ML23041A001
| ML23041A001 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 02/16/2023 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| References | |
| LER 458-2022-003 | |
| Download: ML23041A001 (9) | |
Text
1 Final ASP Analysis - Precursor Accident Sequence Precursor Program - Office of Nuclear Regulatory Research River Bend Station Division 1 Standby Diesel Generator Speed Sensor Power Supply Failure LER:
458-2022-003 Event Date:
7/4/2022 IR:
1x10-5 Plant Type:
General Electric Type 6 Boiling-Water Reactor (BWR) with Mark III Containment Plant Operating Mode (Reactor Power Level):
Mode 1 (100% Reactor Power)
Analyst:
Reviewer:
Completion Date:
Christopher Hunter Latonia Enos-Sylla 2/16/2023 1
EXECUTIVE
SUMMARY
On July 4, 2022, a failure was identified in the speed control circuitry of the division 1 standby diesel generator during monthly surveillance testing. The licensee subsequently declared the diesel generator inoperable due to the failure of both power supplies in the speed control circuit.
These redundant power supplies provide power to individual speed switches, tachometers, and the emergency response information system. The speed control circuit is designed such that either one of the power supplies is sufficient to allow field flash and loading of the diesel generator. These power supplies were refurbished and installed in December 2020.
The division 1 standby diesel generator was last tested satisfactorily on June 1st. The emergency response information system indicated that the power supplies likely failed on June 8th. The power supplies were replaced, and the division 1 standby diesel generator was returned to service after post-maintenance testing was completed on July 7th.
The mean core damage probability (CDP) for this event is 1x10-5 and, therefore, this event is a precursor. The dominant hazards for this accident sequence precursor (ASP) analysis are internal events, which contribute approximately 92 percent of the total CDP. High winds (including hurricanes and tornados) and internal floods contribute approximately 5 percent and 3 percent of the total CDP, respectively. Seismic events are minimal risk contributors for this analysis. Due to the lack of modeling of internal fires, the impact of internal fires is considered qualitatively.
2 EVENT DETAILS 2.1 Event Description On July 4, 2022, a failure was identified in the speed control circuitry of the division 1 standby diesel generator during monthly surveillance testing. The licensee subsequently declared the diesel generator inoperable due to the failure of both power supplies in the speed control circuit.
These redundant power supplies provide power to individual speed switches, tachometers, and the emergency response information system. The speed control circuit is designed such that either one of the power supplies is sufficient to allow field flash and loading of the diesel generator. These power supplies were refurbished and installed in December 2020.
LER 458-2022-003 2
The division 1 standby diesel generator was last tested satisfactorily on June 1st. The emergency response information system indicated that the power supplies likely failed on June 8th. The power supplies were replaced, and the division 1 standby diesel generator was returned to service after post-maintenance testing was completed on July 7th. Additional information is provided in licensee event report (LER) 485-2022-003, Division 1 Standby Diesel Generator Speed Sensor Power Supply Failure (ML22244A098).
2.2 Cause The direct cause of this event is currently unknown. Licensee troubleshooting identified blown fuses internally to both power supplies. Both of the failed power supplies were sent to an outside vendor for a detailed failure analysis.
3 MODELING 3.1 Basis for ASP Analysis The ASP Program uses SDP results for degraded conditions when available (and applicable).
Discussions with Region 4 staff indicated that no licensee performance deficiency associated with this condition has been identified; however, the LER remains open. Therefore, an independent ASP analysis was performed because there was no performance deficiency identified and its potential risk significance. A search of River Bend Station LERs did not reveal any windowed events.1 3.2 Analysis Type This degraded condition was evaluated using a test and limited use (TLU) version of the River Bend Station SPAR model created in December 2022. This TLU model includes revisions to the version 8.80 SPAR model of record based on the review of the licensees final integrated plan (FIP) for the FLEX mitigation strategies (ML15279A345). This SPAR model includes the following hazards:
Internal events, Internal floods, High winds (including hurricanes and tornados), and Seismic events.
The lack of modeling of internal fires scenarios is a key uncertainty and is considered qualitatively in Section 4.4.
3.3 SPAR Model Modifications The following additional SPAR model modifications were required for this initiating event analysis:
FLEX Reliability Parameters. The base SPAR models currently use the reliability parameters of permanently installed equipment as placeholders for FLEX equipment because FLEX-specific reliability parameters were not available when the FLEX logic 1
Windowed events are when multiple structures, systems, and components are unable to perform their safety function at the same time.
LER 458-2022-003 3
was incorporated into the SPAR models. Updated FLEX reliability parameters are provided in Table 6-1 in Pressurized Water Reactor Owners Group (PWROG) 18042NP, FLEX Equipment Data Collection and Analysis, Revision 1 (ML22123A259).
This analysis uses this data because it is more representative of the as-built, as-operated plant.
Crediting the FLEX N+1 Diesel Generator. During an extended loss of AC power (ELAP), it may not be possible for licensee personnel to transport, connect, and start the FLEX N+1 diesel generator to provided battery charging prior to battery depletion in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. Therefore, this analysis eliminates this base SPAR model credit. This modeling assumption is potentially conservative and is a key uncertainty of this analysis. The effects of the uncertainty are evaluated in Section 4.4.
3.4 Exposure Time The emergency response information system indicated that the power supplies likely failed on June 8th. The power supplies were replaced on July 7th. Because the time of failure is known, the exposure time was calculated using the following equation from Section 2.3 of Volume 1 (internal events) of the Risk Assessment of Operational Events (or RASP) Handbook (ML17348A149):
= + = 27 + 3 = 30 3.5 Analysis Assumptions The following modeling assumptions were required to reflect the plant status and event circumstances for this initiating event assessment:
Basic event EPS-DGN-FS-DGA (diesel generator A fails to start) was set to TRUE because the division 1 standby diesel generator would have failed to start due to the unavailable field flash as a result of the failure of both power supplies in the speed control circuit. No credit for recovery of the division 1 standby diesel generator was provided in this analysis.
4 ANALYSIS RESULTS 4.1 Results The mean CDP for this analysis is calculated to be 1.2x10-5. The ASP Program threshold is 1x10-6 for degraded conditions; therefore, this event is a precursor. The parameter uncertainty results for this analysis provided below:
Table 1. Parameter Uncertainty Results 5%
Median Point Estimate Mean 95%
2.7x10-6 8.4x10-6 1.4x10-5 1.2x10-5 3.1x10-5
LER 458-2022-003 4
4.2 Dominant Hazards2 The dominant hazard for this analysis is internal events (CDP = 1.2x10-5), which contribute approximately 92 percent of the total CDP. High winds (including hurricanes and tornados) and internal floods contribute approximately 5 percent (CDP = 6.9x10-7) and 3 percent (CDP
x10-7), respectively. Seismic events hazards are minimal contributors to the total CDP for this analysis 4.3 Dominant Sequences The dominant accident sequence is loss of offsite power (LOOP) sequence 53-03-09 (CDP
7.2x10-6), which contributes approximately 53 percent of the total CDP. The sequences that contribute at least 5.0 percent to the total CDP are provided in the following table. These dominant sequences are shown graphically in Figures A-1, A-2, and A-3 of Appendix A.
Table 2. Dominant Sequences Sequence CDP Description LOOP 59-03-09 7.2x10-6 53.1%
LOOP due to internal events (all types including consequential) occurs; successful reactor trip; failures of the emergency power system result in a station blackout (SBO); reactor core isolation cooling (RCIC) and high-pressure coolant spray (HPCS) successfully maintain reactor coolant system (RCS) inventory; operators successfully declare ELAP; FLEX diesel generator fails resulting in the subsequent loss of safety-related DC power; and operator failure to restore AC power within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> results in core damage.
LOOP 59-09-09 1.5x10-6 11.0%
LOOP due to internal events (all types) occurs; successful reactor trip; failures of the emergency power system result in an SBO; HPCS fails, but RCIC successfully maintains RCS inventory; operators successfully declare ELAP; FLEX diesel generator fails resulting in the subsequent loss of safety-related DC power; and operator failure to restore AC power within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> results in core damage.
HWD and HCN 59-03-09 6.9x10-7 5.1%
LOOP due to high winds (including hurricanes) occurs; successful reactor trip; failures of the emergency power system result in an SBO; RCIC and HPCS successfully maintain RCS inventory; operators successfully declare ELAP; FLEX diesel generator fails resulting in the subsequent loss of safety-related DC power; and operator failure to restore AC power within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> results in core damage.
2 The CDPs provided in Sections 4.2 and 4.3 are point estimates.
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Sequence CDP Description LOOP 7 6.9x10-7 5.1%
LOOP due to internal events (all types) occurs; successful reactor trip; emergency power system successfully provides safety-related AC power; RCIC or HPCS successfully maintain RCS inventory; suppression pool cooling fails; operators successfully depressurize the reactor; shutdown cooling fails; and failure of containment heat removal and venting results in core damage.
4.4 Key Uncertainties The following are the key uncertainties of this ASP analysis.
Lack of Internal Fire Modeling in the SPAR Model. The key modeling uncertainty for this analysis is lack of internal fire scenarios in the River Bend SPAR model. To address this uncertainty, the risk information provided by the licensee for various risk-informed applications (e.g., technical specification change) was reviewed. River Bend license amendment requests to date state that the licensee does not have a PRA for internal fires and references its individual plant examination for external events (IPEEE) for a quantitative assessment of the risk associated with internal fires. However, discussion with Region 4 staff revealed that the licensee has recently completed a PRA for internal fires.
A review of the River Bend IPEEE results show that areas identified as important are likely to impact the risk of this condition (i.e., areas in which a fire would result in a LOOP or partial LOOP). However, a quantitative estimate of this risk impact is not possible given the information provided by the IPEEE. However, based on the importance measures of the division 1 standby diesel generator in the licensee fire PRA, its failure is expected to have risk impact in the low 10-6 range for internal fires. Therefore, the overall CDP is would likely remain in the 10-5 range and would not increase to the significant precursor threshold of greater than or equal to 10-3.
Crediting the FLEX N+1 Diesel Generator. A sensitivity calculation was performed to evaluate the uncertainty associated with crediting the FLEX N+1 diesel generator in the best estimate analysis. This calculation resulted in mean CDP of 7x10-6 (a decrease of 37 percent of the best estimate CDP).
LER 458-2022-003 A-1 Appendix A: Key Event Trees IE-LOOPWR LOSS OF OFFSITE POWER INITIATOR (WEATHER-RELATED)
RPS REACTOR SHUTDOWN EPS FS = FTF-SBO EMERGENCY POWER SRV SRV'S CLOSE HCS HPCS RCI RCIC SPC SUPPRESSION POOL COOLING DEP MANUAL REACTOR DEPRESS LCS LPCS LCI LOW PRESSURE COOLANT INJECTION VA FAILURE OF ALTERNATE LOW PRESSURE INJECTION SPC SUPPRESSION POOL COOLING DEP MANUAL REACTOR DEPRESS SDC SHUTDOWN COOLING CHR CONTAINMENT HEAT REMOVAL (FAN COOLERS)
CVS CONTAINMENT VENTING LI LATE INJECTION End State (Phase - CD) 1 OK 2
OK 3
OK 4
OK LI00 5
CD 6
OK LI01 7
CD 8
OK 9
OK LI00 10 CD 11 OK LI01 12 CD 13 OK 14 OK 15 OK 16 OK LI00 17 CD 18 OK LI01 19 CD 20 OK 21 OK 22 OK LI00 23 CD 24 OK LI01 25 CD 26 OK SDC1 27 OK 28 OK LI00 29 CD 30 OK LI01 31 CD 32 CD 33 CD 34 OK 35 OK 36 OK 37 OK LI00 38 CD 39 OK LI01 40 CD 41 OK 42 OK 43 OK 44 OK LI00 45 CD 46 OK LI01 47 CD 48 OK SPC1 49 OK SDC1 50 OK 51 OK LI00 52 CD 53 OK LI01 54 CD 55 CD 56 CD P1 57 LOOP-1 P2 58 LOOP-2 59 SBO 60 ATWS 61 CD Figure A-1. Weather-Related LOOP Event Tree
LER 458-2022-003 A-2 EPS FS = FTF-SBO EMERGENCY POWER SRV SRV'S CLOSE RCPSL RECIRC PUMP SEAL INTEGRITY RCI02 RCIC HCS-SBO FS = FTF-HPCS-SBO HPCS - SBO ET OPR OFFSITE POWER RECOVERY DGR DIESEL GENERATOR RECOVERY CVS CONTAINMENT VENTING End State (Phase - CD)
OPR-01H 1
SBO-OP OPR-01H 2
OK DGR-01H 3
SBO-ELAP OPR-01H 4
SBO-OP OPR-01H 5
OK DGR-01H 6
SBO-ELAP OPR-01H 7
SBO-OP OPR-01H 8
OK DGR-01H 9
SBO-ELAP OPR-30M 10 SBO-OP OPR-30M 11 OK DGR-30M 12 CD 13 SBO-P1 P1 14 SBO-P1 P2 15 SBO-P2 Figure A-2. SBO Event Tree
LER 458-2022-003 A-3 SBO-ELAP SBO - Extended Loss of AC Power ELAP ELAP IS DECLARED WHEN NEEDED FLEX-DLSHED Deep Load Shedding per FSGs OPR OFFSITE POWER RECOVERY FLEX-480 FLEX DIESEL GENERATOR OPERATION AND BUS ALIGNMENT (DC CHARGING)
FLEX-SPC-CL FAILURE OF SUPPRESSION POOL COOLING FLEX-TDP EXTENDED TDP (RCIC/HPCI)
OPERATION FLEX-DEP MANUAL REACTOR DEPRESS DURING ELAP FLEX-RPV FAILURE OF FLEX RPV LOW-PRESSURE INJECTION FLEX-CVS CONTAINMENT VENTING DURING ELAP End State (Phase - CD)
OPR-08H 1
SBO-OP OPR-08H 2
OK FLEX-TDP2 3
CD 4
OK 5
OK 6
CD 7
CD 8
CD 9
CD OPR-04H 10 OK OPR-04H 11 OK FLEX-TDP3 12 CD OPR-04H 13 OK OPR-04H 14 OK FLEX-TDP3 15 CD Figure A-3. SBO-ELAP Event Tree
ML23041A001; ML23041A001 OFFICE RES/DRA/PRB RES/DRA NAME CHunter JTappert DATE Feb 10, 2023 Feb 16, 2023