ML13322A833

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Final ASP Analysis - River Bend Station
ML13322A833
Person / Time
Site: River Bend Entergy icon.png
Issue date: 11/25/2013
From:
NRC/RES/DRA
To:
Division of Operating Reactor Licensing
Hunter C
Shared Package
ML13322A414 List:
References
IR-12-009, IR-12-010, LER 12-003-00
Download: ML13322A833 (9)
v • d • e


Text

Final Precursor Analysis Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Loss of Normal Service Water, Circulating Water, and River Bend Station Feedwater Due to Electrical Fault LER: 458/12-003 Event Date: 05/24/2012 CCDP = 2x10-4 IR: 50-458/12-09, 50-458/12-10 Plant Type: Boiling Water Reactor (BWR); General Electric-6 with a Mark III Containment Plant Operating Mode (Reactor Power Level): Mode 1 (33% Reactor Power)

EXECUTIVE

SUMMARY

Due to an electrical feeder cable fault to nonsafety-related 4.16 kV Bus NNS-SWG2A that occurred on May 21, 2012, the licensee was powering all circulating water (CW) pumps, feedwater (FW) pumps, and normal service water (NSW) pumps from a single source (Bus NNS-SWG2B) while repairs were being performed. This electrical alignment left the plant susceptible to a loss of all three systems (CW, FW, and NSW) given a single failure causing the unavailability of nonsafety-related 4.16 kV Bus NNS-SWG2B. On May 24th, a fault on FW Pump B motor termination box was not isolated by the associated motor feeder breaker (due to failed lockout relay) causing the loss of Bus NNS-SWG2B leading to loss of CW, FW, and NSW, and a subsequent reactor trip. The loss of feedwater and circulating water pumps caused the unavailability of the normal source of reactor inventory control and decay heat removal via the main condenser, respectively.

Given the loss of FW, reactor core isolation cooling (RCIC) or high-pressure core spray (HPCS) must supply makeup to the reactor or operators must depressurize the reactor coolant system (RCS) to allow low-pressure injection systems to provide inventory control. The safety relief valves (SRVs) provide RCS pressure control if the main condenser is unavailable. However, given the loss of NSW, at least 2 of the 4 standby service water pumps must successfully start and run to provide the capability to transfer heat to the ultimate heat sink. With no standby service water to transfer heat to the ultimate heat sink, suppression pool cooling (required for RCIC, HPCS, and successful pressure control via the SRVs), shutdown cooling [via the residual heat removal (RHR) system], and containment heat removal are rendered unavailable.

According to the risk analysis modeling assumptions used in this ASP analysis, the most likely core damage sequences [accounting for approximately 96% of the conditional core damage probability (CCDP)] are the loss of power to all NSW, FW, and CW pumps and subsequent reactor trip (which occurred during the event) combined with postulated failures/unavailabilities of standby service water components causing system failure. If the standby service water system had failed (leading to the loss of the ultimate heat sink) without recovery of power to NSW pumps (not possible during the event; and therefore, not credited), core damage could have occurred.

Enclosure 1

LER 458/12-003 EVENT DETAILS Event Description. On May 24th, the reactor plant was operating at 33 percent power with one FW pump (Pump C) and two CW pumps (Pumps B and D) in service. At approximately 1:48 p.m., operators started the FW Pump B and a subsequent fault occurred in the FW pump motor termination box. The fault was not isolated by the motor feeder breaker due to a failed lockout relay. As a result, the supply breaker for the nonsafety-related 4.16 kV Supply Bus NNS-SWG2B tripped to clear the fault. This resulted in the loss of power to all running FW, CW, and NSW pumps.1 The operators initiated a manual reactor scram due to the trip of all running FW and CW pumps, and immediately began implementing procedures EOP-1, Reactor Pressure Vessel Control, and EOP-3, Secondary Containment and Radioactive Release Control. The operators initiated the RCIC system per procedure EOP-1 for reactor level control. Soon after the manual scram, the control room received a report of smoke from the circuitry board for FW Pump B. At 1:58 p.m., the fire brigade reported that the smoke was dissipating and there was physical damage to the connection box, but no visible fire was identified. At 2:05 p.m., the operators closed the main steam isolation valves.

At 3:01 p.m., the licensee began restoring power to the nonsafety-related Train B Buses by connecting them to the associated Train A Buses. With the exception of the bus providing power to the CW and NSW pumps, the Train A Busses were available and could provide power to the Train B Buses. The Train A Bus (NNS-SWG2A) providing power to the CW and NSW pumps remained unavailable due to a cable failure that occurred on May 21st.2 During the cooldown, the operators used the RCIC system for level control and the SRVs for pressure control. At 3:34 p.m., reactor coolant level reached a Level 8 condition, and the RCIC system automatically stopped running. The SRVs were subsequently closed, and RCIC was restarted after reactor vessel water level was reduced below Level 8. The licensee continued to cooldown the plant using the RCIC system and the SRVs until the RHR system could be placed in service. The licensee reached Cold Shutdown conditions at 2:00 am on May 25. Additional information is provided in References 1-3.

MODELING ASSUMPTIONS Analysis Type. The River Bend SPAR model, created in April 2012, was used for this event analysis. This event was modeled as a loss of NSW initiating event.

Analysis Rules. The ASP program uses Significance Determination Process results for degraded conditions when available. However, the ASP Program performs independent analyses for initiating events.

1 On May 22nd, the licensee closed a tie breaker to power nonsafety-related 4160 V Bus NNS-SWG2A from the opposite train switchgear. This resulted in all CW pumps and NSW water pumps receiving power through nonsafety-related 4160 V Bus NNS-SWG2B. This power alignment was made due to a Bus NNS-SWG2A feeder cable failure experienced on May 21st that resulted in the trip of two of the four CW pumps and a subsequent reactor scram due to low condenser vacuum.

2 With both buses (NNS-SWG2A and NNS-SWG2B) that supply power to the CW pumps unavailable, recovery of the condenser heat sink was not possible for this event.

2

LER 458/12-003 Key Modeling Assumptions. The following modeling assumptions were determined to be significant to the modeling of this event analysis:

  • The normal service water pumps were not recoverable due to both nonsafety-related 4.16 kV supply buses (NNS-SWG2A and NNS-SWG2B) being damaged.

Fault Tree Modification. Added Basic Event IEFT-LONSW-NONVITAL (Loss of Non-Vital Bus Supplying Power to NSW and SWC Pumps) in the IEFT-LONSW Fault Tree to account for the non-recoverable loss of NSW due to the NSW and service water cooling (SWC) pumps losing power. See Figure B-1 for the modified IEFT-LONSW Fault Tree.

Basic Event Probability Changes. The following initiating event frequencies and basic event probabilities were modified for this event analysis:

  • The probability of IE-LONSW (Loss of Normal Service Water Initiating Event) was set to 1.0; all other initiating event probabilities were set to zero.
  • The basic event IEFT-LONSW-NONVITAL was set to TRUE to account for the loss of electrical power to all NSW and SWC pumps.3
  • Basic Event RCI-RESTART (Restart of RCIC is Required) was set to TRUE because operators needed to manually restart RCIC after reactor coolant level reached a Level 8 and RCIC automatically stopped, as designed.

ANALYSIS RESULTS Conditional Core Damage Probabilities. The point estimate CCDP for this event is 1.8x10-4.

The Accident Sequence Precursor Program acceptance threshold is a CCDP of 1x10-6 or the CCDP equivalent of a reactor trip with a non-recoverable loss of secondary plant systems (e.g.,

feed water and condensate), whichever is greater. This CCDP equivalent for River Bend is 3x10-6.

Dominant Sequence. The dominant accident sequence is LONSW Sequence 10 (CCDP = 1.1x10-4), which contribute approximately 63% of the total internal events CCDP.

Additional sequences that contribute greater than 1% of the total internal events CCDP are provided in Appendix A.

The dominant sequences are shown graphically in Figure B-2 in Appendix B. The events and important component failures in LONSW Sequence 10 are:

  • Non-recoverable loss of NSW occurs,
  • Steam relief valves succeed (reclose if opened),
  • Suppression pool cooling fails,
  • Manual reactor depressurization succeeds, 3

Although not modeled because the effect on the analysis results would be negligible, recovery of power to SWC pumps was possible by aligning power for the pumps via the Train A buses.

3

LER 458/12-003

  • Containment heat removal fails (i.e., containment coolers).

REFERENCES

1. River Bend Station, "LER 458/12-003- Reactor Scram Following a Loss of Main Reactor Feedwater Pump Due to Electrical Fault, dated July 23, 2012 (ML12209A097).
2. U.S. Nuclear Regulatory Commission, River Bend Station - NRC Augmented Inspection Team Report 05000458/2012009, dated August 7, 2012 (ML12221A233).
3. U.S. Nuclear Regulatory Commission, River Bend Station - NRC Augmented Inspection Team Follow-Up Inspection Report 05000458/2012010, dated November 23, 2012 (ML12328A178).
4. Idaho National Laboratory, NUREG/CR-6883, The SPAR-H Human Reliability Analysis Method, August 2005 (ML051950061).
5. Idaho National Laboratory, INL/EXT-10-18533, SPAR-H Step-by-Step Guidance, May 2011 (ML112060305).

4

LER 458/12-003 Appendix A: Analysis Results Summary of Conditional Event Changes Cond. Nominal Event Description Value Value IEFT-LONSW-NONVITAL LOSS OF NON-VITAL BUS SUPPLYING POWER TRUE NOT TO NSW AND SWC PUMPS MODELED IE-LONSWa LOSS OF NORMAL SERVICE WATER 1.00E+0 1.00E+0 RCI-RESTART RESTART OF RCIC IS REQUIRED TRUE 1.50E-1

a. All other initiating probabilities are 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 IEFT-LONSW, /RPS, /SRV, /HCS, SPC, /DEP, LONSW 10 1.14E-04 62.58%

SDC, CHR, SWSR IEFT-LONSW, /RPS, /SRV, HCS, /RCI, SPC, LONSW 34 6.19E-05 33.98%

/DEP, LPI, /VA, SDC1, CHR, SWSR2 LONSW 63 3.66E-06 2.01% IEFT-LONSW, /RPS, /SRV, HCS, RCI, DEP IEFT-LONSW, /RPS, /SRV, HCS, /RCI, SPC, LONSW 31 2.60E-06 1.43% /DEP, LPI, /VA, SDC1, CHR, /SWSR2, PCSR,

/CVS01, LI00 Total 1.82E-04 100%

Referenced Fault Trees Fault Tree Description CHR CONTAINMENT HEAT REMOVAL (FAN COOLERS)

DEP MANUAL REACTOR DEPRESS HCS HIGH-PRESSURE COOLANT SPRAY IEFT-LONSW LOSS OF NORMAL SERVICE WATER (IE FAULT TREE)

LPI LOW-PRESSURE INJECTION (LPCS OR LPCI)

RCI REACTOR CORE ISOLATION COOLING SDC SHUTDOWN COOLING SDC1 RIVER BEND SHUTDOWN COOLING FAULT TREE SRV SAFETY RELIEF VALVES (RECLOSE IF OPENED)

SPC SUPPRESSION POOL COOLING SPC1 RIVER BEND SUPPRESSION POOL COOLING FAULT TREE SWSR SERVICE WATER SYSTEM RECOVERY VA ALTERNATE LOW-PRESSURE INJECTION Cut Set Report - LONSW 10 Only items contributing at least 1% to the total are displayed.

  1. CCDP Total% Cut Set 1.14E-4 100 Displaying 2082 of 2082 Cut Sets.

1 1.32E-5 11.7 IE-LONSW,SSW-MDP-TM-P2B,SSW-XHE-XR-3302 2 1.32E-5 11.7 IE-LONSW,SSW-MDP-TM-P2D,SSW-XHE-XR-3302 3 1.28E-5 11.2 IE-LONSW,SSW-MDP-TM-P2B,SSW-MOV-CC-F055A 4 1.28E-5 11.2 IE-LONSW,SSW-MDP-TM-P2D,SSW-MOV-CC-F055A 5 6.62E-6 5.83 IE-LONSW,RHR-XHE-XM-ERROR,SSW-MDP-TM-P2D 6 6.62E-6 5.83 IE-LONSW,RHR-XHE-XM-ERROR,SSW-MDP-TM-P2B A-1

LER 458/12-003

  1. CCDP Total% Cut Set 7 5.74E-6 5.05 IE-LONSW,RHR-MDP-TM-PC002B,SSW-XHE-XR-3302 8 5.53E-6 4.87 IE-LONSW,RHR-MDP-TM-PC002B,SSW-MOV-CC-F055A 9 1.36E-6 1.2 IE-LONSW,SSW-MDP-FS-P2B,SSW-XHE-XR-3302 10 1.36E-6 1.2 IE-LONSW,SSW-MDP-FS-P2D,SSW-XHE-XR-3302 11 1.31E-6 1.16 IE-LONSW,SSW-MDP-FS-P2B,SSW-MOV-CC-F055A 12 1.31E-6 1.16 IE-LONSW,SSW-MDP-FS-P2D,SSW-MOV-CC-F055A 13 1.25E-6 1.1 IE-LONSW,CHR-ACX-TM-FANA,RHR-XHE-XM-ERROR 14 1.25E-6 1.1 IE-LONSW,CHR-ACX-TM-FANB,RHR-XHE-XM-ERROR Cut Set Report - LONSW 34 Only items contributing at least 1% to the total are displayed.
  1. CCDP Total% Cut Set 6.19E-5 100 Displaying 891 of 891 Cut Sets.

1 3.05E-5 49.3 IE-LONSW,SSW-MOV-CF-DISCH 2 1.27E-5 20.4 IE-LONSW,SSW-MDP-CF-START 3 7.01E-6 11.3 IE-LONSW,SSW-MDP-CF-RUN 4 6.29E-6 10.2 IE-LONSW,NSW-XHE-XL-NOREC2,SSW-MOV-CF-F055AB 5 1.00E-6 1.62 IE-LONSW,SSW-XHE-XR-3302,SSW-XHE-XR-3303 6 9.63E-7 1.56 IE-LONSW,SSW-MOV-CC-F055B,SSW-XHE-XR-3302 Cut Set Report - LONSW 63 Only items contributing at least 1% to the total are displayed.

  1. CCDP Total% Cut Set 3.66E-6 100 Displaying 413 of 413 Cut Sets.

1 1.11E-6 30.5 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-TM-HPCS,RCI-TDP-FR-TRAIN 2 9.36E-7 25.6 IE-LONSW,ADS-XHE-XM-MDEPR,CDS-XVM-OC-F001 3 5.64E-7 15.4 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-TM-HPCS,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 4 1.83E-7 5.01 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-TM-HPCS,RCI-TDP-FS-TRAIN 5 1.52E-7 4.16 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MOV-CC-F004,RCI-TDP-FR-TRAIN 6 1.50E-7 4.09 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-FS-HPCS,RCI-TDP-FR-TRAIN 7 7.70E-8 2.11 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MOV-CC-F004,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 8 7.58E-8 2.07 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-FS-HPCS,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 9 5.73E-8 1.57 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-FR-HPCS,RCI-TDP-FR-TRAIN 10 3.92E-8 1.07 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MOV-CC-F004,RCI-TDP-TM-TRAIN 11 3.85E-8 1.05 IE-LONSW,ADS-XHE-XM-MDEPR,HCS-MDP-FS-HPCS,RCI-TDP-TM-TRAIN Cut Set Report - LONSW 31 Only items contributing at least 1% to the total are displayed.

  1. CCDP Total% Cut Set 2.60E-6 100 Displaying 60 of 60 Cut Sets.

1 2.31E-6 89 IE-LONSW,CVENT,SSW-MOV-CF-F055AB 2 1.20E-7 4.63 IE-LONSW,CVENT,SSW-MOV-CC-F055A,SSW-XHE-XR-3303 3 1.16E-7 4.46 IE-LONSW,CVENT,SSW-MOV-CC-F055A,SSW-MOV-CC-F055B A-2

LER 458/12-003 Referenced Events Event Description Probability ADS-XHE-XM-MDEPR OPERATOR FAILS TO DEPRESSURIZE THE REACTOR 4.00E-3 CDS-XVM-OC-F001 CST SUPPLY TO RCIC/HPCS ISOLATION VALVE F001 FAILS 2.34E-4 CLOSED CHR-ACX-TM-FANA CHR FAN COOLER A OUT FOR TEST OR MAINTENANCE 2.50E-3 CHR-ACX-TM-FANB CHR FAN COOLER B OUT FOR TEST OR MAINTENANCE 2.50E-3 CVENT CONTAINMENT VENTING CAUSES LOSS OF OPERATING 1.25E-1 INJECTION SOURCE HCS-MDP-FR-HPCS HPCS PUMP FAILS TO RUN 3.62E-4 HCS-MDP-FS-HPCS HPCS PUMP FAILS TO START 9.47E-4 HCS-MDP-TM-HPCS HPCS IS UNAVAILABLE BECAUSE OF MAINTENANCE 7.05E-3 HCS-MOV-CC-F004 HPCS INJECTION VALVE F004 FAILS TO OPEN 9.63E-4 IE-LONSW LOSS OF NORMAL SERVICE WATER 1.00E+0 NSW-XHE-XL-NOREC2 OPERATOR FAILS TO RECOVER NORMAL SERVICE WATER 3.40E-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 8.00E-2 RUN RCI-TDP-FS-TRAIN RCIC PUMP FAILS TO START 6.49E-3 RCI-TDP-TM-TRAIN RCIC PUMP TRAIN IS UNAVAILABLE BECAUSE OF 1.02E-2 MAINTENANCE RCI-XHE-XL-RSTRT OPERATOR FAILS TO RECOVER RCIC FAILURE TO RESTART 2.50E-1 RHR-MDP-TM-PC002B RHR TRAIN PC002B IS UNAVAILABLE DUE TO T & M 5.74E-3 RHR-XHE-XM-ERROR OPERATOR FAILS TO START/CONTROL RHR 5.00E-4 SSW-MDP-CF-RUN SSW PUMPS FAIL FROM COMMON CAUSE TO RUN 7.01E-6 SSW-MDP-CF-START SSW PUMPS FAIL FROM COMMON CAUSE TO START 1.26E-5 SSW-MDP-FS-P2B SSW PUMP P2B FAILS TO START 1.36E-3 SSW-MDP-FS-P2D SSW PUMP P2D FAILS TO START 1.36E-3 SSW-MDP-TM-P2B SSW PUMP P2B IS UNAVAILABLE DUE TO T & M 1.32E-2 SSW-MDP-TM-P2D SSW PUMP P2D IS UNAVAILABLE DUE TO T & M 1.32E-2 SSW-MOV-CC-F055A SSW BASIN OUTLET ISOLATION VLV F055A FAILS TO OPEN 9.63E-4 SSW-MOV-CC-F055B SSW BASIN OUTLET ISOLATION VLV F055B FAILS TO OPEN 9.63E-4 SSW-MOV-CF-DISCH SSW PUMP DISCHARGE VALVES F040A/B/C/D FAIL TO OPEN 3.05E-5 SSW-MOV-CF-F055AB SSW BASIN DISCHARGE VALVES F055A/B FAIL TO OPEN 1.85E-5 SSW-XHE-XR-3302 OPERATOR FAILS TO RESTORE SSW TRAIN A DISCHARGE 1.00E-3 VALVE SWP-3302 FOLLOWING T&M SSW-XHE-XR-3303 OPERATOR FAILS TO RESTORE SSW TRAIN B DISCHARGE 1.00E-3 VALVE SWP-3303 FOLLOWING T&M A-3

LER 458/12-003 Appendix B: Modified Fault Tree and Key Event Trees LOSS OF NORMAL SERVICE WATER (IE FAULT TREE)

IEFT-LONSW LOSS OF SERVICE WATER - HEAT LOSS OF SERVICE WATER - SWC LOSS OF SERVICE WATER - NSW Loss of Nonvital Bus Supplying Power EXCHANGERS IE PUMPS IE PUMPS IE to NSW and SWC Pumps IEFT-LONSW-HX IEFT-LONSW-SWC IEFT-LONSW-NSW IEFT-LONSW-NONVITAL Ignore CCF OF ALL SWC HTXs (IE per year) LOSS OF SERVICE WATER - SWC LOSS OF SERVICE WATER - NSW PUMP A IE PUMP A IE IE-SWC-HTX-CF-ALL 2.07E-06 IEFT-LONSW-SWC-1 Ext IEFT-LONSW-NSW-1 Ext CCF OF ALL SWC HTXs (IE per year, LOSS OF SERVICE WATER - SWC LOSS OF SERVICE WATER - NSW environmental) PUMP B IE PUMP B IE IE-SWC-HTX-CF-ALLE 5.71E-08 IEFT-LONSW-SWC-2 Ext IEFT-LONSW-NSW-2 Ext LOSS OF SERVICE WATER - SWC LOSS OF SERVICE WATER - NSW PUMP C IE PUMP C IE IEFT-LONSW-SWC-3 Ext IEFT-LONSW-NSW-3 Ext COMMON CAUSE SWC PUMP IE COMMON CAUSE NSW PUMP IE IEFT-LONSW-SWC-4 Ext IEFT-LONSW-NSW-4 Ext Figure B-1. Modified IE-LONSW Fault Tree.

B-1

LER 458/12-003 LOSS OF NORMAL SERVICE LOSS OF NORMAL SERVICE REACTOR SHUTDOWN SRV'S CLOSE HPCS RCIC SUPPRESSION POOL MANUAL REACTOR CRD INJECTION (2 PUMPS) LOW PRESSURE INJECTION ALTERNATE LOW PRESSURE SUPPRESSION POOL MANUAL REACTOR SHUTDOWN COOLING CONTAINMENT HEAT SERVICE WATER SYSTEM POWER CONVERSION CONTAINMENT VENTING LATE INJECTION # End State WATER WATER (IE FAULT TREE) COOLING DEPRESS (LPCS OR LPCI) INJECTION COOLING DEPRESS REMOVAL (FAN COOLERS) RECOVERY SYSTEM RECOVERY (Phase - CD)

IE-LONSW IEFT-LONSW RPS SRV HCS RCI SPC DEP CRD LPI VA SPC DEP SDC CHR SWSR PCSR CVS LI 1 OK 2 OK 3 OK 4 OK 5 OK 6 OK LI00 7 CD 8 OK CVS01 LI01 9 CD 10 CD 11 OK 12 OK 13 OK LI00 14 CD 15 OK CVS01 LI01 16 CD 17 CD 18 OK 19 OK 20 OK 21 OK 22 OK LI00 23 CD 24 OK CVS01 LI01 25 CD 26 CD 27 OK 28 OK 29 OK 30 OK LI00 31 CD 32 OK CVS01 LI01 33 CD 34 CD 35 CD 36 OK 37 OK 38 OK LI00 39 CD 40 OK CVS01 LI01 41 CD 42 CD 43 CD 44 OK 45 OK 46 OK 47 OK 48 OK LI00 49 CD 50 OK CVS01 LI01 51 CD 52 CD 53 OK 54 OK 55 OK 56 OK 57 OK SPC1 SDC1 LI00 58 CD 59 OK CVS01 LI01 60 CD 61 CD 62 CD 63 CD P1 64 LONSW-1 P2 65 LONSW-2 66 ATWS Figure B-2. Loss of NSW event tree.

B-2

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