ML17325B667

Transmittal of Final Accident Sequence Precursor Report (Licensee Event Report 2017-001-00)

ML17325B667
Person / Time
Site:	Vogtle
Issue date:	11/29/2017
From:	Michael Orenak Plant Licensing Branch II
To:	Hutto J Southern Nuclear Operating Co
Orenak M DORL/LPL2-1 415-3229
Shared Package
ML17325B676	List: ML17325B667
References
IR 2017002 LER 17-001-00
Download: ML17325B667 (15)

Text

{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, O.C. 20555-0001 Mr. James J. Hutto Regulatory Affairs Director Vogtle Electric Generating Plant November 29, 2017 Southern Nuclear Operating Company, Inc. 40 Inverness Center Parkway, Bin 038 Birmingham, AL 35201-1295

SUBJECT:

VOGTLE ELECTRIC GENERATING PLANT, UNIT 2 -TRANSMITTAL OF FINAL ACCIDENT SEQUENCE PRECURSOR REPORT (LICENSEE EVENT REPORT 2017-001-00)

Dear Mr. Hutto:

By letter dated April 27, 2017 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML17117A347), Southern Nuclear Operating Company, Inc. (the licensee), submitted Licensee Event Report (LER) 2017-001-00, "Power Supply Failure Results in Operation in a Condition Prohibited by Technical Specifications," [annotated in this Accident Sequence Precursor (ASP) report as 425-2017-01] to the U.S. Nuclear Regulatory Commission (NRC) staff pursuant to Title 10 of the Code of Federal Regulations Section 50.73, "Licensee event report system." As part of the ASP Program, the NRC staff reviewed the event to identify potential precursors and to determine the probability of the event leading to a core damage state. The results of the analysis are provided in the enclosure to this letter. The NRC does not request a formal analysis review, in accordance with Regulatory Issue Summary 2006-24, "Revised Review and Transmittal Process for Accident Sequence Precursor Analyses," dated December 6, 2006 (ADAMS Accession No. ML060900007), because the analysis resulted in an increase in core damage probability (LiCDP) of less than 1x10* 4 Final ASP Analysis Summary. A brief summary of the final ASP analysis, including the results, is provided below. Power Supply Failures Result in a Failure of an Emergency Diesel Generator during Testing. This event is documented in LER 2017-001-00 and Inspection Report 05000425/2017002 dated July 21, 2017 (ADAMS Accession No. ML17202G465). Executive Summary. On March 8, 2017, emergency diesel generator (EOG) 2A was started for a monthly surveillance; however, the local tachometer indication was not working and the operators shut down the EOG and subsequently declared it inoperable. A review by the licensee determined that the first of the two power supplies that support the ventilation supply fans for EOG 2A failed on January 19, 2017, and the second failed on February 22, 2017, resulting in both ventilation supply fans being inoperable. When both power supplies are unavailable, an EOG can start; however, it will not automatically load onto the electrical bus due to an unavailable electrical permissive that is normally provided by the speed sensing circuit. Both power supplies were replaced on March 9, 2017, and EOG 2A successfully ran during

post-maintenance testing. The power supplies for EOG 28 and the Unit 1 EDGs were checked; none were failed. According to the risk analysis modeling assumptions used in this ASP analysis, the most likely core damage scenarios involve a non-recoverable loss of offsite power and subsequent station blackout due to the failure of both EDGs, operators failing to recover offsite power within 2 hours, and the failure of operators to maintain turbine-driven auxiliary feedwater pump operation after depletion of the safety-related batteries (4 hours). These accident sequences account for approximately 42 percent of the increase in core damage probability (~CDP) for this event. The point estimate ~CDP for this event is 8x10*5, which is considered a precursor in the ASP Program. The dominant cut sets reveal the potential for common-cause failure of all four EDG fans contributes approximately 84 percent of the risk for this ASP analysis. To date, no performance deficiency associated with this event has been identified, hence, no Significance Determination Process analysis was completed. Therefore, an independent ASP analysis was performed. Summary of Analysis Results. This operational event resulted in a best estimate D.CDP of 8x1 o*5* The detailed ASP analysis can be found in the enclosure. If you have any questions, please contact me at 301-415-3229 or at Michael.Orenak@nrc.gov. Docket No. 50-425

Enclosure:

Michael D. Orenak, Project Manager Plant Licensing Branch 11-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Final Accident Sequence Precursor Analysis cc: Listserv

ENCLOSURE Final Accident Sequence Precursor Analysis - Vogtle Electric Generating Plant (Unit 2), Power Supply Failures Result in a Failure of an Emergency Diesel Generator during Testing (LEA 425-2017-001) - Precursor

Final Precursor Anal - Precursor Vogtle Electric Generating Plant, Unit 2 Event Date: 3/9/2017 Power Supply Failures Result in a Failure of an Emergency Diesel Generator during Testing LERs: 425-2017-001 IRs: 05000425/2017002 L1CDP = 8x10-5 Plant Type: Pressurized-Water Reactor (PWR); Westinghouse Four-Loop with a Large, Dry Containment Plant Operating Mode (Reactor Power Level): Mode 1 (100% Reactor Power) Analyst: Reviewer: Christopher Hunter Keith Tetter EXECUTIVE

SUMMARY

Contributors: David Aird and Ian Gifford BC Review Date: 9/1/2017 On March 8, 2017, emergency diesel generator (EOG) 2A was started for a monthly surveillance; however, the local tachometer indication was not working and the operators shut down the EOG and subsequently declared it inoperable. A review by the licensee determined that the first of the two power supplies that support the ventilation supply fans for EOG 2A failed on January 19, 2017, and the second failed on February 22, 2017, resulting in both ventilation supply fans being inoperable. When both power supplies are unavailable, an EOG can start; however, it will not automatically load onto the electrical bus due to an unavailable electrical permissive that is normally provided by the speed sensing circuit. Both power supplies were replaced on March 9, 2017, and EOG 2A successfully ran during post-maintenance testing. The power supplies for EOG 28 and the Unit 1 EDGs were checked; none were failed. According to the risk analysis modeling assumptions used in this accident sequence precursor (ASP) analysis, the most likely core damage scenarios involve a non-recoverable loss of offsite power (LOOP) and subsequent station blackout (SBO) due to the failure of both EDGs, operators failing to recover offsite power within 2 hours, and the failure of operators to maintain turbine-driven auxiliary feedwater (AFW) pump operation after depletion of the safety-related batteries (4 hours). These accident sequences account for approximately 42 percent of the increase in core damage probability (~CDP) for this event. The point estimate ~CDP for this event is 8x1 o-5, which is considered a precursor in the ASP Program. The dominant cut sets reveal the potential for common-cause failure (CCF) of all four EOG fans contributes approximately 84 percent of the risk for this ASP analysis. To date, no performance deficiency associated with this event has been identified, hence, no Significance Determination Process (SOP) analysis was completed. Therefore, an independent ASP analysis was performed. EVENT DETAILS Event Description. On March 8, 2017, EOG 2A was started for a monthly surveillance; however, the local tachometer indication was not working and the operators shut down the EOG and subsequently declared it inoperable. A review by the licensee determined that the first of 1

LER 425-2017-001 two power supplies that support the ventilation supply fans for EOG 2A failed on January 19, 2017. In addition, the second power supply failed on February 22, 2017, resulting in both ventilation supply fans being inoperable. These power supply failures were not identified by the operators due to the lack of indication for these failures. Both power supplies were replaced on March 9, 2017, and EOG 2A successfully ran during post-maintenance testing. The power supplies for EOG 28 and the Unit 1 EOGs were checked; none were failed. Each Vogtle EOG has two power supplies, which support a common speed sensing circuit and the ventilation fans. An EOG can successfully operate with one power supply supporting the speed sensing circuit. Each EOG also has two ventilation fans. Ventilation fan A automatically starts upon the start of the EOG. Ventilation fan B will start if thermostat temperature reaches 93 degrees Fahrenheit {°F}. Except in extreme temperatures, only one fan is needed to keep the temperature in the EOG room below the limit of 120 °F. When both power supplies are unavailable, an EOG can start; however, it will not automatically load onto the electrical bus due to an unavailable electrical permissive that is normally provided by the speed sensing circuit. Additional information is provided in licensee event report (LER) 425-2017-001 (Ref. 1) and inspection report (IR) 05000425/2017002 (Ref. 2). Cause. The cause of the power supply failures is currently unknown; an investigation is currently ongoing. MODELING ASSUMPTIONS Analysis Type. The Vogtle Standardized Plant Analysis Risk (SPAR) Model Version 8.50, dated April 2017, was used for this condition assessment. Note that this model is set up for Unit 1; therefore, when using it to perform an analysis of the Unit 2 event, Unit 1 basic events are used as surrogates that represent applicable basic events on the other unit. SDP Results/Basis for ASP Analysis. IR 05000425/2017002 provides some information on this event as part of a review of licensee corrective actions on repetitive power supply failures. No performance deficiency was identified because inspectors determined that given the information available, it was not reasonable for the licensee to have identified that the power supplies had failed prior to March 8, 2017; however, the LER remains open. An independent ASP analysis was performed given the lack of an identified performance deficiency (i.e., no SOP analysis was completed). A search for additional Vogtle (Unit 2) LERs was performed to determine if additional unavailabilities existed during the exposure period of the EOG 2A power supply failures. To date, there were no additional LERs issued in 2017 associated with Vogtle Electric Generating Station, Unit 2. SPAR Model Modifications. The following base SPAR model modifications were required for this condition assessment: Significant modifications were made to the SBO event tree. The current SPAR models for Westinghouse PWRs, including Vogtle, implement the expanded Westinghouse Owner's Group (WOG) 2000 reactor coolant pump (RCP) seal leakage model as evaluated and accepted by the NRC in "Safety Evaluation of Topical Report WCAP-15603, Revision 1, WOG 2000 Reactor Coolant Pump Seal Leakage Model for Westinghouse PWRs." This safety evaluation report states the following, 2

LER 425-2017-001 "NRC staff accepts the use of a zero failure probability for 0-ring extrusion failure of high-temperature 0-rings as long as the licensee documents the justification and supporting analyses and bases in the licensee controlled PAA documentation. Such documentation should show that the plant's cooldown will result in a RCS pressure of less than 171 O psi within 2 hours." Vogtle MELCOR calculations performed for an RCP seal leakage rate of 21 gallon per minute (gpm) with continuous AFW, and no depressurization, indicate that reactor coolant system (RCS) pressure would be well below 1,71 O pounds per square inch (psi) at approximately 2 hours. In addition, Vogtle has a unique RCP seal design that does not include 0-rings. Therefore, the 0-ring extrusion failure mode is determined to not be applicable to Vogtle. To remove this RCP seal failure mode, the RSD-B (rapid secondary depressurization), 01 (RCP seal stage 1 integrity (0-ring extrusion)), and,02 (RCP seal stage 2 integrity (0-ring extrusion) top events (including applicable event tree branching) were eliminated from the SBO event tree.1 In ASP analyses, EOG recovery credit is limited to cases where event information supports credit for EOG recovery. No recovery credit for the failure of the EOG 2A fan power supplies was provided in this analysis. Therefore, the DGR-4H (diesel generator recovery (in 4 hours)) top event (including applicable event tree branching) was eliminated from the SBO event tree. In the SPAR models, the time allowed for offsite power recovery to the safety-related alternating-current (AC) buses is typically limited to the time to uncover the core or the battery depletion time of safety-related batteries, whichever is less. At Vogtle, the safety-related batteries have a 4-hour battery life. However, at Vogtle, other batteries are needed to manipulate the breakers/switchers required to restore offsite power to the safety-related AC buses. In addition to the safety-related batteries, the turbine building batteries (2 hour battery life) supply direct-current (DC) power to circuit switchers directly upstream of the reserve auxiliary transformers (RATs); and the switchyard batteries (4 hour battery life) supply DC control power to the circuit breakers that are located in the switchyard (immediately downstream of 230 kilovolt (kV) buses 1 and 2). A review of the Vogtle SBO procedure (ECA-0.0, "Loss of All AC Power") provides direction for shedding unnecessary DC loads on the safety-related batteries; however, there are no actions to prolong the life of the turbine building batteries. In addition, procedure prerequisites for the realignment of offsite power, system procedures 13427 A(B), "4160V AC Bus AA02 (BA03) 1 E Electrical Distribution System," require that DC power be available to operate some of the breaker/switchers. It is not fully clear how operators will respond if the prerequisite for DC power is not met. Some of the breakers/switchers that are needed to restore offsite power to safety-related AC buses can be operated manually (e.g., switchers upstream of the RATs can be closed manually with a local hand crank), while some cannot be operated manually (e.g., DC power is required to close the bus AA02 and BA03 feeder breakers). However, there are no procedures that direct the manual operation of these breakers. Given these considerations, the turbine building batteries have the limiting depletion time for realigning offsite power to the safety-related AC buses. Therefore, the top event OPR-4H (offsite power recovery (in The end state for revised sequence 6 (76 gpm/RCP seal leakage) was revised to result in core damage because offsite power recovery is needed to restore emergency core cooling systems to provide inventory makeup to the RCS. 3

LER 425-2017-001 4 hours)) has been changed to OPR-2H ( offsite power recovery (in 2 hours)) in the SBO event tree.2 The licensee PRA assumes that Plant Wilson is unavailable for grid-and weather-related LOOPs; therefore, basic event EPS-SWYO-UA-WILSON (Plant Wilson unavailability due to grid-related LOOP) was set to TRUE. Exposure Periods. The following two exposure periods were identified for this condition assessment: Exposure Period #1: The first power supply for EOG 2A failed on January 19, 2017. Therefore, EOG 2A had one of its two available power supply to support its ventilation fans for approximately 34 days. Exposure Period #2. The second power supply for EOG 2A failed on February 22, 2017. Both power supplies were replaced on March 9, 2017. Therefore, EOG 2A had no available power supplies to support its ventilation fans for approximately 15 days. Key Modeling Assumptions. The following modeling assumptions were determined to be significant to the modeling of this event: Exposure Period #1 (-34 days) Basic event EPS-FAN-FS-VENT1 (failure of OG-A room vent fan 1 to start) was set to TRUE due to the unavailability of one of the two EOG 2A fans caused by the first failed power supply.3* 4* 5 Exposure Period #2 (-15 days) Basic events EPS-FAN-FS-VENT1 and EPS-FAN-FS-VENT2 (failure of OG-A room vent fan 2 to start) were set to TRUE to represent the unavailabilities of both EOG 2A fans caused by failure of both power supplies.6 ANALYSIS RESULTS .6CDP. The point estimate LiCOP for this event is 8.4x1 o-s, which is the sum of both exposure periods. The ASP Program acceptance threshold is a LiCOP of 1x10*6 for degraded conditions. The LiCOP for this event exceeds this threshold; therefore, this event is a precursor. The dominant initiating events for this analysis are provided in the following table: 2 3 4 5 6 The time for offsite power recovery for SBO sequences with failure of either AFW or a stuck-open pressurizer power-operated relief valve(s) was not modified (assumed to be approximately 1 hour). This Unit 1 basic event is used as a surrogate for this analysis of the Unit 2 degraded condition. The licensee PAA considers the power supplies that failed during the event as part of the EOG fan component boundary. SAPHIRE automatically recalculates the probability for basic events associated with CCF of EOG fans. For the first exposure period, the CCF probability for failure-to-start increases from 2. 7x1 o-6 to 4.15x10*3, given the observed failure of one out of four EOG fans. For the second exposure period, the CCF probability for failure-to-start increases from 2. 7x1 o-s to 2.39x 10*1 given the observed failure of two out of four EOG fans. 4

LER 425-2017-001 Event Tree ACDP Percentage Description LOOPGR 4.71 x10*5 56.0% Loss of Offsite Power: (Grid-Related) LOOPWR 3.16x10-5 37.5% Loss of Offsite Power (Weather-Related) LOOPSC 4.09x10*6 4.9% Loss of Offsite Power (Switchyard-Centered) Dominant Sequence. The dominant accident sequence is grid-related LOOP sequence 16-02-10 (b.CDP = 1.7x10*5), which contributes approximately 20 percent of the total internal events b.CDP. The dominant sequences that contribute at least 1.0 percent to the total internal events b.CDP are provided in the following table. The dominant sequence is shown graphically in Figure A-1, Figure A-2, and Figure A-3 in Appendix A. Sequence ACDP Percentage Description Grid-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPGR 16-02-10 1.69x10*5 20.1% AFW succeeds; RCP seal integrity is maintained; operators fail to restore offsite power within 2 hours; and operators fail to continue turbine-driven AFW pump operation after battery depletion Grid-related LOOP initiating event; successful reactor LOOPGR 16-12 1.44x10*5 17.1% trip; emergency power system failure results in SBO; AFW fails; and operators fail to restore offsite power within 1 hour Grid-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; LOOPGR 16-04 1.41 x10*5 16.8% AFW succeeds; RCP seal integrity fails (i.e., 182 gpm/RCP LOCA); operators fail to restore offsite power within 2 hours Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP seal integrity is LOOPWR 16-02-10 1.20x10*5 14.3% maintained; operators fail to restore offsite power within 2 hours; and operators fail to continue turbine-driven AFW pump operation after battery depletion Weather-related LOOP initiating event; successful reactor trip; emergency power system failure results in LOOPWR 16-04 1.01x10-5 12.0% SBO; AFW succeeds; RCP seal integrity fails (i.e., 182 gpm/RCP LOCA); operators fail to restore offsite power within 2 hours Weather-related LOOP initiating event; successful LOOPWR 16-12 8.52x10*6 10.1% reactor trip; emergency power system failure results in SBO; AFW fails; and operators fail to restore offsite power within 1 hour Switchyard-centered LOOP initiating event; successful reactor trip; emergency power system failure results in SBO; AFW succeeds; RCP seal integrity is LOOPSC 16-02-10 8.52x10*6 1.7% maintained; operators fail to restore offsite power within 2 hours; and operators fail to continue turbine-driven AFW pump operation after battery depletion 5

LER 425-2017-001 Sequence ACDP Percentage Description Switchyard-centered LOOP initiating event; successful LOOPSC 16-12 8.52x10*6 1.5% reactor trip; emergency power system failure results in SBO; AFW fails; and operators fail to restore offsite power within 1 hour Switchyard-centered LOOP initiating event; successful reactor trip; emergency power system failure results in LOOPSC 16-04 8.52x10*6 1.4% SBO; AFW succeeds; RCP seal integrity fails (i.e., 182 gpm/RCP LOCA); operators fail to restore offsite power within 2 hours Modeling Uncertainties. There are certain LOOP/SBO modeling assumptions for which there is no consensus approach. The lack of a consensus approach can lead to potentially conservative (or non-conservative) results in an event and/or condition assessment, depending on the assumptions used by the risk analyst. Some key examples of these modeling assumptions include: Manual operation of the turbine-driven AFW pump without DC power and steam generator level indication (for the entire PAA mission time); Manipulation of breakers without DC power to allow recovery of offsite power after battery depletion; Battery depletion times of nonsafety-related batteries (e.g., switchyard batteries) required for realigning offsite power to the safety-related buses; Requirement of AC power recovery to reach a safe/stable end state; and Credit for EOG repair and recovery. As a general rule in the base SPAR models, the time allowed for successful offsite power recovery is limited to the battery depletion time of the safety-related batteries. The only exception to this assumption is if credit for continued turbine-driven AFW pump operation after depletion of the safety-related batteries is provided in the applicable SPAR model.7 If continued operation of the turbine-driven AFW pump is successful (as it is in the base Vogtle SPAR model), recovery of offsite power at 24 hours is required to reach a safe/stable end state. This credit is provided, given the large time window available, assuming that operators would be able to use other resources (e.g., portable generators) to allow manipulation of the breakers needed to restore offsite power to a safety-related buses.8 The base SPAR models provide credit for EOG repair and recovery; however, it is up to the analyst to determine whether credit should be applied given the specific circumstances surrounding the event being analyzed.9 7 8 9 Credit in the base SPAR model is provided if the licensee credits this mode of operation and direction for the operator action is in their emergency procedures. The requirement of successful offsite power recovery to reach a safe/stable end state has a negligible effect on the analysis results because the applicable scenarios are dominated by the human error probability (0.3) of operators failing to feed the SGs using the turbine-driven AFW pump with neither DC power nor SG level indication. The base SPAR models credit repair and recovery of the EDGs using the mean time to repair data. However, this repair data is not collected under SBO conditions. In addition, during postulated SBO scenarios, multiple EOG failures would have occurred, thus further complicating troubleshooting activities. 6

LER 425-2017-001 In this ASP analysis (best estimate case), the time to recover offsite power was changed from the base SPAR model assumption of depletion of the safety-related batteries to the most limiting battery depletion time (i.e., turbine building batteries-2 hours).10 This analysis credits continued turbine-driven AFW pump operation after depletion of the safety-related batteries (4 hours). 11 In addition, the base SPAR model assumption that AC power recovery to a safety-related bus is required to reach a safe/stable end state was adopted. This ASP analysis does not credit EOG repair and recovery. See the SPAR Model Modifications Section for additional information on the modeling assumption changes from the base SPAR model. Note that other mitigation strategies (e.g., FLEX) were not considered as part of this ASP analysis. These alternate strategies are being considered as part of other efforts within the SPAR Model Development Program; however, they are not typically considered in ASP analyses. Sensitivity analyses were performed to show the effects of some of these modeling assumptions. The following table provides a brief description of each sensitivity case, including the results and notes/observations. Description ACDP Notes/Observations Similar dominant sequences and cut sets to the best estimate analysis. The 2 hours of additional time for offsite power recovery prior Use of the standard SPAR modeling to battery depletion has a limited impact on weather-and grid-related LOOPs. Additional assumption of the limiting battery depletion 6.5x10*5 time allowed for recovery is not expected to time associated with the safety-related result in further reduction in the ~CDP batteries (i.e., 4 hours). because the dominant scenarios (e.g., 182 gpm/RCP seal leakage, initial failure of turbine-driven AFW pump) have core uncovery times of s 4 hours. No credit provided for continued As expected, the 2-hour sequences with no turbine-driven AFW pump operation after 1.5x10*4 RCP seal failure become more dominant. depletion of the safety-related batteries. Decrease in ~CDP is limited by EOG Credit provided for EOG repair and recovery12 5.5x1Q*S recovery data for s 4 hours (i.e., probabilities of recovery are<! 0.73). 10 This may be conservative for plant-centered and some switchyard-centered LOOPS, depending on the fault location. Since plant-and switchyard-centered LOOPS are small contributors to risk, this assumption has a minimal effect on the analysis results. 11 This modeling assumption remains unchanged from the base Vogtle SPAR model. 12 The time available for EOG repair and recovery is limited to the battery depletion time of the safety-related batteries (i.e., 4 hours) or core uncovery, whichever occurs first. 7

LER 425-2017-001 REFERENCES

1. Vogtle Electric Generating Plant, "LEA 425/17-001 - Power Supply Failure Results in Operation in a Condition Prohibited by Technical Specifications," dated April 27, 2017 (ADAMS Accession No. ML17117A347).

2. U.S. Nuclear Regulatory Commission, "Vogtle Electric Generating Plant - NRC Integrated Inspection Report 05000424/2017002 and 05000425/2017002," July 21, 2017 (ADAMS Accession No. ML17202G465).

2

LER 425-2017-001 Appendix A: Key Event Trees IOSIOf(JISm~ Rf.lC10ll9ll100WN I !IBaYPOWIII II.IIIIIIRYlll!llllll IONSIIE CIOSID.,=- 1*--1 flll).111)111) CIIIRIOIBIIICIJl8IY Oll'SIIIOIBIIICIJl8IY ~*,lll!SIUl-- IGlllll!BIIIDC, &,j51't! 111111110ll(~1!111

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~ '---<,----{) ~ ~ 0 0 r1 *1 ~ o o o H -151~ ,-~--~a H ~dl~~: 0 ~ ~ tdl I CD Cl( CD ()I; CD -- CD - SB) ) AlWS > CD I Figure A-1. Vogtle LOOP (Grid-Related) Event Tree A-1

EPS EMERGENCY POWER AUXILIARY FEEDNA"TER I YOGT1.E PORVS,'SR\\'S OPEN (llACKOVT) WUr<<. STATION llACl<<)UT" FTF*S80 I AFW*B FTF-580 I PORV-B FTf-SIIO I BP1 ( 0 0 ( RCP SEAL STAGE l INTBiUTY (l!DaNG,'POPPING [ 0 en RCP SEAL STAGE 2 I OFfSlTE JNTEGRITY (IIIMING,'FOPAN 21 gpm'RCP OPR-o2H --0 I 182 g pm'FO> 0 I 76g pm'FCP 0 I pm'FCP 0 I 0 I 0 I Figure A-2. Vogtle Revised SBO Event Tree A-2 LEA 425-2017-001 POWER RBXMRY End State INlt§ (Phase

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- 0 l OK I - 0 2 580-4 > - 0 3 580*1 > - 0 4 co I - 0 5 SBO* l > - 0 6 I CD I - 0 7 580*1 > - 0 8 co I - 0

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580*2 > OPA-01H - 0 10 co I OPR-01H - 0 11 580-3 > OPR-01H - 0 12 CD I OPR-01H

LER 425-2017-001 t'FW HIGH PRESSURE MANUAL CONTROLAFW CONDENSA'TE SJ0RAGE ~SGS lA'TE POWER RECOVERY End State PORTION TANC REFILL U:>NG-'TERM (Phase - CD) MFW-PUMPS AFW-MAN CST-REFILL-LT SG-OEP-LT PWR-REC 0 I 1 I OK I 0 24hrs I 2 I SG-DEP-LT2 0 CD CST-REF1LL-LT1 I 0 I 3 I OK I 0 Tcu or 24 hrs I 4 I SG-DEP-LT2 0 CD 0 0 I s I OK 0 I ' " " Tosi,S>ly 16 I OK CST-REFILL-L T1 0 I 7 I CD 0 0 I 8 I OK 0 I Tcu or 24 hrs I 9 I 0 0 I SG-DEP-LT1 0 CD 0 0 I 10 I CD SG-DEP-LT1 Figure A-3. Vogtle SB0-4 Event Tree A-3

Package: ML173258676; Letter: ML173258667; ASP R ML172508343 eport: OFFICE NRR/DORULPL2-1/PM NRR/DORULPL2-1/LA NRR/DORULPL2-1/BC NRR/DORULPL2-1/PM NAME MOrenak KGoldstein (JBurkhardt for) MMarkley MOrenak DATE 11/28/17 11/27/17 11/29/17 11/29/17}}