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:	Jason Kozal NRC/RGN-IV/DRP/RPB-C
To:	Dent J Nebraska Public Power District (NPPD)
Kozal J
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

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 in NRC Inspection Report 05000298/2017011, 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 NRCs 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 NRCs Rules of Practice, a copy of this letter and its enclosures will be available electronically for public inspection in the NRCs Public Document Room or from the Publicly Available Records (PARS) component of the NRCs 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

1 COOPER NUCLEAR STATION REGULATORY CONFERENCE Emergency Station Service Transformer Phase-to-Phase Busbar Fault, January 17, 2017 November 7, 2017

2 OPENING REMARKS John Dent Site Vice President and Chief Nuclear Officer, Cooper Nuclear Station

3 Agenda Introduction Dan Buman, Director, Nuclear Safety Assurance Electrical System & Station Jerry Long, Senior Reactor Operator, Response Assistant Operations Manager (Shift)

Causal Evaluation Dan Buman / Khalil Dia, Director, Engineering New Information Dan Buman, Director, Nuclear Safety Assurance Risk Significance Wayne Schmidt, PRA Consultant Concluding Statement John Dent, Site Vice President and Chief Nuclear Officer

4 Introduction On January 17, 2017, CNS experienced a Phase-to-Phase fault on the busbars for the Emergency Station Service Transformer

• The Emergency Transformer was in a standby, energized condition, but not loaded

• No switching activities were occurring related to the Emergency Transformer at the time of the fault

• No grid or plant electrical instabilities existed at the time of the fault

• The NPPD Operations Center promptly analyzed the fault and determined that the circuit fault protection operated correctly

• The plant responded as designed with no additional malfunctions, human performance errors, or procedural deviations that complicated the condition

5 Efforts to Understand the Issue As a result of the January 17, 2017, Emergency Transformer Bus Fault, CNS has:

• Developed a greater understanding of the issue

• CNS investigated whether the Startup Bus was susceptible to the fault condition through inspection, testing, and analysis

• Revised testing procedures and inspected non-segregated buses

• Completed extensive engineering and Probabilistic Risk Assessment (PRA) evaluations

6 Emergency Transformer Bus Startup Buses Normal Transformer Turbine Building Railroad Airlock Door

7 Emergency Transformer Bus (2000 A)

Startup Bus (3000 A)

Startup Bus (2000 A)

8 Emergency Transformer & Startup Buses

9 ELECTRICAL SYSTEM &

STATION RESPONSE Jerry Long Senior Reactor Operator, Assistant Operations Manager (Shift),

Cooper Nuclear Station

10 Location of Generator Flexible Links Location of Bus Fault Fault Terminating Breaker

11 4160V Distribution System

• Supplies and distributes AC power to both the normal and safety related station electrical auxiliary loads

• Made 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 robust

12 4160V Distribution System Normal Lineup Location of Bus Fault Safety Related Equipment Powered from Normal Transformer Powered From Startup Transformer

13 Station Response

• Plant continued to run at 100% power as designed at the time of the fault

• The Control Room received annunciators for Emergency Transformer and switchyard trouble as designed

• The Station entered 7 day LCO 3.8.1 Condition A to restore the Emergency Transformer bus to Operable status

• No abnormal plant or system response resulted from Emergency Transformer bus loss

14 Operations Response Monitored redundant electrical sources via hourly panel walkdowns Protected the following equipment in accordance with plant procedures:

• Startup Transformer

• HPCI

• RCIC

• Critical DC Buses

• Main Power Transformer Yard

• Both Diesel Generators Fire Risk Management Actions were established for the following areas:

• Cable Spreading Room

• RHR SWBP and Service Air Compressor Area Formed an Emergent Issues Team

15 Response Timeline Emergency Emergency Post Work Restored 7-Day Transformer Transformer Testing and LCO Bus LCO Exit Bus Fault Complete Energized Expiry Repaired

^ ^ ^ ^ ^ ^

Jan 17, Jan 22, Jan 22, Jan 22, Jan 23, Jan 24, 16:44 01:04 16:50 23:42 01:38 16:44

• Following entry into the LCO CNS conducted a complete inspection of the Emergency Transformer buswork, including cleaning and repairing all damaged components

• The total time to restore the Emergency Transformer bus was 127 hours0.00147 days <br />0.0353 hours <br />2.099868e-4 weeks <br />4.83235e-5 months <br />

• All actions and return to operable status were completed within the 7-day LCO time requirement

16 Timeline & Plant Response for Hypothetical Concurrent Emergency Transformer & Startup Bus Failures T=0 T=0+ T=Initial T= 0-24 Hours T= 24-36 Hours (immediate) Response Emergency

• Trip of One RR Pump

• TS 3.8.1 Action

• Backfeed Installation

• Prepare for Reactor Transformer with Corresponding Statement (36 hrs to Preps Hot Shutdown Bus Fault with Reactor Power Hot SD) Should Emergency

• Repair of Startup Consequential Reduction or Startup

• Backfeed directed via and/or Emergency Startup Bus Transformers

• Normal Operations ~ Shutdown Procedure Transformer Buses Failure Remain Inoperable 60% Power 2.1.4.1

• Manual Start of DGs

• DGs Power 4160

• Emergent Issues per Procedure 2.0.3 VAC Buses (post Team 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />

• Reactor Shutdown Scram) coverage for Bus Commences if Offsite Repair and Backfeed Power Not Restored Prep within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

17 CAUSAL EVALUATION Dan Buman / Khalil Dia, Director, Nuclear Safety Assurance / Director, Engineering Cooper Nuclear Station

18 Causal Evaluation CR-CNS-2017-00223 - Phase-to-Phase Fault of Emergency Transformer Bus

• Assessed 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 Configuration

• Evaluated the basis for the hi-pot testing conducted on the non-segregated buses and ensured alignment with industry standards

19 Example of corona damage to the Emergency Transformer bus insulation which occurred at a support piece

20 Corrective Actions

• Corrections to CNS Procedure 7.3.41, Examination and High Pot Testing of Non-Segregated Buses and Associated Equipment

• Examination requirements increased

• High Pot testing requirements brought in line with applicable standards

• Full inspection and testing of the Startup bus

• Analysis of and corrections to training for personnel inspecting the Busbars

• Planned replacement of insulation on Emergency Transformer Bus (to be completed in October of 2018)

21 NEW INFORMATION Dan Buman Director, Nuclear Safety Assurance, Cooper Nuclear Station

22 New Information

1. Startup Bus Inspection Results

2. Hypothetical Consequential Startup Bus fault not credible

a. 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 Implementation

23 New Information #1 Startup Bus Inspection Results

24 Startup Bus Inspection Results

• The environment and material condition of the Startup Bus is different than the Emergency Transformer Bus based on the physical installation

• Startup Bus inspection showed it to be overall in a very good condition with no issues discovered that would threaten ability of the bus to perform its function

• Any non-superficial defects were repaired

• Passed the revised post-maintenance Hi-Pot and Low-Resistance testing

• No indications of corona induced tracking were found

25 As-Found As-Left (Startup 2000A bus) (Startup 2000A bus)

26 As-Found As-Left Startup 3000A Bus Startup 3000A Bus

27 New Information #2 Hypothetical 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 Testing

28 Independent Engineering Evaluation

• Determined that a fault on the Emergency Transformer bus would not result in discernable damage to the Startup bus

• Used actual fault data and analyzed the potential effects on equipment and materials installed at CNS

29 Independent Testing

• Mockup testing performed of two representative regions - one away from a duct support, one at the expansion joint location

• Testing confirmed the results of the independent evaluation and determined that a Emergency Transformer bus fault would not cause a secondary fault on the Startup bus

• Damage evidenced by testing was comparable to that from the actual Emergency Transformer bus fault

30 Configuration for Emergency Transformer bus fault test at non-supported section of bus duct

31 Configuration for Emergency Transformer bus fault test at booted connection of bus duct

32 Configuration of Glastic box at each booted connection which separates and surrounds each phase of both the Emergency and Startup buses Emergency Transformer Bus Startup Bus

33 Test 1 Test 2 January CNS Fault

34 Test 1 Test 2 January CNS Fault

35 New Information #3 Station Blackout Risk Mitigation Efforts

a. Implementation of FLEX and Associated EOPs

b. Backfeed Implementation

36 EOP/FLEX SBO Improvements & Insights

• Portable FLEX DGs extend DC capability >24 hrs

• Two 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 hrs

37 EOP/FLEX SBO Improvements & Insights

• Containment Venting Improvements

• Dedicated 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.

38 Backfeed Lineup Following SBO

• The 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 Transformer

• This is driven by the SBO Emergency Procedure

39 Timeline to Backfeed Normal Transformer

40 41 Flexible Links Location in Turbine Building Basement

42 43 44 RISK SIGNIFICANCE Wayne Schmidt PRA Consultant, Cooper Nuclear Station

45 Key Risk Assessment Refinements Preliminary Assessment Refined Risk Assessment

1. Assumed Hypothetical Failure of the Startup 1. Actual Emergency Transformer bus fault did not Transformer Bus result in a Startup bus fault.

1. Validated with engineering analysis and mockup testing

2. No Hypothetical Conditions in SDP

2. SPAR Limits Credit for Current SBO Mitigation 2. Additional analysis updates SPAR to reflect current Strategies/Equipment SBO mitigation capabilities in place at the time of

• SPAR not updated to reflect FLEX capabilities the Emergency Transformer bus fault

• SPAR model of RR seal leak treated as SORV

3. Assumed backfeed of offsite power was not timely 3. Additional field demonstrations and event timeline support crediting the backfeed power supply

46 Refined Risk Assessment

• SPAR SBO modeling updated to reflect

• High pressure injection capabilities with FLEX

• New 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 leak

• Current FLEX analysis demonstrates no impact for conservative RR seal leak

• Low pressure injection capability for one stuck open relief valve situations

• Backfeed updated modeling

• Maintained conservative 17 hr estimate for backfeed

• New 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)

47 CNS Best Estimate

• Significance based solely on actual occurrence:

• Fault caused loss of Emergency Transformer bus and Supplemental Diesel Generator for 151 hours0.00175 days <br />0.0419 hours <br />2.496693e-4 weeks <br />5.74555e-5 months <br /> Increase in CDP Preliminary Significance Determination 6.3E-06 No Consequential Startup Bus Failure, 3.8E-07 FLEX and Backfeed Refined

48 Additional Risk Significance Conservatisms

• Common Cause

• No failure data for passive component failure

• Startup Bus Environment different than Emergency Transformer Bus

• SORV Likelihood

• Generic SPAR SORV Likelihood Refined to Reflect CNS Derivation

• Exposure Time for situations that include EDG Test and Maintenance

• TS 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 hours0.00175 days <br />0.0419 hours <br />2.496693e-4 weeks <br />5.74555e-5 months <br />

49 SAFETY SIGNIFICANCE

SUMMARY

• Best estimate result is very low safety significance Preliminary Increase in Core Additionally Informed Increase in Final Finding Significance Damage Probability Core damage Probability Characterization (Internal/External Events) (Internal/External Events) 6.3E-06 3.8E-07 Green

• Cooper PRA found the SPAR model to be bounding Increase in Case Name CDP Refined SPAR Internal Events 2.3E-07 Cooper Internal Events 1.3E-07

50 CONCLUDING STATEMENT John Dent Site Vice President and Chief Nuclear Officer, Cooper Nuclear Station

Cooper Nuclear Station Regulatory Conference Nuclear Regulatory Commission - Region IV Arlington, TX November 7, 2017 1

Agenda

• Introduction of Participants

• NRC Opening Remarks

• Licensee Presentation

• Questions and Discussion

• NRC Caucus

• Final Questions

• NRC Closing Remarks

• Conference Conclusion

• Members of the Public Questions / Comments

• Adjournment 2

ML17324A280 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