ML23104A013
| ML23104A013 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 04/19/2023 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| References | |
| LER 327-2022-001 IR 2022004 | |
| Download: ML23104A013 (8) | |
Text
1 Final ASP Analysis - Precursor Accident Sequence Precursor Program - Office of Nuclear Regulatory Research Sequoyah Nuclear Plant, Unit 1 Failure of 1B-B Centrifugal Charging Pump Results in Condition Prohibited by Technical Specifications LER:
327-2022-001 Event Date:
7/22/2022 IR: 05000327/2022004 CDP =
2x10-6 Plant Type:
Westinghouse Four-Loop Pressurized Water Reactor with Wet, Ice Condenser Containment Plant Operating Mode (Reactor Power Level):
Mode 1 (100% Reactor Power)
Analyst:
Reviewer:
Completion Date:
Christopher Hunter Mehdi Reisi Fard 4/19/2023 1
EXECUTIVE
SUMMARY
On July 18, 2022, main control room (MCR) operators determined the centrifugal charging pump (CCP) 1B-B was unable to maintain pressurizer level with downstream charging flow control valve fully open. A subsequent MCR alarm was received for low flow to the reactor coolant pump (RCP) 1 seal. RCP seal flow was adjusted, and the alarm cleared. Approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> later, the low pressurizer level alarm was received in the MCR. Operators manually started CCP 1A-A to recover pressurizer level. The CCP 1B-B was declared inoperable, and the plant entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.5.2, Emergency Core Cooling Systems (ECCS), Condition A. Licensee troubleshooting activities revealed the degradation of CCP 1B-B was caused by severe wear of multiple impellers.
The estimated repair time associated with CCP 1B-B was expected to exceed the TS 72-hour allowed outage time (AOT). On July 21st, the licensee requested a notice of enforcement discretion (NOED) for 69 hours7.986111e-4 days <br />0.0192 hours <br />1.140873e-4 weeks <br />2.62545e-5 months <br /> additional hours to repair the pump, which the U.S. Nuclear Regulatory Commission (NRC) approved. Repairs were completed and CCP 1B-B was declared operable on July 24th.
The mean core damage probability (CDP) for this event is 2x10-6 and, therefore, this event is a precursor. The dominant hazard for this accident sequence precursor (ASP) analysis is internal fires, which contribute approximately 64 percent of the total CDP. The risk from internal events is approximately 24 percent, seismic events contribute approximately 10 percent, and internal floods contribute approximately 2 percent. External floods and high winds (including and tornados) are minimal contributors to the total CDP for this analysis.
2 EVENT DETAILS 2.1 Event Description On July 18, 2022, MCR operators determined the CCP 1B-B was unable to maintain pressurizer level with downstream charging flow control valve fully open. A subsequent MCR alarm was received for low flow to the RCP 1 seal. RCP seal flow was adjusted, and the alarm cleared. Approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> later, the low pressurizer level alarm was received in the MCR.
Operators manually started CCP 1A-A to recover pressurizer level. The CCP 1B-B was
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declared inoperable, and the plant entered TS LCO 3.5.2, ECCS, Condition A. Licensee troubleshooting activities revealed the degradation of CCP 1B-B was caused by severe wear of multiple impellers.
The estimated repair time associated with CCP 1B-B was expected to exceed the TS 72-hour AOT. On July 21st, the licensee requested a NOED for 69 hours7.986111e-4 days <br />0.0192 hours <br />1.140873e-4 weeks <br />2.62545e-5 months <br /> additional hours to repair the pump, which the U.S. NRC approved. Repairs were completed and CCP 1B-B was declared operable on July 24th. Additional information is provided in licensee event report (LER) 327-2022-001, Failure of 1B-B Centrifugal Charging Pump Results in Condition Prohibited by Technical Specifications, (ML22258A065) and inspection report (IR) 05000327/2022004, Sequoyah, Units 1 and 2 - Integrated Inspection Report 05000327/2022004 and 05000328/2022004, (ML23041A069).
2.2 Cause The CCP 1B-B impellers showed significant wear, which is indicative of bowing of the pump shaft. A dye penetrant examination of the shaft revelated a crack in the split-ring keeper key groove at the 11th stage impeller. The most likely cause of the failure is low-stress, high-cycle fatigue.
3 MODELING 3.1 Basis for ASP Analysis The ASP Program uses SDP results for degraded conditions when available (and applicable).
No licensee performance deficiency associated with this condition was identified. The LER was reviewed and closed in IR 05000327/2022004 (ML23041A069). Therefore, an independent ASP analysis was performed because there was no performance deficiency identified and its potential risk significance. A search of Sequoyah Nuclear Plant (Unit 1) LERs did not reveal any windowed events.
3.2 Analysis Type A condition assessment event analysis was performed using a test and limited use (TLU) of the version 8.80 standardized plant analysis risk (SPAR) model for Sequoyah Nuclear Plant (Unit 1) created on April 5, 2022. This SPAR model includes the following hazards:
Internal events, Internal fires, Internal floods, External floods, Seismic events, and High winds (including tornados).
3.3 SPAR Model Modifications No modifications were made to the TLU version of the base SPAR model to support this analysis.
3.4 Exposure Time The following table provides the key dates and times associated with the failure of CCP 1B-B:
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Table 1. Key Dates and Time Associated with the Failure of CCP 1B-B Date Time Description June 9, 2022
Surveillance test of the ECCS function for CCP 1B-B is completed satisfactorily. With the exception of a 2-hour period surveillance test on CCP 1A-A, CCP 1B-B is running until the failure experienced on July 18th.
July 18, 2022 2130 MCR alarm received for low RCP seal 1 flow. Operators manually adjust RCP seal flow to restore adequate flow.
July 18, 2022 2230 MCR alarm received for lowering pressurizer level. Operators start CCP 1A-A and restore pressurizer level to normal. CCP 1B-B is declared inoperable, and the plant entered TS LCO 3.5.2, Condition A.
July 24, 2022 1635 Repairs completed; CCP 1B-B declared operable.
Based on this information, the following two exposure times were identified for this condition analysis:
Exposure Time 1. This period represents the time from when the observed failure CCP 1B-B occurred on July 18th until repairs were completed on July 24th, which is approximately 139 hours0.00161 days <br />0.0386 hours <br />2.29828e-4 weeks <br />5.28895e-5 months <br />. During this period, CCP 1B-B was unable to fulfil both its RCP seal injection and ECCS functions.
Exposure Time 2. This period represents the time from when the ECCS TS surveillance was completed on June 9th until the observed failure CCP 1B B on July 18th, which is approximately 39 days. During this period, CCP 1B-B was assumed to be unable to fulfil its ECCS functions only. Since the exact time when CCP 1B-B was unable to perform its safety function is unknown, the 39-day time period is divided by two, which results in an exposure time of 468 hours0.00542 days <br />0.13 hours <br />7.738095e-4 weeks <br />1.78074e-4 months <br />. The assumption that CCP 1B-B was unable to fulfil its ECCS function during this exposure time is potentially conservative and is a key uncertainty for this analysis, which is discussed further in Section 4.4.
3.5 Analysis Assumptions The following modeling assumptions were required to reflect the plant status and event circumstances for this condition assessment:
Basic event CVC-MDP-FR-1B (CVC MDP train B failure to run) was set to TRUE for Exposure Time 1 due to the failure of CCP 1B-B on July 18th. This basic event fails both the RCP seal injection and ECCS functions for the pump.
Basic events CVC-MDP-AP-RUN1A (CVC pump 1A-A is normally running) was set to TRUE and CVC-MDP-AP-RUN1B (CVC pump 1B-B is normally running) was set to FALSE to account for operating status (i.e., which pump was running) during Exposure Time 1.
Housed event HE-TLU-SENS (house event - TLU sensitivity) was set to TRUE to activation the fault tree logic that allows to failure ECCS high-pressure injection function for CCP 1B-B only.
Basic event CVC-MDP-FR-1B-TLU (CVC MDP train B failure to run - TLU sensitivity) was set to TRUE for Exposure Time 2. This basic event only fails the ECCS function for the pump.
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Basic events CVC-MDP-AP-RUN1A (CVC pump 1A-A is normally running) was set to FALSE and CVC-MDP-AP-RUN1B (CVC pump 1B-B is normally running) was set to TRUE to account for operating status (i.e., which pump was running) during Exposure Time 2.
4 ANALYSIS RESULTS 4.1 Results The overall mean CDP for this analysis is calculated to be 1.7x10-6, which is the sum of the two exposure times. The ASP Program threshold is 1x10-6 for degraded conditions; therefore, this event is a precursor. The parameter uncertainty results for both exposure times of this analysis are provided below:
Table 2. Parameter Uncertainty Results for Exposure Times 1 and 2 Exposure Time 5%
Median Point Estimate Mean 95%
1 8.4x10-8 5.5x10-7 9.4x10-7 1.1x10-6 3.9x10-6 2
1.3x10-7 4.4x10-7 5.6x10-7 6.0x10-7 1.6x10-6 4.2 Dominant Hazards1 The dominant hazard for this analysis is internal fires (CDP = 9.6x10-7), which contribute approximately 64 percent of the total CDP. Internal events contribute approximately 24 percent (CDP = 3.6x10-7), seismic events contribute approximately 10 percent (CDP = 1.5x10-7), and internal floods contribute approximately 2 percent (CDP = 2.6x10-8). External floods and high winds (including and tornados) are minimal contributors to the total CDP for this analysis.
4.3 Dominant Sequences The dominant accident sequence is FRI-ERCW-1A 2-2-2-4 (CDP = 5.0x10-7), which contributes approximately 33 percent of the total CDP. The sequences that contribute at least 5 percent to the total CDP are provided in the following table. These dominant sequences are shown graphically in Figures A-1 though and A-4 of Appendix A.
1 The CDPs provided in Sections 4.2 and 4.3 are point estimates.
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Table 3. Dominant Sequences Sequence CDP Description FRI-ERCW-1A 2-2-2-4 5.0x10-7 33.3%
Fire in the emergency raw cooling water (ERCW) building results in a loss in ERCW train A initiating event; auxiliary feedwater (AFW) is successful; RCP seal cooling fails; operators successfully trip the RCPs, but the stage 2 seals fail resulting in a small loss-of-coolant accident (SLCOA); high-pressure injection is successful; operators successfully cooldown and depressurize the reactor coolant system (RCS); shutdown cooling fails; and cold leg recirculation fails resulting in core damage.
LOERCW-A 4-2-4 1.3x10-7 8.4%
A loss of ERCW train A initiating event occurs; AFW is successful; RCP seal cooling fails; operators successfully trip the RCPs, but the stage 2 seals fail resulting in a SLCOA; high-pressure injection is successful; operators successfully cooldown and depressurize the RCS; shutdown cooling fails; and cold leg recirculation fails resulting in core damage.
4.4 Key Uncertainties The following are the key uncertainties of this ASP analysis.
Loss of ECCS Function of CCP 1B-B Prior to Observed Failure. The CCPs have two safety-related functions. First, they provide RCP seal injection flow. In addition, they provide an ECCS source of high-pressure injection. CCP 1B-B failed on July 18th resulting in a loss of both functions for the pump. This analysis also assumed that the high-pressure injection function was lost prior to the observed failure. This assumption is potentially conservative; however, due to the different system dynamics between the two functions (i.e., different flow rates, temperatures, etc.) the high-pressure injection function for CCP 1B-B could have been unavailable since the last successful surveillance test completed on June 9th. Since the exact time that the high-pressure injection was potentially lost is unknown, the Exposure Time 2 in this analysis used the t/2 approximation. Although this assumption is still potentially conservative (i.e., CCP 1B-B could have maintained its high-pressure injection function until the observed failure on July 18th), the CDP for this condition still exceeds the precursor threshold if the high-pressure injection function is assumed to be available.
LER 327-2022-001 A-1 Appendix A: Key Event Trees IE-FRI-ERCW-1A Fire in the ERCW Building causing loss of ERCW Train A TRANSFER Event set to TRUE for transfer End State (Phase - CD) 1 OK 2
TRANS Figure A-1. IE-FRI-ERCW-1A Event Tree IE-TRANS GENERAL PLANT TRANSIENT RPS REACTOR SHUTDOWN OEP OFFSITE ELECTRICAL POWER AFW AUXILIARY FEEDWATER MFW MAIN FEEDWATER PORV PORVS ARE CLOSED LOSC FS = FTF-LOSC RCP SEAL COOLING MAINTAINED HPI HIGH PRESSURE INJECTION BLEED PORVS OPEN SSCR SECONDARY SIDE COOLING RECOVERY SSC RCS COOLDOWN TO RHR PRESSURE (SEC / PRIMARY)
RHR RESIDUAL HEAT REMOVAL HPR INADEQUATE HIGH PRESSURE RECIRC End State (Phase - CD) 1 OK 2
LOSC 3
OK 4
OK 5
CD 6
OK 7
CD 8
CD PORV1 9
OK 10 LOSC PORV1 11 OK 12 OK 13 CD 14 OK 15 CD 16 CD 17 OK 18 OK 19 CD 20 CD 21 CD 22 LOOPPC 23 ATWS 24 CD Figure A-2. TRANS Event Tree
LER 327-2022-001 A-2 LOSC FS = FTF-LOSC RCP SEAL COOLING MAINTAINED RCPT REACTOR COOLANT PUMPS TRIPPED RSD FS = FTF-RSD RAPID SECONDARY DEPRESSURIZATION (<1710 PSI IN 2 HR)
BP1 RCP SEAL STAGE 1 INTEGRITY O1 RCP SEAL STAGE 1 INTEGRITY BP2 RCP SEAL STAGE 2 INTEGRITY O2 RCP SEAL STAGE 2 INTEGRITY End State (Phase - CD) 1 OK 0.200 2
SLOCA 0.0125 3
SLOCA 0.200 4
MLOCA 5
OK 0.500 6
SLOCA 0.200 7
SLOCA 0.500 8
SLOCA 0.500 9
SLOCA 0.200 10 SLOCA 0.0125 11 SLOCA 0.500 12 SLOCA 0.200 13 MLOCA 14 MLOCA Figure A-3. LOSC Event Tree IE-SLOCA SMALL LOCA RPS REACTOR SHUTDOWN FW FEEDWATER (AFW OR MFW)
HPI HIGH PRESSURE INJECTION BLEED PORVS OPEN SSCR SECONDARY SIDE COOLING RECOVERY SSC RCS COOLDOWN TO RHR PRESSURE (SEC / PRIMARY)
LPI LOW PRESS INJECTION LATE RHR RESIDUAL HEAT REMOVAL HPR INADEQUATE HIGH PRESSURE RECIRC LPR LOW PRESS RECIRCULATION End State (Phase - CD) 1 OK 2
OK 3
OK 4
CD 5
OK 6
CD SSC1 7
OK 8
OK 9
CD 10 CD SSC1 11 CD 12 OK 13 OK 14 CD 15 OK 16 CD 17 OK 18 CD 19 CD 20 CD 21 ATWS Figure A-4. SLOCA Event Tree
ML23104A013; ML23104A013 OFFICE RES/DRA/PRB RES/DRA NAME CHunter CAraguas VHall for DATE Apr 14, 2023 Apr 19, 2023