NRC Generic Letter 1995-05

NRC Generic Letter 1995-005: Voltage Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking

ML031470641
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Issue date:	08/03/1995
From:	Office of Nuclear Reactor Regulation
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GL-95-005
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NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... Index Site Map FAQ Help Glossary Contact Us l[Sa ]A'x$) (U.S. Nuclear Regulatory Commissionll 11 Reactors ll Nuclear llRadioactive ll Public Home Who We Are flWhat We Do fl Nuclear Nula f aiaciePbiI 0 0 R~~~~eactors fl Materials Waste flInvolvementHome > Electronic Reading Room > Document Collections > Generic Communications > Generic Letters> 1995 > GL95005UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001August 3, 1995NRC GENERIC LETTER 95-05: VOLTAGE-BASED REPAIR CRITERIA FOR WESTINGHOUSESTEAM GENERATOR TUBES AFFECTED BY OUTSIDE DIAMETERSTRESS CORROSION CRACKING

Addressees

All holders of operating licenses or construction permits forpressurized-water reactors (PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter togive guidance to licensees who may wish to request a license amendment to theplant technical specifications to implement alternate steam generator tuberepair criteria applicable specifically to outside diameter stress corrosioncracking (ODSCC) at the tube-to-tube support plate intersections inWestinghouse-designed steam generators having drilled-hole tube support plates(TSPs) and alloy 600 steam generator tubing. In the past, the NRC has allowedsome licensees to implement alternate steam generator repair criteria for thisparticular degradation mechanism on an operating cycle-specific basis. Thisgeneric letter does not restrict the approval of such repair criteria to acycle specific basis. It is expected that recipients will review thisinformation for applicability to their facilities and consider actions, asappropriate, to implement the alternate criteria. However, suggestionscontained in this generic letter are not NRC requirements; therefore, nospecific action or written response is required.BackgroundThe tubing of the steam generator constitutes more than half of the reactorcoolant pressure boundary (RCPB). Design of the RCPB for purposes ofstructural and leakage integrity is a requirement under Title 10 of the Codeof Federal Regulations Part 50 (10 CFR Part 50), Appendix A. Specificrequirements governing the maintenance of steam generator tube integrity arein plant technical specifications and in Section XI of the American Society ofMechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code).These include requirements for periodic inservice inspection of the tubing,flaw acceptance criteria (i.e., repair limits for plugging or sleeving), andprimary-to-secondary leakage limits. These requirements, coupled with thebroad scope of plant operational and maintenance programs, have formed thebasis for ensuring adequate steam generator tube integrity.9507310085. GL 95-05August 3, 1995 Flaw acceptance criteria, termed plugging' or "repair limits," are specifiedin the plant technical specifications. Current plant technical specificationsrequire that flawed tubes be removed from service by plugging or be repairedby sleeving, if the depths of the flaws exceed the repair limit, typically40 percent through-wall. The technical specification repair limits ensurethat tubes accepted for continued service will retain adequate structural andleakage integrity during normal operating, transient, and postulated accidentconditions, consistent with General Design Criteria (GDCs) 14, 15, 30, 31, andhttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... 32 of 10 CFR Part 50, Appendix A. Structural integrity refers to maintainingadequate margins against gross failure, rupture, and collapse of the steamgenerator tubing. Leakage integrity refers to limiting primary-to-secondaryleakage to within acceptable limits.The traditional strategy for achieving the objectives of the GDCs related tosteam generator tube integrity has been to establish a minimum wall thicknessrequirement in accordance with the structural criteria of Regulatory Guide(RG) 1.121, Bases for Plugging Degraded PWR Steam Generator Tubes.'Development of minimum wall thickness requirements to satisfy RG 1.121 wasgoverned by analyses for uniform thinning of the tube wall in the axial andcircumferential directions. The assumption of uniform thinning results indevelopment of a repair limit that is conservative for all flaw typesoccurring in the field. The resultant 40-percent depth-based repair limittypically incorporated into the technical specifications is conservative forhighly localized flaws such as pits, short cracks, and in particular ODSCCthat occurs at TSPs.This generic letter offers guidance on the implementation of an alternaterepair criterion to be applied to predominantly axially oriented ODSCC at TSPlocations. This criterion does not set limits on the depth of ODSCCindications to ensure tube integrity margins; instead, it relies oncorrelating the eddy current voltage amplitude from a bobbin coil probe withthe more specific measurement of burst pressure and leak rate. The staffrecognizes that although total margin may be reduced following application ofthe voltage-based repair guidance of this generic letter, this guidance doesensure that structural and leakage integrity continues to be maintained withan acceptable level of margin consistent with the applicable GDCs of 10 CFRPart 50, Appendix A and the limits of 10 CFR Part 100. Since thevoltage-based repair criteria do not incorporate minimum wall thicknessrequirements, there is a possibility that tubes with up to 100-percentthrough-wall cracks can remain in service. Because of the increasedlikelihood of through-wall cracks, the staff has included provisions foraugmented steam generator tube inspections and more restrictive operationalleakage limits in this generic letter guidance.In taking the action described in this letter, the NRC staff is approving theuse of the specific voltage-based repair criteria described herein as anacceptable measure for dealing with ODSCC tube degradation. This actionGL 95-05August 3, 1995 should not be construed to discourage licensees from using better or furtherrefined data acquisition techniques, eddy current technology, and eddy currentdata analysis techniques as they become available. The staff stronglyencourages the industry to continue its efforts to improve the nondestructiveexamination (NDE) of steam generator tubes and continues to believe thatinspection methods and repair criteria based on physical dimensions (e.g.,length and depth) of defects are the most desirable when they can be achieved.This generic letter is intended to provide relief while maintaining anacceptable level of safety for licensees having steam generators experiencingthis particular degradation mechanism while the staff pursues a longer termresolution to the issue of steam generator degradation through the developmentof a steam generator rule. The staff recognizes that licensees may wish topursue various alternatives to the guidance of this generic letter [e.g.,alternative probability of detection (POD) calculations]. However, licenseesshould recognize that pursuing such alternatives could have significantschedular implications, since the NRC staff would be required to review andapprove the associated information and analyses.Discussion1. Overview of the Voltage-Based ApproachIn order to use the voltage-based repair criteria, licensees should completethe following actions:Perform an enhanced inspection of tubes, particularly at the TSPintersections.Utilize NDE data acquisition and analysis procedures that are consistenthttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... with the methodology used to develop the voltage-based repair criteria..Repair tubes that exceed the voltage limits..Determine the beginning-of-cycle (BOC) voltage distribution..Project the end-of-cycle (EOC) voltage distribution..For the projected EOC voltage distribution, calculate both the primary-to-secondary leakage under postulated accident conditions and the conditionalburst probability. As an alternative, the actual measured EOC voltagedistribution can be used when it is impractical to complete the projectedGL 95-05August 3, 1995 EOC calculation prior to returning the steam generators to service for thepurpose of determining whether the reporting criteria in GL Sections 6.a.1and 6.a.3 apply.2. Generic Letter ApplicabilityThe criteria in this generic letter are only applicable to predominantlyaxially oriented ODSCC indications located at the tube-to-TSP intersections inWestinghouse-designed steam generators with alloy 600 steam generator tubing.These criteria are not applicable to other forms of steam generator tubedegradation, nor are they applicable to ODSCC that occurs at other locationswithin a steam generator. The voltage-based repair criteria can be appliedonly under the following constraints:The repair criteria of this generic letter apply only to Westinghouse-designed steam generators with 1.9-cm [3/4-inch} and 2.2-cm [7/8-inch]diameter alloy 600 tubes and drilled-hole TSPs.The repair criteria of this generic letter apply only to predominantlyaxially oriented ODSCC confined within the tube-to-TSP intersection (referto Section l.a of Attachment 1 for further guidance).Certain intersections are excluded from the application of thevoltage-based repair criteria, as discussed in Section l.b ofAttachment 1.3. Voltage Repair LimitsThe voltage repair limits are:Indications less than the lower voltage repair limit, as measured bybobbin coil, may remain in service. For 2.2-cm [7/8-inch] diameter tubes,the lower voltage repair limit is 2.0 volts. For 1.9-cm [3/4-inch]diameter tubes, the lower voltage repair limit is 1.0 volt.Indications greater than the lower limit and less than or equal to theupper voltage repair limit, as measured by bobbin coil, can remain inservice if rotating pancake coil (RPC) inspections do not confirm theindications. The methodology for calculating the upper voltage repairlimit is specified in Section 2.a.2 and 2.a.3 of Attachment 1.Indications greater than the lower limit and less than or equal to theupper voltage repair limit, as measured by bobbin coil, that are confirmedby RPC, and indications greater than the upper voltage repair limit, asmeasured by bobbin coil, must be repaired.The voltage-based repair limits in this generic letter were determinedconsidering the entire range of design basis events that could challenge tubeintegrity. The voltage-based repair criteria ensure structural and leakageGL 95-05August 3, 1995 integrity for all postulated design basis events. The structural criteria areintended to ensure that indications subjected to the voltage repair limitswill be able to withstand pressure loadings consistent with the criteria ofhttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... RG 1.121. The leakage criteria ensure that for degradation subjected to thevoltage repair criteria, induced leakage under worst-case MSLB conditionscalculated using licensing basis assumptions, will not result in offsite andcontrol room dose releases that exceed the applicable limits of 10 CFRPart 100 and GDC 19.Requested ActionsImplementation of the guidance in this generic letter is voluntary. Alicensee that chooses to implement these criteria should include all of thefollowing in the proposed program:.Implementation of the applicability requirements discussed in Section 1 ofAttachment 1. The applicability requirements ensure that the repaircriteria are applied only to those intersections for which the voltage-basedrepair criteria were developed..Implementation of the inspection guidance discussed in Section 3 ofAttachment 1. The inspection guidance ensures that the techniques used toinspect the steam generator tubes are consistent with the techniques used todevelop the voltage-based repair criteria..Calculation of leakage according to the guidance discussed in Section 2.b ofAttachment 1. This calculation, in conjunction with the use of licensingbasis assumptions for calculating offsite and control room doses, enableslicensees to demonstrate that the applicable limits of 10 CFR Part 100 andGDC 19 continue to be met. This calculation is performed using theprojected EOC voltage distribution for the next cycle of operation. If itis not practical to complete this calculation prior to returning the steamgenerators to service, the measured EOC voltage distribution can be used(from the previous cycle of operation) as an alternative (refer toSection 2.c of Attachment 1) for the purposes of determining whether thereporting criteria of Section 6.a.1 apply..Calculation of conditional burst probability according to the guidancediscussed in Section 2.a of Attachment 1. This is a calculation to assessthe voltage distribution for the next cycle of operation. The results arecompared against a threshold value. This calculation is performed using theprojected EOC voltage distribution for the next cycle of operation. If itis not practical to complete this calculation prior to returning the steamgenerators to service, the measured EOC voltage distribution can be used(from the previous cycle of operation) as an alternative (refer toSection 2.c) for the purposes of determining whether the reporting criteriaof Section 6.a.3 apply.GL 95-05August 3, 1995 .Implementation of the operational leakage monitoring program according tothe guidance discussed in Section 5 of Attachment 1. The operational leakrate monitoring program is a defense-in-depth measure that provides a meansfor identifying leaks during operation to enable repair before such leaksresult in tube failure..Acquisition of tube pull data according to the guidance discussed inSection 4 of Attachment 1..Reporting of results according to the guidance discussed in Section 6 ofAttachment 1..Submittal of a technical specification (TS) amendment request that commitsto the preceding actions and provides TS pages according to the guidancediscussed in Attachment 2, including the associated no significant hazardsconsideration' (10 CFR 50.92) and supporting safety analysis.Licensees planning to adopt this TS amendment are encouraged to follow theguidance discussed in Attachments 1 and 2. Whenever practical, the staffrequests that licensees following the guidance of this generic letter submittheir TS amendment request at least 90 days prior to the beginning of theoutage during which the alternate repair criteria are to be implemented.

Backfit Discussion

http://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005 .html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... Licensee action to propose TS changes under the guidance of this genericletter is voluntary; therefore, such action is not a backfit under theprovisions of 10 CFR 50.109.

Paperwork Reduction Act Statement

The voluntary information collections contained in this request are covered bythe Office of Management and Budget clearance number 3150-0011, which expiresJuly 31, 1997. The public reporting burden for this voluntary collection ofinformation is estimated to average 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> per response, including the timefor reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection ofinformation. Send comments regarding this burden estimate or any other aspectof this voluntary collection of information, including suggestions forreducing this burden, to the Information and Records Management Branch(T-6 F33), U.S. Nuclear Regulatory Commission, Washington, D.C. 20555-0001,and to the Desk Officer, Office of Information and Regulatory Affairs,NEOB-10202 (3150-0011), Office of Management and Budget, Washington, D.C.20503.GL 95-05August 3, 1995 This generic letter requires no specific action or written response. If youhave any questions about this matter, please contact the technical contactlisted below or the appropriate Office of Nuclear Reactor Regulation (NRR)project manager./s/'d by DMCrutchfieldDennis M. Crutchfield, DirectorDivision of Reactor Program ManagementOffice of Nuclear Reactor RegulationTechnical contact: Emmett Murphy, NRR(301) 415-2710Lead project manager: Gordon E. Edison, NRR(301) 415-1448Attachments:1. Guidance for a Proposed License Amendment to Implement an Alternate SteamGenerator Tube Repair Limit for Outside Diameter Stress Corrosion Crackingat the Tube Support Plate Intersections2. Model Technical Specifications3. List of Recently Issued NRC Generic Letters. Guidance for a Proposed License Amendment toan Alternate Steam Generator Tube Repair Limit for Outside Diameter StressCorrosion Cracking at the Tube Support Plate Intersections1. IntroductionThis guidance is the NRC staff position on the implementation of thevoltage-based repair criteria in steam generators designed by Westinghouse foroutside diameter stress corrosion cracking (ODSCC) located at the tube-to-TSPintersections. This guidance is not applicable to other forms of steamgenerator (SG) tube degradation nor is it applicable to ODSCC that occurs atother locations within the SG. The voltage-based repair criteria have beendeveloped for, and are currently applicable only to, Westinghouse designed SGswith 2.2-cm [7/8-inch] or 1.9-cm [3/4-inch] diameter alloy 600 tubes withdrilled-hole TSPs. Application of the alternate repair criteria to othervendor designed SGs would require both the development and NRC staff reviewand approval of a comparable data base and the associated correlations foreach vendor's SG type.The NRC staff emphasizes that although the NRC has approved the implementationof the voltage-based repair criteria (described in this generic letter) as ashort-term measure, this guidance should not be construed as discouraging thedevelopment and use of better acquisition techniques, eddy current technology,and eddy current data analysis techniques. The staff strongly encourages theindustry to continue to improve the nondestructive examination (NDE) of SGhttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... tubes.l.a ODSCCThe voltage-based repair criteria are applicable only to indications at TSPintersections where the degradation mechanism is dominantly axial ODSCC withno NDE detectable cracks extending outside the thickness of the support plate.For purposes of this guidance, ODSCC refers to degradation whose dominantmorphology consists of axial stress corrosion cracks which occur eithersingularly or in networks of multiple cracks, sometimes with limited patchesof general intergranular attack (IGA). Circumferential cracks may sometimesoccur in the IGA affected regions producing a grid-like pattern of axial andcircumferential cracks, termed "cellular corrosion." Cellular corrosion isassumed to be relatively shallow (based on data from tube specimens removedfrom the field), transitioning to dominantly axial cracks as the crackingprogresses in depth. The circumferential cracks are assumed (based onavailable data) to be of insufficient size to produce a discrete, crack-likecircumferential indication during field NDE inspections. Thus, the failuremode of ODSCC is axial and the burst pressure is controlled by the geometry ofthe most limiting axial crack or array of axial cracks.It is also assumed for purposes of this guidance that the ODSCC is confined towithin the thickness of the TSP, based on data from tube specimens removedfrom the field. Very shallow microcracks are sometimes observed on thesespecimens to initiate at locations slightly outside the thickness of the TSP;however, these microcracks are small compared to the cracks within thethickness of the TSP and are too small to produce an eddy current response.The degradation mechanism should be confirmed as dominantly axial ODSCC byperiodically removing tube specimens from the SGs and by examining and testingthem as specified in Section 4 of this guidance. The acceptance criteriashould consist of demonstrating that the dominant degradation mechanismaffecting the burst and leakage properties of the tube is axially orientedODSCC. In addition, results of inservice inspections with rotating pancakecoil (RPC) probes should be evaluated in accordance with Section 3.b of thisguidance to confirm the absence of detectable crack-like circumferentialindications and detectable ODSCC indications extending outside the TSPthickness.l.b Exclusion of IntersectionsThe voltage-based repair criteria of this guidance do not apply tointersections meeting the criteria specified below.l.b.l The repair criteria do not apply to tube-to-TSP intersections where thetubes with degradation may potentially collapse or deform as a result of thecombined postulated loss-of-coolant accident and safe shutdown earthquakeloadings (e.g., intersections near the wedge supports at the upper TSPs).Licensees should perform or reference an analysis that identifies whichintersections are to be excluded.l.b.2 The repair criteria do not apply to tube-to-TSP intersections havingdent signals greater than 5.0 volts as measured with the bobbin probe.l.b.3 The repair criteria do not apply to intersections at which there aremixed residuals of sufficient magnitude to cause a 1.0 volt ODSCC indication(as measured with a bobbin probe) to be missed or misread.l.b.4 The repair criteria do not apply to intersections with interferingsignals from copper deposits.l.b.5 The repair criteria do not apply to the tube-to-flow distributionbaffle plate intersections except as discussed in Section 2.a.3.2. Tube Integrity EvaluationLicensees should perform an evaluation to confirm that the SG tubes willretain adequate structural and leakage integrity until the next scheduledinspection. The first portion of this evaluation, referred to as the"conditional burst probability calculation," assesses the voltage distributionleft in service against a threshold value of 1 p 10-2 probability of ruptureunder postulated main steamline break (MSLB) conditions. The conditionalburst probability calculation is intended to provide a conservative assessmenthttp://www.nrc.gov/reading-rm/doc-collections/gen-conmgen-letters/I 995/gl95005.htrnl 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... of tube structural integrity during a postulated MSLB occurring at the end ofcycle (EOC). It is used to determine whether the NRC needs to focusadditional attention on the particular voltage repair limit application. Ifthe calculated conditional burst probability exceeds 1 10-2, the licenseeshould notify the NRC according to the guidance in Section 6.The second portion of the tube integrity evaluation is intended to ensure thatthe total leak rate from the affected SG during a postulated MSLB occurring atEOC would be less than a rate that could lead to radiological releases inexcess of the licensing basis for the plant. If calculated leakage exceedsthe allowable limit determined by the licensing basis dose calculation,licensees can either repair tubes, beginning with the largest voltageindications until the leak limit is met, reduce reactor coolant systemspecific iodine activity [refer to example technical specification (TS) pagesof Attachment 2], or reduce the length of the operating cycle. The analysesdiscussed above may incorporate or reference previous analyses, or portionsthereof, to the extent that they continue to bound the conditions of the SG asdetermined by inspection.For plants in which the TSs do not require the pressurizer power-operatedrelief valves (PORVs) to be operable during power operation, these tubeintegrity analyses should be conducted for an assumed differential pressureacross the tube walls equal to the pressurizer safety valve setpoint plus3 percent for the valve accumulation, less atmospheric pressure in faultedSGs. For plants in which the TSs do require the PORVs to be operable, theassumed differential pressure for the conditional burst probabilitycalculation may be based on the PORV setpoint in lieu of the safety valvesetpoint with similar adjustments. The TS requirements for operation withPORV block valves closed due to leaking PORVs should be in accordance withEnclosure A of Generic Letter 90-06, 'Resolution of Generic Issue 70, Power-Operated Relief Valve and Block Valve Reliability,' and Generic Issue 94,'Additional Low-Temperature Overpressure Protection for Light-Water Reactors,'Pursuant to 10 CFR 50.54(f)." That is, electrical power to the block valvesmust be maintained to allow continued operation with the block valves closed,as required in the sample technical specification Section 3.4.4 of GL 90-06.2.a Conditional Probability of Burst During an MSLBFor this generic letter, the conditional probability of burst refers to theprobability that the burst pressures associated with one or more indicationsin the faulted SG will be less than the maximum pressure differentialassociated with a postulated MSLB assumed to occur at EOC. A methodologyshould be submitted for NRC approval for calculating this conditional burstprobability. After the NRC approves a method for calculating conditionalprobability of burst, licensees may reference the approved method. Thismethodology should involve (1) determining the distribution of indications asa function of their voltage response at the beginning of cycle (BOC) asdiscussed in Section 2.b.1, (2) projecting this BOC distribution to an EOCvoltage distribution based on consideration of voltage growth due to defectprogression between inspections as discussed in Section 2.b.2(2) and voltagemeasurement uncertainty as discussed in Section 2.b.2(l), and (3) evaluatingthe conditional probability of burst for the projected EOC voltagedistribution using the correlation between burst pressure and voltagediscussed in Section 2.a.l. The solution methodology should account foruncertainties in voltage measurement [Section 2.b.2(l)], the distribution ofpotential voltage growth rates applicable to each indication[Section 2.b.2(2)], and the distribution of potential burst pressures as afunction of voltage (Section 2.a.1). Monte Carlo simulations are anacceptable approach for accounting for these various sources of uncertainty.2.a.1 Burst Pressure Versus Bobbin VoltageAn empirical model, for 7/8-inch or 3/4-inch diameter tubing, as applicable,should be used to relate burst pressure to bobbin voltage response forpurposes of estimating the conditional probability of burst during apostulated MSLB. The model should consider, at a minimum, the scale factorsfor the coordinate system (e.g., linear or logarithmic), the detection andtreatment of outliers, the order of the regression equation, the potentialinfluence of measurement errors in the variables, and the evaluation of theresiduals following the development of a relationship. This model shouldexplicitly account for burst pressure uncertainty as indicated by scatter ofthe supporting test data and should also account for the parametric (i.e.,slope and intercept) uncertainty of the regression fit of the data.Currently, an approved model consists of determining a linear first-orderequation between the burst pressure and the logarithm (base 10) of the bobbinhttp:/www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... voltage amplitude with standard least-squares linear regression analysis. Themodel may need to be changed as additional information is acquired; however,such changes should be submitted to the NRC staff for approval. Thesupporting data sets for 7/8-inch diameter and 3/4-inch diameter tubing shouldcontain all applicable data consistent with the latest revision of theindustry data base as approved by the NRC. The currently approved data basefor burst pressure as a function of voltage is given in Reference 7, assupplemented by References 1 and 2.2.a.2 Determination of the Upper Voltage Repair Limit for TSP IntersectionsFrom the regression relationship (discussed above in Section 2.a.1), a lower95-percent prediction bound should be determined for the burst pressure as afunction of bobbin voltage amplitude. The lower 95-percent predictioninterval is further reduced to account for the lower 95/95-percent tolerancebound for tubing material properties at 650 oF. Using this reduced lowerprediction bound curve, the structural limit is determined for a free spanburst pressure of 1.4 times MSLB differential pressure kPMSLB) consistent withthe structural limits in RG 1.121.To determine the upper voltage repair limit, the structural limit is reducedto account for flaw growth and voltage measurement uncertainty. The methodfor determining the flaw growth allowance is discussed in Section 2.b.2(2) andshould be a plant-specific average growth rate or 30-percent per effectivefull power year (EFPY), whichever is larger. The voltage measurementuncertainty allowance should be the 95-percent cumulative probability valuefor the voltage measurement uncertainty models. Eddy current voltagemeasurement uncertainty is discussed in Section 2.b.2(l). Currently the95-percent cumulative probability value is 20 percent of the BOC voltageamplitude. The upper voltage repair limit should be determined prior to eachoutage, using the most recently approved NRC data base.2.a.3 Determination of the Upper Voltage Repair Limit for Flow DistributionBaffle Plate IntersectionsBecause of the greater diametral clearances at the flow distribution baffleplate and the potential for higher voltage growth rates at theseintersections, different tube repair limits may be necessary for indicationsat the flow distribution baffle plate. To determine if the voltage-basedrepair criteria can be applied to flow distribution baffle plateintersections, the causal factors for the high voltage growth rates and theapplicability of these conditions at the plant should be assessed. Thisassessment should be provided to the NRC for approval along with the originalamendment request.If the assessment indicates that the voltage-based repair criteria can beapplied to indications at the flow distribution baffle plate, the methodologyused for calculating the upper voltage repair limit for the flow distributionbaffle plate intersections should be identical to the methodology discussed inSection 2.a.2, except as noted below:The tube structural limit should be determined for a free span burstpressure of 3 times normal operational differential pressure (PNO) or1.4kPMSLB, whichever is more limiting, because of the greater diametralclearances at the flow distribution baffle plate.The growth rate of the indications at the flow distribution baffle plateshould be determined and should be compared to the growth rates ofindications at TSP elevations to determine if the growth rate at the flowdistribution baffle is considerably different from the growth rate at theTSPs. If the growth rate of indications at the flow distribution baffleplate is higher than the remainder of the indications, this growth rate, ora higher one, should be use for determining the upper voltage repair limitfor flow distribution baffle plate indications. The growth rate should notbe less than 30-percent per EFPY, as discussed in Section 2.a.2. The uppervoltage repair limit for the flow distribution baffle should be determinedbefore each outage using the most recently approved NRC data base.2.b Total Leak Rate During MSLBA leak rate methodology is approved in Reference 8 and is described inReference 9. Licensees may reference the approved method. The leak ratemethodology involves (1) determining the distribution of indications as afunction of their voltage response at BOC as discussed in Section 2.b.1, (2)projecting this BOC distribution to an EOC voltage distribution based onhttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005 .html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Gener... consideration of voltage growth due to defect progression between inspectionsas discussed in Section 2.b.2(2) and voltage measurement uncertainty asdiscussed in Section 2.b.2(l), and (3) evaluating the total leak rate for theprojected EOC voltage distribution using a probability of leakage (POL) model,as discussed in Section 2.b.3(l), and the conditional leak rate model, asdiscussed in Section 2.b.3(2). The solution methodology should account foruncertainties in voltage measurement [Section 2.b.2(l)], the distribution ofpotential voltage growth rates applicable to each indication[Section 2.b.2(2)], the uncertainties in the probability of leakage as afunction of voltage [Section 2.b.3(l)], and the distribution of potentialconditional leak rates as a function of voltage [Section 2.b.3(2)]. MonteCarlo simulations are an acceptable method for accounting for these sources ofuncertainty, provided that the calculated total leak rate reflects an upper95-percent quantile value at an upper 95-percent confidence bound.2.b.1 Distribution of Bobbin Indications as a Function of Voltage at BOCThe frequency distribution by voltage of bobbin indications actually foundduring inspection should be scaled upward by a factor of 1/POD to account fornon-detected cracks which can potentially leak or rupture under postulatedMSLB conditions during the next operating cycle. This adjusted frequencydistribution minus detected indications for tubes that have been plugged orrepaired should constitute, for purposes of the tube integrity analyses, theassumed frequency distribution of bobbin indications at BOC as a function ofvoltage. This can also be expressed asNl = (/POD)(Nd) -Nr (1)where:Nl = assumed frequency distribution of bobbin indicationsNd = frequency distribution of indications actually detectedNr = frequency distribution of repaired indicationsPOD = probability of detection of ODSCC flawsPOD should be assumed to have a value of 0.6, or as an alternative, an NRCapproved POD function can be used, if such a function becomes available.Nd includes all flaw indications detected by the bobbin coil, regardless ofwhether these indications are confirmed by rotating pancake coil (RPC)inspection. Alternatively, a fraction of bobbin indications at locationswhich have been inspected with an RPC probe, but where the RPC failed toconfirm the bobbin indication, may be excluded from Nd subject to NRCapproval.If the steam generators have been chemically cleaned, the impact of thechemical cleaning on the BOC voltage distribution needs to be evaluated.2.b.2 Projected End-of-Cycle (EOC) Voltage DistributionAs discussed above, the calculation of both conditional burst probability andleakage (during a postulated MSLB) requires the generation of the projectedEOC voltage distribution. To project an EOC voltage distribution from the BOCvoltage distribution determined above, requires consideration of (1) eddycurrent voltage measurement uncertainty and (2) the addition of voltage growthto account for defect progression. Monte Carlo techniques are an acceptablemeans for sampling eddy current measurement uncertainty and the voltage growthdistribution to determine the projected EOC voltage distribution. Eddycurrent measurement uncertainty and voltage growth are discussed below.2.b.2(l) Eddy Current Voltage Measurement UncertaintyUncertainty in eddy current voltage measurements stems primarily from twosources:voltage response variability (i.e., test repeatability error) which stemsprimarily from probe wearvoltage measurement variability among data analysts (i.e., measurementrepeatability error)Each of these uncertainties should be quantified. An acceptablecharacterization of these uncertainties is contained in EPRI TR-100407,Revision 1, Draft Report August 1993, "PWR Steam Generator Tube RepairLimits-Technical Support Document for Outside Diameter Stress CorrosionCracking at the Tube Support Plates" (Reference 3), Sections 2.4.1, 2.4.2, andD.4.2.3, with the exception that no distribution cutoff should be applied tohttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... the voltage measurement variability distribution. (However, the assumed15 percent cutoff for the voltage response variability distribution inReference 3 is acceptable.)2.b.2(2) Voltage Growth Due to Defect ProgressionPotential voltage growth rates during the next inspection cycle (i.e.,operating cycle between two scheduled SG inspections) should be based onvoltage growth rates observed during the last one or two inspection cycles.For a given inspection, previous inspection results at tube-to-TSPintersections currently exhibiting a bobbin indication should be evaluatedconsistent with the data analysis guidelines in Section 3 below. In cases inwhich data acquisition guidelines employed during previous inspections differfrom those discussed in Section 3, the evaluation of the previous data shouldbe adjusted to compensate for the difference. Voltage growth rates shouldonly be evaluated for those intersections at which bobbin indications can beidentified at two successive inspections, except if an indication changes fromnon-detectable to a relatively high voltage (e.g., 2.0 volts).The distribution of voltage growth rates (based on the change in voltage on anintersection-to-intersection basis) should be determined for each of the lastone or two inspection cycles. When only the current or only the current andprevious inspections employed data acquisition guidelines similar to thosediscussed in Section 3, only the growth rate distribution for the previouscycle should be used to estimate the voltage growth rate distribution for thenext inspection cycle. If both of the two previous inspections employed suchsimilar guidelines, the most limiting of the two previous growth ratedistributions should be used to estimate the voltage growth rate distributionfor the next inspection cycle. However, the two distributions should becombined if one or both of the distributions is based on a minimal number(i.e., < 200) of indications. If the growth rate distribution, or combineddistribution from two cycles, consists of fewer than 200 indications, abounding probability distribution function of growth rates should be usedbased on consideration of experience to date at similarly designed andoperated units.It is acceptable to use a statistical model fit of the observed growth ratedistribution as part of the tube integrity analysis provided that thestatistical model conservatively accounts for the tail of the distribution.It is also acceptable that the voltage growth distribution be in terms ofb volts rather than percent k volts, provided the conservatism of thisapproach continues to be supported by operating experience. For the purposesof assessing the conditional probability of burst and conditional leak rate,negative growth rates should be included as zero growth rates in the assumedgrowth rate distribution. However, for the purposes of determining the uppervoltage repair limit in accordance with Sections 2.a.2 and 2.a.3, it isappropriate to consider negative growth rates as part of the estimate foraverage growth rate.If the steam generators have been chemically cleaned, the impact of thechemical cleaning on voltage growth rates needs to be evaluated.2.b.3 Calculation of Projected MSLB LeakageOnce the projected EOC voltage distribution is determined, the leakage for thepostulated MSLB is calculated utilizing the EOC voltage distribution and theuse of two models: (1) the probability of leakage model and (2) theconditional leak rate model. As previously discussed in Section 2.b, MonteCarlo techniques are an acceptable approach for accounting for theuncertainties implicit in these models. These models are discussed below.2.b.3(l) Probability of Leakage as a Function of VoltageAn empirical model, for 7/8-inch and 3/4-inch diameter tubing as applicable,should be used to relate the probability of leakage (POL) to the bobbinvoltage response. This model should explicitly account for parameteruncertainty of the POL functional fit of the data (i.e., 'model fit"uncertainty). Currently, the staff has approved a model which uses a log-logistic function to fit the data. This model may need to be changed asadditional leakage data is acquired. Revisions to this model should besubmitted to the NRC for review and approval.The supporting data sets for 2.2-cm (7/8-inch) diameter and 1.9-cm (3/4-inch)diameter tubing should include all applicable data consistent with the latestrevision of the industry data base as approved by the NRC. The currentlyhttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... approved data base for POL as a function of voltage is given in Reference 7,as supplemented by References 1 and 2.2.b.3(2) Conditional Leakage Rate under MSLB ConditionsAn empirical model, for 7/8-inch or 3/4-inch diameter tubing as applicable,should be used to relate the conditional leak rate to the bobbin voltageresponse. This empirical model should account for both data scatter andparameter uncertainty of the empirical fit. Currently, an approved modelconsists of determining a linear first-order equation between the logarithm(base 10) of the conditional leak rate and the logarithm (base 10) of thebobbin voltage amplitude with standard least-squares linear regressionanalysis. The model may need to be changed as additional information isacquired; such changes should be submitted to the NRC staff for review andapproval.Use of the linear regression fit of the logarithm of the conditional leak rateto the logarithm of the bobbin voltage is subject to demonstrating that thelinear regression fit is valid at the 5-percent level with a p-value" test.If this condition is not satisfied, the linear regression fit should beassumed to have zero slope (i.e., the linear regression fit should be assumedto be constant with voltage).The supporting data sets for 2.2-cm (7/8-inch) diameter and 1.9-cm (3/4-inch)diameter tubing should include all applicable data consistent with the latestrevision of the industry data base as approved by the NRC. The currentlyapproved data base for conditional leak rate as a function of voltage is givenin Reference 7, as supplemented by References 1 and 2, with certainexceptions. Specifically, data excluded under criteria 3a, 3b, and 3c inReferences 1 and 2 should not be excluded pending NRC review and approval ofthese criteria. In addition, an MSLB leak rate of 2496 liters per hour shouldbe utilized for the data point obtained from V.C. Summer tube R28C41, pendingstaff review and approval of any proposed alternative estimate.2.b.4 Calculation of Offsite and Control Room DosesFor the MSLB leak rate calculated above, offsite and control room doses shouldbe calculated utilizing currently accepted licensing basis assumptions.Licensees should note that Attachment 2 to this generic letter providesexample TS pages for reducing reactor coolant system specific iodine activitylimits. Licensees who wish to take credit for reduced reactor coolant systemiodine activities (below 0.35 microcuries per gram dose equivalent I-131) inthe radiological dose calculation should provide a justification supportingthe request that evaluates the release rate data described in Reference 6.Reduction of reactor coolant iodine activity is an acceptable means foraccepting higher projected leakage rates and still meeting the applicablelimits of Title 10 of the Code of Federal Regulations Part 100 and GDC 19utilizing licensing basis assumptions.2.c Alternative Tube Integrity CalculationAs discussed above, licensees should calculate (1) primary-to-secondaryleakage under postulated accident conditions and (2) conditional probabilityof burst given an MSLB, to confirm that the SG will retain adequate structuraland leakage integrity until the next scheduled SG inspection. Section 6 ofthis attachment contains reporting guidance that recommends that licenseesnotify the staff prior to returning the SGs to service when conditional burstprobability exceeds 1 x 10-2 or when calculated accident leakage exceeds thelicensing limit. These calculations are to be performed using the projectedEOC voltage distribution; however, it may not always be practical to completethese calculations prior to returning the SGs to service. Under thesecircumstances, it is acceptable to use the actual measured bobbin voltagedistribution instead of the projected EOC voltage distribution to determinewhether the reporting criteria in Section 6 of this guidance are satisfied.The actual measured bobbin voltage distribution should contain all bobbinindications detected, regardless of whether the RPC probe confirmed thedegradation to be present and the NDE uncertainty distribution should besampled. The postulated accident leakage and the conditional probability ofburst should be calculated in accordance with Sections 2.a. and 2.b.3 of thisattachment. This calculation is intended to assess whether the SGs can bereturned to service and the plant can be operated until the full assessment(submitted within 90 days of restart) from the projected EOC voltagedistribution is performed.3. Inspection Criteriahttp://www.nrc.gov/reading-rmdoc-collections/gen-comm/gen-letters/1995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... The inspection scope, data acquisition, and data analysis should be performedin a manner consistent with the methodology utilized to develop the voltagelimits (e.g., the methodology described in Reference 4, Appendix A, andReference 5, Appendix A) with the exceptions and clarifications noted below.3.a Bobbin Coil Inspection Scope and SamplingThe bobbin coil inspection should include 100 percent of the hot-leg TSPintersections and cold-leg intersections down to the lowest cold-leg TSP withknown ODSCC. The determination of TSPs having ODSCC should be based on theperformance of at least a 20-percent random sampling of tubes inspected overtheir full length.3.b Rotating Pancake Coil (RPC) InspectionRPC inspections should be conducted as discussed below for purposes ofobtaining additional characterization of ODSCC flaws found with the bobbinprobe and to inspect intersections with significant bobbin interferencesignals (due to copper, dents, large mix residuals) which may impair thedetectability of degradation with the bobbin probe or which may undulyinfluence the bobbin voltage measurement. With respect to ODSCC flawcharacterization, a key purpose of the RPC inspections is to ensure theabsence of detectable crack-like circumferential indications and detectableindications extending outside the thickness of the TSP. The voltage-basedrepair criteria are not applicable to intersections exhibiting suchindications, and special reporting requirements pertaining to the finding ofsuch indications are described in Section 6.3.b.1 RPC inspection should be performed for all indications exceeding2.0 volts as measured by bobbin coil for 2.2-cm [7/8-inch] diameter tubes or1.0 volt as measured by bobbin coil for 1.9-cm [3/4-inch] diameter tubes.3.b.2 All intersections with interfering signals from copper deposits shouldbe inspected with RPC. Any indications found at such intersections with RPCshould cause the tube to be repaired.3.b.3 All intersections with dent signals greater than 5 volts should beinspected with RPC. Any indications found at such intersections with RPCshould cause the tube to be repaired. If circumferential cracking or primarywater stress corrosion cracking indications are detected, it may be necessaryto expand the RPC sampling plan to include dents less than 5.0 volts.3.b.4 All intersections with large mixed residuals should be inspected withRPC. For purposes of this guidance, large mixed residuals are those thatcould cause a 1.0 volt bobbin signal to be missed or misread. Any indicationsfound at such intersections with RPC should cause the tube to be repaired.3.c Data Acquisition and Analysis3.c.1 The bobbin coil should be calibrated against the reference standardused in the laboratory as part of the development of the voltage-basedapproach by direct testing or through use of a transfer standard.3.c.2 Once the probe has been calibrated on the 20-percent through-wallholes, the voltage response of new bobbin coil probes for the 40-percent to100-percent American Society of Mechanical Engineers (ASME) through-wall holesshould not differ from the nominal voltage by more than 10 percent.3.c.3 Probe wear should be controlled by either an inline measurement deviceor through the use of a periodic wear measurement. When utilizing theperiodic wear measurement approach, if a probe is found to be out ofspecification, all tubes inspected since the last successful calibrationshould be reinspected with the new calibrated probe. Alternatives to thisapproach, which provide equivalent detection and sizing and are consistentwith the tube integrity analyses discussed in Section 2, may be permittedsubject to NRC approval.3.c.4 Data analysts should be trained and qualified in the use of theanalyst's guidelines and procedures. Data analyst performance should beconsistent with the assumptions for analyst measurement variability[Section 2.b.2()] utilized in the tube integrity evaluation (Section 2).3.c.5 Quantitative noise criteria (resulting from electrical noise, tubehttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/ 1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... noise, calibration standard noise) should be included in the data analysisprocedures. Data failing to meet these criteria should be rejected, and thetube should be reinspected.3.c.6 Data analysts should review the mixed residuals on the standard itselfand take action as necessary to minimize these residuals.3.c.7 Smaller and larger diameter probes can be used to inspect tubes whereit is impractical to utilize a nominal-size probe (i.e., 0.610 inch diameterfor 3/4-inch tubing and 0.720 inch diameter for 7/8-inch tubing) provided thatthe probes and procedures have been demonstrated on a statisticallysignificant basis to give an equivalent voltage response and detectioncapability when compared to the nominal-size probe. This can be demonstratedon a plant-specific or generic basis. Data supporting the use of alternateprobe sizes should be submitted for NRC approval.3.c.8 Data analysts should be trained on the potential for primary waterstress corrosion cracking to occur at TSP intersections. The analysts shouldbe sensitized to identifying indications attributable to primary water stresscorrosion cracking.4. Tube Removal and Examination/TestingImplementation of voltage-based repair criteria should include a program oftube removals for testing and examination as described below. The purpose ofthis program is to () confirm axial ODSCC as the dominant degradationmechanism as discussed in Section l.a; (2) monitor the degradation mechanismover time; (3) provide additional data to enhance the burst pressure,probability of leakage, and conditional leak rate correlations described inSections 2.a.1, 2.b.3(l), and 2.b.3(2), respectively; and (4) assessinspection capability.4.a Number and Frequency of Tube PullsTwo pulled tube specimens with an objective of retrieving as manyintersections as is practical (a minimum of four intersections) should beobtained for each plant either during the plant SG inspection outage thatimplements the voltage-based repair criteria or during an inspection outagepreceding initial application of these criteria. On an ongoing basis, anadditional (follow-up) pulled tube specimen with an objective of retrieving asmany intersections as is practical (minimum of two intersections) should beobtained at the refueling outage following accumulation of 34 effective fullpower months of operation or at a maximum interval of three refueling outages,whichever is shorter, following the previous tube pull.Alternatively, the request to acquire pulled tube specimens may be met byparticipating in an industry sponsored tube pull program endorsed by the NRCthat meets the objectives of this guidance. Such a program would have tosatisfy the following objectives: (1) to confirm the degradation mechanism forplants utilizing the generic letter for the first time, (2) to continuemonitoring the ODSCC mechanism over time, (3) to enhance the burst pressure,probability of leakage, and conditional leak rate correlations, and (4) toassess inspection capability.4.b Selection CriteriaSelection of the tubes to be removed should consider the following criteria:4.b.1 There should be an emphasis on removing tube intersections with largevoltage indications.4.b.2 Where possible, the removed tube intersections should cover a range ofvoltages, including intersections with no detectable degradation.4.b.3 As a minimum, selected intersections should ensure that the total dataset includes a representative number of intersections with RPC signaturesindicative of a single dominant crack as compared to intersections with RPCsignatures indicative of two or more dominant cracks about the circumference.4.c Examination and TestingRemoved tube intersections should be subjected to leak and burst tests undersimulated MSLB conditions to confirm that the failure mode is axial and tohttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/ 1 995/gl95005 .html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... permit enhancement of the supporting data sets for the burst pressure andleakage correlations. The systems for future tests should accommodate, andpermit the measurement of, as high a leak rate as is practical, including leakrates that may be in the upper tail of the leak rate distribution for a givenvoltage. Leak rate data should be collected at temperature for thedifferential pressure loadings associated with the maximum postulated MSLB.When it is not practical to perform hot temperature leak tests, roomtemperature leak rate testing may be performed as an alternative. Bursttesting may be performed at room temperature. The burst and leak ratecorrelations and/or data should be normalized to reflect the appropriatepressure and temperature assumptions for a postulated MSLB.Subsequent to burst testing, the intersections should be destructivelyexamined to confirm that the degradation morphology is consistent with theassumed morphology for ODSCC at the tube-to-TSP intersections. Thedestructive examinations should include techniques such as metallography andscanning electron microscope (SEM) fractography as necessary to characterizethe degradation morphology (e.g., axial ODSCC, circumferential ODSCC, IGAinvolvement, cellular IGA, and combinations thereof) and to characterize thelargest crack networks with regard to their orientation, length, depth, andligaments. The purpose of these examinations is to verify that thedegradation morphology is consistent with the assumptions made in Section l.aof this attachment. This includes demonstrating that the dominant degradationmechanism affecting the tube burst and leakage properties is axially oriented,ODSCC.5. Operational Leakage5.a The operational leakage limit should be reduced in the TSs to 150 gallonsper day (gpd) through each SG.5.b Licensees should review their leakage monitoring measures to ensure thatshould a significant leak occur in service, it will be detected and the plantwill be shut down in a timely manner to reduce the likelihood of a potentialtube rupture. Specifically, the effectiveness of these procedures forensuring the timely detection, trending, and response to rapidly increasingleaks should be assessed. The licensee should consider the appropriateness ofalarm setpoints on the primary-to-secondary leakage detection instrumentationand the various criteria for operator actions in response to detected leakage.5.c SG tubes with known leaks should be repaired prior to returning the SGsto service following an SG inspection outage.6. Reporting Requirements6.a Threshold Criteria for Requiring Prior Staff Notification To ContinueWith Voltage-Based CriteriaThis guidance allows licensees to implement the voltage-based repair criteriaon a continuing basis after the NRC staff has approved the initial TSamendment. However, in several situations, the NRC staff must receivenotification to enable the staff to assess whether a licensee can continuewith the implementation of the voltage-based repair criteria:6.a.1 If the projected EOC voltage distribution results in an estimatedleakage greater than the leakage limit (determined from the licensing basiscalculation), then the licensee should notify the NRC of this occurrence andprovide an assessment of its significance prior to returning the SGs toservice. If it is not practical to complete this calculation prior toreturning the SGs to service, the measured EOC voltage distribution can beused (from the previous cycle of operation) as an alternative (refer toSection 2.c). If it is determined that the projected calculated leakage willexceed the leakage limit (during the operating cycle) after the SGs arereturned to service, then licensees should provide an assessment of the safetysignificance of the occurrence, describe the compensatory measures being takento resolve the issue, and follow any other applicable reportabilityregulations.6.a.2 If indications are identified that (1) extend beyond the confines ofthe TSP, or (2) appear to be circumferential in nature, or (3) areattributable to primary water stress corrosion cracking, the NRC staff shouldbe notified prior to returning the SGs to service.6.a.3 If the calculated conditional probability of rupture under postulatedMSLB conditions based on the projected EOC voltage distribution exceedshttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005 .html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... 1 10-2, licensees should notify the NRC and provide an assessment of thesignificance of this occurrence prior to returning the SGs to service. Thisassessment should address the safety significance of the calculatedconditional probability and can account for operator actions to preventprimary pressure from reaching the PORV or safety valve setpoint provided thatthe assessment includes a probabilistic assessment of the operator actions.If it is not practical to complete this calculation prior to returning the SGsto service, the measured EOC voltage distribution can be used (from theprevious cycle of operation) as an alternative (refer to Section 2.c).6.b Information To Be Provided Following Each RestartThe following information should be submitted to the NRC staff within 90 daysof each restart following an SG inspection:(a) The results of metallurgical examinations performed for tubeintersections removed from the SG. If it is not practical to provideall the results within 90 days, as a minimum, the burst test, leakagetest and morphology conclusions should be provided within 90 days. Theremaining information should be submitted when it becomes available.(b) The following distributions should be provided in both tabular andgraphical form. This information will enable the staff to assess theeffectiveness of the methodology, determine whether the degradation ischanging significantly, determine whether the data supports differentvoltage repair limits, and perform confirmatory calculations. Thevoltages reported should be adjusted to account for differences betweenthe laboratory standard and the standard used in the field (i.e.,transfer standard corrections should be made).(i) EOC voltage distribution -all indications found during theinspection regardless of RPC confirmation(ii) cycle voltage growth rate distribution (i.e., from BOC to EOC) -the data should indicate whether the distribution has beenadjusted for the length of the operating interval, and the lengthof the operating interval should be provided (i.e., in EFPYs).The planned length of the next operating interval should also beprovided (in EFPYs).(iii) voltage distribution for EOC repaired indications -distributionof indications presented in (i) above that were repaired (i.e.,plugged or sleeved)(iv) voltage distribution for indications left in service at thebeginning of the next operating cycle regardless of RPCconfirmation -obtained from (i) and (iii) above(v) voltage distribution for indications left in service at thebeginning of the next operating cycle that were confirmed by RPCto be crack-like or not RPC inspected(vi) non-destructive examination uncertainty distribution used inpredicting the EOC (for the next cycle of operation) voltagedistribution(c) The results of the tube integrity evaluation (calculated accidentleakage and conditional burst probability) described in Section 2,including the repair limits that were implemented (i.e., the uppervoltage repair limit, the average growth rate at the tube support platesand flow distribution baffle, if applicable, the measurement variabilityallowance, and the correlation used). Note that if the leakage andconditional burst probability were calculated using the measured EOCvoltage distribution for the purposes of addressing the 6.a.1 and 6.a.3reporting criteria, then the results of the projected EOC voltagedistribution should be given in this report..7. References1. Letter dated April 22, 1994, to Jack Strosnider, NRC, from David A.Steininger, EPRI, 'Exclusion of Data from Alternate Repair Criteria (ARC) DataBases Associated with 7/8 inch Tubing Exhibiting ODSCC.-2. Letter dated June 9, 1994, to Brian Sheron, NRC, from David J. Modeen,Nuclear Energy Institute.3. EPRI TR-100407, Revision 1, Draft Report August 1993, PWR Steam GeneratorTube Repair Limits-Technical Support Document for Outside Diameter StressCorrosion Cracking at the Tube Support Plates.'http://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/ 1 995/gl95005 .html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... 4. WCAP-12985, Revision 1, "Kewaunee Steam Generator Tube Plugging Criteriafor ODSCC at Tube Support Plates,' Westinghouse Electric Corporation, January1993, Westinghouse Proprietary Class 2.5. WCAP-13522, V.C. Summer Steam Generator Tube Plugging Criteria forIndications at Tube Support Plates," Westinghouse Electric Corporation,Westinghouse Proprietary Class 2.6. J. P. Adams and C. L. Atwood, "The Iodine Spike Release Rate During aSteam Generator Tube Rupture,' Nuclear Technology, Vol. 94, p. 361 (1991).7. EPRI Draft Report, NP-7480-L, "Steam Generator Tubing Outside DiameterStress Corrosion Cracking at Tube Support Plates -Data Base for AlternateRepair Limits,' Volume 1, Revision 1, September 1993, "7/8 Inch DiameterTubing," and Volume 2, October 1993, "3/4 Inch Diameter Tubing.'8. Safety Evaluation by the Office of Nuclear Reactor Regulation Related toAmendment No. 54 to Facility Operating License NPF-72, Commonwealth EdisonCompany, Braidwood Station, Unit 1, Docket No. STN 50-456, as documented in anamendment package titled, "Issuance of Amendment (TAC NO. M89697)' datedAugust 18, 1994.9. WCAP-14046, Braidwood Unit 1 Technical Support for Cycle 5 SteamGenerator Interim Plugging Criteria," Westinghouse Electric Corporation,Westinghouse Proprietary Class 2.. ABBREVIATIONSalternate repair criteriaAmerican Society of Mechanical Engineersbeginning of cycleeffective full power yearend of cycleElectric Power Research InstituteGeneral Design Criteriaintergranular attackmain steamline breaknondestructive examinationNuclear Energy InstituteNuclear Regulatory Commissionoutside diameter stress corrosion crackingprobability of detectionprobability of leakagepower-operated relief valvepressurized-water reactorreactor coolant pressure boundaryregulatory guiderotating pancake coilscanning electron microscopesteam generatorstandard technical specificationstechnical specificationtube support plate.Model Technical SpecificationsThe model technical specifications (TSs) are based on the "Standard TechnicalSpecifications (STS) for Westinghouse Pressurized Water Reactors,' NUREG-0452,Revision 4a. The changes are indicated in italics. Note that the model TSchanges described below also contain an example change to reduce reactorcoolant system specific activity. The model TSs that appear below should beadopted consistent with the licensing basis. It should be noted that in theimproved STS, some of these surveillance requirements have been relocated tothe 'Administrative Controls" section.3/4.4.5 REACTOR COOLANT SYSTEM4.4.5.2 Steam Generator Tube Selection and Inspection[Add the following paragraphs:]b. 4. Indications left in service as a result of application ofthe tube support plate voltage-based repair criteria shallbe inspected by bobbin coil probe during all futurerefueling outages.d. Implementation of the steam generator tube/tube support plate repaircriteria requires a 100-percent bobbin coil inspection for hot-legand cold-leg tube support plate intersections down to the lowesthttp:/www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003ARCASMEBOCEFPYEOCEPRIGDCsIGAMSLBNDENEINRCODSCCPODPOLPORVPWRRCPBRGRPCSEMSGSTSTSTSP

NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... cold-leg tube support plate with known outside diameter stresscorrosion cracking (ODSCC) indications. The determination of thelowest cold-leg tube support plate intersections having ODSCCindications shall be based on the performance of at least a20-percent random sampling of tubes inspected over their fulllength.4.4.5.4 Acceptance Criteriaa. As used in this specification:6. Plugging Limit means the imperfection depth at or beyondwhich the tube shall be removed from service and is equal to40 percent of the nominal wall thickness. This definitiondoes not apply to tube support plate intersections for whichthe voltage-based repair criteria are being applied. Referto 4.4.5.4.a.10 for the repair limit applicable to theseintersections.10. Tube Support Plate Plugging Limit is used for thedisposition of an alloy 600 steam generator tube forcontinued service that is experiencing predominantly axiallyoriented outside diameter stress corrosion cracking confinedwithin the thickness of the tube support plates. At tubesupport plate intersections, the plugging (repair) limit isbased on maintaining steam generator tube serviceability asdescribed below:a. Steam generator tubes, whose degradation isattributed to outside diameter stress corrosioncracking within the bounds of the tube supportplate with bobbin voltages less than or equal tothe lower voltage repair limit [Note 1], will beallowed to remain in service.b. Steam generator tubes, whose degradation isattributed to outside diameter stress corrosioncracking within the bounds of the tube supportplate with a bobbin voltage greater than thelower voltage repair limit [Note 1], will berepaired or plugged, except as noted in4.4.5.4.a.10.c below.c. Steam generator tubes, with indications ofpotential degradation attributed to outsidediameter stress corrosion cracking within thebounds of the tube support plate with a bobbinvoltage greater than the lower voltage repairlimit [Note 1] but less than or equal to theupper voltage repair limit [Note 2], may remainin service if a rotating pancake coil inspectiondoes not detect degradation. Steam generatortubes, with indications of outside diameterstress corrosion cracking degradation with abobbin voltage greater than the upper voltagerepair limit [Note 2] will be plugged orrepaired.d. [If applicable] Certain intersections asidentified in [reference report] will beexcluded from application of the voltage-basedrepair criteria as it is determined that theseintersections may collapse or deform following apostulated LOCA + SSE event.e. If an unscheduled mid-cycle inspection isperformed, the following mid-cycle repair limitsapply instead of the limits identified in4.4.5.4.10.a, 4.4.5.4.10.b, and 4.4.5.4.10.c.The mid-cycle repair limits are determined fromthe following equations:http://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... where:VURL = upper voltage repairlimitVLRL = lower voltage repairlimitVMURL = mid-cycle upper voltagerepair limit based ontime into cycleVMLRL = mid-cycle lower voltagerepair limit based onVMURL and time into cyclekt = length of time sincelast scheduledinspection during whichVURL and VLRL wereimplementedCL = cycle length (the timebetween two scheduledsteam generatorinspections)VSL = structural limit voltageGr = average growth rate percycle lengthNDE = 95-percent cumulativeprobability allowancefor nondestructiveexamination uncertainty(i.e., a value of 20-percent has beenapproved by NRC)Implementation of these mid-cycle repair limits shouldfollow the same approach as in TS 4.4.5.4.10.a,4.4.5.4.10.b, and 4.4.5.4.10.c.Note 1: The lower voltage repair limit is 1.0 volt for 3/4-inch diametertubing or 2.0 volts for 7/8-inch diameter tubingNote 2: The upper voltage repair limit is calculated according to themethodology in Generic Letter 95-xx as supplemented. VRL maydiffer at the TSPs and flow distribution baffle.4.4.5.5 Reportsd. For implementation of the voltage-based repair criteria to tubesupport plate intersections, notify the staff prior to returning thesteam generators to service should any of the following conditionsarise:1. If estimated leakage based on the projected end-of-cycle (or if notpractical, using the actual measured end-of-cycle) voltagedistribution exceeds the leak limit (determined from the licensingbasis dose calculation for the postulated main steamline break) forthe next operating cycle.2. If circumferential crack-like indications are detected at the tubesupport plate intersections.3. If indications are identified that extend beyond the confines of thetube support plate.4. If indications are identified at the tube support plate elevationsthat are attributable to primary water stress corrosion cracking.5. If the calculated conditional burst probability based on theprojected end-of-cycle (or if not practical, using the actualmeasured end-of-cycle) voltage distribution exceeds 1 X 10-2, notifyhttp://www.nrc.gov/reading-rm/doc-collections/gen-commlgen-letters/1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria forWestinghouse Steam Ge... the NRC and provide an assessment of the safety significance of theoccurrence.REACTOR COOLANT SYSTEM3/4.4.6 REACTOR COOLANT SYSTEM LEAKAGE3.4.6.2 Reactor Coolant System leakage shall be limited to:a. No PRESSURE BOUNDARY LEAKAGE,b. 1 GPM UNIDENTIFIED LEAKAGE,c. 150 gallons per day of primary-to-secondary leakage through any onesteam generator,d. 10 GPM IDENTIFIED LEAKAGE from the Reactor Coolant System, ande. GPM CONTROLLED LEAKAGE at a Reactor Coolant System pressure of2235 ft 20 psig.f. 1 GPM leakage at a Reactor Coolant System pressure of 2235 f 20 psigfrom any Reactor Coolant System Pressure Isolation Valve specifiedin Table 3.4-1.For licensees who want to reduce RCS specific iodine activity, the followingTS pages apply:REACTOR COOLANT SYSTEM3/4.4.8 SPECIFIC ACTIVITY3.4.8 The specific activity of the primary coolant shall be limited to:a. Less than or equal to [reduced value] microcurie per gram DOSEEQUIVALENT I-131, andb. Less than or equal to 100/1 microcuries per gram.APPLICABILITY: MODES 1, 2, 3, 4, and 5.ACTION:MODES 1 2, and 3*:a. With the specific activity of the primary coolant greater than[reduced value] microcurie per gram DOSE EQUIVALENT I-131 for morethan 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> MODES 1, 2, 3, 4, and 5:a. With the specific activity of the primary coolant greater than[reduced value] microcurie per gram DOSE EQUIVALENT I-131 or greater[Revise Figure 3.4-1 to lower the line by a factor corresponding to thereduction in specific activity. The lowered line should parallel theoriginal.].REACTOR COOLANT SYSTEMBASES3/4.4.5 STEAM GENERATORSThe voltage-based repair limits of SR 4.4.5 implement the guidance in GL 95-XXand are applicable only to Westinghouse-designed steam generators (SGs) withhttp://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/ 1 995/gl95005.html 03/13/2003 NRC Generic Letter 95-05: Voltage Based Repair Criteria for Westinghouse Steam Ge... outside diameter stress corrosion cracking (ODSCC) located at the tube-to-tubesupport plate intersections. The voltage-based repair limits are notapplicable to other forms of SG tube degradation nor are they applicable toODSCC that occurs at other locations within the SG. Additionally, the repaircriteria apply only to indications where the degradation mechanism isdominantly axial ODSCC with no significant cracks extending outside thethickness of the support plate. Refer to GL 95-XX for additional descriptionof the degradation morphology.Implementation of SR 4.4.5 requires a derivation of the voltage structurallimit from the burst versus voltage empirical correlation and then thesubsequent derivation of the voltage repair limit from the structural limit(which is then implemented by this surveillance).The voltage structural limit is the voltage from the burst pressure/bobbinvoltage correlation, at the 95-percent prediction interval curve reduced toaccount for the lower 95/95-percent tolerance bound for tubing materialproperties at 650 oF (i.e., the 95-percent LTL curve). The voltage structurallimit must be adjusted downward to account for potential flaw growth during anoperating interval and to account for NDE uncertainty. The upper voltagerepair limit; VURL, is determined from the structural voltage limit byapplying the following equation:VURL = VSL -VGr -VNDEwhere VGr represents the allowance for flaw growth between inspections and VNDErepresents the allowance for potential sources of error in the measurement ofthe bobbin coil voltage. Further discussion of the assumptions necessary todetermine the voltage repair limit are discussed in GL 95-XX.The mid-cycle equation in SR 4.4.5.4.10.e should only be used during unplannedinspections in which eddy current data is acquired for indications at the tubesupport plates.SR 4.4.5.5 implements several reporting requirements recommended by GL 95-XXfor situations which the NRC wants to be notified prior to returning the SGsto service. For the purposes of this reporting requirement, leakage andconditional burst probability can be calculated based on the as-found voltagedistribution rather than the projected end-of-cycle voltage distribution(refer to GL 95-XX for more information) when it is not practical to completethese calculations using the projected EOC voltage distributions prior toreturning the SGs to service. Note that if leakage and conditional burstprobability were calculated using the measured EOC voltage distribution forthe purposes of addressing the GL section 6.a.1 and 6.a.3 reporting criteria,then the results of the projected EOC voltage distribution should be providedper the GL section 6.b (c) criteria.3/4.4.6 LEAKAGE LIMITSThe leakage limits incorporated into SR 4.4.6 are more restrictive than thestandard operating leakage limits and are intended to provide an additionalmargin to accommodate a crack which might grow at a greater than expected rateor unexpectedly extend outside the thickness of the tube support plate.Hence, the reduced leakage limit, when combined with an effective leak ratemonitoring program, provides additional assurance that should a significantleak be experienced in service, it will be detected, and the plant shut downin a timely manner.3/4.4.8 SPECIFIC ACTIVITYReduction of the RCS specific activity to levels less than 0.35 microcurie pergram DOSE EQUIVALENT I-131 requires an evaluation (provided with the TSamendment request) of the release rate data described in Reference 6 ofGL 95-XX.http://www.nrc.gov/reading-rn/doc-collections/gen-comm/gen-letters/1 995/gl95005 .html 03/13/2003