ML13085A028

San Onofre, Unit 2 - Response to Request for Additional Information (RAI 18), Revision 1 Regarding Confirmatory Action Letter Response

ML13085A028
Person / Time
Site:	San Onofre
Issue date:	03/22/2013
From:	St.Onge R J Southern California Edison Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
TAC ME9727
Download: ML13085A028 (7)

Text

SOUTHERN CALIFORNIA Richard 1. St. OngeEDISON° Director, Nuclear Regulatory Affairs andEmergency PlanningAn EDISON INTERNATIONAL CompanyMarch 22, 201310 CFR 50.4U.S. Nuclear Regulatory CommissionATTN: Document Control DeskWashington, DC 20555-0001Subject: Docket No. 50-361Response to Request for Additional Information (RAI 18), Revision IRegarding Confirmatory 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. James R. Hall (USNRC) to Mr. Peter T. Dietrich (SCE), datedDecember 26, 2012, Request for Additional Information Regarding Responseto Confirmatory Action Letter, San Onofre Nuclear Generating Station, Unit 24. Letter from Mr. Richard J. St. Onge (SCE) to NRC Document Control Desk,dated January 24, 2013, Response to Request for Additional Information(RAI 18) Regarding Confirmatory Action Letter Response, San Onofre NuclearGenerating Station, 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.By letter dated December 26, 2012 (Reference 3), the NRC issued Requests for AdditionalInformation (RAIs) regarding the CAL response. SCE provided the response to RAI 18 in aletter dated January 24, 2013 (Reference 4). Enclosure 1 of this letter provides Revision 1 tothe RAI 18 response.P.O. Box 128San Clemente, CA 92672 Document Control Desk-2-March 22, 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,Enclosures:1. Response to RAI 18, Revision 1cc: E. E. Collins, 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 18, Revision 1 RAI 18Reference 1, Section 11.1, page 52 -SCE proposes to upgrade the vibration and loose partsmonitoring system (VLPMS) as a defense-in-depth measure to enhance plant monitoringcapability to facilitate early detection of a steam generator tube leak and ensure immediate andappropriate plant operator and management response.Fluid Elastic Instability (FEI) was identified as a main cause of the tube wear for both the Unit 2and 3 steam generators. The FEI experienced is due to a combination of the conditions ofsteam quality, secondary side fluid velocity in the vicinity of the tube bundle, and steam voidfraction, and the degree of such fluid elastic instability is related to the damping provided byinternal support structures. According to your report, "steam quality directly affects the fluiddensity outside the tube, affecting the level of hydrodynamic pressure that provides the motiveforce for tube vibration. When the energy imparted to the tube from hydrodynamic pressure(density times velocity squared, or pv2) is greater than the energy dissipated through damping,FEI will occur." However, the proposed plant VLPMS enhancement does not appear to directlymonitor steam quality, secondary side fluid velocity, or steam void fraction.Please provide the following information to address the effectiveness of the enhanced VLPMS:a. Describe the specific purpose of using the enhanced VLPMS equipment for monitoringsteam generator performance. For example, is it to be used for monitoring acousticnoise indicative of flow velocity, steam quality, and void fraction, or for the measurementof metallic noise indicative of vibration of tubes against each other or against tubesupport structures? Exactly how will this be done? What is the theory of operation? If itwill be used to monitor an increase in pv2 leading to the onset of FEI, provide adescription of the correlation of the velocity of steam voids through the secondary side ofthe steam generator and the relative changes in characteristics of the signal output fromthe various VLPMS accelerometers. If it is to be used for detecting actual tube vibration,provide a description of the process that will be used for discerning actual tube vibrationnoise from background noise, and the required threshold identification criteria that will beapplied to reach the conclusion that tube vibration is occurring.b. Identify the ranges of amplitudes and frequencies of the acoustic noise signals fromeach accelerometer that are indicative of an approach to the conditions leading to FEI oractual tube vibration, and the reasons for selection of the more sensitive accelerometers.Also, discuss the required response time of the signal processing equipment needed todetect and continuously monitor either fluid velocities within the steam generator or tubeimpact noise, depending on the intended use of the enhanced VLPMS, and the actualresponse time capabilities of the equipment, from sensor through processed signaloutput, that is being proposed for use.c. Discuss the acceptance criteria (e.g., magnitude of signal, plant power level, etc.) thatwill be used to establish the setpoints for the alarms described in Section 11 of yourreport: "The signals from these sensors are compared with preset alarm setpoints."Provide a description of how the alarm setpoints were established, and at what pointduring the start-up of Unit 2 will these alarm setpoints be calibrated into the VLPMS. Ifthe setpoints have not yet been determined, provide a description of your plan fordetermining and implementing these settings.Page 2 of 5

d. Describe the planned operator actions and any changes to the procedures forresponding to alarms or signals potentially indicative of tube-to-tube contact, includingtime limits for analyzing the signals and taking any necessary action including plantshutdown. Describe the lessons learned that have been drawn from the signals ofpotential metal-to-metal contact experienced in Unit 3 and how these lessons have beenfactored into current procedures.e. A description of how you determined that acoustic noise monitoring and predictive signalprocessing was the best method for monitoring either the onset of FEI or actual tubevibration, including a list of other methods (e.g., time domain reflectivity probescalibrated for steam void propagation monitoring) that had been considered forenhancing steam generator tube monitoring during start-up of Unit 2, and the reasons fortheir rejection.RESPONSE -Revision IThe purpose of the upgraded vibration and loose parts monitoring system (VLPMS) is to provideadditional monitoring capabilities for steam generator (SG) secondary side acoustics. Theupgraded VLPMS is capable of recording secondary side acoustic signals via accelerometersmounted on the external shell of the SGs at locations near the upper tube bundle andtubesheet. The upgraded VLPMS will be used as a backward looking analysis tool insubsequent inspection outages should unexpected wear be discovered. The upgraded VLPMSwill enable SCE to evaluate historical SG secondary side acoustic signal data for events whichmay help with the understanding of the causes of unexpected tube wear. To gain experiencewith the analysis tools during the next cycle, the recorded data will be reviewed on a monthlybasis in accordance with plant procedures.The Unit 2 Return to Service (RTS) report describes the upgrades to the VLPMS in Section 11.1as an additional action to provide monitoring capabilities for secondary side acoustic signals.SCE did not propose the upgrade of the VLPMS as a defense-in-depth measure nor as ameans of monitoring steam quality, secondary side fluid velocity, or steam void fraction.Corrective measures to control these secondary side parameters are addressed in Section 8 ofthe RTS report. The defense-in-depth measures being taken in support of Unit 2 return toservice are described in Section 9 of the RTS report.The theory of operation of the VLPMS data acquisition equipment is provided as follows:The Vibration and Loose Parts Monitoring System (V&LPMS) is a stand-alone systemdesigned to perform loose parts detection and vibration monitoring functions. The loosepart detection function is designed to fulfill the requirements of the loose parts detectionsystem as set forth in Regulatory Guide 1.133. The design objective of the loose partsdetection portion of the V&LPMS is to detect the presence of loose parts in the reactorcoolant system (RCS) and annunciate an alarm in the control room when a loose part isdetected.The system consists of loose parts and vibration sensors, preamplifiers and acomputerized data acquisition and processing system. The sensors and preamplifiersare located inside the containment and the data acquisition and processing equipment ishoused in a cabinet located in the control room cabinet area. The equipment locatedinside containment consists of piezo-electric sensors, preamplifiers and associatedPage 3 of 5 cabling at each of the following natural collection regions of each unit to detect looseparts:* Upper reactor vessel; vessel head on head lift rig stopper" Lower reactor vessel; outside wall* Steam generator E088; outside wall* Steam generator E089; outside wallThe RCS component vibration monitoring, reactor internal vibration monitoring, andvibration data analysis features are on-demand functions. The on-demand featuresprovided with the VLPMS allow the selection of any two loose parts, vibration or reactorinternal vibration channels for vibration monitoring or analysis. The on-demand dataacquisition and analysis features also allow a live channel signal or historical data fromthe historical data file to be selected for time domain and/or frequency domain analysis,displayed, stored and/or printed.The upgrades to the VLPMS consist of:* Relocation of existing VLPMS accelerometers (2 per SG) from the support skirt tolocations above and below the SG tubesheet. These will remain as VLPMS sensors tomeet Regulatory Guide 1.133, "Loose-Part Detection Program for the Primary System ofLight-Water-Cooled Reactors"* Installation of increased sensitivity accelerometers (2 per SG) at locations above andbelow the tubesheet" Installation of increased sensitivity accelerometers (2 per SG) on an 8 inch hand hole onthe side of the SGs to monitor for secondary side noises at the upper tube bundleRelocation of the accelerometers enhances the acoustic monitoring of the SG secondary sideby placing accelerometers closer to the upper tube bundle.The new accelerometers have an increased sensitivity of 25 pC/g compared with 10 pC/g forthe existing accelerometers. The acceptance criteria used to establish the setpoints for thealarms associated with the upgraded VLPMS accelerometers is the same as used with theRegulatory Guide 1.133 accelerometers. After Unit 2 reaches 70% power, background data iscollected and alarm thresholds are established to compensate for high background noise asdiscussed in Regulatory Guide 1.133, section C.1.b, "System Sensitivity."Operator actions for VLPMS alarms are controlled by the VLPMS trouble annunciatoroperations alarm response procedure. Upon an alarm, the VLPMS automatically records datafor all of the VLPMS accelerometer channels. The Control Room Shift Technical Advisor isinstructed by this procedure to immediately notify the system engineer supervisor for any looseparts channel alarms associated with the SGs. The Control Room Operator documents theVLPMS alarm in the site Corrective Action Program (CAP). The CAP requires an operabilitydetermination to be made within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of event discovery. Each loose parts alarmassociated with SGs will be independently reviewed by an offsite vendor.Following identification of tube-to-tube wear (TTW) caused by Fluid Elastic Instability (FEI) inUnit 3 and two indications of TTW in Unit 2, a review of the VLPMS alarms for the previousoperating cycle of both units was performed. No potential metal-to-metal contact alarms wererecorded for Unit 2. Potential metal-to-metal contact alarms were recorded for Unit 3. AnalysisPage 4 of 5 of data from Unit 3 VLPMS events concluded most of the events were the result of RCStemperature changes. A number of events were not directly associated with RCS temperaturechanges and were reviewed by on-site as well as independent off site personnel. Theindependent review concluded these alarms were caused by: "...true metallic impacts and notfalse indications from electrical noise or fluctuations in background noise." The review found theacoustic signals were similar to those that occur when the SGs shift during RCS temperaturetransients. None of the VLPMS alarms were attributed to SG tube vibration.Since the VLPMS is not designed to detect tube to tube contact, the absence of tube vibrationrelated VLPMS alarms is consistent with the capabilities of its design. The approach in theUnit 2 RTS plan is to eliminate the causes of TTW caused by FEI. Reducing power reduces SGsecondary side thermal-hydraulic parameters to values within the industry's experience. Whilethe RTS plan does not require a direct method to measure tube vibration, SCE determined itwas appropriate to upgrade the existing VLPMS as discussed above.Page 5 of 5