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{{#Wiki_filter:Final ASP Program Analysis - Reject Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Susquehanna Steam Electric Station, Unit 2 HPCI Manually Overridden Prior to a Manual Scram During a Plant Transient  Event Date:  5/13/2016 LER(s):  388-2016-005 and 388-2016-004-01 IR(s):  05000387/2016002 CCDP = 4x10-7 Plant Type:  General Electric, BWR-4 with a Wet Mark II Containment Plant Operating Mode (Reactor Power Level):  Mode 1 (69% Reactor Power) Analyst: David Aird Reviewer: Keith Tetter Contributors: N/A BC Approved Date: 12/6/2016  EVENT DETAILS              Event Description. At 11:55 pm on May 12, 2016, Susquehanna Steam Electric Station (SSES) Unit 2 experienced an electrical transient resulting in a loss of 2B246 Reactor Building Engineered Safeguard System Division 2 480V Motor Control Center (MCC) and 2Y246 208/120V Alternate Current Instrument Panel. With the loss of the MCC, several drywell cooling fans were lost leading to an increase in drywell pressure. Operators reduced reactor power by reducing reactor recirculation flow. Maintenance personnel reported that the 2B246 bus faulted and was not able to be re-energized. Operators determined that a manual scram was required based upon a loss of drywell cooling.
On May 13, 2016 at 12:55 am, the High Pressure Coolant Injection (HPCI) system was overridden (placed in manual) prior to the manual scram to prevent the trip of all feedwater pumps on reactor pressure vessel high water level alarm. Although HPCI is not normally expected to initiate during a reactor scram if the non-safety related integrated control system (ICS) responds appropriately to control reactor water level, taking HPCI out of automatic mode rendered the system incapable of automatic initiation in the event of a failure of the ICS.
Technical Specification 3.5.1 was entered for HPCI inoperable due to manual override. At 1:10 am, with drywell pressure increasing to 1.3 psig, operators placed the mode switch in the shutdown position to manually scram the reactor. All control rods inserted as expected. Reactor water level lowered and was immediately restored by normal feedwater level control.
Primary Containment Isolation System isolations occurred, along with an initiation of the Reactor Core Isolation Cooling (RCIC) system. Once adequate level was verified, RCIC was overridden. Reactor pressure was controlled with turbine bypass valves, and subsequently main steam line drains. HPCI was restored to automatic mode at 1:22 am. All other safety systems functioned as expected. Additional information related to the override of HPCI and the electrical transient is available in licensee event reports (LERs) 388-2016-005 (Ref. 1), 388-2016-004-01 (Ref.2), and Inspection Report (IR) 05000387/2016002 (Ref. 3). Causes. The direct cause of the transient was found to be a phase to ground short between a cable and a protruding screw in the MCC. The protruding screw damaged and/or abraded the wire insulation until the short occurred.
LER 388-2016-005 2  There were two contributing causes to the incorrect override of HPCI prior to the manual reactor scram. First, the Unit Supervisor made a decision to prematurely override HPCI to minimize distractions later in the shutdown without procedural guidance to do so. Second, weaknesses in teamwork and oversight prevented the mistake from being corrected by the crew. A crew update was not conducted to announce the placing of HPCI in manual override, preventing the crew from providing a peer check of the Unit Supervisor's decision. MODELING              Basis for ASP Analysis/SDP Results. The ASP Program uses Significance Determination Process (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  This event was addressed and evaluated in IR 05000387/2016002 (Ref. 3). Inspectors identified a performance deficiency for the licensee's failure to implement procedures for controlling the HPCI system. A detailed risk evaluation (condition assessment) was performed that resulted in a change in core damage frequency per year of less than 1x10-9. Therefore, the issue was determined to be of very low safety significance (Green).
An independent ASP analysis is required because this is an initiating event with a concurrent unavailability of a safety-related system designed to mitigate the consequences of an accident.
The analysis performed in support of the SDP was not an initiating event analysis and only analyzed the performance deficiency for the short duration that HPCI was not in automatic mode.
Analysis Type. An initiating event analysis was performed using the SSES Unit 2 Standardized Plant Analysis Risk (SPAR) model Revision 8.21, created in May 2014.
SPAR Model Modifications. No SPAR model modifications were required as part of this analysis.
Key Modeling Assumptions. The following assumptions were determined to be significant to the modeling of this event:
* This event is modeled as a transient initiating event. Therefore, the probability of IE-TRANS (General Plant Transient) was set to 1.0. All other initiating event probabilities were set to zero.
* Basic event HCI-XHE-XO-ERROR (Operator fails to start/control HPCI injection) was set to TRUE. This is a conservative assumption because 1) operators could have assumed manual control of HPCI at any time if needed, and 2) HPCI was restored to automatic mode 27 minutes into the event.
* All other safety systems responded as designed.                                                              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.
LER 388-2016-005 3  ANALYSIS RESULTS            CCDP/Rejection Basis. The point estimate conditional core damage probability (CCDP) for this event is 4.2x10-7. The ASP Program acceptance threshold is a CCDP of 1x10-6. Therefore, this event is not a precursor and is screened out of the ASP Program.
Dominant Sequence. The dominant accident sequence is TRANS Sequence 49 (CCDP = 3.4x10-7) that contributes approximately 81% of the total internal events CCDP. Figure 1 in Appendix B illustrates this sequence. The cut sets/sequences that contribute to the top 95% and/or at least 1% of the total internal events CCDP are provided in Appendix A. The events and important component/system failures in TRANS Sequence 49 are:
* Reactor shutdown success,
* Safety relief valve close if opened,
* Power conversion system (condenser) is unavailable,
* High pressure injection sources (HPCI and RCIC) are unavailable, and
* Manual reactor depressurization fails. REFERENCES              1. Susquehanna Nuclear, LLC, "LER 388-2016-005 - Susquehanna Steam Electric Station Unit 2 HPCI Manually Overridden Prior to a Manual Scram During a Plant Transient," dated July 12, 2016 (ML16194A251). 2. Susquehanna Nuclear, LLC, "LER 388-2016-004-01 - Susquehanna Steam Electric Station Unit 2 experienced an electrical transient resulting in a manual SCRAM," dated September 29, 2016 (ML16273A157). 3. U.S. Nuclear Regulatory Commission, "Susquehanna Steam Electric Station - NRC Integrated Inspection Report 05000387/2016002 and 05000388/2016002, dated August 11, 2016 (ML16225A000).
LER 388-2016-005 A-1  Appendix A: SAPHIRE 8 Worksheet  Summary of Conditional Event Changes Event Description Cond Value Nominal Value HCI-XHE-XO-ERROR OPERATOR FAILS TO START/CONTROL HPCI INJECTION True 1.00E-3 IE-TRANS GENERAL PLANT TRANSIENT 1.00E+0a 7.62E-1 a. All other initiating event probabilities were set to zero. Dominant Sequence Results Only items contributing at least 1.0% to the total CCDP are displayed. Event Tree  Sequence  CCDP  %Contribution Description TRANS 49 3.39E-7 81.4% /RPS, /SRV, PCS, HPI, DEP TRANS 34 4.54E-8 10.9% /RPS, /SRV, PCS, /HPI, SPC, DEP, /CRD, RHR, PCSR, CVS, LI08 TRANS 52-10 1.19E-8 2.8% RPS, /PPR, RRS TRANS 09 5.96E-9 1.4% /RPS, /SRV, PCS, /HPI, SPC, /DEP, /CRD, RHR, PCSR, CVS, LI08 TRANS 52-05 5.10E-9 1.2% RPS, /PPR, /RRS, /PCS1, SLC Total  4.17E-7 100.0%  Referenced Fault Trees Fault Tree Description CVS CONTAINMENT VENTING DEP MANUAL REACTOR DEPRESS HPI HIGH PRESSURE INJECTION (HPCI or RCIC) LI08 LATE INJECTION PCS POWER CONVERSION SYSTEM PCSR POWER CONVERSION SYSTEM RECOVERY RHR RESIDUAL HEAT REMOVAL RPS REACTOR SHUTDOWN RRS RECIRC PUMP TRIP SLC STANDBY LIQUID CONTROL SPC SUPPRESSION POOL COOLING  Cut Set Report - TRANS 49 Only items contributing at least 1% to the total are displayed. #  CCDP  Total%  Cut Set    3.39E-7 100 Displaying 108 Cut Sets. (108 Original) 1 8.30E-8 24.45 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-TDP-FR-TRAIN 2 3.98E-8 11.72 IE-TRANS,DCP-BDC-CF-ALL 3 2.30E-8 6.78 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-TDP-TM-TRAIN 4 1.98E-8 5.82 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-XHE-XO-ERROR,RCI-TDP-FR-TRAIN 5 1.98E-8 5.82 IE-TRANS,ADS-XHE-XM-MDEPR,RCI-TDP-FR-TRAIN,SWS-XHE-XO-ERROR 6 1.88E-8 5.54 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-AOV-CC-START,RCI-TDP-FR-TRAIN 7 1.88E-8 5.54 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP1,RCI-TDP-FR-TRAIN 8 1.88E-8 5.54 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP2,RCI-TDP-FR-TRAIN 9 1.36E-8 4.02 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-TDP-FS-TRAIN 10 1.19E-8 3.51 IE-TRANS,DCP-BCH-CF-CHRS,RCI-TDP-FR-TRAIN 11 6.30E-9 1.86 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-RESTART,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 12 5.48E-9 1.61 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-XHE-XO-ERROR,RCI-TDP-TM-TRAIN 13 5.48E-9 1.61 IE-TRANS,ADS-XHE-XM-MDEPR,RCI-TDP-TM-TRAIN,SWS-XHE-XO-ERROR 14 5.21E-9 1.53 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-AOV-CC-START,RCI-TDP-TM-TRAIN 15 5.21E-9 1.53 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP1,RCI-TDP-TM-TRAIN 16 5.21E-9 1.53 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP2,RCI-TDP-TM-TRAIN LER 388-2016-005 A-2  Cut Set Report - TRANS 34 Only items contributing at least 1% to the total are displayed. #  CCDP  Total%  Cut Set    4.54E-8 100 Displaying 2 Cut Sets. (2 Original) 1 4.52E-8 99.67 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,DCP-BCH-CF-CHRS  Cut Set Report - TRANS 52-10 Only items contributing at least 1% to the total are displayed. #  CCDP  Total%  Cut Set    1.19E-8 100 Displaying 10 Cut Sets. (10 Original) 1 4.07E-9 34.30 IE-TRANS,RPS-SYS-FC-PSOVS,RRS-CRB-CC-PUMP1 2 4.07E-9 34.30 IE-TRANS,RPS-SYS-FC-PSOVS,RRS-CRB-CC-PUMP2 3 9.09E-10 7.67 IE-TRANS,RPS-SYS-FC-RELAY,RRS-CRB-CC-PUMP1 4 9.09E-10 7.67 IE-TRANS,RPS-SYS-FC-RELAY,RRS-CRB-CC-PUMP2 5 5.98E-10 5.04 IE-TRANS,RPS-SYS-FC-CRD,RRS-CRB-CC-PUMP2 6 5.98E-10 5.04 IE-TRANS,RPS-SYS-FC-CRD,RRS-CRB-CC-PUMP1 7 2.63E-10 2.22 IE-TRANS,RPS-SYS-FC-HCU,RRS-CRB-CC-PUMP1 8 2.63E-10 2.22 IE-TRANS,RPS-SYS-FC-HCU,RRS-CRB-CC-PUMP2  Cut Set Report - TRANS 09 Only items contributing at least 1% to the total are displayed. #  CCDP  Total%  Cut Set    5.97E-9 100 Displaying 31 Cut Sets. (31 Original) 1 1.05E-9 17.60 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,MSS-TBV-CC-BYPAS,RHR-XHE-XM-ERROR 2 5.50E-10 9.21 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-XHE-XM-ERROR 3 5.50E-10 9.21 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-XHE-XM-ERROR 4 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-MOV-OO-HXBPA,RSW-XHE-XL-SPVB 5 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-MOV-OO-HXBPB,RSW-XHE-XL-SPVA 6 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MOV-OO-1222A,RSW-XHE-XL-SPVB 7 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MOV-OO-1222B,RSW-XHE-XL-SPVA 8 3.48E-10 5.84 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,ESW-MDP-CF-RUN,SWS-XHE-XO-ERROR 9 2.92E-10 4.90 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,ESW-MDP-CF-START,SWS-XHE-XO-ERROR 10 2.50E-10 4.19 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,MFW-XHE-XO-ERROR,RHR-XHE-XM-ERROR 11 2.50E-10 4.19 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR,SWS-XHE-XO-ERROR 12 2.38E-10 3.99 IE-TRANS,CDS-AOV-CC-MKUP1,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR 13 2.38E-10 3.99 IE-TRANS,CDS-AOV-CC-MKUP2,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR 14 2.38E-10 3.99 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,MFW-AOV-CC-START,RHR-XHE-XM-ERROR 15 1.11E-10 1.86 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,RHR-XHE-XM-ERROR,TBC-MDP-CF-RUN,TBC-MDP-FC-ARUN 16 1.11E-10 1.86 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,RHR-XHE-XM-ERROR,TBC-MDP-CF-RUN,TBC-MDP-FC-BRUN 17 6.55E-11 1.10 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MDP-TM-PUMP1B,RSW-MDP-TM-PUMP2B,RSW-XHE-XL-SPVA LER 388-2016-005 A-3  18 6.55E-11 1.10 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MDP-TM-PUMP1A,RSW-MDP-TM-PUMP2A,RSW-XHE-XL-SPVB  Cut Set Report - TRANS 52-05 Only items contributing at least 1% to the total are displayed. #  CCDP  Total%  Cut Set    5.10E-9 100 Displaying 11 Cut Sets. (11 Original) 1 3.40E-9 66.63 IE-TRANS,RPS-SYS-FC-PSOVS,SLC-XHE-XM-ERROR 2 7.60E-10 14.89 IE-TRANS,RPS-SYS-FC-RELAY,SLC-XHE-XM-ERROR 3 5.00E-10 9.80 IE-TRANS,RPS-SYS-FC-CRD,SLC-XHE-XM-ERROR 4 2.20E-10 4.31 IE-TRANS,RPS-SYS-FC-HCU,SLC-XHE-XM-ERROR 5 1.09E-10 2.14 IE-TRANS,RPS-SYS-FC-PSOVS,SLC-EPV-CF-VALVS  Referenced Events Event Description ProbabilityACP-BAC-LP-2A201 DIVISION I AC POWER BUS 2A201 FAILS 3.33E-5 ACP-BAC-LP-2A202 DIVISION II AC POWER BUS 2A202 FAILS 3.33E-5 ADS-XHE-XM-MDEPR OPERATOR FAILS TO MANUALLY DEPRESSURIZE THE REACTOR 5.00E-4 CDS-AOV-CC-MKUP1 HOTWELL MAKEUP VALVE FAILS TO OPEN 9.51E-4 CDS-AOV-CC-MKUP2 HOTWELL MAKEUP VALVE FAILS TO OPEN 9.51E-4 CFAILED CONTAINMENT FAILURE CAUSES LOSS OF ALL INJECTION 5.00E-1 CVS-XHE-XM-VENT OPERATOR FAILS TO VENT CONTAINMENT 1.00E-3 CVS-XHE-XM-VENTL OPERATOR FAILS TO VENT CONTAINMENT LOCALLY 3.00E-1 DCP-BCH-CF-CHRS BATTERY CHARGERS FAIL FROM COMMON CAUSE 3.02E-7 DCP-BDC-CF-ALL 4-OF-4 125 VDC BUSES FAIL FROM COMMON CAUSE 3.98E-8 ESW-MDP-CF-RUN ESW PUMPS FAIL FROM COMMON CAUSE TO RUN 2.32E-6 ESW-MDP-CF-START ESW PUMPS FAIL FROM COMMON CAUSE TO START 1.95E-6 IAS-XHE-XM-IACIG OPERATOR FAILS TO OPEN IA-CIG CROSSTIE VALVES (PSA VALUE) 2.20E-1 IE-TRANS GENERAL PLANT TRANSIENT 1.00E+0 MFW-AOV-CC-START FEEDWATER STARTUP VALVE FAILS TO OPERATE 9.51E-4 MFW-XHE-XO-ERROR OPERATOR FAILS TO START/CONTROL FEEDWATER 1.00E-3 MSS-TBV-CC-BYPAS TURBINE BYPASS VALVES FAIL TO OPEN 4.20E-3 RCI-RESTART RESTART OF RCIC IS REQUIRED 1.50E-1 RCI-TDP-FR-TRAIN RCIC PUMP FAILS TO RUN GIVEN THAT IT STARTED 3.95E-2 RCI-TDP-FS-RSTRT RCIC FAILS TO RESTART GIVEN START AND SHORT-TERM RUN 8.00E-2 RCI-TDP-FS-TRAIN RCIC PUMP FAILS TO START 6.49E-3 RCI-TDP-TM-TRAIN RCIC TRAIN IS UNAVAILABLE BECAUSE OF MAINTENANCE 1.09E-2 RCI-XHE-XL-RSTRT OPERATOR FAILS TO RECOVER RCIC FAILURE TO RESTART 2.50E-1 RHR-MOV-OO-HXBPA RHR LOOP A HTX BYPASS MOV F048A FAILS TO CLOSE 9.63E-4 RHR-MOV-OO-HXBPB RHR LOOP B HTX BYPASS MOV F048B FAILS TO CLOSE 9.63E-4 RHR-XHE-XM-ERROR OPERATOR FAILS TO START/CONTROL RHR 5.00E-4 RPS-SYS-FC-CRD CONTROL ROD DRIVE MECHANICAL FAILURE 2.50E-7 RPS-SYS-FC-HCU HCU COMPONENTS FAIL 1.10E-7 RPS-SYS-FC-PSOVS HCU SCRAM PILOT SOVS FAIL 1.70E-6 RPS-SYS-FC-RELAY TRIP SYSTEM RELAYS FAIL 3.80E-7 RRS-CRB-CC-PUMP1 RECIRC PUMP 1 FIELD BREAKER FAILS TO OPEN 2.39E-3 RRS-CRB-CC-PUMP2 RECIRC PUMP 2 FIELD BREAKER FAILS TO OPEN 2.39E-3 RSW-MDP-TM-PUMP1A RHRSW PUMP TRAIN 1A IS IN TEST OR MAINT 1.32E-2 RSW-MDP-TM-PUMP1B RHRSW PUMP TRAIN 1B IS IN TEST OR MAINT 1.32E-2 RSW-MDP-TM-PUMP2A RHRSW PUMP TRAIN 2A IS IN TEST OR MAINT 1.32E-2 RSW-MDP-TM-PUMP2B RHRSW PUMP TRAIN 2B IS IN TEST OR MAINT 1.32E-2 LER 388-2016-005 A-4  RSW-MOV-OO-1222A RHRSW/ESW SPRAY POND VALVE HV-01222A FAILS TO CLOSE 9.63E-4 RSW-MOV-OO-1222B RHRSW/ESW SPRAY POND VALVE HV-01222B FAILS TO CLOSE 9.63E-4 RSW-XHE-XL-SPVA OPERATOR FAILS TO MANUALLY STROKE SPRAY POND NETWORK A VLVS 3.40E-1 RSW-XHE-XL-SPVB OPERATOR FAILS TO MANUALLY STROKE SPRAY POND NETWORK B VLVS 3.40E-1 SLC-EPV-CF-VALVS SLC SQUIB VALVES FAIL FROM COMMON CAUSE 6.42E-5 SLC-XHE-XM-ERROR OPERATOR FAILS TO START/CONTROL SLC 2.00E-3 SWS-XHE-XO-ERROR OPERATOR FAILS TO MAINTAIN SERVICE WATER FLOW 1.00E-3 TBC-MDP-CF-RUN TBCCW PUMPS FAIL FROM COMMON CAUSE TO RUN 2.95E-6 TBC-MDP-FC-ARUN TBCCW PUMP A IS RUNNING, PUMP B IS IN STANDBY 5.00E-1 TBC-MDP-FC-BRUN TBCCW PUMP B IS RUNNING, PUMP A IS IN STANDBY 5.00E-1 LER 388-2016-005 B-1  Appendix B: Key Event Tree  Figure 1:  Transient Event Tree (Sequence 49 in bold) 
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Revision as of 13:18, 1 May 2018

ASP Analysis-Reject-Final Susquehanna-HPCI Manually Overridden Prior to a Manual Scram During a Plant Transient (LER 388-2016-005)
ML16344A443
Person / Time
Site: Susquehanna Talen Energy icon.png
Issue date: 12/06/2016
From:
Office of Nuclear Regulatory Research
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References
IR 2016002, LER 16-004-01, LER 16-005-00
Download: ML16344A443 (8)
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Final ASP Program Analysis - Reject Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Susquehanna Steam Electric Station, Unit 2 HPCI Manually Overridden Prior to a Manual Scram During a Plant Transient Event Date: 5/13/2016 LER(s): 388-2016-005 and 388-2016-004-01 IR(s): 05000387/2016002 CCDP = 4x10-7 Plant Type: General Electric, BWR-4 with a Wet Mark II Containment Plant Operating Mode (Reactor Power Level): Mode 1 (69% Reactor Power) Analyst: David Aird Reviewer: Keith Tetter Contributors: N/A BC Approved Date: 12/6/2016 EVENT DETAILS Event Description. At 11:55 pm on May 12, 2016, Susquehanna Steam Electric Station (SSES) Unit 2 experienced an electrical transient resulting in a loss of 2B246 Reactor Building Engineered Safeguard System Division 2 480V Motor Control Center (MCC) and 2Y246 208/120V Alternate Current Instrument Panel. With the loss of the MCC, several drywell cooling fans were lost leading to an increase in drywell pressure. Operators reduced reactor power by reducing reactor recirculation flow. Maintenance personnel reported that the 2B246 bus faulted and was not able to be re-energized. Operators determined that a manual scram was required based upon a loss of drywell cooling.

On May 13, 2016 at 12:55 am, the High Pressure Coolant Injection (HPCI) system was overridden (placed in manual) prior to the manual scram to prevent the trip of all feedwater pumps on reactor pressure vessel high water level alarm. Although HPCI is not normally expected to initiate during a reactor scram if the non-safety related integrated control system (ICS) responds appropriately to control reactor water level, taking HPCI out of automatic mode rendered the system incapable of automatic initiation in the event of a failure of the ICS.

Technical Specification 3.5.1 was entered for HPCI inoperable due to manual override. At 1:10 am, with drywell pressure increasing to 1.3 psig, operators placed the mode switch in the shutdown position to manually scram the reactor. All control rods inserted as expected. Reactor water level lowered and was immediately restored by normal feedwater level control.

Primary Containment Isolation System isolations occurred, along with an initiation of the Reactor Core Isolation Cooling (RCIC) system. Once adequate level was verified, RCIC was overridden. Reactor pressure was controlled with turbine bypass valves, and subsequently main steam line drains. HPCI was restored to automatic mode at 1:22 am. All other safety systems functioned as expected. Additional information related to the override of HPCI and the electrical transient is available in licensee event reports (LERs) 388-2016-005 (Ref. 1), 388-2016-004-01 (Ref.2), and Inspection Report (IR) 05000387/2016002 (Ref. 3). Causes. The direct cause of the transient was found to be a phase to ground short between a cable and a protruding screw in the MCC. The protruding screw damaged and/or abraded the wire insulation until the short occurred.

LER 388-2016-005 2 There were two contributing causes to the incorrect override of HPCI prior to the manual reactor scram. First, the Unit Supervisor made a decision to prematurely override HPCI to minimize distractions later in the shutdown without procedural guidance to do so. Second, weaknesses in teamwork and oversight prevented the mistake from being corrected by the crew. A crew update was not conducted to announce the placing of HPCI in manual override, preventing the crew from providing a peer check of the Unit Supervisor's decision. MODELING Basis for ASP Analysis/SDP Results. The ASP Program uses Significance Determination Process (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 This event was addressed and evaluated in IR 05000387/2016002 (Ref. 3). Inspectors identified a performance deficiency for the licensee's failure to implement procedures for controlling the HPCI system. A detailed risk evaluation (condition assessment) was performed that resulted in a change in core damage frequency per year of less than 1x10-9. Therefore, the issue was determined to be of very low safety significance (Green).

An independent ASP analysis is required because this is an initiating event with a concurrent unavailability of a safety-related system designed to mitigate the consequences of an accident.

The analysis performed in support of the SDP was not an initiating event analysis and only analyzed the performance deficiency for the short duration that HPCI was not in automatic mode.

Analysis Type. An initiating event analysis was performed using the SSES Unit 2 Standardized Plant Analysis Risk (SPAR) model Revision 8.21, created in May 2014.

SPAR Model Modifications. No SPAR model modifications were required as part of this analysis.

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

  • This event is modeled as a transient initiating event. Therefore, the probability of IE-TRANS (General Plant Transient) was set to 1.0. All other initiating event probabilities were set to zero.
  • Basic event HCI-XHE-XO-ERROR (Operator fails to start/control HPCI injection) was set to TRUE. This is a conservative assumption because 1) operators could have assumed manual control of HPCI at any time if needed, and 2) HPCI was restored to automatic mode 27 minutes into the event.
  • All other safety systems responded as designed. 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.

LER 388-2016-005 3 ANALYSIS RESULTS CCDP/Rejection Basis. The point estimate conditional core damage probability (CCDP) for this event is 4.2x10-7. The ASP Program acceptance threshold is a CCDP of 1x10-6. Therefore, this event is not a precursor and is screened out of the ASP Program.

Dominant Sequence. The dominant accident sequence is TRANS Sequence 49 (CCDP = 3.4x10-7) that contributes approximately 81% of the total internal events CCDP. Figure 1 in Appendix B illustrates this sequence. The cut sets/sequences that contribute to the top 95% and/or at least 1% of the total internal events CCDP are provided in Appendix A. The events and important component/system failures in TRANS Sequence 49 are:

  • Reactor shutdown success,
  • Power conversion system (condenser) is unavailable,
  • High pressure injection sources (HPCI and RCIC) are unavailable, and

LER 388-2016-005 A-1 Appendix A: SAPHIRE 8 Worksheet Summary of Conditional Event Changes Event Description Cond Value Nominal Value HCI-XHE-XO-ERROR OPERATOR FAILS TO START/CONTROL HPCI INJECTION True 1.00E-3 IE-TRANS GENERAL PLANT TRANSIENT 1.00E+0a 7.62E-1 a. All other initiating event probabilities were set to zero. Dominant Sequence Results Only items contributing at least 1.0% to the total CCDP are displayed. Event Tree Sequence CCDP %Contribution Description TRANS 49 3.39E-7 81.4% /RPS, /SRV, PCS, HPI, DEP TRANS 34 4.54E-8 10.9% /RPS, /SRV, PCS, /HPI, SPC, DEP, /CRD, RHR, PCSR, CVS, LI08 TRANS 52-10 1.19E-8 2.8% RPS, /PPR, RRS TRANS 09 5.96E-9 1.4% /RPS, /SRV, PCS, /HPI, SPC, /DEP, /CRD, RHR, PCSR, CVS, LI08 TRANS 52-05 5.10E-9 1.2% RPS, /PPR, /RRS, /PCS1, SLC Total 4.17E-7 100.0% Referenced Fault Trees Fault Tree Description CVS CONTAINMENT VENTING DEP MANUAL REACTOR DEPRESS HPI HIGH PRESSURE INJECTION (HPCI or RCIC) LI08 LATE INJECTION PCS POWER CONVERSION SYSTEM PCSR POWER CONVERSION SYSTEM RECOVERY RHR RESIDUAL HEAT REMOVAL RPS REACTOR SHUTDOWN RRS RECIRC PUMP TRIP SLC STANDBY LIQUID CONTROL SPC SUPPRESSION POOL COOLING Cut Set Report - TRANS 49 Only items contributing at least 1% to the total are displayed. # CCDP Total% Cut Set 3.39E-7 100 Displaying 108 Cut Sets. (108 Original) 1 8.30E-8 24.45 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-TDP-FR-TRAIN 2 3.98E-8 11.72 IE-TRANS,DCP-BDC-CF-ALL 3 2.30E-8 6.78 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-TDP-TM-TRAIN 4 1.98E-8 5.82 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-XHE-XO-ERROR,RCI-TDP-FR-TRAIN 5 1.98E-8 5.82 IE-TRANS,ADS-XHE-XM-MDEPR,RCI-TDP-FR-TRAIN,SWS-XHE-XO-ERROR 6 1.88E-8 5.54 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-AOV-CC-START,RCI-TDP-FR-TRAIN 7 1.88E-8 5.54 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP1,RCI-TDP-FR-TRAIN 8 1.88E-8 5.54 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP2,RCI-TDP-FR-TRAIN 9 1.36E-8 4.02 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-TDP-FS-TRAIN 10 1.19E-8 3.51 IE-TRANS,DCP-BCH-CF-CHRS,RCI-TDP-FR-TRAIN 11 6.30E-9 1.86 IE-TRANS,ADS-XHE-XM-MDEPR,MSS-TBV-CC-BYPAS,RCI-RESTART,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 12 5.48E-9 1.61 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-XHE-XO-ERROR,RCI-TDP-TM-TRAIN 13 5.48E-9 1.61 IE-TRANS,ADS-XHE-XM-MDEPR,RCI-TDP-TM-TRAIN,SWS-XHE-XO-ERROR 14 5.21E-9 1.53 IE-TRANS,ADS-XHE-XM-MDEPR,MFW-AOV-CC-START,RCI-TDP-TM-TRAIN 15 5.21E-9 1.53 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP1,RCI-TDP-TM-TRAIN 16 5.21E-9 1.53 IE-TRANS,ADS-XHE-XM-MDEPR,CDS-AOV-CC-MKUP2,RCI-TDP-TM-TRAIN LER 388-2016-005 A-2 Cut Set Report - TRANS 34 Only items contributing at least 1% to the total are displayed. # CCDP Total% Cut Set 4.54E-8 100 Displaying 2 Cut Sets. (2 Original) 1 4.52E-8 99.67 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,DCP-BCH-CF-CHRS Cut Set Report - TRANS 52-10 Only items contributing at least 1% to the total are displayed. # CCDP Total% Cut Set 1.19E-8 100 Displaying 10 Cut Sets. (10 Original) 1 4.07E-9 34.30 IE-TRANS,RPS-SYS-FC-PSOVS,RRS-CRB-CC-PUMP1 2 4.07E-9 34.30 IE-TRANS,RPS-SYS-FC-PSOVS,RRS-CRB-CC-PUMP2 3 9.09E-10 7.67 IE-TRANS,RPS-SYS-FC-RELAY,RRS-CRB-CC-PUMP1 4 9.09E-10 7.67 IE-TRANS,RPS-SYS-FC-RELAY,RRS-CRB-CC-PUMP2 5 5.98E-10 5.04 IE-TRANS,RPS-SYS-FC-CRD,RRS-CRB-CC-PUMP2 6 5.98E-10 5.04 IE-TRANS,RPS-SYS-FC-CRD,RRS-CRB-CC-PUMP1 7 2.63E-10 2.22 IE-TRANS,RPS-SYS-FC-HCU,RRS-CRB-CC-PUMP1 8 2.63E-10 2.22 IE-TRANS,RPS-SYS-FC-HCU,RRS-CRB-CC-PUMP2 Cut Set Report - TRANS 09 Only items contributing at least 1% to the total are displayed. # CCDP Total% Cut Set 5.97E-9 100 Displaying 31 Cut Sets. (31 Original) 1 1.05E-9 17.60 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,MSS-TBV-CC-BYPAS,RHR-XHE-XM-ERROR 2 5.50E-10 9.21 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-XHE-XM-ERROR 3 5.50E-10 9.21 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-XHE-XM-ERROR 4 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-MOV-OO-HXBPA,RSW-XHE-XL-SPVB 5 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RHR-MOV-OO-HXBPB,RSW-XHE-XL-SPVA 6 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MOV-OO-1222A,RSW-XHE-XL-SPVB 7 3.60E-10 6.03 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MOV-OO-1222B,RSW-XHE-XL-SPVA 8 3.48E-10 5.84 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,ESW-MDP-CF-RUN,SWS-XHE-XO-ERROR 9 2.92E-10 4.90 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,ESW-MDP-CF-START,SWS-XHE-XO-ERROR 10 2.50E-10 4.19 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,MFW-XHE-XO-ERROR,RHR-XHE-XM-ERROR 11 2.50E-10 4.19 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR,SWS-XHE-XO-ERROR 12 2.38E-10 3.99 IE-TRANS,CDS-AOV-CC-MKUP1,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR 13 2.38E-10 3.99 IE-TRANS,CDS-AOV-CC-MKUP2,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR 14 2.38E-10 3.99 IE-TRANS,CFAILED,CVS-XHE-XM-VENT,MFW-AOV-CC-START,RHR-XHE-XM-ERROR 15 1.11E-10 1.86 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,RHR-XHE-XM-ERROR,TBC-MDP-CF-RUN,TBC-MDP-FC-ARUN 16 1.11E-10 1.86 IE-TRANS,CFAILED,CVS-XHE-XM-VENTL,RHR-XHE-XM-ERROR,TBC-MDP-CF-RUN,TBC-MDP-FC-BRUN 17 6.55E-11 1.10 IE-TRANS,ACP-BAC-LP-2A201,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MDP-TM-PUMP1B,RSW-MDP-TM-PUMP2B,RSW-XHE-XL-SPVA LER 388-2016-005 A-3 18 6.55E-11 1.10 IE-TRANS,ACP-BAC-LP-2A202,CFAILED,CVS-XHE-XM-VENTL,IAS-XHE-XM-IACIG,RSW-MDP-TM-PUMP1A,RSW-MDP-TM-PUMP2A,RSW-XHE-XL-SPVB Cut Set Report - TRANS 52-05 Only items contributing at least 1% to the total are displayed. # CCDP Total% Cut Set 5.10E-9 100 Displaying 11 Cut Sets. (11 Original) 1 3.40E-9 66.63 IE-TRANS,RPS-SYS-FC-PSOVS,SLC-XHE-XM-ERROR 2 7.60E-10 14.89 IE-TRANS,RPS-SYS-FC-RELAY,SLC-XHE-XM-ERROR 3 5.00E-10 9.80 IE-TRANS,RPS-SYS-FC-CRD,SLC-XHE-XM-ERROR 4 2.20E-10 4.31 IE-TRANS,RPS-SYS-FC-HCU,SLC-XHE-XM-ERROR 5 1.09E-10 2.14 IE-TRANS,RPS-SYS-FC-PSOVS,SLC-EPV-CF-VALVS Referenced Events Event Description ProbabilityACP-BAC-LP-2A201 DIVISION I AC POWER BUS 2A201 FAILS 3.33E-5 ACP-BAC-LP-2A202 DIVISION II AC POWER BUS 2A202 FAILS 3.33E-5 ADS-XHE-XM-MDEPR OPERATOR FAILS TO MANUALLY DEPRESSURIZE THE REACTOR 5.00E-4 CDS-AOV-CC-MKUP1 HOTWELL MAKEUP VALVE FAILS TO OPEN 9.51E-4 CDS-AOV-CC-MKUP2 HOTWELL MAKEUP VALVE FAILS TO OPEN 9.51E-4 CFAILED CONTAINMENT FAILURE CAUSES LOSS OF ALL INJECTION 5.00E-1 CVS-XHE-XM-VENT OPERATOR FAILS TO VENT CONTAINMENT 1.00E-3 CVS-XHE-XM-VENTL OPERATOR FAILS TO VENT CONTAINMENT LOCALLY 3.00E-1 DCP-BCH-CF-CHRS BATTERY CHARGERS FAIL FROM COMMON CAUSE 3.02E-7 DCP-BDC-CF-ALL 4-OF-4 125 VDC BUSES FAIL FROM COMMON CAUSE 3.98E-8 ESW-MDP-CF-RUN ESW PUMPS FAIL FROM COMMON CAUSE TO RUN 2.32E-6 ESW-MDP-CF-START ESW PUMPS FAIL FROM COMMON CAUSE TO START 1.95E-6 IAS-XHE-XM-IACIG OPERATOR FAILS TO OPEN IA-CIG CROSSTIE VALVES (PSA VALUE) 2.20E-1 IE-TRANS GENERAL PLANT TRANSIENT 1.00E+0 MFW-AOV-CC-START FEEDWATER STARTUP VALVE FAILS TO OPERATE 9.51E-4 MFW-XHE-XO-ERROR OPERATOR FAILS TO START/CONTROL FEEDWATER 1.00E-3 MSS-TBV-CC-BYPAS TURBINE BYPASS VALVES FAIL TO OPEN 4.20E-3 RCI-RESTART RESTART OF RCIC IS REQUIRED 1.50E-1 RCI-TDP-FR-TRAIN RCIC PUMP FAILS TO RUN GIVEN THAT IT STARTED 3.95E-2 RCI-TDP-FS-RSTRT RCIC FAILS TO RESTART GIVEN START AND SHORT-TERM RUN 8.00E-2 RCI-TDP-FS-TRAIN RCIC PUMP FAILS TO START 6.49E-3 RCI-TDP-TM-TRAIN RCIC TRAIN IS UNAVAILABLE BECAUSE OF MAINTENANCE 1.09E-2 RCI-XHE-XL-RSTRT OPERATOR FAILS TO RECOVER RCIC FAILURE TO RESTART 2.50E-1 RHR-MOV-OO-HXBPA RHR LOOP A HTX BYPASS MOV F048A FAILS TO CLOSE 9.63E-4 RHR-MOV-OO-HXBPB RHR LOOP B HTX BYPASS MOV F048B FAILS TO CLOSE 9.63E-4 RHR-XHE-XM-ERROR OPERATOR FAILS TO START/CONTROL RHR 5.00E-4 RPS-SYS-FC-CRD CONTROL ROD DRIVE MECHANICAL FAILURE 2.50E-7 RPS-SYS-FC-HCU HCU COMPONENTS FAIL 1.10E-7 RPS-SYS-FC-PSOVS HCU SCRAM PILOT SOVS FAIL 1.70E-6 RPS-SYS-FC-RELAY TRIP SYSTEM RELAYS FAIL 3.80E-7 RRS-CRB-CC-PUMP1 RECIRC PUMP 1 FIELD BREAKER FAILS TO OPEN 2.39E-3 RRS-CRB-CC-PUMP2 RECIRC PUMP 2 FIELD BREAKER FAILS TO OPEN 2.39E-3 RSW-MDP-TM-PUMP1A RHRSW PUMP TRAIN 1A IS IN TEST OR MAINT 1.32E-2 RSW-MDP-TM-PUMP1B RHRSW PUMP TRAIN 1B IS IN TEST OR MAINT 1.32E-2 RSW-MDP-TM-PUMP2A RHRSW PUMP TRAIN 2A IS IN TEST OR MAINT 1.32E-2 RSW-MDP-TM-PUMP2B RHRSW PUMP TRAIN 2B IS IN TEST OR MAINT 1.32E-2 LER 388-2016-005 A-4 RSW-MOV-OO-1222A RHRSW/ESW SPRAY POND VALVE HV-01222A FAILS TO CLOSE 9.63E-4 RSW-MOV-OO-1222B RHRSW/ESW SPRAY POND VALVE HV-01222B FAILS TO CLOSE 9.63E-4 RSW-XHE-XL-SPVA OPERATOR FAILS TO MANUALLY STROKE SPRAY POND NETWORK A VLVS 3.40E-1 RSW-XHE-XL-SPVB OPERATOR FAILS TO MANUALLY STROKE SPRAY POND NETWORK B VLVS 3.40E-1 SLC-EPV-CF-VALVS SLC SQUIB VALVES FAIL FROM COMMON CAUSE 6.42E-5 SLC-XHE-XM-ERROR OPERATOR FAILS TO START/CONTROL SLC 2.00E-3 SWS-XHE-XO-ERROR OPERATOR FAILS TO MAINTAIN SERVICE WATER FLOW 1.00E-3 TBC-MDP-CF-RUN TBCCW PUMPS FAIL FROM COMMON CAUSE TO RUN 2.95E-6 TBC-MDP-FC-ARUN TBCCW PUMP A IS RUNNING, PUMP B IS IN STANDBY 5.00E-1 TBC-MDP-FC-BRUN TBCCW PUMP B IS RUNNING, PUMP A IS IN STANDBY 5.00E-1 LER 388-2016-005 B-1 Appendix B: Key Event Tree Figure 1: Transient Event Tree (Sequence 49 in bold)

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