ML17108A730

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Final ASP Analysis-Wolf Creek (LER 482-2016-001-01)
ML17108A730
Person / Time
Site: Wolf Creek Wolf Creek Nuclear Operating Corporation icon.png
Issue date: 04/12/2017
From: Michael Cheok
NRC/RES/DRA
To: Ross-Lee M
Division of Operating Reactor Licensing
Shared Package
ML17108A729 List:
References
LER-482-2016-001-01
Download: ML17108A730 (14)
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Final ASP Program Analysis - Precursor Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Wolf Creek Power Potential Transformer Overloading Results in Emergency Generating Station Diesel Generator Inoperability LER: 482-2016-001-01 Event Date: 10/6/2014 CDP= 2x10-6 IR(s): 05000482/2016008 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 (EDG) 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' EDG was repaired and declared operable on October 9th at 5:17 p.m.

A failure investigation concluded that the smoking and eventual failure of the PPT on October 6th was most likely due to overloading caused from a failed diode that occurred during load transients on June 9, 2014. Therefore, the 'B' EDG would not have been able to fulfill its safety function for the complete mission time from June 9th until October 9th. Note that the B EDG had run successfully in the previous three monthly surveillance tests (for a total of approximately 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />). During the June-October time period, the 'A' EDG 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 unavailabilities. When the 'A' and 'B' EDGs are inoperable, there are no remaining safety-related onsite standby 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, RCP seal integrity is maintained, operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />), operators successfully maintain AFW without DC power, and operators fail to restore offsite power within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. This accident sequence accounts for approximately 27% 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.

1

LER 482-2016-001-01 EVENT DETAILS Event Description. On October 6, 2014, at 1:26 p.m. during a scheduled 24-hour run, the 'B' EDG 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' EDG 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 6th was most likely due to overloading. The overloading of the PPT resulted from failure of a diode in the power rectifier of the EDG excitation system. Failure of the diode occurred during load transients on June 9, 2014, resulting from a governor actuator malfunction. Therefore, the 'B' EDG would not have been able to fulfill its safety function for the completed mission time from June 9th until October 9th; however, note that the EDG had run successfully in the previous three monthly surveillance tests (for a total of approximately 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />).

During the June-October time period, the 'A' EDG 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 (Ref. 1) and inspection reports (IRs) 05000482/2016004 (Ref. 2) and 05000482/2016008 (Ref. 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 licensees 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/SDP Results. The ASP Program uses SDP 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 unavailabilities (e.g., equipment out for test/maintenance) that occurred during the event, regardless of licensee performance.1 An independent ASP analysis was required because both safety-related EDGs were unavailable at the same time due to different causes.

1 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.

2

LER 482-2016-001-01 NRC 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 EDG 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 EDG unavailability that was originally considered as concurrent SBO diesel generators and EDG unavailability which resulted in the White finding. NRC 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 13.8kV) and ACP-NB02-5 (Loss of Offsite Power to Bus NB02 from 13.8kV) 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 EDG run times on different dates, referred to as EP-1 through EP-7:

  • EP-1 consists of the 3-day period from June 9, 2014 to June 11, 2014 when the B EDG would have failed to run for its full 24-hour mission time, but could have possibly run for 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br />.
  • EP-2 consists of the 28-day period from June 11, 2014 to July 9, 2014 when the B EDG would have failed to run for its full 24-hour mission time, but could have possibly run for 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />.
  • 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 EDG would have failed to run for its full 24-hour mission time, but could have possibly run for 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />.

2 In addition, basic events EPS-XHE-XL-NR01H (Operator Fails to Recover Emergency Diesel in 1 Hour),

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 Hour), EPS-XHE-XL-NR04H (Operator Fails to Recover Emergency Diesel in 4 Hour), 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 EDG recovery.

3

LER 482-2016-001-01

  • EP-4 consists of the 33-day period from August 6, 2014 to September 8, 2014 when the B EDG would have failed to run for its full 24-hour mission time, but could have possibly run for 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />
  • EP-5 consists of the 28-day period from September 8, 2014 to October 6, 2014 when the B EDG would have failed to run for its full 24-hour mission time, but could have possibly run for 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />.
  • EP-6 consists of the 3-day period from October 6, 2014 to October 9, 2014 when the B EDG was failed and undergoing repairs.
  • EP-7 consists of the one day period July 21, 2014 when the A EDG was inoperable and the B EDG would have failed to run for its full 24-hour mission time, but could have possibly run for 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />.

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 EDG is modeled by setting basic event EPS-DGN-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 EDG could potentially have run for 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> 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 <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> additional time.

Basic Event Initial Probability Revised Probability OEP-XHE-XL-NR01HGR 6.587E-1 8.455E-3 OEP-XHE-XL-NR01HPC 3.309E-1 7.157E-3 OEP-XHE-XL-NR01HSC 4.015E-1 9.867E-3 OEP-XHE-XL-NR01HWR 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 1.117E-1 5.731E-3 OEP-XHE-XL-NR03HSC 1.453E-1 7.909E-3 OEP-XHE-XL-NR03HWR 4.800E-1 1.705E-1 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 4

LER 482-2016-001-01 Basic Event Initial Probability Revised Probability 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 2.781E-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 EDG could potentially have run for 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> 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 <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> additional time.

Basic Event Initial Probability Revised Probability OEP-XHE-XL-NR01HGR 6.587E-1 3.111E-2 OEP-XHE-XL-NR01HPC 3.309E-1 1.911E-2 OEP-XHE-XL-NR01HSC 4.015E-1 2.610E-2 OEP-XHE-XL-NR01HWR 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-NR03HWR 4.800E-1 2.334E-1 OEP-XHE-XL-NR04HGR 1.685E-1 1.684E-2 OEP-XHE-XL-NR04HPC 7.753E-2 1.195E-2 OEP-XHE-XL-NR04HSC 1.024E-1 1.641E-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 2.781E-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 EDG could potentially have run for 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> 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 <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> additional time.

5

LER 482-2016-001-01 Basic Event Initial Probability Revised Probability OEP-XHE-XL-NR01HGR 6.587E-1 5.005E-2 OEP-XHE-XL-NR01HPC 3.309E-1 2.781E-2 OEP-XHE-XL-NR01HSC 4.015E-1 3.774E-2 OEP-XHE-XL-NR01HWR 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 1.911E-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 1.641E-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 2.781E-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 <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> 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 <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> additional time.

Basic Event Initial Probability Revised Probability OEP-XHE-XL-NR01HGR 6.587E-1 8.686E-2 OEP-XHE-XL-NR01HPC 3.309E-1 4.365E-2 OEP-XHE-XL-NR01HSC 4.015E-1 5.866E-2 OEP-XHE-XL-NR01HWR 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 2.781E-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 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 6

LER 482-2016-001-01 Basic Event Initial Probability Revised Probability 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 2.781E-2 1.195E-2 OEP-XHE-XL-NR08HSC 3.774E-2 1.641E-2 OEP-XHE-XL-NR08HWR 2.982E-1 2.216E-1 For EP-5:

  • The B EDG could potentially have run for 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> 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 <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> additional time.

Basic Event Initial Probability Revised Probability OEP-XHE-XL-NR01HGR 6.587E-1 1.685E-1 OEP-XHE-XL-NR01HPC 3.309E-1 7.753E-2 OEP-XHE-XL-NR01HSC 4.015E-1 1.024E-1 OEP-XHE-XL-NR01HWR 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 2.781E-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 EDG was undergoing repair, technical specifications would prevent test/maintenance activities that would render certain equipment unavailable. Therefore, basic events AFW-TDP-TM-PAL02 (Feed Pump PAL02 is in Test or Maintenance),

EPS-DGN-TM-NE01 (Diesel Generator NE01 Unavailable due to Test and Maintenance), ESW-MDP-TM-1A (ESW Train A MDP 1A Unavailable due to Test or Maintenance), and ESW-SYS-TM-TRAINA (Service Water Train A Unavailable Due To Maintenance) were set to FALSE.

7

LER 482-2016-001-01 For EP-7:

  • The failure of A EDG is modeled by setting basic event EPS-DGN-TM-NE01 to TRUE.
  • The B EDG could potentially have run for 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> 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 <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> additional time.

Basic Event Initial Probability Revised Probability OEP-XHE-XL-NR01HGR 6.587E-1 5.005E-2 OEP-XHE-XL-NR01HPC 3.309E-1 2.781E-2 OEP-XHE-XL-NR01HSC 4.015E-1 3.774E-2 OEP-XHE-XL-NR01HWR 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 1.911E-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 1.641E-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 2.781E-2 9.127E-3 OEP-XHE-XL-NR08HSC 3.774E-2 1.257E-2 OEP-XHE-XL-NR08HWR 2.982E-1 2.014E-1 ANALYSIS RESULTS CDP. The point estimate CDP for this event is 1.9x10-6. The ASP Program acceptance threshold is a CDP of greater than 1x10-6. Therefore, this event is a precursor. The following table provides the CDP breakdown for each exposure period.

Exposure Period CDP EP-1 3.0x10-8 EP-2 3.3x10-7 EP-3 3.4x10-7 EP-4 4.7x10-7 EP-5 4.8x10-7 EP-6 3.9x10-8 EP-7 2.2x10-7 TOTAL 1.9x10-6 8

LER 482-2016-001-01 Dominant Sequence. The dominant accident sequence is weather-related LOOP (LOOPWR)

Sequence 16-06-4 (CDP = 5.2x10-7) that contributes 27% of the total internal events CDP.

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.

Sequence CDP Percentage Description 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 <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />); operators successfully maintain AFW without DC power; operators fail to restore offsite power within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 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 <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> 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 2.1E-07 10.8% LOCA; operators successfully restore offsite power within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, 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 1.8E-07 9.2% LOCA; operators successfully restore offsite power within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, HPI succeeds, secondary cooldown/depressurization succeeds, low-pressure recirculation fails Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in LOOPWR 16-48 1.5E-07 7.6%

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

AFW fails; operators fail to restore offsite power within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> Grid-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPGR 16-09 7.1E-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 <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP seal integrity is LOOPWR 16-06-10 7.1E-08 3.8%

maintained; operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />); operators fail to maintain AFW without DC power 9

LER 482-2016-001-01 Sequence CDP Percentage Description 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 <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, HPI succeeds, secondary cooldown/

depressurization succeeds, low-pressure recirculation fails Switchyard LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPSC 16-48 4.5E-08 2.4%

AFW fails; operators fail to restore offsite power within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> Switchyard LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPSC 16-09 5.1E-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 <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> 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 <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, 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 4.1E-08 2.1% operators fail to recover power prior to battery depletion (8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />); operators successfully maintain AFW without DC power; operators fail to restore offsite power within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 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 <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, 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 <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />); operators successfully maintain AFW without DC power; operators fail to restore offsite power within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> REFERENCES

1. Wolf Creek Generating Station, Power Potential Transformer Overloading Results in Emergency Diesel Generator Inoperability, LER 482-2016-001-01, dated November 10, 2016 (ML16326A175).
2. U.S. Nuclear Regulatory Commission, Wolf Creek Generating Station - NRC Integrated Inspection Report 05000482/2016004, dated February 13, 2017 (ML17045A201).

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LER 482-2016-001-01

3. U.S. Nuclear Regulatory Commission, Wolf Creek Generating Station - NRC Inspection Report 05000482/2016008; Preliminary White Finding, dated August 19, 2016 (ML16235A132).

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LER 482-2016-001-01 Appendix A: Key Event Trees Figure A-1. Wolf Creek LOOP Event Tree (Weather-Related)

A-1

LER 482-2016-001-01 Figure A-2. Wolf Creek SBO Event Tree A-2

LER 482-2016-001-01 Figure A-3. Wolf Creek SBO-4 Event Tree A-3

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