ML15238B710

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Final ASP Analysis-Calvert Cliffs 2 (LER 318-14-001)
ML15238B710
Person / Time
Site: Calvert Cliffs Constellation icon.png
Issue date: 09/11/2015
From: Correia R
NRC/RES/DRA
To: Louise Lund
Office of Nuclear Reactor Regulation
Tetter Keith 301-415-2407
Shared Package
ML15238B535 List:
References
LER-318-2014-001
Download: ML15238B710 (9)
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Final Precursor Analysis Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Calvert Cliffs, Reactor Trip Due to Loss of 13 kV Service Bus 21 and Subsequent Unit 2 Partial Loss of Offsite Power LER: 318/14-001 Event Date: 1/21/2014 CCDP = 5x10-6 IRs: 50-318/14-004 & 50-318/14-008 Plant Type: Combustion Engineering Generation II two-loop PWR with Dry Ambient Pressure Containment Plant Operating Mode (Reactor Power Level): Mode 1 (100 Percent Reactor Power)

Analyst: Gary Wang Reviewer: Chris Hunter Contributors: Song-hua Shen EXECUTIVE

SUMMARY

A reactor trip occurred at the Calvert Cliffs Nuclear Power Plant Unit 2 on January 21, 2014, at 9:25 pm. The feeder breaker to the Service Bus 21 tripped that resulted in a partial loss of offsite power. The preliminary cause of the loss of the Service Bus 21 is the weather-related impacts of ice and snow that caused a fault on its associated ground work.

A loss of the Service Bus 21 resulted in a loss of Unit 2 plant equipment including the motor generator sets for the control element drive mechanisms (CEDM), circulating water pumps, and main feedwater (MFW) pumps. The loss of the Service Bus 21 also caused a loss of a single safety-related 4kV bus on both units, resulting in an emergency diesel generator (EDG) starting on each unit to supply power to its respective 4kV bus. The Unit 2 scram is considered complicated since the normal heat sink was lost and was not recoverable; however, the atmospheric dump valves (ADVs) functioned as designed.

According to the analysis modeling assumptions, the most likely core damage sequence for this initiating event at Unit 2 is a non-recoverable loss of the 13kV Bus 21 resulting in a reactor trip, followed by subsequent failures of the auxiliary feedwater (AFW) and once-through cooling (OTC). This accident sequence accounts for approximately 58 percent of the event conditional core damage probability (CCDP) for Unit 2. This event was determined to be a precursor with the CCDP for this event being 5x10-6.

EVENT DETAILS Event Description. On January 21, 2014, at 9:25 pm, the Calvert Cliffs Nuclear Power Plant Unit 1 and Unit 2 reactors automatically scrammed from full power operation. The scram occurred as a result of a partial loss of offsite power (LOOP) caused by a ground fault on 13kV Service Bus 21 Feeder Breaker 252-2104. The ground fault resulted in a loss of power to 13kV Service Bus 21. Service Bus 11 was unaffected.

The normal electrical alignment has the 13kV Service Bus 21 powering the Unit 2 safety-related 4kV Bus 24 and non-safety-related 4kV Buses 22, 23, 25, and 26. Loss of power to safety-related 4kV Bus 24 resulted in a loss of power to the motor control sets that led to an automatic reactor trip as expected. The loss of non-safety-related 4kV buses resulted in a loss of power to the circulating water, condensate, condensate booster, and condenser air removal pumps 1

LER 318/14-001 leading to a loss of main condenser vacuum. As a result, the operating crew maintained RCS temperature using the main steam atmospheric dump valves (ADVs) and auxiliary feedwater (AFW) [secondary side once through cooling (OTC)]. The turbine-driven and motor-driven AFW pumps started automatically, as designed, on low steam generator water level and operated properly. The loss of normal power to safety-related 4kV Bus 24 caused an automatic start of the EDG 2B to restore power to Bus 24, as designed. The loss of 4kV Bus 24 also resulted in 120 volt instrument Bus 2Y 10 being de-energized causing the loss of letdown flow through the Chemical and Volume Control System (CVCS). All other safety-related equipment operated as designed. Additional information is provided in Licensee Event Report (LER) 318/14-001 (Reference 1), NRC Integrated Inspection Report (IR) 50-318/2014-04 (Reference 2), and NRC Special IR 50-318/2014-08 (Reference 3).

Cause. The direct cause of the event was the loss of power to non-safety-related 13kV Service Bus 21 due to water intrusion. An air filter assembly located at the back of Breaker 252-2104 cubicle became dislodged during a snow storm. The dislodged air filter assembly allowed snow to enter the cubicle that subsequently melted causing the ground fault.

MODELING ASSUMPTIONS Analysis Type. The Calvert Cliffs Unit 2 SPAR Model Revision 8.23, dated May 2014, was used for this event analysis.

SDP Results/Basis for ASP Analysis. The inspectors reviewed this issue and did not identify any performance deficiencies. The LER is closed. For more details, please see Reference 2 and Reference 3.

The SDP assesses the risk significance of individual inspection findings considering only those systems, structures, and components (SSCs) directly associated with the licensee performance deficiency. The ASP Program assesses the risk significance of operational events considering all SSC failures and unavailabilities (e.g., equipment out for test and maintenance), regardless of whether performance deficiencies were identified. The ASP Program uses SDP results for degraded conditions when possible; however, an independent ASP analysis is performed for initiating events. An ASP analysis was performed for this event because an SDP analysis was not performed, no performance deficiencies were identified, and it is an initiating event.

Fault Tree Modification. The OTC Fault Tree contains unnecessary AFW logic intended to parse the power operated relief valve (PORV) success criteria.1 The SPAR model success criteria already uses the limiting criteria of 2 of 2 PORVs, as such, Transfer Gates AFW-FCNTRL (AFW Flow Control Available) and LPF-OPER (Operator Fails to Establish Low-Pressure Feedwater in the Short and Long Term), as well as Gate OTC-4 (OPs Fails Long Term AFW Flow) and Basic Event AFW-SYS-FC-AP2311 (Global Failure of Decay Heat Removal when not in EOP-8 (PSA)) have been eliminated. The modified OTC Fault Tree is shown in Figure B-1 in Appendix B.

1 The licensee PORV success criteria for OTC is 1 of 2 PORVs given successful AFW flow for a certain period of time.

2

LER 318/14-001 Key Modeling Assumptions. The following modeling assumptions were determined to be significant to the modeling of this event analysis:

The initiating event IE-LOFACB (Loss of 13KV AC BUS 21) was set to 1.0, and all other initiating event probabilities were set to zero.

ANALYSIS RESULTS CCDP. As a result of SPAR model analyses, the CCDP for Calvert Cliffs Unit 2 is 5.0x10-6, which is greater than the ASP Program acceptance threshold of 1x10-6 or the CCDP equivalent of an uncomplicated reactor trip with a non-recoverable loss of feedwater and condenser heat sink (for Calvert Cliffs Unit 2 this CCDP equivalent is 1.6x10-6), whichever is greater. Therefore, this event is a precursor.

Dominant Sequence. The dominant accident sequence is Sequence LOFACB 15 (CCDP =

2.9 x10-6) that contributes approximately 58 percent of the total internal events CCDP. The cut sets and sequences that contribute to the top 95 percent and/or at least 1 percent of the total internal events CCDP are provided in Appendix A.

The dominant sequence is shown graphically in Figure B-2 in Appendix B. The events and important component failures in Sequence LOFACB 15 are:

  • Loss of facility power
  • Once through cooling fails REFERENCES
1. Constellation Energy, "LER 318/14-001- Calvert Cliffs Nuclear Power Plant, Unit 2 -

Reactor Trip Due to Inadequate Protection Against Weather Related Water Intrusion, dated March 20, 2014 (ML14080A309).

2. U.S. Nuclear Regulatory Commission, Calvert Cliffs Nuclear Power Plant - NRC Integrated Inspection Report 05000317/2014004 and 05000318/2014004 and Independent Spent Fuel Storage Inspection report 07200008/2014001, dated October 27, 2014 (ML14301A010).
3. U.S. Nuclear Regulatory Commission, Calvert Cliffs Nuclear Power Plant Unit 1 and 2 -

NRC Special Inspection Report 05000317/2014008 and 05000318/2014008, dated March 13, 2014 (ML14072A470).

3

LER 318/14-001 Appendix A: Unit 2 Analysis Results Summary of Conditional Event Changes Event Description Cond. Value Nominal Value IE-LOFACB LOSS OF 13KV AC BUS 21 1.00E+0a 9.21E-2

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 LOFACB 15 2.93E-6 58.3% IEFT-LOFACB, /RPS, SGC, OTC LOFACB 16-09 1.23E-6 24.5% IEFT-LOFACB, RPS, /RCSPRESS, /SGC-A, /BORATION, PORV-A, HPI LOFACB 03-9-4 3.71E-7 7.4% IEFT-LOFACB, /RPS, /SGC, /PORV, LOSC-F, RCPT, HPI LOFACB 16-12 1.54E-7 3.1% IEFT-LOFACB, RPS, RCSPRESS LOFACB 13 1.35E-7 2.7% IEFT-LOFACB, /RPS, SGC, /OTC, /HPR, CSR LOFACB 16-10 6.99E-8 1.4% IEFT-LOFACB, RPS, /RCSPRESS, /SGC-A, BORATION LOFACB 03-8-4 6.37E-8 1.3% IEFT-LOFACB, /RPS, /SGC, /PORV, LOSC-F, /RCPT, CBO, RSUB, RCPSI04, HPI Total 5.03E-6 100%

Referenced Fault Trees Fault Tree Description BORATION EMERGENCY BORATION CBO CONTROLLED BLEEDOFF ISOLATED CSR CONTAINMENT COOLING HPI HIGH PRESSURE INJECTION IEFT-LOFACB LOSS OF FACILITY B POWER LOSC-F RCP SEAL INTEGRITY MAINTAINED OTC ONCE THROUGH COOLING PORV-A PORVs AND SRVs ARE CLOSED RCPSI04 RCP SEALS FROM LOSS OF ALL COOLING RCPT REACTOR COOLANT PUMPS TRIPPED RCSPRESS RCS PRESSURE LIMITED RPS REACTOR TRIP RSUB REACTOR COOLANT SUBCOOLING MAINTAINED SGC STEAM GENERATOR COOLING (AFW or MFW) 4

LER 318/14-001 Cut Set Report - LOFACB 15

  1. CCDP Total % Cut Set 2.93E-6 100 Displaying 14284 Cut Sets. (14284 Original) 1 2.40E-7 8.20 IE-LOFACB,AFW-XHE-XM-HXNORM,AFW-XHE-XM-UQ,HPI-XHE-XM-OTC 2 1.24E-7 4.25 IE-LOFACB,ACP-XHE-XM-M2A,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFOC 3 8.40E-8 2.87 IE-LOFACB,AFW-XHE-XM-HXNORM,AFW-XHE-XM-UQ,PPR-SRV-CC-2ERV402 4 8.40E-8 2.87 IE-LOFACB,AFW-XHE-XM-HXNORM,AFW-XHE-XM-UQ,PPR-SRV-CC-2ERV404 5 6.22E-8 2.12 IE-LOFACB,ACP-XHE-XM-M2A,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFOB 6 5.29E-8 1.81 IE-LOFACB,ACP-XHE-XM-M2A,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,EPS-DGN-FR-2B,EPS-DGN-TM-1B,EPS-XHE-XE-DG0C45,IAS-XHE-XM-2CAFOB 7 5.29E-8 1.81 IE-LOFACB,ACP-XHE-XM-M2A,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,EPS-DGN-FR-1B,EPS-DGN-TM-2B,EPS-XHE-XE-DG0C45,IAS-XHE-XM-2CAFOB 8 3.08E-8 1.05 IE-LOFACB,ACP-XHE-XM-M2A,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,EPS-DGN-TM-1B,EPS-XHE-XE-DG0C45,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-22 9 2.96E-8 1.01 IE-LOFACB,ACP-XHE-XM-M2A,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,EPS-DGN-FR-1B,EPS-DGN-FR-2B,EPS-DGN-FR-DG0C,IAS-XHE-XM-2CAFOB Cut Set Report - LOFACB 16-09
  1. CCDP Total % Cut Set Total 1.23E-6 100 Displaying 12 Cut Sets. (12 Original) 1 1.60E-7 12.99 IE-LOFACB,PPR-SRV-OO-2RV200LIQ,RPS-RTB-FC-FTO 2 1.60E-7 12.99 IE-LOFACB,PPR-SRV-OO-2RV201LIQ,RPS-RTB-FC-FTO 3 1.60E-7 12.99 IE-LOFACB,PPR-SRV-OO-2ERV404LIQ,RPS-RTB-FC-FTO 4 1.60E-7 12.99 IE-LOFACB,PPR-SRV-OO-2ERV402LIQ,RPS-RTB-FC-FTO 5 1.20E-7 9.74 IE-LOFACB,PPR-SRV-OO-2RV200LIQ,RPS-VCF-FO-MECH 6 1.20E-7 9.74 IE-LOFACB,PPR-SRV-OO-2RV201LIQ,RPS-VCF-FO-MECH 7 1.20E-7 9.74 IE-LOFACB,PPR-SRV-OO-2ERV404LIQ,RPS-VCF-FO-MECH 8 1.20E-7 9.74 IE-LOFACB,PPR-SRV-OO-2ERV402LIQ,RPS-VCF-FO-MECH 9 2.80E-8 2.27 IE-LOFACB,PPR-SRV-OO-2RV200LIQ,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 10 2.80E-8 2.27 IE-LOFACB,PPR-SRV-OO-2RV201LIQ,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 11 2.80E-8 2.27 IE-LOFACB,PPR-SRV-OO-2ERV404LIQ,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 12 2.80E-8 2.27 IE-LOFACB,PPR-SRV-OO-2ERV402LIQ,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM Cut Set Report - LOFACB 03-9-4
  1. CCDP Total % Cut Set 3.71E-7 100 Displaying 1542 Cut Sets. (1542 Original) 1 8.25E-8 22.22 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XM-KBI,RCP-XHE-XM-TRIP,SWS-VCF-TM-22 2 8.25E-8 22.22 IE-LOFACB,CCW-HX22STBY-21OP,CCW-XHE-XM-KBI,RCP-XHE-XM-TRIP,SWS-VCF-TM-21 3 1.03E-8 2.78 IE-LOFACB,CCW-HTX-TM-21,CCW-HX21STBY-22OP,RCP-XHE-XM-TRIP,SWS-VCF-TM-22 4 1.03E-8 2.78 IE-LOFACB,CCW-HTX-TM-22,CCW-HX22STBY-21OP,RCP-XHE-XM-TRIP,SWS-VCF-TM-21 5 5.76E-9 1.55 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XM-KBI,EPS-DGN-TM-2B,EPS-XHE-XE-DG0C45,RCP-XHE-XM-TRIP,SWS-FLAG-23SB22IS 6 4.13E-9 1.11 IE-LOFACB,CCW-HX22STBY-21OP,CCW-XHE-XR-HTX22,RCP-XHE-XM-TRIP,SWS-VCF-TM-21 7 4.13E-9 1.11 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XR-HTX21,RCP-XHE-XM-TRIP,SWS-VCF-TM-22 5

LER 318/14-001 Cut Set Report - LOFACB 16-12

  1. CCDP Total % Cut Set 1.54E-7 100 Displaying 145 Cut Sets. (145 Original) 1 2.24E-8 14.52 IE-LOFACB,RCS-PHN-MODPOOR,RPS-RTB-FC-FTO 2 1.68E-8 10.89 IE-LOFACB,RCS-PHN-MODPOOR,RPS-VCF-FO-MECH 3 1.28E-8 8.30 IE-LOFACB,PPR-SRV-CC-2RV200,RPS-RTB-FC-FTO 4 1.28E-8 8.30 IE-LOFACB,PPR-SRV-CC-2RV201,RPS-RTB-FC-FTO 5 1.12E-8 7.26 IE-LOFACB,PPR-SRV-CC-2ERV404,RPS-RTB-FC-FTO 6 1.12E-8 7.26 IE-LOFACB,PPR-SRV-CC-2ERV402,RPS-RTB-FC-FTO 7 9.60E-9 6.22 IE-LOFACB,PPR-SRV-CC-2RV200,RPS-VCF-FO-MECH 8 9.60E-9 6.22 IE-LOFACB,PPR-SRV-CC-2RV201,RPS-VCF-FO-MECH 9 8.40E-9 5.45 IE-LOFACB,PPR-SRV-CC-2ERV404,RPS-VCF-FO-MECH 10 8.40E-9 5.45 IE-LOFACB,PPR-SRV-CC-2ERV402,RPS-VCF-FO-MECH 11 3.92E-9 2.54 IE-LOFACB,RCS-PHN-MODPOOR,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 12 2.30E-9 1.49 IE-LOFACB,EPS-DGN-TM-2B,EPS-XHE-XE-DG0C45,RPS-RTB-FC-FTO 13 2.24E-9 1.45 IE-LOFACB,PPR-SRV-CC-2RV200,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 14 2.24E-9 1.45 IE-LOFACB,PPR-SRV-CC-2RV201,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 15 1.96E-9 1.27 IE-LOFACB,PPR-SRV-CC-2ERV404,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 16 1.96E-9 1.27 IE-LOFACB,PPR-SRV-CC-2ERV402,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 17 1.73E-9 1.12 IE-LOFACB,EPS-DGN-TM-2B,EPS-XHE-XE-DG0C45,RPS-VCF-FO-MECH Cut Set Report - LOFACB 13
  1. CCDP Total % Cut Set 1.35E-7 100 Displaying 1193 Cut Sets. (1193 Original) 1 8.55E-9 6.34 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFOC,SWS-VCF-TM-21 2 8.55E-9 6.34 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFOC,SWS-VCF-TM-21 3 4.28E-9 3.17 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 4 4.28E-9 3.17 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 5 3.64E-9 2.70 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,EPS-DGN-FR-2B,EPS-DGN-TM-1B,EPS-XHE-XE-DG0C45,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 6 3.64E-9 2.70 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,EPS-DGN-FR-2B,EPS-DGN-TM-1B,EPS-XHE-XE-DG0C45,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 7 2.03E-9 1.51 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,EPS-DGN-FR-1B,EPS-DGN-FR-2B,EPS-DGN-FR-DG0C,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 8 2.03E-9 1.51 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,EPS-DGN-FR-1B,EPS-DGN-FR-2B,EPS-DGN-FR-DG0C,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 9 1.71E-9 1.27 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,DCP-BDC-LP-21,IAS-XHE-XM-2CAFOC,SWS-VCF-TM-21 10 1.71E-9 1.27 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,DCP-BDC-LP-21,IAS-XHE-XM-2CAFOC,SWS-VCF-TM-21 11 1.71E-9 1.27 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFO3,SWS-VCF-TM-21 12 1.71E-9 1.27 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,DCP-BAT-LP-BATT21,IAS-XHE-XM-2CAFO3,SWS-VCF-TM-21 13 1.65E-9 1.22 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX21STBY-22OP,EPS-DGN-CF-ALL5FR,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 14 1.65E-9 1.22 IE-LOFACB,AFW-XHE-XM-F1Q,AFW-XHE-XM-HX,CCW-HX22STBY-21OP,EPS-DGN-CF-ALL5FR,IAS-XHE-XM-2CAFOB,SWS-VCF-TM-21 6

LER 318/14-001 Cut Set Report - LOFACB 16-10

  1. CCDP Total % Cut Set 6.98E-8 100 Displaying 147 Cut Sets. (147 Original) 1 3.20E-8 45.82 IE-LOFACB,CVC-XHE-XM-BOR,RPS-RTB-FC-FTO 2 2.40E-8 34.36 IE-LOFACB,CVC-XHE-XM-BOR,RPS-VCF-FO-MECH 3 5.60E-9 8.02 IE-LOFACB,CVC-XHE-XM-BOR,RPS-VCF-FO-ELEC,RPS-XHE-XM-SCRAM 4 7.92E-10 1.13 IE-LOFACB,RCS-PHN-RPS0KI1432-1,RPS-RTB-FC-FTO,SWS-VCF-TM-22 Cut Set Report - LOFACB 03-8-4
  1. CCDP Total % Cut Set 6.37E-8 100 Displaying 216 Cut Sets. (216 Original) 1 2.10E-8 32.88 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XM-KBI,RCS-MDP-LK-SEALS04,SWS-VCF-TM-22 2 2.10E-8 32.88 IE-LOFACB,CCW-HX22STBY-21OP,CCW-XHE-XM-KBI,RCS-MDP-LK-SEALS04,SWS-VCF-TM-21 3 2.62E-9 4.11 IE-LOFACB,CCW-HTX-TM-22,CCW-HX22STBY-21OP,RCS-MDP-LK-SEALS04,SWS-VCF-TM-21 4 2.62E-9 4.11 IE-LOFACB,CCW-HTX-TM-21,CCW-HX21STBY-22OP,RCS-MDP-LK-SEALS04,SWS-VCF-TM-22 5 1.46E-9 2.30 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XM-KBI,EPS-DGN-TM-2B,EPS-XHE-XE-DG0C45,RCS-MDP-LK-SEALS04,SWS-FLAG-23SB22IS 6 1.05E-9 1.64 IE-LOFACB,CCW-HX22STBY-21OP,CCW-XHE-XR-HTX22,RCS-MDP-LK-SEALS04,SWS-VCF-TM-21 7 1.05E-9 1.64 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XR-HTX21,RCS-MDP-LK-SEALS04,SWS-VCF-TM-22 8 8.18E-10 1.28 IE-LOFACB,CCW-HX21STBY-22OP,CCW-XHE-XM-KBI,EPS-DGN-FR-2B,EPS-DGN-FR-DG0C,RCS-MDP-LK-SEALS04,SWS-FLAG-23SB22IS 9 6.75E-10 1.06 IE-LOFACB,CCW-HX22STBY-21OP,EPS-DGN-FR-2B,EPS-DGN-FR-DG0C,RCS-MDP-LK-SEALS04,SWS-FLAG-23SB22IS,SWS-VCF-TM-21 10 6.75E-10 1.06 IE-LOFACB,CCW-HX21STBY-22OP,EPS-DGN-FR-2B,EPS-DGN-FR-DG0C,RCS-MDP-LK-SEALS04,SWS-FLAG-23SB22IS,SWS-VCF-TM-21 Referenced Events Event Description Probability OPERATIONS FAILS TO CROSS-CONNECT THE REACTOR MCCS WITHIN ACP-XHE-XM-M2A 1.20E-1 1HR AFW-XHE-XM-F1Q OPERATIONS FAILS TO RECOVER AFW FROM AN SG OVERFILL 3.00E-1 AFW-XHE-XM-HX OP FAILS TO CONTROL AFW FLOW - SPAR 1.20E-1 AFW-XHE-XM-HXNORM OP FAILS TO CONTROL AFW FLOW - SPAR 1.00E-3 OPS ERRONEOUSLY UNDERFILLS S/G DURING LOSS OF FLOW AFW-XHE-XM-UQ 1.20E-2 CONTROL CCW-HTX-TM-21 CCW HEAT EXCHANGER 21 IN T & M 2.50E-3 CCW-HTX-TM-22 CCW HTX-22 UNAVAILABLE DUE TO T & M 2.50E-3 CCW-HX21STBY-22OP CCW HTX-21 IN STANDBY CCW HTX-22 IN OPERATION 5.00E-1 CCW-HX22STBY-21OP CCW HTX-22 IN STANDBY CCW HTX-21 IN OPERATION 5.00E-1 OPS FAILS TO PLACE STANDBY CCW HX IN SERVICE W/IN 10 MINUTES CCW-XHE-XM-KBI 2.00E-2 OF HX FAILURE PRIOR TO TRIP CCW-XHE-XR-HTX21 OPERATOR FAILS TO RESTORE CCW HTX-21 AFTER T & M 1.00E-3 CCW-XHE-XR-HTX22 OPERATOR FAILS TO RESTORE CCW HTX-22 AFTER T & M 1.00E-3 OPS FAILS TO EMERGENCY BORATE W/IN 5 MINS FOLLOWING ATWS CVC-XHE-XM-BOR 2.00E-2 WHERE ALL CEAS FAIL TO DROP DCP-BAT-LP-BATT21 BATTERY (DC) FAIL TO OPERATE 4.80E-5 DCP-BDC-LP-21 BUS FAILS TO OPERATE 9.60E-6 EPS-DGN-CF-ALL5FR CCF OF ALL FIVE DIESEL GENERATORS TO RUN 1.85E-5 EPS-DGN-FR-1B DIESEL GENERATOR 1B FAILS TO RUN 2.84E-2 EPS-DGN-FR-2B DIESEL GENERATOR 2B FAILS TO RUN 2.84E-2 EPS-DGN-FR-DG0C SBO DIESEL GENERATOR DG0C FAILS TO RUN 2.84E-2 EPS-DGN-TM-1B DIESEL GENERATOR 1B UNAVAILABLE DUE TO T & M 1.20E-2 7

LER 318/14-001 Event Description Probability DIESEL GENERATOR 2A UNAVAILABLE DUE TO T & M DIESEL EPS-DGN-TM-2B 1.20E-2 GENERATOR 2B UNAVAILABLE DUE TO T & M EPS-XHE-XE-DG0C45 OPERATIONS ALIGN THE 0C DG IN 45 MINS 1.20E-1 HPI-XHE-XM-OTC OPERATOR FAILS TO INITIATE FEED AND BLEED COOLING 2.00E-2 OP FAILS TO ALIGN U2SWAC & U2AFW GIVEN U1SWAC OK, NO U2 IAS-XHE-XM-2CAFO3 1.20E-1 ANNUN, U2AFW FLOW CONT AVAIL OP FAILS TO ALIGN U1SWAC & U1AFW GIVEN U2 SWACS FAIL, U1 IAS-XHE-XM-2CAFOB 3.00E-1 ANNUN & U1 AFW FLOW CNT OK OP FAILS TO ALIGN U1SWAC & AFW GIVEN U2 SWAC OK NO U1 ANNUN IAS-XHE-XM-2CAFOC 6.00E-1 NO U1AFW FLOW CONTROL IE-LOFACB LOSS OF 13KV AC BUS 21 1.00E+0 PPR-SRV-CC-2ERV402 2ERV-402 FAILS TO OPEN ON DEMAND 7.00E-3 PPR-SRV-CC-2ERV404 2ERV-404 FAILS TO OPEN ON DEMAND 7.00E-3 PPR-SRV-CC-2RV200 FAILURE OF SRV 2RV-200 TO OPEN 8.00E-3 PPR-SRV-CC-2RV201 FAILURE OF SRV 2RV-201 TO OPEN 8.00E-3 PPR-SRV-OO-2ERV402LIQ 2ERV 402 FAILS TO RECLOSE AFTER PASSING LIQUID 1.00E-1 PPR-SRV-OO-2ERV404LIQ 2ERV 2 FAILS TO RECLOSE AFTER PASSING LIQUID 1.00E-1 PPR-SRV-OO-2RV200LIQ SRV 2RV-200 FAILS TO RECLOSE AFTER PASSING LIQUID 1.00E-1 PPR-SRV-OO-2RV201LIQ SRV RV-201 FAILS TO RECLOSE AFTER PASSING LIQUID 1.00E-1 OPERATIONS FAILS TO TRIP THE RCPS WHEN THE CCW PUMPS WERE RCP-XHE-XM-TRIP 5.00E-4 NOT AVAIL FOR RECOVERY - ASA - U2 TRIP NEOP8 RCS-MDP-LK-SEALS04 RCP SEALS FAIL W/O COOLING AND INJECTION 1.27E-4 RCS-PHN-MODPOOR MODERATOR TEMP COEFFICIENT NOT ENOUGH NEGATIVE 1.40E-2 FRACTION OF THE CYCLE OPERATIONS CAN NOT MITIGATE AN ATWS RCS-PHN-RPS0KI1432-1 3.00E-2 WHEN THE TBVS DO NOT QUICK OPEN (PSA)

RPS-RTB-FC-FTO TRIP CIRCUIT BREAKERS FAIL TO OPEN 1.60E-6 ELECTRICAL (UV & ST) RPS FAILURE TO OPEN TRIP CIRCUIT RPS-VCF-FO-ELEC 1.40E-5 BREAKERS RPS-VCF-FO-MECH CONTROL ROD ASSEMBLIES FAIL TO INSERT 1.20E-6 OPS FAILS TO MANUALLY TRIP THE RX WITHIN 5 MINUTES FOLLOWING RPS-XHE-XM-SCRAM 2.00E-2 THE FAILURE OF RPS AND DSS SWS-FLAG-23SB22IS SW PUMP22 IN-SERVICE WITH 23 IN STAND-BY (PSA) 8.00E-1 SWS-VCF-TM-21 SW HDR 21 IS UNAVAILABLE WHEN UNIT 1 IS AT-POWER (PSA) 1.65E-2 SWS-VCF-TM-22 SW HDR 22 IS UNAVAILABLE WHEN UNIT 2 IS AT-POWER (PSA) 1.65E-2 8

LER 318/14-001 Appendix B: Modified Fault Tree and Key Event Tree ONCE THROUGH COOLING OTC HIGH PRESSURE INJECTION Ops fails to establish Unit 2 Ops fails to establish Unit 1 Ops fails to establish Unit 1 LOW PRESSURE FEED IS Ops fails to establish Unit 1 Once Through Core Cooling OTCC and Open PORVs given OTCC and Open PORVs LATE UNAVAILABLE OTCC and Open PORVs EARLY TLOFW at Rx Trip (six hours) after TLOFW (one hour) after TLOFW HPI Ext OTC-0 OTC-1 OTC-2 OTC-5 OTC-3 CALVERT CLIFFS 1 & 2 PWR G BLEED PORTION OF FAB BLEED Ext Ops fails to establish Unit 2 Once Through Core Cooling and Open PORVs OTC-17 Int PRESSURIZED THERMAL SHOCK; UNIT 1 RX VESSELS FAILS TO REMAIN INTACT PTS Ext Figure B Modified OTC Fault Tree LOSS OF 13KV AC BUS Loss fo Facility B Power REACTOR TRIP STEAM GENERATOR PORVs ARE ISOLATED RCP SEAL INTEGRITY HIGH PRESSURE ONCE THROUGH SECONDARY SIDE SHUTDOWN COOLING HIGH PRESSURE CONTAINMENT # End State 21 COOLING (AFW or MFW) MAINTAINED INJECTION COOLING COOLDOWN RECIRC COOLING (Phase - CD)

IE-LOFACB IEFT-LOFACB RPS SGC PORV LOSC-F LOSC-LOFACB-FTF HPI LOOP-RECOVERY OTC LOOP-RECOVERY SSC SDC HPR LOOP-RECOVERY CSR LOOP-RECOVERY 1 @

2 OK 3 LOSC 4 OK 5 OK 6 CD 7 CD 8 OK 9 CD 10 CD 11 CD 12 OK 13 CD 14 CD 15 CD 16 ATWS Figure B LOFACB (Loss of 13kV Bus 21) Event Tree 9

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