ML112411361
| ML112411361 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 09/23/2011 |
| From: | NRC/RES/DRA |
| To: | |
| Hunter C, 251-7575 RES/DRA | |
| References | |
| IR-10-004, IR-10-008, LER-10-003-01 | |
| Download: ML112411361 (7) | |
Text
1 Final Precursor Analysis Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Susquehanna, Unit 1 Manual Reactor Scram due to Leakage from the Circulating Water System and Subsequent Flooding of the Condenser Bay Event Date: 07/16/2010 LER: 387/10-003-01 IRs: 50-387/10-04, 50-387/10-08 CDP = 4x10-6 EVENT
SUMMARY
Brief Event Description. On July 16, 2010, at approximately 1520, Unit 1 received a condenser bay flood alarm. Plant operators verified that flooding was occurring into the 656' elevation of the condenser bay. Reactor power was reduced to 40 percent via control rod insertions and a recirculation runback. Operator attempts to isolate condenser waterboxes remotely were unsuccessful. Unit 1 was subsequently manually scrammed, main steam isolation valves (MSIVs) were shut, and the main condenser was isolated so that the circulating water (CW) system could be shutdown. Concurrently, plant operators manually closed waterbox isolation valves and isolated the leak.
Plant response following the manual reactor scram was not as expected. The integrated control system (ICS) feedwater level control (FWLC) is designed to switch to single element control on low main steam flow. Due to steam condensation and flashing on the flow instrument, measured main steam flow remained above the transition point and ICS FWLC remained in three element control. The effect of this was that while feedwater (FW) Pumps B and C automatically switched to the idle mode and the level setpoint set-down occurred as expected, FW Pump A underwent demand oscillations prior to its transition to discharge pressure mode.
Inventory continued to be added to the reactor vessel until level reached the high level turbine trip setpoint and peaked at 55 inches. Exceeding the setpoint resulted in a trip of all FW pump turbines, the high-pressure coolant injection (HPCI) turbine, the reactor core isolation cooling (RCIC) turbine, and the main turbine. It took approximately 14 minutes for reactor vessel water level to steam down less than the trip setpoint. Once level was restored below the setpoint, HPCI and RCIC were manually reinitiated for pressure and level control respectively.
Additional details are provided in References 1, 2, and 3.
Key Event Details. The following event details are significant to the modeling of this event analysis:
A reactor scram and subsequent loss of condenser heat sink occurred due to circulating water leak. Operators manually scrammed the plant and isolated the condenser by shutting the MSIVs and closed the waterbox isolation valves locally. No recovery of the condenser is credited in this analysis.
HPCI and RCIC both tripped on high reactor vessel water level. Operators could restore these two pumps to operation when reactor vessel water level dropped below the trip set point (operators were successful at restarting HPCI and RCIC during the event).
Additional Event Information. The following event details are provided as additional information about the event. This additional information was not factored in the modeling of this analysis due to the negligible risk impact.
In an attempt to dewater the turbine building, operations personnel transferred water from the condenser area to the condensate storage tank (CST) berm using temporary pumping equipment. However, the procedure provided no guidance as to a maximum level that should be transferred to the berm to limit interactions with other safety-related equipment. On July 17, 2010, the inspectors informed PPL that water was entering the buildings housing the B and 'D' Emergency Diesel Generators through conduit and a junction box (which contained instrumentation cables associated with suction transfer of HPCI and RCIC from the CST to the suppression pool). It was determined that the switches and associated unscheduled junction boxes in the berm had been submerged when water was transferred from the condenser area and that these switches and associated junction boxes were not qualified for submergence.
On July 20, 2010, RCIC swapped its suction from the CST to the suppression pool, with CST level at 24% (transfer should occur at a level of 7.5%). It was subsequently determined that the short-term failure mechanism of the level switches controlling the Technical Specifications (TS) required function was a simulated low-level condition (i.e.,
a fail-safe condition that ensures that the TS safety function is preserved).
ANALYSIS RESULTS Change in Core Damage Probability. The conditional core damage probability (CCDP) for this event is 3.7x10-6.
The Accident Sequence Precursor (ASP) Program acceptance threshold is a CCDP of 1x10-6 or the CCDP equivalent of an uncomplicated reactor trip with a non-recoverable loss of secondary plant systems (e.g., feed water and condensate), whichever is greater. This CCDP equivalent for Susquehanna, Unit 1 is 1.1x10-6.
Dominant Sequence. The dominant accident sequence, Loss of Condenser Heat Sink (LOCHS) 44 (CCDP = 3.3x10-6) contributes 87% 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 sequence is shown graphically in Figure B-1 in Appendix B. The events and important component failures in LOCHS Sequence 44 are:
Loss of condenser heat sink transient occurs, Reactor trip succeeds, Safety relief valves reclose (if challenged),
High-pressure injection (HPCI and RCIC) fails, and RCS depressurization fails.
SAPHIRE 8 Report. The SAPHIRE 8 Worksheets (Appendix A) provide the following:
Summary of conditional event changes, including base and change case probabilities/frequencies.
Event tree dominant results Dominant sequences (including CCDPs).
Sequence logic for all dominant sequences.
Referenced fault trees (including definitions).
Cutset report for each dominant sequence.
Referenced events (including definitions and probabilities for key basic events)
MODELING ASSUMPTIONS Analysis Type. The Revision 8.17 of the Susquehanna, Unit 1 Standardized Plant Analysis Risk (SPAR) Model created in September 2010 was used for this event analysis. This event was modeled as a loss of condenser heat sink initiating event with complications.
Analysis Rules. The ASP program uses Significance Determination Process results for degraded conditions when available. However, the ASP Program performs independent initiating event analysis when an initiator occurs.
Key Modeling Assumptions. The following modeling assumptions and associated basic event modifications were required for this event analysis:
The probability of IE-LOCHS (Initiating Event-Loss of Condenser Heat Sink) was set to 1.0; all other initiating event frequencies were set to zero.
The basic events HCI-MULTIPLE-INJECT (Probability of Multiple HPCI Injections) and RCI-RESTART (Restart of RCIC is Required) were set to TRUE because both the HPCI and RCIC pumps tripped automatically due high reactor vessel water level and were restarted manually by operators.
REFERENCES
- 1. Susquehanna Steam Electric Station, Unit 1, "LER 387/10-003-001 - Unit 1 Manual Reactor Scram due to Leakage from the Unit 1 Circulating Water System and Subsequent Flooding of the Unit 1 Condenser Bay, dated September 14, 2010.
- 2. U.S. Nuclear Regulatory Commission, Susquehanna Steam Electric Station - NRC Integrated Inspection Report 05000387/2010004 and 05000388/2010004; Preliminary White Finding, dated November 12, 2010
- 3. U.S. Nuclear Regulatory Commission, Susquehanna Steam Electric Station - NRC Inspection Report 05000387/2010008 and 05000388/2010008; Final Significance Determination of White Finding with Assessment Follow-up, dated December 16, 2010.
LER 387/10-003-01 A-1 Appendix A: SAPHIRE 8 Worksheets Summary of Conditional Event Changes Event Description Cond. Value Nominal Value HCI-MULTIPLE-INJECT PROBABILITY OF MULTIPLE HPCI INJECTIONS True 1.500E-1 IE-LOCHS LOSS OF CONDENSER HEAT SINKa 1.000E+0 2.000E-1 RCI-RESTART RESTART OF RCIC IS REQUIRED True 1.500E-1
- a.
All other initiating events frequencies were set to zero.
Dominant Sequence Results Only items contributing at least 1.0% to the total CCDP are displayed.
E V E N T T R E E S E Q U E N C E CCDP
% C O N T R I B U T I O N D E S C R I P T I O N LOCHS 44 3.270E-6 87.2%
/RPS, /SRV, HPI, DEP LOCHS 14 2.682E-7 7.2%
/RPS, /SRV, /HPI, SPC, /DEP, CDS, /LPI, CSS, PCSR, CVS, LI08 LOCHS 47-06-16 1.415E-7 3.8%
RPS, /PPR, /RRS, PCS1, /SLC, /NX, /TAF, DE2 Total 3.749E-6 100.0%
Referenced Fault Trees Fault Tree Description CDS CONDENSATE CSS CONTAINMENT SPRAY CVS CONTAINMENT VENTING DE2 MANUAL REACTOR DEPRESS DEP MANUAL REACTOR DEPRESS HPI HIGH PRESSURE INJECTION (HPCI or RCIC)
LI08 LATE INJECTION PCS1 POWER CONVERSION SYSTEM PCSR POWER CONVERSION SYSTEM RECOVERY RPS REACTOR SHUTDOWN SPC SUPPRESSION POOL COOLING Cutset Report - LOCHS 44 Only items contributing at least 1% to the total are displayed.
CCDP T O T A L%
C U T S E T 3.270E-6 100 Displaying 171 of 171 Cutsets.
1 1.200E-6 36.69 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-MOV-CC-IVFRO,HCI-XHE-XL-INJECT,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 2
6.000E-7 18.35 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-MOV-CC-IVFRO,HCI-XHE-XL-INJECT,RCI-TDP-TM-TRAIN 3
4.200E-7 12.84 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-MOV-CC-IVFRO,HCI-XHE-XL-INJECT,RCI-TDP-FS-TRAIN 4
2.461E-7 7.53 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-MOV-CC-IVFRO,HCI-XHE-XL-INJECT,RCI-TDP-FR-TRAIN 5
1.200E-7 3.67 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-TDP-TM-TRAIN,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 6
7.185E-8 2.2 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-XHE-XO-ERROR1,RCI-XHE-XO-ERROR 7
7.000E-8 2.14 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-TDP-FS-TRAIN,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 8
6.000E-8 1.83 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-MOV-CC-IVFRO,HCI-XHE-XL-
LER 387/10-003-01 A-2 CCDP T O T A L%
C U T S E T INJECT,RCI-MOV-CC-INJEC 9
6.000E-8 1.83 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-MOV-CC-IVFRO,HCI-XHE-XL-INJECT,RCI-XHE-XO-ERROR 10 4.570E-8 1.4 IE-LOCHS,DCP-BDC-CF-ALL 11 4.200E-8 1.28 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-TDP-TM-TRAIN,RCI-TDP-FS-TRAIN 12 4.102E-8 1.25 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-TDP-FR-TRAIN,RCI-TDP-FS-RSTRT,RCI-XHE-XL-RSTRT 13 3.500E-8 1.07 IE-LOCHS,ADS-XHE-XM-MDEPR,HCI-TDP-FS-TRAIN,RCI-TDP-TM-TRAIN Cutset Report - LOCHS 14 Only items contributing at least 1% to the total are displayed.
CCDP T O T A L%
C U T S E T 2.682E-7 100 Displaying 31 of 31 Cutsets.
1 2.500E-7 93.23 IE-LOCHS,CFAILED,CVS-XHE-XM-VENT,RHR-XHE-XM-ERROR 2
4.644E-9 1.73 IE-LOCHS,CFAILED,CVS-XHE-XM-VENT,RHR-MDP-CF-START 3
3.785E-9 1.41 IE-LOCHS,CFAILED,CVS-XHE-XM-VENT,RHR-MOV-CF-HXBPS Cutset Report - LOCHS 47-06-16 Only items contributing at least 1% to the total are displayed.
CCDP T O T A L%
C U T S E T 1.415E-7 100 Displaying 29 of 29 Cutsets.
1 1.700E-8 12.01 IE-LOCHS,ADS-XHE-XM-MDEPR2,RPS-SYS-FC-PSOVS 2
1.360E-8 9.61 IE-LOCHS,ADS-SRV-CC-VALV1,RPS-SYS-FC-PSOVS 3
1.360E-8 9.61 IE-LOCHS,ADS-SRV-CC-VALV2,RPS-SYS-FC-PSOVS 4
1.360E-8 9.61 IE-LOCHS,ADS-SRV-CC-VALV3,RPS-SYS-FC-PSOVS 5
1.360E-8 9.61 IE-LOCHS,ADS-SRV-CC-VALV4,RPS-SYS-FC-PSOVS 6
1.360E-8 9.61 IE-LOCHS,ADS-SRV-CC-VALV5,RPS-SYS-FC-PSOVS 7
1.360E-8 9.61 IE-LOCHS,ADS-SRV-CC-VALV6,RPS-SYS-FC-PSOVS 8
3.800E-9 2.68 IE-LOCHS,ADS-XHE-XM-MDEPR2,RPS-SYS-FC-RELAY 9
3.040E-9 2.15 IE-LOCHS,ADS-SRV-CC-VALV1,RPS-SYS-FC-RELAY 10 3.040E-9 2.15 IE-LOCHS,ADS-SRV-CC-VALV2,RPS-SYS-FC-RELAY 11 3.040E-9 2.15 IE-LOCHS,ADS-SRV-CC-VALV3,RPS-SYS-FC-RELAY 12 3.040E-9 2.15 IE-LOCHS,ADS-SRV-CC-VALV4,RPS-SYS-FC-RELAY 13 3.040E-9 2.15 IE-LOCHS,ADS-SRV-CC-VALV5,RPS-SYS-FC-RELAY 14 3.040E-9 2.15 IE-LOCHS,ADS-SRV-CC-VALV6,RPS-SYS-FC-RELAY 15 2.500E-9 1.77 IE-LOCHS,ADS-XHE-XM-MDEPR2,RPS-SYS-FC-CRD 16 2.000E-9 1.41 IE-LOCHS,ADS-SRV-CC-VALV1,RPS-SYS-FC-CRD 17 2.000E-9 1.41 IE-LOCHS,ADS-SRV-CC-VALV2,RPS-SYS-FC-CRD 18 2.000E-9 1.41 IE-LOCHS,ADS-SRV-CC-VALV3,RPS-SYS-FC-CRD 19 2.000E-9 1.41 IE-LOCHS,ADS-SRV-CC-VALV4,RPS-SYS-FC-CRD 20 2.000E-9 1.41 IE-LOCHS,ADS-SRV-CC-VALV5,RPS-SYS-FC-CRD 21 2.000E-9 1.41 IE-LOCHS,ADS-SRV-CC-VALV6,RPS-SYS-FC-CRD Referenced Events Event Description Probability ADS-SRV-CC-VALV1 ADS VALVE FAILS TO OPEN 8.000E-3 ADS-SRV-CC-VALV2 ADS VALVE FAILS TO OPEN 8.000E-3 ADS-SRV-CC-VALV3 ADS VALVE FAILS TO OPEN 8.000E-3
LER 387/10-003-01 A-3 Event Description Probability ADS-SRV-CC-VALV4 ADS VALVE FAILS TO OPEN 8.000E-3 ADS-SRV-CC-VALV5 ADS VALVE FAILS TO OPEN 8.000E-3 ADS-SRV-CC-VALV6 ADS VALVE FAILS TO OPEN 8.000E-3 ADS-XHE-XM-MDEPR OPERATOR FAILS TO MANUALLY DEPRESSURIZE THE REACTOR 5.000E-4 ADS-XHE-XM-MDEPR2 OPERATOR FAILS TO DEPRESSURIZE THE REACTOR (ATWS) 1.000E-2 CFAILED CONTAINMENT FAILURE CAUSES LOSS OF ALL INJECTION 5.000E-1 CVS-XHE-XM-VENT OPERATOR FAILS TO VENT CONTAINMENT 1.000E-3 DCP-BDC-CF-ALL 4-OF-4 125 VDC BUSES FAIL FROM COMMON CAUSE 4.570E-8 HCI-MOV-CC-IVFRO HPCI INJECTION VALVE FAILS TO REOPEN 1.500E-1 HCI-TDP-FR-TRAIN HPCI PUMP TRAIN FAILS TO RUN GIVEN IT STARTED 4.102E-3 HCI-TDP-FS-TRAIN HPCI PUMP FAILS TO START 7.000E-3 HCI-TDP-TM-TRAIN HPCI TRAIN IS UNAVAILABLE BECAUSE OF MAINTENANCE 1.200E-2 HCI-XHE-XL-INJECT OPERATOR FAILS TO RECOVER HPCI INJ. VALVE REOPENING 8.000E-1 HCI-XHE-XO-ERROR1 OPERATOR FAILS TO START/CONTROL HPCI INJECTION 1.437E-1 IE-LOCHS LOSS OF CONDENSER HEAT SINK 1.000E+0 RCI-MOV-CC-INJEC RCIC INJECTION VALVE CAUSES FAILURE TO START 1.000E-3 RCI-TDP-FR-TRAIN RCIC PUMP FAILS TO RUN GIVEN THAT IT STARTED 4.102E-3 RCI-TDP-FS-RSTRT RCIC FAILS TO RESTART GIVEN START AND SHORT-TERM RUN 8.000E-2 RCI-TDP-FS-TRAIN RCIC PUMP FAILS TO START 7.000E-3 RCI-TDP-TM-TRAIN RCIC TRAIN IS UNAVAILABLE BECAUSE OF MAINTENANCE 1.000E-2 RCI-XHE-XL-RSTRT OPERATOR FAILS TO RECOVER RCIC FAILURE TO RESTART 2.500E-1 RCI-XHE-XO-ERROR OPERATOR FAILS TO START/CONTROL RCIC INJECTION 1.000E-3 RHR-MDP-CF-START RHR PUMPS FAIL FROM COMMON CAUSE TO START 9.288E-6 RHR-MOV-CF-HXBPS RHR HTX BYPASS VALVES FAIL FROM COMMON CAUSE 7.570E-6 RHR-XHE-XM-ERROR OPERATOR FAILS TO START/CONTROL RHR 5.000E-4 RPS-SYS-FC-CRD CONTROL ROD DRIVE MECHANICAL FAILURE 2.500E-7 RPS-SYS-FC-PSOVS HCU SCRAM PILOT SOVS FAIL 1.700E-6 RPS-SYS-FC-RELAY TRIP SYSTEM RELAYS FAIL 3.800E-7
LER 387/10-003-01 B-1 Appendix B: Key Event Tree Figure B-1. Susquehanna, Unit 1 LOCHS event tree.
IE-LOCHS LOSS OF CONDENSER HEAT SINK RPS REACTOR SHUTDOWN SRV SRV'S CLOSE HPI-STT-FTF HPI HIGH PRESSURE INJECTION (HPCI or RCIC)
SPC SUPPRESSION POOL COOLING DEP MANUAL REACTOR DEPRESS CRD CRD INJECTION CDS CONDENSATE LPI LOW PRESSURE INJECTION (CS or LPCI)
VA ALTERNATE LOW PRESS INJECTION SPC SUPPRESSION POOL COOLING CSS CONTAINMENT SPRAY PCSR POWER CONVERSION SYSTEM RECOVERY CVS CONTAINMENT VENTING LI LATE INJECTION End State (Phase - CD) 1 OK LI09 2
CD 3
OK 4
OK 5
OK 6
CD 7
OK LI08 8
CD 9
OK 10 OK 11 OK 12 CD 13 OK LI08 14 CD 15 OK 16 OK 17 OK 18 CD 19 OK LI08 20 CD 21 CD 22 OK 23 OK 24 OK 25 CD 26 OK LI08 27 CD 28 CD 29 OK 30 OK 31 OK 32 OK 33 CD 34 OK LI08 35 CD 36 OK 37 OK 38 OK 39 OK 40 CD 41 OK LI08 42 CD 43 CD 44 CD P1 45 1SORV P2 46 2SORVS 47 ATWS