Text

Regarding Accident Sequence Precursor Analysis
	ML17125A291
Person / Time
Site:	Wolf Creek
Issue date:	05/12/2017
From:	Balwant Singal; Plant Licensing Branch IV
To:	Heflin A; Wolf Creek
	Singal B, NRR/DORL/LPL4, 415-3016
References
	LER 16-001-01
	Download: ML17125A291 (18)
	v • d • e

Mr. Adam C. Heflin UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 May 12, 2017 President, Chief Executive Officer, and Chief Nuclear Officer Wolf Creek Nuclear Operating Corporation Post Off ice Box 411 Burlington, KS 66839

SUBJECT:

WOLF CREEK GENERATING STATION-ACCIDENT SEQUENCE PRECURSOR ANALYSIS (LICENSEE EVENT REPORT 2016-001-01)

Dear Mr. Heflin:

By letter dated March 28, 2016 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML16095A207), as supplemented by letter dated November 10, 2016 (ADAMS Accession No. ML16326A175), Wolf Creek Nuclear Operating Corporation (WCNOC) submitted Licensee Event Report (LER) 2016-001-01, "Power Potential Transformer Overloading Results in Emergency Diesel Generator lnoperability," to the U.S. Nuclear Regulatory Commission (NRG) staff pursuant to Title 1 O of the Code of Federal Regulations (1 O CFR) paragraph 50.73(a)(2)(i)(B) and 10 CFR 50.73(a)(2)(v)(B), (C), and (D). The LER describes the event of October 6, 2014, when a fire in one of the electrical cabinets resulted in an unexpected trip of one of the emergency diesel generators at Wolf Creek Generating Station (WCGS). The event also resulted in an unplanned entry into a 72-hour shutdown limiting condition for operation and an ALERT emergency classification.

As per the guidance in NRG Regulatory Issue Summary (RIS) 2006-24, "Revised Review and Transmittal Process for Accident Sequence Precursor Analyses" (ADAMS Accession No. ML060900007), the NRG staff reviewed the event at WCGS based on information provided in the LER to identify potential precursors and to determine the probability of an event leading to a core damage state resulting from the potential precursors.

The results of the NRG staff accident sequence precursor (ASP) analysis for the subject operational event on October 6, 2014, at WCGS, are provided in the enclosure to this letter for WCNOC's information. The NRG staff did not request a formal review by WCNOC since the results of the ASP analysis determined that the operational even had a best estimate increase in core damage probability (~CDP) of 2x1 o*6, which is less than threshold of ~CDP of 1x10-4 for a low risk event provided by RIS 2006-04.

If you have any questions, please contact me at 301-415-3016 or via e-mail at Balwant.Sinqal@nrc.gov.

Docket No. 50-482 Enclosure WCGS Final ASP Program Analysis - Precursor cc w/encl: Distribution via Listserv Sincerely,

~+-C-2.i1~

Balwant K. Singal, Senior Project Manager Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

ENCLOSURE FINAL ACCIDENT SEQUENCE PROGRAM ANALYSIS - PRECURSOR WOLF CREEK NUCLEAR OPERATING CORPORATION WOLF CREEK GENERATING STATION DOCKET NO. 50-482

Final ASP Pro sis - Precursor Wolf Creek Generating Station Event Date: 10/6/2014 Power Potential Transformer Overloading Results in Emergency Diesel Generator lnoperability LER: 482-2016-001-01 IR s : 05000482/2016008

~CDP= 2x10-5 Plant Type: Westinghouse. PWR-4 with a Dry, Ambient Pressure Containment Plant Operating Mode (Reactor Power Level): Mode 1 (100% Reactor Power)

Analyst:

Reviewer:

Contributors:

BC Approved Date:

Keith Tetter Chris Hunter David Aird 04/12/2017 EXECUTIVE

SUMMARY

On October 6, 2014, at 1 :26 p.m. during a scheduled 24-hour run, the 'B' emergency diesel generator (EOG) unexpectedly tripped and a fire was observed in control cabinet NE106. Just prior to the trip, operations personnel observed vapor coming from NE106 and identified the source as the power potential transformer (PPT). The vapor was a deficiency first identified during a post maintenance test run on June 11, 2014. The PPT exhibited the same symptoms during the subsequent surveillances after June 11, 2014. The 'B' EOG was repaired and declared operable on October gth at 5:17 p.m.

A failure investigation concluded that the smoking and eventual failure of the PPT on October 61h was most likely due to overloading caused from a failed diode that occurred during load transients on June 9, 2014. Therefore, the 'B' EOG would not have been able to fulfill its safety function for the complete mission time from June gth until October 91h. Note that the

'B' EOG had run successfully in the previous three monthly surveillance tests (for a total of approximately 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months ). During the June-October time period, the 'A' EOG was taken out of service for maintenance on July 21, 2014, creating a condition where both trains may have been inoperable. Therefore, an independent accident sequence precursor (ASP) analysis was performed given the windowed unavailability. When the 'A' and 'B' EDGs are inoperable, there are no remaining safety-related onsite standby alternating current (AC) sources.

According to the risk analysis modeling assumptions used in this ASP analysis, the most likely core damage scenario is weather-related loss of offsite power (LOOP) initiating event, successful reactor trip, emergency power system failure results in station blackout (SBO),

auxiliary feedwater (AFW) succeeds, reactor coolant pump (RCP) seal integrity is maintained, operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ), operators successfully maintain AFW without direct current (DC) power, and operators fail to restore offsite power within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months . This accident sequence accounts for approximately 27 percent of the increase in core damage probability (~CDP) for the event. The point estimate ~CDP for this event is 2x10-6, which is considered a precursor in the ASP Program.

EVENT DETAILS Event Description. On October 6, 2014, at 1 :26 p.m. during a scheduled 24-hour run, the

'B' EOG unexpectedly tripped and a fire was observed in control cabinet NE106. Just prior to the trip, operations personnel observed vapor coming from NE106 and identified the source as the PPT. The vapor was a deficiency first identified during a post maintenance test run on June 11, 2014. The PPT exhibited the same symptoms during the subsequent surveillances after June 11, 2014. The 'B' EOG was repaired and declared operable on October 9th at 5:17 p.m.

A failure investigation was performed using available site information as well as offsite hardware failure analysis, modeling, testing, and third party reviews. It was concluded that the smoking and eventual failure of the PPT on October 61h was most likely due to overloading. The overloading of the PPT resulted from failure of a diode in the power rectifier of the EOG excitation system. Failure of the diode occurred during load transients on June 9, 2014, resulting from a governor actuator malfunction. Therefore, the 'B' EOG would not have been able to fulfill its safety function for the completed mission time from June 9th until October 91h; however, note that the EOG had run successfully in the previous three monthly surveillance tests (for a total of approximately 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months ).

During the June-October time period, the 'A' EOG was taken out of service for maintenance on July 21, 2014, creating a condition where both trains may have been inoperable. When the 'A' and 'B' EDGs are inoperable, there are no remaining safety-related onsite standby AC sources.

Additional information is provided in licensee event report (LER) 482-2016-001-01 (Reference 1) and inspection reports (I Rs) 05000482/2016004 (Reference 2) and 05000482/2016008 (Reference 3).

Cause. The direct cause of the event is identified as a single diode failure, induced by the governor actuator malfunction on June 9, 2014. The diode failure led to the thermal failure of the PPT.

Additional Event Information. This event occurred on October 6, 2014, but the LER was not available until 2016. There was a delay between the event and LER report because the licensee's evaluation of the issue and cause evaluation/conclusions initially determined that the failure was as a result of thermal degradation of the failed diode(s). Considering this failure mechanism, it was not initially clear that the station was in a reportable condition. The licensee completed additional hardware failure analyses on January 28, 2016, and revised its conclusions (specifically, the licensee determined that a diode had failed in June 2014) so the licensee submitted Revision 0 of LER 482-2016-001 on March 28, 2016. Revision 1 of LER 482-2016-001 was issued on November 10, 2016.

MODELING Basis for ASP Analysis/Significant Determination Process Results. The ASP Program uses significant determination process results for degraded conditions when available and applicable. The ASP Program performs independent analyses for initiating events. ASP analyses of initiating events account for all failures/degraded conditions and unavailability (e.g., equipment out for test/maintenance) that occurred during the event, regardless of licensee 2

performance. 1 An independent ASP analysis was required because both safety-related EDGs were unavailable at the same time due to different causes.

NRG inspectors, in IR 05000482/2016004, re-characterized the preliminary White finding (as documented in IR 05000482/2016008) for failure to adequately develop and adjust preventative maintenance activities for EOG excitation system diodes to a Green finding due to new testing results that showed that over half of the excitation system diodes that were originally installed in the emergency diesel generators had manufacturing defect flaws that reduced their ability to withstand load transients such as the transients experienced on June 9, 2014, and for the lack of overlap in SBO diesel generators unavailability with EOG unavailability that was originally considered as concurrent SBO diesel generators and EOG unavailability, which resulted in the White finding. NRG inspectors discovered additional overpower transients that occurred on June 29, 1999, and December 20, 2007, which were in excess of the 1.4 megawatt threshold for damage as described by the licensee and that these previous overpower transients would have been expected to cause damage to excitation system diodes with pre-existing flaws, and determined that a transient-related aging mechanism caused degradation of the diodes and contributed to the failure of a diode on June 9, 2014, as predicted by the operating experience.

Analysis Type. A condition assessment was performed using the Wolf Creek Standardized Plant Analysis Risk (SPAR) model Revision 8.26, created in February 2015.

SPAR Model Modifications. The following SPAR model modifications were required for this condition assessment. 2 To prevent potential double counting of consequential LOOP events, the ACP-NB01-4 (Loss of Offsite Power to Bus NB01 from 13.BkV) and ACP-NB02-5 (Loss of Offsite Power to Bus NB02 from 13.BkV) gates were removed from the ACP-NB01 (Wolf Creek Power from 4160V AC Bus NB01) and ACP-NB02 (Wolf Creek Power from 4160V AC Bus NB02) faults trees, respectively.

Exposure Periods. In order to model this event, the analyst identified the following seven distinct exposure periods, determined with assistance from Table 1 of IR 05000482/2016008, which listed EOG run times on different dates, referred to as EP-1 through EP-7:

2 EP-1 consists of the 3-day period from June 9, 2014, to June 11, 2014, when the

'B' EOG would have failed to run for its full 24-hour mission time, but could have possibly run for 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months .

EP-2 consists of the 28-day period from June 11, 2014, to July 9, 2014, when the

'B' EOG would have failed to run for its full 24-hour mission time, but could have possibly run for 9 hours1.041667e-4 days 0.0025 hours 1.488095e-5 weeks 3.4245e-6 months .

ASP analyses also account for any degraded condition(s) that were identified after the initiating event occurred if the failure/degradation exposure period(s) overlapped the initiating event date.

In addition, basic events EPS-XHE-XL-NR01 H (Operator Fails to Recover Emergency Diesel in 1 Houi),

EPS-XHE-XL-NR02H (Operator Fails to Recover Emergency Diesel in 2 Hours), EPS-XHE-XL-NR03H (Operator Fails to Recover Emergency Diesel in 3 Houi), EPS-XHE-XL-NR04H (Operator Fails to Recover Emergency Diesel in 4 Houi), EPS-XHE-XL-NR6H (Operator Fails to Recover Emergency Diesel in 6 Hours),

EPS-XHE-XL-NR8H (Operator Fails to Recover Emergency Diesel in 8 Hours), and EPS-XHE-XL-NR24H8 (Operator Fails to Recover Emergency Diesel in 24 Hours (Given Failure at 8) were set to TRUE in the base model. These basic events are set to TRUE for applicable ASP condition assessments and their use is limited to cases where event information supports credit for EOG recovery.

3

EP-3 consists of the 27-day period from July 9, 2014, to August 6, 2014, (subtracting the day for EP-7) when the 'B' EOG would have failed to run for its full 24-hour mission time, but could have possibly run for 7 hours8.101852e-5 days 0.00194 hours 1.157407e-5 weeks 2.6635e-6 months .

EP-4 consists of the 33-day period from August 6, 2014, to September 8, 2014, when the 'B' EOG would have failed to run for its full 24-hour mission time, but could have possibly run for 5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months .

EP-5 consists of the 28-day period from September 8, 2014, to October 6, 2014, when the 'B' EOG would have failed to run for its full 24-hour mission time, but could have possibly run for 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months .

EP-6 consists of the 3-day period from October 6, 2014, to October 9, 2014, when the

'B' EOG was failed and undergoing repairs.

EP-7 consists of the one day period July 21, 2014, when the 'A' EOG was inoperable and the 'B' EOG would have failed to run for its full 24-hour mission time, but could have possibly run for 7 hours8.101852e-5 days 0.00194 hours 1.157407e-5 weeks 2.6635e-6 months .

Key Modeling Assumptions. The following assumptions were determined to be significant to the modeling of this event:

For all exposure periods:

The failure of 'B' EOG is modeled by setting basic event EPS-OGN-FR-NE02 (Diesel Generator NE02 Fails to Run) to TRUE because the 'B' EDG would have failed to run for its 24-hour mission during a postulated LOOP initiating event.

For EP-1:

The 'B' EOG could potentially have run for 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months during this exposure period; therefore additional time may be available to operators to recover offsite power (prior to either core uncovery or battery depletion) for postulated SBO scenarios. This credit is provided by setting the probability of basic events in the following table to probabilities equivalent to the offsite power non-recovery probabilities for those with 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months additional time.

Basic Event Initial Pr9babllltv Revised PrC)l)abllitv OEP-XHE-XL-NR01 HGR 6.587E-1 8.455E-3 OEP-XHE-XL-NR01 HPC 3.309E-1 7.157E-3 OEP-XHE-XL-NR01 HSC 4.015E-1 9.867E-3 OEP-XHE-XL-NR01 HWA 6.868E-1 1.846E-1 OEP-XHE-XL-NR02HGR 3.915E-1 7.240E-3 OEP-XHE-XL-NR02HPC 1.763E-1 6.389E-3 OEP-XHE-XL-NR02HSC 2.240E-1 8.814E-3 OEP-XHE-XL-NR02HWR 5.589E-1 1.773E-1 OEP-XHE-XL-NR03HGR 2.496E-1 6.236E-3 OEP-XHE-XL-NR03HPC 1.11 ?E-1 5.731 E-3 OEP-XHE-XL-NR03HSC 1.453E-1 7.909E-3 OEP-XHE-XL-NR03HWR 4.800E-1 1.705E-1 4

S.slc Event ln.it~I Probabllitv Revlse<t,Prc)babllltv OEP-XHE-XL-NR04HGR 1.685E-1 5.399E-3 OEP-XHE-XL-NR04HPC 7.753E-2 5.163E-3 OEP-XHE-XL-NR04HSC 1.024E-1 7.127E-3 OEP-XHE-XL-NR04HWR 4.244E-1 1.642E-1 OEP-XHE-XL-NR06HGR 8.686E-2 4.106E-3 OEP-XHE-XL-NR06HPC 4.365E-2 4.240E-3 OEP-XHE-XL-NR06HSC 5.866E-2 5.854E-3 OEP-XHE-XL-NR06HWR 3.487E-1 1.529E-1 OEP-XHE-XL-NR08HGR 5.005E-2 3.175E-3 OEP-XHE-XL-NR08HPC 2.781 E-2 3.528E-3 OEP-XHE-XL-NR08HSC 3.774E-2 4.873E-3 OEP-XHE-XL-NR08HWR 2.982E-1 1.431E-1 For EP-2:

The 'B' EOG could potentially have run for 9 hours1.041667e-4 days 0.0025 hours 1.488095e-5 weeks 3.4245e-6 months during this exposure period; therefore additional time may be available to operators to recover offsite power (prior to either core uncovery or battery depletion) for postulated SBO scenarios. This credit is provided by setting the probability of basic events in the following table to probabilities equivalent to the offsite power non-recovery probabilities for those with 9 hours1.041667e-4 days 0.0025 hours 1.488095e-5 weeks 3.4245e-6 months additional time.

- - . D*tcsivent filtthll ProilabllitY RevrsectPrc:SbJJ1bilttv OEP-XHE-XL-NR01 HGR 6.587E-1 3.111 E-2 OEP-XHE-XL-NR01 HPC 3.309E-1 1.911 E-2 OEP-XHE-XL-NR01 HSC 4.015E-1 2.610E-2 OEP-XHE-XL-NR01 HWR 6.868E-1 2.616E-1 OEP-XHE-XL-NR02HGR 3.915E-1 2.507E-2 OEP-XHE-XL-NR02HPC 1.763E-1 1.617E-2 OEP-XHE-XL-NR02HSC 2.240E-1 2.214E-2 OEP-XHE-XL-NR02HWR 5.589E-1 2.466E-1 OEP-XHE-XL-NR03HGR 2.496E-1 2.044E-2 OEP-XHE-XL-NR03HPC 1.117E-1 1.383E-2 OEP-XHE-XL-NR03HSC 1.453E-1 1.897E-2 OEP-XHE-XL-N R03HW R 4.800E-1 2.334E-1 OEP-XHE-XL-N R04HG R 1.685E-1 1.684E-2 OEP-XHE-XL-NR04HPC 7.753E-2 1.195E-2 OEP-XHE-XL-NR04HSC 1.024E-1 1.641 E-2 OEP-XHE-XL-NR04HWR 4.244E-1 2.216E-1 OEP-XHE-XL-NR06HGR 8.686E-2 1.175E-2 OEP-XHE-XL-NR06HPC 4.365E-2 9.127E-3 OEP-XHE-XL-NR06HSC 5.866E-2 1.257E-2 OEP-XHE-XL-NR06HWR 3.487E-1 2.014E-1 OEP-XHE-XL-NR08HGR 5.005E-2 8.455E-3 OEP-XHE-XL-NR08HPC 2.781 E-2 7.157E-3 OEP-XHE-XL-NR08HSC 3.774E-2 9.867E-3 OEP-XHE-XL-NR08HWR 2.982E-1 1.846E-1 For EP-3:

The 'B' EOG could potentially have run for 7 hours8.101852e-5 days 0.00194 hours 1.157407e-5 weeks 2.6635e-6 months during this exposure period; therefore additional time may be available to operators to recover offsite power (prior to either core 5

uncovery or battery depletion) for postulated SBO scenarios. This credit is provided by setting the probability of basic events in the following table to probabilities equivalent to the offsite power non-recovery probabilities for those with 7 hours8.101852e-5 days 0.00194 hours 1.157407e-5 weeks 2.6635e-6 months additional time.

hale Event Initial. Ptobabillty RevlsecU"r~bffitv OEP-XHE-XL-NR01 HGR 6.587E-1 5.005E-2 OEP-XHE-XL-NR01 HPC 3.309E-1 2.781 E-2 OEP-XHE-XL-NR01 HSC 4.015E-1 3.774E-2 OEP-XHE-XL-NR01 HWR 6.868E-1 2.982E-1 OEP-XHE-XL-NR02HGR 3.915E-1 3.915E-2 OEP-XHE-XL-NR02HPC 1.763E-1 2.287E-2 OEP-XHE-XL-NR02HSC 2.240E-1 3.116E-2 OEP-XHE-XL-NR02HWR 5.589E-1 2.786E-1 OEP-XHE-XL-NR03HGR 2.496E-1 3.111 E-2 OEP-XHE-XL-NR03HPC 1.117E-1 1.911 E-2 OEP-XHE-XL-NR03HSC 1.453E-1 2.610E-2 OEP-XHE-XL-NR03HWR 4.800E-1 2.616E-1 OEP-XHE-XL-NR04HGR 1.685E-1 2.507E-2 OEP-XHE-XL-NR04HPC 7.753E-2 1.617E-2 OEP-XHE-XL-NR04HSC 1.024E-1 2.214E-2 OEP-XHE-XL-NR04HWR 4.244E-1 2.466E-1 OEP-XHE-XL-NR06HGR 8.686E-2 1.684E-2 OEP-XHE-XL-NR06HPC 4.365E-2 1.195E-2 OEP-XHE-XL-NR06HSC 5.866E-2 1.641 E-2 OEP-XHE-XL-NR06HWR 3.487E-1 2.216E-1 OEP-XHE-XL-NR08HGR 5.005E-2 1.175E-2 OEP-XHE-XL-NR08HPC 2.781 E-2 9.127E-3 OEP-XHE-XL-NR08HSC 3.774E-2 1.257E-2 OEP-XHE-XL-NR08HWR 2.982E-1 2.014E-1 For EP-4:

The 'B' EDG could potentially have run for 5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months during this exposure period; therefore additional time may be available to operators to recover offsite power (prior to either core uncovery or battery depletion) for postulated SBO scenarios. This credit is provided by setting the probability of basic events in the following table to probabilities equivalent to the offsite power non-recovery probabilities for those with 5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months additional time.

Bast~ Event Initial.Pr(>bibUitY Revh~tc1.. Probabil1tv OEP-XHE-XL-NR01 HGR 6.587E-1 8.686E-2 OEP-XHE-XL-NR01 HPC 3.309E-1 4.365E-2 OEP-XHE-XL-NR01 HSC 4.015E-1 5.866E-2 OEP-XHE-XL-NR01 HWR 6.868E-1 3.487E-1 OEP-XHE-XL-NR02HGR 3.915E-1 6.520E-2 OEP-XHE-XL-NR02HPC 1.763E-1 3.444E-2 OEP-XHE-XL-NR02HSC 2.240E-1 4.654E-2 OEP-XHE-XL-NR02HWR 5.589E-1 3.213E-1 OEP-XHE-XL-NR03HGR 2.496E-1 5.005E-2 OEP-XHE-XL-NR03HPC 1.117E-1 2.781 E-2 OEP-XHE-XL-NR03HSC 1.453E-1 3.774E-2 OEP-XHE-XL-NR03HWR 4.800E-1 2.982E-1 OEP-XHE-XL-NR04HGR 1.685E-1 3.915E-2 OEP-XHE-XL-NR04HPC 7.753E-2 2.287E-2 6

B1slcEvent Initial Probability Revised Probability OEP-XHE-XL-NR04HSC 1.024E-1 3.116E-2 OEP-XHE-XL-NR04HWR 4.244E-1 2.786E-1 OEP-XHE-XL-NR06HGR 8.686E-2 2.507E-2 OEP-XHE-XL-NR06HPC 4.365E-2 1.617E-2 OEP-XHE-XL-NR06HSC 5.866E-2 2.214E-2 OEP-XHE-XL-NR06HWR 3.487E-1 2.466E-1 OEP-XHE-XL-NR08HGR 5.005E-2 1.684E-2 OEP-XHE-XL-NR08HPC 2.781 E-2 1.195E-2 OEP-XHE-XL-NR08HSC 3.774E-2 1.641 E-2 OEP-XHE-XL-NR08HWR 2.982E-1 2.216E-1 For EP-5:

The 'B' EOG could potentially have run for 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months during this exposure period; therefore additional time may be available to operators to recover offsite power (prior to either core uncovery or battery depletion) for postulated SBO scenarios. This credit is provided by setting the probability of basic events in the following table to probabilities equivalent to the offsite power non-recovery probabilities for those with 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months additional time.

.. \\c~llll*tc~.nt... *..... tnitlaflt mw Revised Probability OEP-XHE-XL-NR01 HGR 6.587E-1 1.685E-1 OEP-XHE-XL-NR01 HPC 3.309E-1 7.753E-2 OEP-XHE-XL-NR01 HSC 4.015E-1 1.024E-1 OEP-XHE-XL-NR01 HWR 6.868E-1 4.244E-1 OEP-XHE-XL-NR02HGR 3.915E-1 1.189E-1 OEP-XHE-XL-NR02HPC 1.763E-1 5.702E-2 OEP-XHE-XL-NR02HSC 2.240E-1 7.605E-2 OEP-XHE-XL-NR02HWR 5.589E-1 3.822E-1 OEP-XHE-XL-NR03HGR 2.496E-1 8.686E-2 OEP-XHE-XL-NR03HPC 1.117E-1 4.365E-2 OEP-XHE-XL-NR03HSC 1.453E-1 5.866E-2 OEP-XHE-XL-NR03HWR 4.800E-1 3.487E-1 OEP-XHE-XL-NR04HGR 1.685E-1 6.520E-2 OEP-XHE-XL-NR04HPC 7.753E-2 3.444E-2 OEP-XHE-XL-NR04HSC 1.024E-1 4.654E-2 OEP-XHE-XL-NR04HWR 4.244E-1 3.213E-1 OEP-XHE-XL-NR06HGR 8.686E-2 3.915E-2 OEP-XHE-XL-NR06HPC 4.365E-2 2.287E-2 OEP-XHE-XL-NR06HSC 5.866E-2 3.116E-2 OEP-XHE-XL-NR06HWR 3.487E-1 2.786E-1 OEP-XHE-XL-NR08HGR 5.005E-2 2.507E-2 OEP-XHE-XL-NR08HPC 2.781 E-2 1.617E-2 OEP-XHE-XL-NR08HSC 3.774E-2 2.214E-2 OEP-XHE-XL-NR08HWR 2.982E-1 2.466E-1 For EP-6:

While the 'B' EOG was undergoing repair, technical specifications would prevent test/maintenance activities that would render certain equipment unavailable. Therefore, basic events AFW-TOP-TM-PAL02 (Feed Pump PAL02 is in Test or Maintenance),

EPS-OGN-TM-NE01 (Diesel Generator NE01 Unavailable due to Test and 7

Maintenance), ESW-MOP-TM-1A (ESW Train A MOP 1A Unavailable due to Test or Maintenance), and ESW-SYS-TM-TRAINA (Service Water Train A Unavailable Due To Maintenance) were set to FALSE.

For EP-7:

The failure of 'A' EOG is modeled by setting basic event EPS-OGN-TM-NE01 to TRUE.

The 'B' EOG could potentially have run for 7 hours8.101852e-5 days 0.00194 hours 1.157407e-5 weeks 2.6635e-6 months during this exposure period; therefore additional time may be available to operators to recover offsite power (prior to either core uncovery or battery depletion) for postulated SBO scenarios. This credit is provided by setting the probability of basic events in the following table to probabilities equivalent to the offsite power non-recovery probabilities for those with 7 hours8.101852e-5 days 0.00194 hours 1.157407e-5 weeks 2.6635e-6 months additional time.

Basic Ev.ent lnltltf c~~babllity Revised Probability OEP-XHE-XL-NR01 HGR 6.587E-1 5.005E-2 OEP-XHE-XL-NR01 HPC 3.309E-1 2.781 E-2 OEP-XHE-XL-NR01 HSC 4.015E-1 3.774E-2 OEP-XHE-XL-NR01 HWR 6.868E-1 2.982E-1 OEP-XHE-XL-NR02HGR 3.915E-1 3.915E-2 OEP-XHE-XL-NR02HPC 1.763E-1 2.287E-2 OEP-XHE-XL-NR02HSC 2.240E-1 3.116E-2 OEP-XHE-XL-NR02HWR 5.589E-1 2.786E-1 OEP-XHE-XL-NR03HGR 2.496E-1 3.111E-2 OEP-XHE-XL-NR03HPC 1.117E-1 1.911 E-2 OEP-XHE-XL-NR03HSC 1.453E-1 2.610E-2 OEP-XHE-XL-NR03HWR 4.800E-1 2.616E-1 OEP-XHE-XL-NR04HGR 1.685E-1 2.507E-2 OEP-XHE-XL-NR04HPC 7.753E-2 1.617E-2 OEP-XHE-XL-NR04HSC 1.024E-1 2.214E-2 OEP-XHE-XL-NR04HWR 4.244E-1 2.466E-1 OEP-XHE-XL-NR06HGR 8.686E-2 1.684E-2 OEP-XHE-XL-NR06HPC 4.365E-2 1.195E-2 OEP-XHE-XL-NR06HSC 5.866E-2 1.641 E-2 OEP-XHE-XL-NR06HWR 3.487E-1 2.216E-1 OEP-XHE-XL-NR08HGR 5.005E-2 1.175E-2 OEP-XHE-XL-NR08HPC 2.781 E-2 9.127E-3 OEP-XHE-XL-NR08HSC 3.774E-2 1.257E-2 OEP-XHE-XL-NR08HWR 2.982E-1 2.014E-1 8

ANALYSIS RESULTS

~CDP. The point estimate iiCDP for this event is 1.9x1 o-6. The ASP Program acceptance threshold is a liCDP of greater than 1 x1 o-6* Therefore, this event is a precursor. The following table provides the iiCDP breakdown for each exposure period.

EXT~

Pff: ~a' ACDP EP-1 3.0x10-s EP-2 3.3x10-7 EP-3 3.4x10-7 EP-4 4.7x10-7 EP-5 4.8x10-7 EP-6 3.9x10-s EP-7 2.2x10-7 TOTAL 1.9x10-s Dominant Sequence. The dominant accident sequence is weather-related LOOP (LOOPWR)

Sequence 16-06-4 (LiCDP = 5.2x1 o-7) that contributes 27 percent of the total internal events liCDP. Figures A-1 through A-3 in Appendix A illustrates this sequence. The dominant sequences that contribute at least 1 percent of the total internal events CCDP are provided in the following table.

&Kf uence ACDP p C9QjQe

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Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP seal integrity is LOOPWR 16-06-04 5.2E-07 27.2%

maintained; operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ); operators successfully maintain AFW without DC power; operators fail to restore offsite power within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in LOOPWR 16-09 2.2E-07 11.7%

SBO; AFW succeeds; RCP stage 2 seals fail resulting in a loss-of-coolant accident (LOCA); operators fail to restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months Grid-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP stage 2 seals fail resulting in a LOOPGR 16-07-2 2.1 E-07 10.8%

LOCA; operators successfully restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months , high-pressure injection (HPI) succeeds, secondary cooldown/depressurization succeeds, low-pressure recirculation fails Switchyard LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP stage 2 seals fail resulting in a LOOPSC 16-07-2 1.BE-07 9.2%

LOCA; operators successfully restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months , HPI succeeds, secondary cooldown/depressurization succeeds, low-pressure recirculation fails 9

s~uence ACDP Percentage Qeaqrt.llti9n Weather-related LOOP initiating event; successful LOOPWR 16-48 1.5E-07 7.6%

reactor trip; emergency power system failure results in SBO; AFW fails; operators fail to restore offsite power within 1 hour1.157407e-5 days 2.777778e-4 hours 1.653439e-6 weeks 3.805e-7 months Grid-related LOOP initiating event; successful reactor LOOPGR 16-48 7.8E-08 4.1%

trip; emergency power system failure results in SBO; AFW fails; operators fail to restore offsite power within 1 hour1.157407e-5 days 2.777778e-4 hours 1.653439e-6 weeks 3.805e-7 months Grid-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPGR 16-09 7.1 E-08 3.7%

AFW succeeds; RCP stage 2 seals fail resulting in a LOCA; operators fail to restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in LOOPWR 16-06-10 7.1 E-08 3.8%

SBO; AFW succeeds; RCP seal integrity is maintained; operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ); operators fail to maintain AFW without DC power Transient initiating event with consequential LOOP; emergency power system failure results in SBO; AFW succeeds; RCP stage 2 seals fail resulting in a LOCA; TRANS 21-16-07-2 6.3E-08 3.3%

operators successfully restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months , HPI succeeds, secondary cooldown/depressurization succeeds, low-pressure recirculation fails Switchyard LOOP initiating event; successful reactor LOOPSC 16-48 4.5E-08 2.4%

trip; emergency power system failure results in SBO; AFW fails; operators fail to restore offsite power within 1 hour1.157407e-5 days 2.777778e-4 hours 1.653439e-6 weeks 3.805e-7 months Switchyard LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPSC 16-09 5.1 E-08 2.7%

AFW succeeds; RCP stage 2 seals fail resulting in a LOCA; operators fail to restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP stage 2 seals fail resulting LOOPWR 16-07-2 5.0E-08 2.6%

in a LOCA; operators successfully restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months , HPI succeeds, secondary cooldown/depressurization succeeds, low-pressure recirculation fails Switchyard LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP seal integrity is maintained; LOOPSC 16-06-04 4.1 E-08 2.1%

operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ); operators successfully maintain AFW without DC power; operators fail to restore offsite power within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months 10

Sequenc;,

ACDP Percentage

~eriP*ll Plant-centered LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP stage 2 seals fail resulting LOOPPC 16-07-2 3.3E-08 1.7%

in a LOCA; operators successfully restore offsite power within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months , HPI succeeds, secondary cooldown/depressurization succeeds, low-pressure recirculation fails Grid-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP seal integrity is maintained; LOOPGR 16-06-04 3.0E-08 1.6%

operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ); operators successfully maintain AFW without DC power; operators fail to restore offsite power within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months REFERENCES

1. Wolf Creek Generating Station, "Power Potential Transformer Overloading Results in Emergency Diesel Generator lnoperability," LER 482-2016-001-01, dated November 10, 2016 (ADAMS Accession No. ML16326A175).

2. U.S. Nuclear Regulatory Commission, 'Wolf Creek Generating Station - NRC Integrated Inspection Report 05000482/2016004," dated February 13, 2017 (ADAMS Accession No. ML17045A201 ).

3. U.S. Nuclear Regulatory Commission, 'Wolf Creek Generating Station - NRC Inspection Report 05000482/2016008; Preliminary White Finding," dated August 19, 2016 (ADAMS Accession No. ML16235A132).

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ML17125A291 via e-mail dated 5/8/17 OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/LA RES/DRA/PRB/BC NAME BSingal PBlechman JLane*

DATE 05/08/2017 05/08/2017 05/08/2017 OFFICE N RR/DORL/LPL4/BC N RR/DORL/LPL4/PM NAME RPascarelli BSingal DATE 05/11/2017 05/12/2017

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