ML13109A454

San Onofre Nuclear Generating Station, Unit 2, Response to Request for Additional Information (RAI 71) Regarding Confirmatory Action Letter Response (TAC Me 9727)

ML13109A454
Person / Time
Site:	San Onofre
Issue date:	04/16/2013
From:	St.Onge R J Edison International Co, Southern California Edison Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
TAC ME9727
Download: ML13109A454 (8)

Text

SOUTHERN CALIFORNIA Richard 1. St. OngeE DISON Director, Nuclear Regulatory Affairs and.EEmergency PlanningAn EDISON INTERNATIONAL CompanyApril 16, 2013 10CFR50.4U.S. Nuclear Regulatory CommissionATTN: Document Control DeskWashington, DC 20555-0001Subject: Docket No. 50-361Response to Request for Additional Information (RAI 71) RegardingConfirmatory Action Letter Response(TAC No. ME 9727)San Onofre Nuclear Generating Station, Unit 2References: 1. Letter from Mr. Elmo E. Collins (USNRC) to Mr. Peter T. Dietrich (SCE), datedMarch 27, 2012, Confirmatory Action Letter 4-12-001, San Onofre NuclearGenerating Station, Units 2 and 3, Commitments to Address Steam GeneratorTube Degradation2. Letter from Mr. Peter T. Dietrich (SCE) to Mr. Elmo E. Collins (USNRC), datedOctober 3, 2012, Confirmatory Action Letter -Actions to Address SteamGenerator Tube Degradation, San Onofre Nuclear Generating Station, Unit 23. Letter from Mr. Richard J. St. Onge (SCE) to Document Control Desk(USNRC), dated February 25, 2013, Response to Request for AdditionalInformation (RAIs 2, 3 and 4) Regarding Confirmatory Action Letter Response,San Onofre Nuclear Generating Station, Unit 24. Email from Mr. James R. Hall (USNRC) to Mr. Ryan Treadway (SCE), datedMarch 15, 2013, Request for Additional Information (RAls 68-72) RegardingResponse to Confirmatory Action Letter, San Onofre Nuclear GeneratingStation, Unit 2Dear Sir or Madam,On March 27, 2012, the Nuclear Regulatory Commission (NRC) issued a Confirmatory ActionLetter (CAL) (Reference 1) to Southern California Edison (SCE) describing actions that the NRCand SCE agreed would be completed to address issues identified in the steam generator tubesof San Onofre Nuclear Generating Station (SONGS) Units 2 and 3. In a letter to the NRC datedOctober 3, 2012 (Reference 2), SCE reported completion of the Unit 2 CAL actions andincluded a Return to Service Report (RTSR) that provided details of their completion.SCE provided the response to RAls 2, 3 and 4 in a letter dated February 25, 2013(Reference 3). By e-mail dated March 15, 2013 (Reference 4), the NRC issued Requests forAdditional Information (RAIs) regarding the response to RAIs 2, 3 and 4. Enclosure 1 of thisletter provides the response to RAI 71.P.O. Box 128San Clemente, CA 92672 Document Control. Desk-2-April 16, 2013There are no new regulatory commitments contained in this letter. If you have any questions orrequire additional information, please call me at (949) 368-6240.Sincerely,Enclosure:1. Response to RAI 71cc:A. T. Howell III, Regional Administrator, NRC Region IVJ. R. Hall, NRC Project Manager, SONGS Units 2 and 3G. G. Warnick, NRC Senior Resident Inspector, SONGS Units 2 and 3R. E. Lantz, Branch Chief, Division of Reactor Projects, NRC Region IV ENCLOSURE 1SOUTHERN CALIFORNIA EDISONRESPONSE TO REQUEST FOR ADDITIONAL INFORMATIONREGARDING RESPONSE TO CONFIRMATORY ACTION LETTERDOCKET NO. 50-361TAC NO. ME 9727Response to RAI 71 RAI 71Reference 1, Response to RAI 2 -It is stated on page 4 of 18 that a median value of initiationtime was selected for each tube based on 1000 trials. For purposes of evaluating aconservative probability estimate that one or more tubes do not meet the 3 delta P criterion, whyis it conservative to consider a median value of initiation time for each tube, rather thansampling from the distribution of initiation times developed for each tube during a given MonteCarlo trial of the tube population? Would sampling the distribution of initiation times for eachtube be a more conservative approach, as it would be expected to stretch out the tails of theresulting overall probability distribution for not meeting the 3 delta P criterion? For aprobabilistic assessment such as this, what is the justification for not considering a potentiallylarge source of uncertainty associated with a key input parameter?RESPONSENote: RAI Reference 1 is SCE's "Response to Request for Additional Information (RAIs 2, 3,and 4) Regarding Confirmatory Action Letter Response," dated February 25, 2013.The industry guidelines were followed in performing the tube integrity calculations to meet thestructural integrity performance criteria (SIPC) margin requirements of three times normaloperating pressure differential (Reference R1). The performance acceptance standard forassessing tube integrity requires the worst-case degraded tube to meet the SIPC marginrequirements with a probability of 0.95 at 50% confidence (Section 2.4 in Reference R1). Thetechnical basis for the 95-50 performance standard is documented in Reference R2. Inputdistributions for probabilistic components of the analysis such as degradation growth rates areallowed to be best estimates.Why is it conservative to consider a median value of initiation time for each tube, ratherthan sampling from the distribution of initiation times developed for each tube during agiven Monte Carlo trial of the tube population?A median value of initiation time was selected to provide a conservative (higher) estimate oftube to tube wear (TTW) growth rate in the model used to respond to RAI 2. Sampling thedistribution of initiation times for each tube would result in an earlier estimate (maximumlikelihood) of TTW initiation and a corresponding lower -T-w growth rate.In response to this RAI, Intertek completed an analysis of the simulation model output results todemonstrate that the use of median TTW initiation times adequately models the distribution ofall TTW initiation times. The analysis simulation results allowed an evaluation of the relativelikelihood for the initiation time for each instance of TTW for each tube. The maximumlikelihood for a distribution is determined from the mode of the distribution. Figure 1 illustratesthe mode of a distribution for the TTW initiation times calculated for a typical tube (R90 C84 in3E-088). For a simulated population, the sample mode represents the maximum likelihoodestimate (greatest chance to occur) and is the peak location of the histogram. The initiationtime defined by the mode is the most likely outcome for that tube from the simulation. Alsoshown in Figure 1 is the median value for the same sample distribution. The sample median isthe value where 50% of the occurrences fall to either side of that value when the results areranked in ascending order. For the majority of the tubes with TTW, the median value of eachtube-specific sample provided an initiation time that occurred later in the operation cyclecompared to the mode value. This resulted in a higher TTW growth rate which is morePage 2 of 6 conservative when calculating the final depth of TTW and the resulting probability of meeting the3 delta P criterion. Approximately 85% of the tubes showed this behavior where the medianinitiation time was typically less than 0.2 years at power. Examples of tubes that had a strongbias towards early initiation times are shown in Figure 2a.Approximately 10% tubes had initiation times mid-way through the cycle as shown in Figure 2b.In this case, the median value and the mode value are essentially the same and the resultingTTW growth rates are no different between the median and mode value cases.Tubes with initiations beyond mid-cycle are shown in Figure 2c. For very few tubes, typicallytubes with very low wear indices and/or few affected AVBs, results give very late initiation timesand unrealistically high TTW growth rates. These occurrences are not strongly related tothe AVB wear index and are more likely caused by impacts from unstable neighbor tubes.There were only a few instances (less than 5%) where this was observed.Would sampling the distribution of initiation times for each tube be a more conservativeapproach, as it would be expected to stretch out the tails of the resulting overallprobability distribution for not meeting the 3 delta P criterion?The corresponding distributions developed from the initiation times (all simulated values vs.median values) are plotted together in Figure 3. Figure 3 illustrates the global distributionof TTW initiation times for each Unit 3 steam generator. It can be seen that the two distributionsare comparable. This is a direct consequence of the diversity in distributional form exhibited bythe individual tube initiation time samples shown in Figure 2.Because of the diversity among the individual tubes, the cumulative distribution for medianinitiation times covers the full range of this key parameter including the upper extremes. It is notexpected that sampling the distribution of individual initiation times for each tube would besignificantly more conservative since the tails of the simulated population are represented by thedistribution of the median times.For a probabilistic assessment such as this, what is the justification for not consideringa potentially large source of uncertainty associated with a key input parameter?The median estimates based on 322 calculated values adequately represent the range ofuncertainty in initiation times from the full set of calculations (322,000 values). The ability of thedistribution of median times to produce a similar distributional behavior for the full simulateddata justifies its use in the OA. The potential source of uncertainty associated with the TTWinitiation times is considered and included in the assessment.REFERENCESRI. "Steam Generator Integrity Assessment Guidelines," Revision 3, Electric Power ResearchInstitute, EPRI Report 1019038, (November 2009).R2. "Technical Basis for Steam Generator Tube Integrity Performance Acceptance Standards,"EPRI, Palo Alto, CA: 2006. 1012984.Page 3 of 6 Histogram of Initiation Times2501I250200SaplModeSamplMediaI ITube R90 C84 Distribution0Ez150100 -so I-n -.0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Initiation Time (Yearn at Power)Figure 1 -Sample Median and Mode for a Typical Tube Histogram1.0Page 4 of 6 300U250o 2001Iso100Ua) .... ............Initiation Times (Years at Power)200180-140-120-100-so-EZ 4Oz Go-4020-0"b)IniUaton Times (Years at Power)200ISO F.Rl124-180a 140a120-o 100so-E4020 lC) &0Initiation Times (Years at Power)Figure 2 -TTW Initiation Times from the Simulation for Selected Tubesin 3E-088Page 5 of 6 3E-088ILU.0Z7NU-0.04 I I i0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Initiation Time, (Years at Power)3E-0891.00.90.8U.0.70z 0.6ILaC0 0.50..o 0.4U5 0.3S0.20.10.00.0 0.1 0.2 0.3 0.4 0.5 0.6Initiation Time, (Years at Power)0.7 0.8 0.9 1.0Figure 3 -Cumulative Distributions for Initiation Times (Median vs. All Data)Page 6 of 6