ML060240375
| ML060240375 | |
| Person / Time | |
|---|---|
| Site: | Kewaunee  |
| Issue date: | 02/26/2003 |
| From: | Office of Nuclear Regulatory Research |
| To: | |
| Shared Package | |
| ML060240240 | List: |
| References | |
| LER 03-002-00 | |
| Download: ML060240375 (10) | |
Text
1 For a conditional assessment, the parameter of interest is the measure of importance. This value is obtained subtracting the baseline core damage probability (CDP) from the conditional core damage probability (CCDP).
1 Enclosure Final Precursor Analysis Accident Sequence Precursor Program -- Office of Nuclear Regulatory Research Kewaunee Nuclear Power Plant Both Emergency Diesel Generators Inoperable for 28 Hours Event Date 2/26/2003 LER 305/03-002-00
)CDP = 3x10-6 August 23, 2005 Event Summary On February 26, 2003, at 0107 hours0.00124 days <br />0.0297 hours <br />1.76918e-4 weeks <br />4.07135e-5 months <br />, a manual reactor shutdown was initiated, according to Technical Specifications (TS), due to both emergency diesel generators (EDG) being unavailable.
At 0239, on February 25, EDG A was removed from service to perform scheduled periodic maintenance. A test run on EDG B was satisfactorily completed at 0159 prior to taking EDG A out of service. TS require that EDG B must be tested daily while EDG A is out of service.
During the daily required test on EDG B (at 0017 on February 26), the diesel failed to start.
Troubleshooting revealed a failed relay in the diesel start circuit. The faulty relay was replaced and EDG B was satisfactorily retested and returned to service at 0624, on February 26. The plant shutdown was terminated at 10-percent reactor power. EDG A maintenance was completed and the diesel was returned to service at 0010, on February 27. The total time that both EDGs were out of service was approximately 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br />.
Further information about the event can be found in References 1 and 2.
Analysis Results Importance1 This event was modeled as a conditional assessment with both EDGs unavailable for 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br />. The importance for this condition is 3x10-6 (mean value). The acceptance threshold for the Accident Sequence Precursor Program is an importance $1x10-6. Therefore, this condition is a precursor.
5%
Mean 95%
Importance 2x10-7 3x10-6 1x10-5
Dominant Sequences The dominant core damage sequences for this assessment are Loss of Offsite Power (LOOP), Station Blackout (SBO), Sequence 18-03 (32% of the total importance), SBO Sequence 18-06 (31% of the total importance), and SBO Sequence 18-45 (24% of the total importance). The LOOP and SBO event trees with the dominant sequences highlighted are shown in Figure 1 and Figure 2.
The events and important component failures in LOOP/SBO Sequence 18-03 are:
LOOP occurs, Reactor shutdown succeeds, Emergency power fails, Auxiliary feedwater succeeds, Power operated relief valves (PORV) close, Rapid secondary depressurization succeeds, Reactor coolant pump (RCP) seal stage 1 integrity succeeds, Reactor coolant pump (RCP) seal stage 2 integrity succeeds, Operators fail to recover offsite power in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, and Operators fail to recover an EDG in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />.
The events and important component failures in LOOP/SBO Sequence 18-06 are:
LOOP occurs, Reactor shutdown succeeds, Emergency power fails, Auxiliary feedwater succeeds, Power operated relief valves (PORV) close, Rapid secondary depressurization succeeds, Reactor coolant pump (RCP) seal stage 1 integrity succeeds, Reactor coolant pump (RCP) seal stage 2 integrity fails, Operators fail to recover offsite power in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, and Operators fail to recover an EDG in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.
The events and important component failures in LOOP/SBO Sequence 18-45 are:
LOOP occurs, Reactor shutdown succeeds, Emergency power fails, Auxiliary feedwater fails, Operators fail to recover offsite power in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, and Operators fail to recover an EDG in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.
Results Tables The importance values for the dominant sequences are shown in Table 1.
The event tree sequence logic for the dominant sequences are presented in Table 2a.
Table 2b defines the nomenclature used in Table 2a.
The most important cut sets for the dominant sequences are listed in Table 3.
Table 4 presents names, definitions, and probabilities of (1) basic events whose probabilities were changed to update the referenced SPAR model, (2) basic events whose probabilities were changed to model this event, and (3) basic events that are important to the total result.
Modeling Assumptions Analysis Type This event was modeled as a conditional assessment of both EDGs unavailable for approximately 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br />. The Kewaunee Revision 3.11 Standardized Plant Analysis Risk (SPAR) model, December 31, 2004 was used.
Modeling Assumptions Summary Key modeling assumptions. The key modeling assumptions are listed below. These assumptions are important contributors to risk.
EDG B failed to start and was recoverable within approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. The EDG B failure was diagnosed and the diesel was restored to service in approximately 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. However, based on judgement of the Senior Resident Inspector (SRI) at the site, recovery of EDG B may have been possible in approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. Therefore, recovery of EDG B was credited, using the SPAR-H Human Reliability Analysis Method (Ref. 4), for all EDG recovery events greater than or equal to 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. For all EDG recovery events less than 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, recovery was not credited in this analysis. Further details on the recovery of EDG B are provided in the following sections.
EDG A was out of service for maintenance and could not be recovered prior to core uncovery. EDG A was out of service for maintenance and was restored to service 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after EDG B failed to start during daily testing. Restoration of EDG A was not possible prior to core uncovery during a postulated SBO event.
Basic Event Probability Changes Table 4 provides all the basic events that were modified to reflect the condition being analyzed. The basis for these changes is provided below.
EDG A is unavailable due to testing and maintenance (EPS-DGN-TM-1A). This probability was set to TRUE. EDG A was out of service for maintenance for approximately 46 hours5.324074e-4 days <br />0.0128 hours <br />7.60582e-5 weeks <br />1.7503e-5 months <br />.
EDG B fails to start (EPS-DGN-FS-1B). This probability was set to TRUE. EDG 1B failed to start during daily testing required per TS.
AFW TDP 1C unavailable due to testing and maintenance (AFW-TDP-TM-1C).
This probability is set to FALSE. Maintenance of the TDP is not allowed during EDG maintenance. Normally the SPAR model rules would prohibit this event from appearing in a cutset, but setting EPS-DGN-TM-1A to TRUE circumvents the proper execution of this rule.
Operator fails to recover an EDG in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> (EPS-XHE-XL-NR01H). This probability was set to True. See Key Modeling Assumptions for further details.
Operator fails to recover an EDG in 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> (EPS-XHE-XL-NR02H). This probability was set to True. See Key Modeling Assumptions for further details.
Operator fails to recover an EDG in 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> (EPS-XHE-XL-NR03H). This probability was set to True. See Key Modeling Assumptions for further details.
Operator fails to recover an EDG in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> (EPS-XHE-XL-NR04H). This probability was set to 0.2 using the SPAR-H method. In both the Diagnosis and Action parts of the SPAR-H analysis, the Stress performance shaping factor (PSF) was set to Extreme and the Complexity PSF was set to Moderately Complex. In the Action part of the SPAR-H analysis, the Available Time PSF was set to Barely Adequate. All other PSFs were set to Nominal. See Attachment A for further details.
Operator fails to recover an EDG in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> (EPS-XHE-XL-NR06H). This probability was set to 0.1 using the SPAR-H method. The Stress PSF was set to Extreme and the Complexity PSF was set to Moderately Complex. All other PSFs were set to Nominal. See Attachment A for further details.
Operator fails to recover an EDG in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> (EPS-XHE-XL-NR08H). This probability was set to 0.1 using the SPAR-H method. The Stress PSF was set to Extreme and the Complexity PSF was set to Moderately Complex. All other PSFs were set to Nominal. See Attachment A for further details.
Sensitivity Analysis Sensitivity analyses were performed to determine the effects of modeling uncertainties on results based on best estimate assumptions. Changes to PSFs used in the analysis do not significantly change the results.
References 1.
LER 305/03-002-00, Shutdown Initiated-Diesel Generator Failed to Start Test-Unusual Event-Caused by Start Relay Failure, dated April 28, 2003.
2.
NRC Special Inspection Report 50-305/03-08, dated January 26, 2004.
3.
Idaho National Engineering and Environmental Laboratory, Simplified Plant Analysis Risk (SPAR) Model for Kewaunee Nuclear Power Plant, Revision 3.11, December 31, 2004.
4.
Idaho National Engineering and Environmental Laboratory, The SPAR-H Human Reliability Analysis Method INEEL/EXT-02-01307, May 2004.
Table 1. Importance values of dominating sequences.
Event tree name Sequence no.
Importance1 (CCDP-CDP)
Contribution SBO 18-03 1.1E-006 32.2 SBO 18-06 9.5E-007 30.6 SBO 18-45 7.3E-007 23.5 Total (all sequences)(2) 3.2E-006 100
- 1. Values are point estimates. (File name: GEM 335-03-002.wpd)
- 2. Total Importance includes all sequences (including those not shown in this table).
Table 2a. Event tree sequence logic for dominating sequences.
Event tree name Sequence no.
Logic
(/ denotes success; see Table 2b for top event names)
SBO 18-03
/RPS EPS /AFW-B /PORV-B /RSD /BP1 /BP2 OPR-08H DGR-08H SBO 18-06
/RPS EPS /AFW-B /PORV-B /RSD /BP1 BP2 OPR-04H DGR-04H SBO 18-45
/RPS EPS AFW-B OPR-01H DGR-01H Table 2b. Definitions of top events listed in Table 2a.
Top Event Definition AFW-B BP1 BP2 DGR-01H DGR-04H DGR-08H EPS OPR-01H OPR-04H OPR-08H PORV-B RPS RSD Auxiliary feedwater (AFW) fails during SBO RCP seal stage 2 integrity fails RCP seal stage 2 integrity fails Operator fails to recover an EDG in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> Operator fails to recover an EDG in 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> Operator fails to recover an EDG in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Emergency power system fails Operator fails to recover offsite power in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> Operator fails to recover offsite power in 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> Operator fails to recover offsite power in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> PORVs fail to close during SBO Reactor protection system fails Rapid secondary depressurization fails
Table 3. Conditional cut sets for the dominant sequence. (See Table 4 for definitions and probabilities for the basic events.)
Importance Percent Contribution Minimum Cut Sets (of basic events)
Event Tree: LOOP/SBO, Sequence 18-03 1.1E-006 100.0 EPS-XHE-XL-NR08H OEP-XHE-XL-NR-08H 1.1E-006 100 Total (all cutsets)1 Importance Percent Contribution Minimum Cut Sets (of basic events)
Event Tree: LOOP/SBO, Sequence 18-06 9.5E-007 100.0 EPS-XHE-XL-NR04H OEP-XHE-XL-NR-04H RCS-MDP-LK-BP2 9.5E-007 100 Total (all cutsets)1 Importance Percent Contribution Minimum Cut Sets (of basic events)
Event Tree: LOOP/SBO, Sequence 18-45 3.4E-007 2.3E-007 46.1 31.8 AFW-TDP-FS-1C AFW-TDP-FR-1C OEP-XHE-XL-NR01H OEP-XHE-XL-NR01H 7.3E-007 100 Total (all cutsets)1
- 1. Total Importance includes all cutsets (including those not shown in this table).
Table 4. Definitions and probabilities for modified and dominant basic events.
Event Name Description Probability/
Frequency (per hour)
Modified AFW-TDP-FR-1C AFW-TDP-FS-1C AFW-TDP-TM-1C EPS-DGN-FS-1B EPS-DGN-TM-1A EPS-XHE-XL-NR01H EPS-XHE-XL-NR02H EPS-XHE-XL-NR03H EPS-XHE-XL-NR04H EPS-XHE-XL-NR06H EPS-XHE-XL-NR08H OEP-XHE-XL-NR01H OEP-XHE-XL-NR04H OEP-XHE-XL-NR08H RCS-MDP-LK-BP2 AFW turbine-driven pump 1C fails to run AFW turbine-driven pump 1C fails to start AFW turbing-driven pump 1C unavailable due to test and maintenance Diesel generator 1B fails to start Diesel generator 1A is unavailable due to test and maintenance Operator fails to recover an EDG in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> Operator fails to recover an EDG in 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> Operator fails to recover an EDG in 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> Operator fails to recover an EDG in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> Operator fails to recover an EDG in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> Operator fails to recover an EDG in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Operator fails to recover offsite power in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> Operator fails to recover offsite power in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> Operator fails to recover offsite power in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> RCP seal stage 2 integrity (binding/popping o-rings) 4.1x10-3 6.0x10-3 False True True True True True 2.0x10-1 1.1x10-1 1.1x10-1 5.3x10-1 2.2x10-1 1.2x10-1 2.0x10-1 No No Yes1 Yes1 Yes1 Yes2 Yes2 Yes2 Yes2 Yes2 Yes2 No No No No
- 1. Set to True to reflect plant conditions. See Basic Event Probability Changes for further details.
- 2. Evaluated per SPAR-H method (Ref. 4). See Key Modeling Assumptions and/or Attachment A for further details.
Attachment A AC Power Revovery Modeling Background and Modeling Details of AC Power Recovery The time required to restore AC power to plant emergency equipment is a significant factor in modeling the importance given a postulated loss of offsite power (LOOP) and/or station blackout (SBO). Standardized Plant Analysis Risk (SPAR) LOOP/SBO models include various sequence-specific AC power recovery factors that are based on the time available to recover power to prevent core damage. For a sequence involving failure of all of the cooling sources, only about 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> would be available to recover power to help avoid core damage. On the other hand, sequences involving successful early inventory control and decay heat removal, but failure of long-term decay heat removal, would accommodate several hours to recover AC power prior to core damage.
In this analysis, offsite power recovery probabilities are based on industry average recovery values for all four LOOP types (plant centered, grid-related, severe weather, and extreme weather). The SPAR-H Human Reliability Analysis Method (Ref. 4) was used to estimate non-recovery probabilities of a single emergency diesel generator (EDG).
Diagnosis and Dependency The SPAR Human Reliability Analysis Method considers the following three factors:
Probability of failure to diagnose the need for action, Probability of failure to successfully perform the desired action, and Dependency on other operator actions involved in the specific sequence of interest.
This analysis determined that the probability of failure to successfully perform action needed to recover a failed EDG is negligible for the 6 and 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> cases because the probability of failing the diagnosis is an order of magnitude higher. Dependency is normally considered when multiple operator actions are present in the same cutset. However, dependency between offsite power and EDG recovery tasks is considered low and does not change the best estimate EDG non-recovery probabilities.
Performance Shaping Factors The probability of failure to properly diagnose a condition or failure is the product of a nominal failure probability (1.0x10-2) and the following eight performance shaping factors (PSFs):
Available time Stress Complexity Experience/training Procedures Ergonomics Fitness for duty Work processes Available Time The PSF for Available Time is nominal for diagnosis because it is much less than the 4, 6, or 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> that are the key recovery times. However, the Available Time PSF for action is judged to be barely adequate for the 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> case, based on the SRIs assessment that the repairs would take
approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. Since the repair was completed within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, with no appreciable hurry on the licensees part, the PSF is nominal for the 4 and 6 hour6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> recovery times.
Stress The PSF for Stress is assigned a value of 5 (corresponding to Extreme Stress) for non-recovery probabilities greater than or equal to 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, EPS-XHE-XL-NR04H, EPS-XHE-XL-NR06H, and EPS-XHE-XL-NR08H. Factors considered in assigning this PSF level are the disruptive stress of a postulated SBO and the eminent core damage if AC power (either an EDG or offsite power) is not restored to a safety bus prior to core uncovery.
Complexity The PSF for Complexity is assigned a value of 2 (corresponding to Moderately Complex) for the three EDG non-recovery probabilities. A postulated SBO would "involve concurrent actions" such as communications and coordination of multiple crews outside the control room: EDG repair crew, offsite power repair crew, and utility switchyard operators.
All Other PSFs For all of the AC power non-recovery probabilities, the PSFs for time, experience/training, procedures, ergonomics, fitness for duty, and work processes are set to be nominal (i.e., are assigned values of 1.0). Details of the condition, plant response, and historical crew performance did not warrant a change from nominal for these PSFs.
Table A.1. EDG non-recovery probabilities.
Non-recovery Factor Nominal Value Performance Shaping Factors Non-recovery Probability Stress Complexity Time EPS-XHE-XL-NR04H Diagnosis:1x10-2 Action: 1x10-3 5
5 2
2 1
10 1x10-1 1x10-1 Total: 2x10-1 EPS-XHE-XL-NR06H Diagnosis:1x10-2 Action: 1x10-3 5
5 2
2 1
1 1x10-1 1x10-2 Total: 1.1x10-1 EPS-XHE-XL-NR08H Diagnosis:1x10-2 Action: 1x10-3 5
5 2
2 1
1 1x10-1 1x10-2 Total: 1.1x10-1
HPR HIGH PRESSURE RECIRC RHR RESIDUAL HEAT REMOVAL PZR RCS DEPRESS FOR LPI/RHR SSC SECONDARY SIDE COOLDOWN OPR-06H OFFSITE POWER RECOVERY IN 6 HRS OPR-02H OFFSITE POWER RECOVERY IN 2 HRS FAB FEED AND BLEED HPI HIGH PRESSURE INJECTION LOSC RCP SEAL COOLING MAINTAINED PORV PORVs ARE CLOSED AFW AUXILIARY FEEDWATER EPS EMERGENCY POWER RPS REACTOR SHUTDOWN IE-LOOP OSS OF OFFSITE POWER END-STATE 1
OK 2
T LOOP-1 3
OK 4
OK 5
CD 6
OK 7
CD 8
OK 9
CD 10 OK 11 CD 12 CD 13 OK 14 CD 15 OK 16 CD 17 CD 18 T
SBO 19 T
ATWS HPR-L HPR-L FAB-L AFW-L PORV-L LOSC-L HPI-L Figure 1: Kewaunee LOOP event tree.
DGR-08H DIESEL GENERATOR RECOVERY (IN 8 HR)
OPR-08H OFFSITE POWER RECOVERY (IN 8 HR)
O2 RCP SEAL STAGE 2 INTEGRITY BP2 RCP SEAL STAGE 2 INTEGRITY O1 RCP SEAL STAGE 1 INTEGRITY BP1 RCP SEAL STAGE 1 INTEGRITY RSD RAPID SECONDARY DEPRESS PORV PORVs ARE CLOSED AFW AUXILIARY FEEDWATER BP1 RCP SEAL STAGE 1 INTEGRITY END-STATE 1
OK 2
OK 3
CD 4
T SBO-1 5
OK 6
CD 7
T SBO-1 8
OK 9
CD 10 T
SBO-1 11 OK 12 CD 13 T
SBO-2 14 OK 15 CD 16 T
SBO-2 17 OK 18 CD 19 T
SBO-2 20 OK 21 CD 22 T
SBO-2 23 OK 24 CD 25 T
SBO-2 26 OK 27 CD 28 T
SBO-2 29 OK 30 CD 31 T
SBO-2 32 OK 33 CD 34 T
SBO-2 35 OK 36 CD 37 T
SBO-2 38 OK 39 CD 40 T
SBO-2 41 OK 42 CD 43 T
SBO-3 44 OK 45 CD OPR-01H OPR-01H 21 gpm/rcp 182 gpm/rcp 76 gpm/rcp 480 gpm/rcp 21 gpm/rcp 172 gpm/rcp 182 gpm/rcp 61 gpm/rcp 300 gpm/rcp 300 gpm/rcp 76 gpm/rcp 300 gpm/rcp 480 gpm/rcp OPR-04H OPR-02H OPR-03H OPR-03H OPR-06H OPR-02H OPR-02H OPR-06H OPR-02H OPR-02H DGR-04H DGR-02H DGR-03H DGR-03H DGR-06H DGR-02H DGR-02H DGR-06H DGR-02H DGR-02H AFW-B PORV-B DGR-01H DGR-01H Figure 2: Kewaunee SBO event tree (with dominant sequences highlighted).