ML17324A280

Summary of Regulatory Conference to Discuss Safety Significance of Cooper Nuclear Station Emergency Station Service Transformer Bus Duct Deficiency

ML17324A280
Person / Time
Site:	Cooper
Issue date:	11/16/2017
From:	Kozal J W NRC/RGN-IV/DRP/RPB-C
To:	Dent J Nebraska Public Power District (NPPD)
Kozal J W
References
EA-17-057, IR 2017011, ML17223A459 IR 2017011
Download: ML17324A280 (58)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION REGION IV 1600 E LAMAR BLVD ARLINGTON, TX 76011

-4511 November 16, 2017 EA-17-057 Mr. John Dent, Jr.

Vice President

-Nuclear and CNO Nebraska Public Power District Cooper Nuclear Station 72676 648A Avenue P.O. Box 98 Brownville, NE 68321

SUBJECT:

SUMMARY OF REGULATORY CONFERENCE TO DISCUSS SAFETY SIGNIFICANCE OF COOPER NUCLEAR STATION EMERGENCY STATION SERVICE TRANSFORMER BUS DUCT DEFICIENCY

Dear Mr. Dent:

On November 7, 2016, members of the U.S. Nuclear Regulatory Commission (NRC) staff met with representatives of the Cooper Nuclear Station to discuss an apparent violation related to an electrical fault that occurred in an emergency station service transformer bus duct as documented i n NRC Inspection Report 05000 298/2017011, issued on August 14, 2017 (ML17223A459). The focus of the regulatory conference was a discussion of information important to characterize the safety significance associated with the failure to perform adequate maintenance and testing of the affected bus bars and associated components. The discussion included results of startup bus inspections, testing to determine the credibility of a consequential failure of the startup bus, and actions that could be taken by the station to mitigate the risk associated with a station blackout event. The discussion also included proposed refinements to the NRC's probabilistic risk assessment methodologies and assumptions used to evaluate the deficiency

. The NRC staff asked questions during this regulatory conference, with some questions requiring additional information that we requested you to provide. The NRC will continue to review the information that you provided during the Regulatory Conference and the subsequent information that was requested in order to reach a final significance determination. We will issue a final significance determination letter to you when that review has been completed. A copy of your presentation slides is included as Enclosure 1.

Copies of the NRC slides (Enclosure 2) and meeting attendance lists (Enclosure 3) are also included.

J. Dent, Jr. 2 In accordance with 10 CFR 2.390 of the NRC's "Rules of Practice," a copy of this letter and its enclosures will be available electronically for public inspection in the NRC's Public Document Room or from the Publicly Available Records (PARS) component of the NRC's ADAMS.

ADAMS is accessible from the NRC web site at http://www.nrc.gov/reading

-rm/adams.html (The Public Electronic Reading Room).

Sincerely,

/RA/ Jason W. Kozal, Chief Project Branch C Division of Reactor Projects

Docket No.: 50-298 License No.: DPR-46

Enclosures:

1. CNS Presentation Slides

2. NRC Slides

3. Meeting Attendance Forms

COOPER NUCLEAR STATION REGULATORY CONFERENCEEmergency Station Service TransformerPhase-to-Phase Busbar Fault, January 17, 2017November 7, 2017 1

OPENING REMARKSJohn DentSite Vice President and Chief Nuclear Officer, Cooper Nuclear Station 2

AgendaIntroductionDan Buman, Director, NuclearSafety AssuranceElectrical System & StationJerry Long, Senior Reactor Operator, ResponseAssistant Operations Manager (Shift)Causal EvaluationDan Buman/ Khalil Dia, Director, EngineeringNew InformationDan Buman, Director, Nuclear Safety AssuranceRisk SignificanceWayne Schmidt, PRA ConsultantConcluding StatementJohn Dent, Site Vice President and Chief Nuclear Officer 3

IntroductionOn January 17, 2017, CNS experienced a Phase

-to-Phase fault on the busbars for the Emergency Station Service TransformerThe Emergency Transformer was in a standby, energized condition, but not loadedNo switching activities were occurring related to the Emergency Transformer at the time of the faultNo grid or plant electrical instabilities existed at the time of the faultThe NPPD Operations Center promptly analyzed the fault and determined that the circuit fault protection operated correctlyThe plant responded as designed with no additional malfunctions, human performance errors, or procedural deviations that complicated the condition 4

Efforts to Understand the IssueAs a result of the January 17, 2017, Emergency Transformer Bus Fault, CNS has:Developed a greater understanding of the issueCNS investigated whether the Startup Bus was susceptible to the fault condition through inspection, testing, and analysisRevised testing procedures and inspected non

-segregated busesCompleted extensive engineering and Probabilistic Risk Assessment (PRA) evaluations 5

6Emergency Transformer BusStartup BusesTurbine Building Railroad Airlock DoorNormal Transformer 7Emergency Transformer Bus (2000 A)Startup Bus (3000 A)Startup Bus (2000 A)

Emergency Transformer & Startup Buses 8

ELECTRICAL SYSTEM & STATION RESPONSE Jerry LongSenior Reactor Operator, Assistant Operations Manager (Shift),

Cooper Nuclear Station 9

10Location of Bus FaultFault Terminating BreakerLocation of Generator Flexible Links 4160V Distribution SystemSupplies and distributes AC power to both the normal and safety related station electrical auxiliary loadsMade up of the following components: Normal Transformer, Startup Transformer, Emergency Transformer, Standby AC Power System (2 Diesel Generators), 4160V Switchgear Buses (1A, 1B, 1C, 1D, 1E, 1F, 1G

)Power sources to these components are diverse and robust11 12Location of Bus Fault4160V Distribution System Normal LineupSafety Related EquipmentPowered from Normal TransformerPowered From Startup Transformer Station Response13Plant continued to run at 100% power as designed at the time of the faultThe Control Room received annunciators for Emergency Transformer and switchyard trouble as designedThe Station entered 7 day LCO 3.8.1 Condition A to restore the Emergency Transformer bus to Operable statusNo abnormal plant or system response resulted from Emergency Transformer bus loss Operations Response14Monitored redundant electrical sources via hourly panel walkdownsProtected the following equipment in accordance with plant procedures:Startup TransformerHPCIRCICCritical DC BusesMain Power Transformer YardBoth Diesel GeneratorsFire Risk Management Actions were established for the following areas:Cable Spreading RoomRHR SWBP and Service Air Compressor AreaFormed an Emergent Issues Team Response TimelineFollowing entry into the LCO CNS conducted a complete inspection of the Emergency Transformer buswork, including cleaning and repairing all damaged componentsThe total time to restore the Emergency Transformer bus was 127 hour0.00147 days <br />0.0353 hours <br />2.099868e-4 weeks <br />4.83235e-5 months <br />sAll actions and return to operable status were completed within the 7

-

day LCO time requirement15Emergency Transformer Bus FaultEmergency Transformer Bus Repaired Post Work Testing CompleteRestored and EnergizedLCO Exit7-Day LCO Expiry^Jan 17, 16:44^Jan 22, 01:04^Jan 22, 16:50^Jan 22, 23:42^Jan 23,01:38^Jan 24, 16:44 Timeline & Plant Response for Hypothetical Concurrent Emergency Transformer & Startup Bus FailuresT=0T=0+(immediate)T=Initial ResponseT= 0-24 HoursT= 24-36 HoursEmergency Transformer BusFault with Consequential Startup Bus FailureTrip of One RR Pump with Corresponding Reactor Power ReductionNormal Operations ~

60% PowerTS 3.8.1 Action Statement (36 hrs to Hot SD) Backfeed directed via Shutdown Procedure 2.1.4.1Emergent Issues Team 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> coverage for Bus Repair and Backfeed PrepBackfeed Installation Preps Repair of Startup and/or Emergency Transformer BusesManual Start of DGs per Procedure 2.0.3Reactor Shutdown Commences if Offsite Power Not Restored within24 hours.Prepare for Reactor Hot Shutdown Should Emergency or Startup Transformers Remain InoperableDGs Power 4160 VAC Buses (post Scram) 16 CAUSAL EVALUATIONDan Buman / Khalil Dia, Director, Nuclear Safety Assurance / Director, EngineeringCooper Nuclear Station17 Causal Evaluation18CR-CNS-2017-00223 -Phase-to-Phase Fault of Emergency Transformer BusAssessed the mechanistic cause of the fault and determined organizational and programmatic weaknesses that have contributed to the condition CR-CNS-2017-02164 -Hi-Pot Testing Values and ConfigurationEvaluated the basis for the hi

-pot testing conducted on the non

-segregated buses and ensured alignment with industry standards 19Example of corona damage to the Emergency Transformer bus insulation which occurred at a support piece 20Corrective ActionsCorrections to CNS Procedure 7.3.41, "Examination and High Pot Testing of Non

-Segregated Buses and Associated Equipment"Examination requirements increasedHigh Pot testing requirements brought in line with applicable standardsFull inspection and testing of the Startup busAnalysis of and corrections to training for personnel inspecting the BusbarsPlanned replacement of insulation on Emergency Transformer Bus (to be completed in October of 2018)

NEW INFORMATIONDan BumanDirector, Nuclear Safety Assurance, Cooper Nuclear Station21 New Information 1.Startup Bus Inspection Results 2.Hypothetical Consequential Startup Bus fault not crediblea.The degraded condition on the Emergency Transformer Bus did not and would not cause a Startup Bus fault b.Validated by Independent Engineering Evaluation and Mockup Testing 3.Station Blackout Risk Mitigation Efforts a.Implementation of FLEX and Associated EOPs b.Backfeed Implementation22 New Information #1Startup Bus Inspection Results23 Startup Bus Inspection ResultsThe environment and material condition of the Startup Bus is different than the Emergency Transformer Bus based on the physical installationStartup Bus inspection showed it to be overall in a very good conditionwith no issues discovered that would threaten ability of the bus to perform its functionAny non-superficial defects were repairedPassed the revised post

-maintenance Hi

-Pot and Low

-Resistance testingNo indications of corona induced tracking were found24 25As-Found(Startup 2000A bus)

As-Left(Startup 2000A bus) 26As-FoundStartup 3000A Bus As-LeftStartup 3000A Bus New Information #2Hypothetical Consequential Startup Bus fault not credible a.The degraded condition on the Emergency Transformer Bus did not cause a consequential Startup Bus fault b.Validated by Independent Engineering Evaluation and Mockup Testing27 Independent Engineering EvaluationDetermined that a fault on the Emergency Transformer bus would not result in discernable damage to the Startup busUsed actual fault data and analyzed the potential effects on equipment and materials installed at

CNS28 Independent TestingMockup testing performed of two representative regions

-one away from a duct support, one at the expansion joint locationTesting confirmed the results of the independent evaluation and determined that a Emergency Transformer bus fault would not cause a secondary fault on the Startup busDamage evidenced by testing was comparable to that from the actual Emergency Transformer bus fault29 30Configuration for Emergency Transformer bus fault test at

non-supported section of bus duct 31Configuration for Emergency Transformer bus fault test at booted connection of bus duct 32Configuration of Glastic box at each booted connection which separates and surrounds each phase of both the Emergency and Startup busesEmergency Transformer Bus Startup Bus 33January CNS FaultTest 1Test 2 34January CNS FaultTest 1Test 2 New Information #3Station Blackout Risk Mitigation Efforts a.Implementation of FLEX and Associated EOPsb.Backfeed Implementation35 EOP/FLEX SBO Improvements & InsightsPortable FLEX DGs extend DC capability >24 hrsTwo FLEX 175KW DGs, either capable of powering battery chargers provides added defense

-in-depth,FLEX validated on

-shift crew capable of aligning in less than 2 hrs (Compared to 9 hr battery capacity),Successful alignment provides DC support for high

-

pressure injection systems (RCIC/HPCI) for >24 hrs36 EOP/FLEX SBO Improvements & InsightsContainment Venting ImprovementsDedicated Uninterruptible Power Supplies (UPS) provide valve, control power, and indication > 24 hrs,Vent operations provided from control room after UPS alignment,Containment vented prior to 15 psig and pressure maintained 5

-15 psig,Successful venting eliminates containment related challenges to high

-pressure injection for > 24 hrs.37 Backfeed Lineup Following SBOThe loss of the Emergency Transformer bus, a reactor scram, and with a subsequent loss of the Startup bus requires the site to pursue Backfeed to the Normal TransformerThis is driven by the SBO Emergency Procedure38 39Timeline to Backfeed Normal Transformer 40 41Flexible Links Location in Turbine Building Basement 42 43 RISK SIGNIFICANC EWayne SchmidtPRA Consultant, Cooper Nuclear Station44 Key Risk Assessment RefinementsPreliminary Assessment 1.Assumed Hypothetical Failure of the Startup Transformer Bus 2.SPAR Limits Credit for Current SBO Mitigation Strategies/EquipmentSPAR not updated to reflect FLEX capabilities SPAR model of RR seal leak treated as SORV 3.Assumed backfeed of offsite power was not timelyRefined Risk Assessment 1.Actual Emergency Transformer bus fault did not result in a Startup bus fault.

1.Validated with engineering analysis and mockup testing 2.No Hypothetical Conditions in SDP 2.Additional analysis updates SPAR to reflect current SBO mitigation capabilities in place at the time of the Emergency Transformer bus fault 3.Additional field demonstrations and event timeline support crediting the backfeed power supply45 Refined Risk AssessmentSPAR SBO modeling updated to reflect High pressure injection capabilities with FLEXNew human reliability analysis (HRA) for alignment of FLEX PDG at 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> (1.1E

-2)Current EOP strategies and FLEX analyses including successful Early Containment Venting allows 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> AC power recoveries.No impact of Reactor Recirculation (RR) seal leakCurrent FLEX analysis demonstrates no impact for conservative RR seal leakLow pressure injection capability for one stuck open relief valve situationsBackfeed updated modelingMaintained conservative 17 hr estimate for backfeedNew HRAs for alignment of Backfeed Post SBO at 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> (0.26) and at 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> (0.21)46 CNS Best Estimate Significance based solely on actual occurrence:Fault caused loss of Emergency Transformer bus and Supplemental Diesel Generator for 151 hour0.00175 days <br />0.0419 hours <br />2.496693e-4 weeks <br />5.74555e-5 months <br />s47Increase in CDPPreliminarySignificance Determination6.3E-06No Consequential StartupBus Failure, FLEX and Backfeed Refined3.8E-07 Additional Risk Significance Conservatisms Common CauseNo failure data for passive component failureStartup Bus Environment different than Emergency Transformer BusSORV LikelihoodGeneric SPAR SORV Likelihood Refined to Reflect CNS DerivationExposure Time for situations that include EDG Test and MaintenanceTS 3.8.1 would limit exposure time to 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> vice 151 hour0.00175 days <br />0.0419 hours <br />2.496693e-4 weeks <br />5.74555e-5 months <br />s48 SAFETY SIGNIFICANCE SUMMARYBest estimate result is very low safety significanceCooper PRA found the SPAR model to be bounding49Preliminary Increase in Core Damage Probability(Internal/ExternalEvents)Additionally Informed Increase in Core damage Probability(Internal/ExternalEvents)Final Finding Significance Characterization6.3E-063.8E-07GreenCase NameIncrease in CDPRefined SPAR Internal Events2.3E-07Cooper Internal Events1.3E-07 CONCLUDING STATEMENTJohn DentSite Vice President and Chief Nuclear Officer, Cooper Nuclear Station50 1Cooper Nuclear StationRegulatory ConferenceNuclear Regulatory Commission

-Region IVArlington, TXNovember 7, 2017 2AgendaIntroduction of ParticipantsNRC Opening RemarksLicensee PresentationQuestions and DiscussionNRC Caucus Final Questions NRC Closing Remarks Conference Conclusion Members of the Public Questions / Comments Adjournment

ML17324A28 0 SUNSI Rev Compl.

Yes No ADAMS Yes No Reviewer Initials CHY Publicly Avail.

Yes No Sensitive Yes No Sens. Type Initials CHY SPE:DRP/C BC:DRP/C CYoung JKozal /RA/ /RA/ 11/16/17 11/16/17