repaired tubes and M

inspect 100% of the repaired tubes in C-2 Plug defective this S.G.

repaired tubes C-3 Perform action for C-3 result of first sample C-3 Inspect all All other NONE repaired tubes in S.G.s arf T-1 this S.G., plug defective tubes and inspect 20% of the Some S.G.s Perform action repaired tubes in C-2 but no for C-2 result of each other S.G.

additional first sample S.G.s are C-3 Additional Inspect all S.G. is C-3 repaired tubes in each S.G. and plug defective tubes.

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

(1)

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

(2)

The inspection of repaired tubes may be performed on tubes from 1 to 3 steam generators based on outage plans.

(3)

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

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5.2 Visual Acceptance Examination of Sleeve Welds The following is imposed to support the licensee's comitment 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 subjrzted 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 comitment 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, Revisior 2), can be accomplished to produce sleeved tubes of acceptable metallurgical properties, corrosion resistance, and structural and leakage integrity. As indicated previously, recent experience has indicated the need to evaluate additional artas that were not applicable to previous CE sleeving amendments or were not addressed in detail in previously approved CE slesving amendments. The staff evaluation of these issues is discussed below.

Visual surface examinations of the tube inside diameter surface will be used to confirm the adequacy of the cleaning process. The licensee indicated in their amendment that this requirement may be relaxed if the cleaning process appears to be effective. The use of the visual examination will enable the vendor to detect inadequately cleaned surfaces prior to sleeve installation and welding. Welding the sleeve to a poorly cleaned tube surface may result in weld zone indications and possible rejection of thL sleeve weld.

Experience has demonstrated that the indications that arise from inadequate cleaning are detectable using eddy current and ultrasonic inspection methods.

Should the licensee elect to eliminate the visual inspections, this may increase the sleeve weld rejection rate; however, the adequacy (i.e.,

structural and leakage integrity) of the sleeve weld will eventually be assessed through post weld examinations. Because the visual inspection of cleaned tube surfaces does not affect the final disposition of sleeve welds, the staff concludes that the licensee's proposal to make these inspections optional is acceptable.

l The licensee's commitment to visually inspect sleese welds after installation addresses potential concerns resulting from experience at another plant where indications were identified only by visual inspection methods.

At this time, industry experience with these indications is limited. The licensee has proposed a reasonable approach to identify indications that could potentially affect the structural and leakage integrity of CE welded sleeves.

The Beaver Valley Steam Generator Tube Surveillance Requirements currently require an examination of at least three percent of the inservice steam 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 pro)osed scope for the int,ervice inspection of sleeves increases the probability t1at 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 useful life 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 Boller and Pressure Vessel Code,Section II 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 III, 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 strength of Alloy 600 and 690 as 80 ksi. Therefore, the staff has concluded that the use of 90 ksi in 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 to amend the BVPS-1 TS to permit the installation of CE Leak Tight Sleeves per topical report CEN-629-P, Revision 2.

In addition, the licensee has also committed to perform visual examinations of sleeve 1

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welds, complete additional examinations of sleeve / tube assemblies during inservice inspections, and perform a PWHT of CE sleeve welds during the installation process. The NRC staff has concluded, based on previous evaluations of CE sleeve re) airs and the discussion included Section 6 of this SE, that the amendment to tme BVPS-1 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 TS changes remains unchanged.

8.0 STATE CON 5L'LTATION In accordance with the Comission's regulations, the Pennsylvania State official was notified of the proposed issuance of the amendments. The State official had no coments.

9.0 ENVIRONMENTAL CONSIDERATI0!i The amendment changes surveillance requirements. The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be releasec offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Comission has previously is ued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public coment on such finding (62 FR 19829). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(?). Pursuant to 10 CFR 51.22(b) no environmental imoact 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 wi)l not be endangered by operation in the proposed manner, (2) such l

activities will be conducted in compliance with the Comission's regulations, and (3) the issuance of the amendment will not be inimical to the comon defense and security or to the health and safety of the public.

Principal Contributor:

P. Rush Date:

November 25, 1997

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