ML25149A224
| ML25149A224 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 05/29/2025 |
| From: | Christopher Hunter Office of Nuclear Regulatory Research |
| To: | |
| References | |
| LER 498-2024-006 | |
| Download: ML25149A224 (1) | |
Text
1 Final ASP Analysis - Precursor Accident Sequence Precursor Program - Office of Nuclear Regulatory Research South Texas Project, Unit 1 Condition Prohibited by Technical Specifications and Potential Loss of Safety Function Due to Inoperable Pressurizer Power-Operated Relief Valve Event Date: 10/5/2024 LERs: 498-2024-006 CDP =
3x10-6 IR:
TBD Plant Type:
Westinghouse Four-Loop Pressurized-Water Reactor (PWR) with Dry Ambient Pressure Containment Plant Operating Mode (Reactor Power Level):
Mode 3 (0% Reactor Power)
Analyst:
Reviewer:
Completion Date:
Christopher Hunter Kyle Warns 5/29/2025 1
EXECUTIVE
SUMMARY
On October 5, 2024, pressurizer power-operated relief valve (PORV) 656A failed to open from the main control room (MCR) hand switch and auxiliary shutdown panel during performance of an operability test. Initial licensee troubleshooting identified a partially disengaged fastener that is used to connect a wire from the PORV solenoid to its power source. This degraded condition was determined to have resulted in inoperability of the valve following installation of a replacement solenoid valve in November 2021. PORV 656A was declared operable on November 4th, following corrective maintenance and successful post-maintenance testing.
During the period when PORV 656A was unable to fulfil its safety function, PORV 655 was also inoperable at various times to support testing and maintenance activities. In addition, a review of South Texas Project (STP) Unit 1 licensee event reports (LERs) revealed six windowed events (i.e., initiating events or degraded conditions that were concurrent with the PORV 656A failure).
An overall mean CDP of 5.6x10-7 for this degraded condition was calculated using a revised STP Unit 1 standardized plant analysis risk (SPAR) model. However, this SPAR model does not include all key hazards, most notably internal fires. Importance measures from the existing licensee probabilistic risk assessment (PRA) results in an estimated CDP of 4.9x10-7 from internal fires. This risk impact, along with those from the windowed events (CDP ~ 1.4x10-6),
indicate the that overall CDP would be in low 10-6 range (i.e., ~2.5x10-6) and, therefore, the degraded condition associated with the failure of PORV 656A is a precursor.
2 EVENT DETAILS 2.1 Event Description On October 5, 2024, pressurizer PORV 656A failed to open from the MCR hand switch and auxiliary shutdown panel during performance of an operability test. Initial licensee troubleshooting identified a partially disengaged fastener that is used to connect a wire from the PORV solenoid to its power source. This degraded condition was determined to have resulted in inoperability of the valve following installation of a replacement solenoid valve in November 2021. PORV 656A was declared operable on November 4th, following corrective maintenance and successful post-maintenance testing. During the period when PORV 656A
LER 498-2024-006 2
was unable to fulfil its safety function, PORV 655 was also inoperable at various times to support testing and maintenance activities. See LER 498-2024-006, Condition Prohibited by Technical Specifications and Potential Loss of Safety Function Due to Inoperable Pressurizer Power Operated Relief Valve, (ML24358A097) for additional information.
2.2 Causes The licensee identified a loose electrical connection associated with the PORV valve solenoid to its power source. A subsequent causal analysis is being performed to determine if there were any additional causes of the PORV failure.
3 MODELING 3.1 SDP Results/Basis for ASP Analysis The ASP Program uses Significance Determination Process results for degraded conditions when available (and applicable). Discussions with Region 4 staff indicated that no licensee performance deficiency associated with this degraded condition have been identified; however, the LER remains open. Therefore, an independent ASP analysis was performed to determine the risk significance of PORV 656A failure. A search of STP LERs revealed six windowed events (i.e., initiating events or degraded conditions that were concurrent with the PORV 656A failure). Evaluations of the risk impact of these windowed events is provided in Section 4.5.
3.2 Analysis Type A degraded condition analysis was performed using a test and limited use revision of the version 8.80 SPAR model for STP Project Unit 1 created on February 25, 2025. The following SPAR model changes were made to support this analysis:
The pressurizer PORV success criteria for feed and bleed cooling was changed to 1 out of 2 valves based on a review of thermal-hydraulic calculations for STP.
The manual and automatic functions of the pressurizer PORVs were separated with the applicable fault tree logic.
The common-cause failure (CCF) events associated with the essential chilled water (ECH) chillers were modified to allow the manual adjustment to the various CCF combinations.
Credit for other steam pathways in addition to the steam generator (SG) PORVs for decay heat removal (e.g., steam dumps and SG safety valves) were provided.
The reactor coolant system pressure relief success criteria given an anticipated transient without scram were modified based on a review of thermal-hydraulic calculations for STP. Specifically, either both pressurizer PORVs or safety valves are required to operate to ensure that RCS pressure does not exceed 3200 psi.
Corrections were made to the auxiliary feedwater (AFW) electrical room cooling fault tree.
This SPAR model includes the following hazards:
Internal events, Seismic events, and High winds (including hurricanes and tornados).
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Internal fire and flood scenarios are not included in the STP SPAR model. Internal floods, based on the results of the licensee PRA, are a minimal contributor to the overall risk at STP and, therefore, the risk impact of internal flood scenarios with these degraded conditions are not expected to be significant. The lack of internal fire hazards modeling is a key uncertainty for this analysis, which is evaluated in Section 4.4.
3.3 SPAR Model Modifications The existing STP SPAR model SBO-ELAP event tree has three transfers to the SBO-1 event tree given successful offsite power recovery. These transfers were changed to OK end states because reactivity control and decay heat removal were successful and are not associated with a loss-of-coolant accident (LOCA), which is consistent with other initiating events in the STP SPAR model and SBO-ELAP event trees in other SPAR models.
3.4 Exposure Time PORV 656A was unable to perform is manual safety function from November 2021 through November 2024. Therefore, the exposure time was set to the maximum of 1 year in accordance with of Volume 1 (internal events) of the Risk Assessment of Operational Events (or RASP)
Handbook (ML17348A149). During this 1-year exposure time, PORV 655 was unavailable for approximately 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> to support various test and maintenance activities. Therefore, the following two exposure times were identified:
Exposure Time 1PORV 656A failure to operate (manual function only) for approximately 8724 hours0.101 days <br />2.423 hours <br />0.0144 weeks <br />0.00332 months <br />.
Exposure Time 2PORV 656A failure to operate (manual function only) and PORV 655 unavailable (both manual and automatic functions) for approximately 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
3.5 Analysis Assumptions The following modeling assumptions were determined to be significant for this analysis:
Basic event PPR-PRV-CC-PCV656M (PORV PCV-656 fails to open on demand (manual)) was set to TRUE due inability of PORV 656A to be manually opened during Exposure Times 1 and 2.
Basic events PPR-PRV-CC-PCV655M (PORV PCV-655 fails to open on demand (manual)) and PPR-PRV-CC-PCV655A (PORV PCV-655 fails to open on demand (automatic)) were set to 1.0 due to the inability to manually open PORV 655 during Exposure Time 2.
4 ANALYSIS RESULTS 4.1 Results1 The overall mean CDP for this analysis is calculated to be 5.6x10-7, which is the sum of the two exposure times. The parameter uncertainty results for both exposure times of this analysis are provided below:
1 The CDPs presented in the following sections are point estimates unless otherwise noted.
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Table 1. Parameter Uncertainty Results for Exposure Times 1 and 2 Exposure Time 5%
Median Point Estimate Mean 95%
1 3.6x10-8 2.1x10-7 4.9x10-7 4.4x10-7 1.5x10-6 2
1.0x10-8 4.2x10-8 7.6x10-8 6.9x10-8 2.1x10-7 The CDP for hazards included in the STP SPAR model is below the ASP Program threshold of 1x10-6 for degraded conditions. However, additional risk impacts estimated for internal fires (CDP = 4.9x10-7; see Section 4.4 for additional information) and windowed events (CDP =
1.4x10-6; see Section 4.5 for additional information) indicate that the overall CDP would be in low 10-6 range (2.5x10-6) and, therefore, the degraded condition associated with the failure of PORV 656A is a precursor.2 4.2 Dominant Hazards The dominant hazard for this analysis is high winds (including hurricanes and tornados) (CDP
= 3.3x10-7), which contribute approximately 59 percent of the total CDP. Internal events contribute approximately 41 percent (2.3x10-7) of the total CDP. Seismic hazards are minimal contributors for this analysis. The lack of internal fire scenarios in the SPAR model is a key uncertainty, which is considered qualitatively in Section 4.4.
4.3 Dominant Sequences The dominant accident sequence is hurricane (bin 2) sequence 4-21 (CDP = 2.8x10-7), which contributes approximately 49 percent of the total CDP. The sequences that contribute at least 5 percent to the total CDP are provided in Table 2. The event tree with the dominant sequence is shown graphically in Figures A-1 and A-2 of Appendix A.
Table 2. Dominant Sequences Sequence CDP Description HCN-BIN-2 4-21 2.8x10-7 49.4% Hurricane (bin 2) occurs that results in a LOOP initiating event; the reactor trips successfully; the emergency power system is successful; AFW fails; and feed and bleed cooling fails resulting in core damage.
PLOMFW 33 9.3x10-8 16.4% Partial loss of main feedwater (MFW) initiating event occurs; the reactor trips successfully; AFW fails; and feed and bleed cooling fails resulting in core damage.
SGTR 9 5.8x10-8 10.3% A SG tube rupture occurs, AFW or MFW is successful; high-pressure safety injection is successful; SG isolation is successful; secondary side cooldown fails; refueling water storage tank refill fails; and additional actions to restore decay heat removal fail resulting in core damage.
2 When adding the contribution from the windowed events, the exposure time associated with the failure of PORV 656A must be adjusted to eliminate double counting. The total exposure time considered in evaluating the risk impact of the windowed events is approximately 49 days. Therefore, Exposure Time 1 associated with the failure of PORV 656A would decrease from 8724 hours0.101 days <br />2.423 hours <br />0.0144 weeks <br />0.00332 months <br /> to 7539 hours0.0873 days <br />2.094 hours <br />0.0125 weeks <br />0.00287 months <br />, which would result in a mean CDP of 3.8x10-7 for this exposure time. However, the overall CDP associated with the failure of PORV 656A, including estimated from internal fires and the windowed events, remains in the low 10-6 range.
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4.4 Key Uncertainties A review of the analysis assumptions and results reveal the following key uncertainties:
Lack of Internal Fire Modeling in the SPAR Model. The STP SPAR model does not include internal fire scenarios. The risk impact from internal fires is likely to be dominated from scenarios that result in a loss of MFW and/or AFW or affect the availability of PORV 655. A quantitative evaluation using the information in the STPs individual plant examination from external events (IPEEE) is not practical; however, the risk achievement worth (RAW) importance measure of 1.78 of pressurizer PORV 656A from the existing licensee PRA results in an additional CDP of 4.9x10-7 from internal fire scenarios.3 This risk estimate is bounding for Exposure Time 1 since the importance measure considers the manual and automatic functions of PORV 656A; however, it is expected that the failure of the manual function to be the dominant risk contributor. In addition, the risk estimate for Exposure Time 2 is likely underestimated because the importance measures only consider a single failure (i.e., the importance measure for PORV 656A was not calculated based on the additional failure of PORV 655). The inclusion of internal fires results in the overall CDP to increase to 1.1x10-6, which is the ASP Program threshold for degraded conditions.
4.5 Windowed Events A search of STP Unit 1 LERs identified the six windowed events/degraded conditions that occurred during the maximum 1-year exposure time of PORV 656A (see Figure 1). Most of these events/degraded conditions were evaluated by previously completed ASP analyses. A brief description of these events/degraded conditions along with a reevaluation of the risk impacts associated with the concurrent unavailability of PORV 656A are provided below.
Figure 1: STP Unit 1 Windowed Events with PORV 656A Failure 3
The licensee fire PRA core damage frequency is 6.3x10-7 per year.
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LER 498-23-003. On November 10, 2023, essential chilled water (ECH) system train B was declared inoperable due to the temperature exceeding limits. Approximately 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> later, the ECH train C chiller was declared inoperable due to exceeding maximum compressor discharge pressure. ECH trains B and C were restored to service after successful repairs and post-maintenance testing were completed on November 11th and 20th, respectively. See LER 498-2023-003, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function, (ML24036A352) for additional information.
An ASP evaluation was performed assuming three exposure times(a.) ECH chiller B was failed for approximately 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, (b.) ECH chillers B and C were failed for approximately 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, and (c.) ECH chiller C was failed for approximately 9 days. The risk impact from the concurrent unavailabilities of the SG PORVs 1A and 1C (LER 498-24-001) and low-head safety injection (LHSI) pump 1A (LER 498-24-003) are negligible and, are not evaluated further as part of the evaluation of these degraded conditions. This analysis resulted in an overall CDP of 1.9x10-7 from internal events, high winds (including hurricanes and tornadoes), and seismic events. Internal flooding and fires scenarios are not included in the South Texas SPAR model. The risk impact due to internal floods is likely to be minimal for this degraded condition because these scenarios are unlikely to result in a LOCA for which the ECH system is needed to support the emergency core cooling systems (ECCS). The potential for LOCAs due to internal fires is expected to be mostly limited to LOCAs resulting from failures of reactor coolant pump (RCP) seals and the pressurizer PORVs. The risk impact from losses of RCP seal cooling/injection are mitigated by the installation of the passive shutdown seals and PORVs failure are likely to be isolable using the block valves. A quantitative evaluation using the information in the IPEEE is not practical; however, the RAW importance measures of 12.3 for the ECH chillers from the existing licensee PRA result in an additional CDP of 2.2x10-7 from internal fires, which would not result in the overall risk impact to exceed the ASP Program threshold. Given these considerations, this degraded condition was determined to not be a precursor.
A subsequent reevaluation considering PORV 656A was concurrently unavailable to be operated manually during the applicable exposure time results in the CDP increasing approximately 11 percent (2.1x10-7) from internal events, high winds (including hurricanes and tornadoes), and seismic events. The RAW importance measure of 1.78 for PORV 656A from the existing licensee fire PRA results in an additional CDP of 1.4x10-8 from internal fires. Although the risk impact from fires is likely underestimated because the available importance measures are limited to single failures, the overall risk of 4.4x10-7 from all hazards is very unlikely to increase to above the ASP Program threshold given the modest increases shown when evaluating the concurrent degraded conditions using the SPAR model. Therefore, this degraded condition is still not a precursor.
LER 498-23-004. On November 11, 2023, a condition report identified ECH train C expansion tank level lowering over the previous 3 months to the low-level alarm setpoint.
On November 21st, ECH train 'C' was declared inoperable due to excessive chilled water leakage. A subsequent licensee engineering evaluation determined that due to the excessive chilled water leakage, ECH train 'C' was considered to have been inoperable from November 20th until December 3rd when ECH train C leakage was corrected. See LER 498-2023-004, Condition Prohibited by Technical Specifications Due to Inoperable Train of Essential Chilled Water, (ML24036A352) for additional information.
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An ASP evaluation was performed assuming ECH train C was unable to perform its safety function for approximately 14 days. The risk impact from the concurrent unavailabilities of the SG PORVs 1A and 1C (LER 498-24-001) and LHSI pump 1A (LER 498-24-003) are negligible and, are not evaluated further as part of the evaluation of this degraded condition. This analysis resulted in an overall CDP of 2.1x10-7 from internal events, high winds (including hurricanes and tornadoes), and seismic events.
Internal flooding and fires scenarios are not included in the South Texas SPAR model.
The risk impact due to internal floods is likely to be minimal for this degraded condition because these scenarios are unlikely to result in a LOCA for which the ECH system is needed to support the ECCS. The potential for LOCAs due to internal fires is expected to be mostly limited to LOCAs resulting from failures of RCP seals and the pressurizer PORVs. The risk impact from losses of RCP seal cooling/injection are mitigated by the installation of the passive shutdown seals and PORVs failure are likely to be isolable using the block valves. A quantitative evaluation using the information in the IPEEE is not practical; however, the RAW importance measure of 12.3 for the ECH chiller from the existing licensee PRA results in an additional CDP of 2.7x10-7 from internal fires, which would not result in the overall risk impact to exceed the ASP Program threshold.
Given these considerations, this degraded condition was determined to not be a precursor.
A subsequent reevaluation considering PORV 656A was concurrently unavailable to be operated manually during the applicable exposure time results in the CDP increasing approximately 10 percent (2.3x10-7) from internal events, high winds (including hurricanes and tornadoes), and seismic events. The RAW importance measure of 1.78 for PORV 656A from the existing licensee fire PRA results in an additional CDP of 1.9x10-8 from internal fires. Although the risk impact from fires is likely underestimated because the available importance measures are limited to single failures, the overall risk of 5.2x10-7 from all hazards is very unlikely to increase to above the ASP Program threshold given the modest increases shown when evaluating the concurrent degraded conditions using the SPAR model. Therefore, this degraded condition is still not a precursor.
LER 498-24-001. On January 22, 2024, steam generator (SG) PORV 1A was declared inoperable due to its failure to operate in automatic or manual mode. On January 23rd, SG PORV 1C was declared inoperable due to its failure to operate in manual mode (only). Repairs on SG PORV 1C were successfully completed and the PORV was declared operable later that day. Unit 1 was restarted on January 30th and returned to full-power operation on January 31st. On March 1, 2024, the station entered a forced outage to replace a SG safety relief valve (SRV) spring. With the unit in Mode 4, after closing SG PORV 1C earlier in the day, operators attempted to open SG PORV 1C, but the valve would not open. The operators observed that the demand signal did not change on the controller. A subsequent licensee investigation found the same fuse blown that resulted in the valve failure identified on January 23rd, and an intermittent electrical failure on one of the A/B solenoid coils. The licensee replaced the A/B solenoid and SG PORV 1C was declared operable following post-maintenance testing on March 2nd. See LER 498-2024-001, Two SG PORVs Inoperable Resulting in a Condition that Could Have Prevented Fulfillment of a Safety Function, (ML24092A190) for additional information.
An ASP analysis was completed for these degraded conditions and is documented in an ASP analysis report (ML24183A224). This analysis resulted in a CDP of 2x10-9. from
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internal events, high winds (including hurricanes and tornadoes), and seismic events.
The dominant scenario from the failed SG PORVs included in the SPAR model is a SGTR initiating event. Internal fire and flood scenarios are very unlikely to result in a SGTR. In addition, internal fire and floods are unlikely to result in similar scenarios for the smaller risk contributors, which include steam line break and seismically small LOCAs with concurrent losses of offsite power.
The concurrent failure of PORV 656A would not affect the same scenarios and, therefore, the risk of these windowed events is only additive, with the failure of SG PORVs being a negligible contributor to the risk associated with the failure of pressurizer PORV 656A (i.e., the overall risk of the concurrent degraded conditions will be approximately equal to the risk associated with the failure of PORV 656A itself).
Therefore, the degraded conditions associated with the failure of SG PORVs is still not a precursor.
LER 498-24-002. On March 9, 2024, essential cooling water (ECW) train C was declared inoperable due to a through-wall leak on ECW pump 1C discharge vent line.
On March 10th, ECH train B was declared inoperable due to chilled water outlet temperature exceeding limits. ECH train B temperature limits were restored in approximately 47 minutes. The ECW pump C discharge vent line was repaired on March 11th. See LER 498-2024-002, Two Essential Chilled Water Trains Inoperable Resulting in a Condition That Could Have Prevented Fulfillment of a Safety Function, (ML24130A271) for additional information.
An ASP evaluation was performed assuming two exposure times(a.) ECW train C was unable to fulfil its safety function for approximately 2 days and (b.) both ECW train C and ECH chiller B failed. The risk impact from the concurrent unavailability of LHSI pump 1A (LER 498-24-003) is negligible and, is not evaluated further as part of the evaluation of these degraded conditions. This analysis resulted in an overall CDP of 9.2x10-8 from internal events, high winds (including hurricanes and tornadoes), and seismic events. Internal flooding and fires scenarios are not included in the South Texas SPAR model. The risk impact due to internal floods is likely to be minimal for this degraded condition because these scenarios are unlikely to result in a LOOP or LOCA for which the ECW system is needed to support the emergency diesel generators and ECCS. The potential for LOCAs due to internal fires is expected to be mostly limited to LOCAs resulting from failures of RCP seals and the pressurizer PORVs. The risk impact from losses of RCP seal cooling/injection are mitigated by the installation of the passive shutdown seals and PORVs failure are likely to be isolable using the block valves. A quantitative evaluation using the information in the IPEEE is not practical; however, the RAW importance measures of 16.1 and 12.3 for the ECW train and ECH chiller from the existing licensee fire PRA result in an additional CDP of 5.4x10-8 from internal fires, which would not result in the overall risk impact to exceed the ASP Program threshold.
Given these considerations, this degraded condition was determined to not be a precursor.
A subsequent reevaluation considering PORV 656A was concurrently unavailable to be operated manually during the applicable exposure time results in the CDP increasing approximately 3 percent (9.5x10-8) from internal events, high winds (including hurricanes and tornadoes), and seismic events. The RAW importance measure of 1.78 for PORV 656A from the existing licensee fire PRA results in an additional CDP of 2.7x10-9 from internal fires. Although the risk impact from fires is likely underestimated because the
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available importance measures are limited to single failures, the overall risk of 1.5x10-7 from all hazards is very unlikely to increase to above the ASP Program threshold given the modest increases shown when evaluating the concurrent degraded conditions using the SPAR model. Therefore, this degraded condition is still not a precursor.
LER 498-24-003. On April 21, 2024, LHSI pump 1A failed to start from the MCR during a valve operability test. An inspection of the 4.16 kilovolt breaker identified an unrestrained washer in the breaker cubicle that prevented the breaker from closing. It was determined that the unrestrained washer adversely impacted the seismic qualification of the breaker and there was no reasonable assurance that LHSI pump 1A breaker would have been able to perform its required safety-related function during a seismic event following breaker installation on April 4, 2023, until the breaker was replaced, and the pump was declared operable on April 25, 2024. In addition, the breaker would not have operated for any demand since it was last operated successfully on April 2, 2024. See LER 498-2024-003, Condition Prohibited by Technical Specifications and Potential Loss of Safety Function Due to Inoperable LHSI Pump, (ML24183A174) for additional information.
An initial ASP evaluation was performed assuming LHSI pump 1A would have failed to start during (a.) any demand for an exposure time of 24 days and (b.) any seismic event for the exposure time of 341 days. This analysis resulted in an overall CDP of 2.6x10-7 from internal events, high winds (including hurricanes and tornadoes), and seismic events. Internal flooding and fires scenarios are not included in the STP SPAR model.
The risk impact due to these unmodeled hazards for exposure time b is likely to be minimal because these scenarios are unlikely to result in a medium or large LOCAs, which are the dominant scenarios for this degraded condition. A quantitative evaluation using the information in the IPEEE is not practical; however, the RAW importance measure of 1.61 for LHSI pump 1A from the existing licensee fire PRA results in an additional CDP of 2.5x10-8 from internal fires, which would not result in the overall risk impact to exceed the ASP Program threshold. Given these considerations, this degraded condition was determined to not be a precursor.
A subsequent reevaluation considering PORV 656A was concurrently unavailable to be operated manually during the applicable exposure times results in the CDP increasing approximately 2 percent (2.7x10-7) from internal events, high winds (including hurricanes and tornadoes), and seismic events. The RAW importance measure of 1.78 for PORV 656A from the existing licensee fire PRA results in an additional CDP of 3.2x10-8 from internal fires. Although the risk impact from fires is likely underestimated because the available importance measures are limited to single failures, the overall risk of 3.2x10-7 from all hazards is very unlikely to increase to above the ASP Program threshold given the modest increases shown when evaluating the concurrent degraded conditions using the SPAR model. Therefore, this degraded condition is still not a precursor.
LER 498-24-004. On July 24, 2024, a LOOP occurred, resulting in an automatic reactor trip and actuation of all three Unit 1 EDGs and all four AFW pumps. All three trains of essential safety feature buses were energized via the EDGs, and all equipment responded as expected without any complications except for SG PORV 1C. Unit 2 experienced a partial LOOP and automatic actuation of EDG 22 and one of four AFW pumps. See LER 498-2024-004, LOOP Resulting in Unit 1 Automatic Reactor Trip and Actuation of EDGs and AFW Pumps, (ML24263A145) and inspection report (IR) 05000498/2024050 South Texas Project Electric Generating Station, Units 1 and 2 -
LER 498-2024-006 10 NRC Special Inspection Report 05000498/2024050 and 05000499/2024050 and Preliminary White Finding, (ML24320A137) for additional information. The risk impact of the LOOP with the concurrent unavailability of PORV 656A is modeled explicitly in the final precursor analysis (ML25086A198). The mean conditional core damage probability (CCDP) of the LOOP and concurrent failure of PORV 656A is 3.6x10-6. The mean CCDP of this event would have been 3.0x10-6 if PORV 656A was able to fulfil its full safety function. Note that the CCDP associated with this initiating event analysis was not added to the overall CDP of the degraded condition analysis documented in this report because different metrics are used and to prevent double counting.
Table 3 provides a summary of the ASP results from windowed events that contribute additional risk to the failure of PORV 656A:4 Table 3: Summary of ASP Results of Windowed Events LER Impacted System(s)
Exposure Time CDP w/o PORV 656A Failure CDP w/ PORV 656A Failure 498-23-003 ECH
~10 days 4.1x10-7 4.4x10-7 498-23-004 ECH
~14 days 4.8x10-7 5.2x10-7 498-24-002 ECW; ECH
~2 days 1.5x10-7 1.5x10-7 498-24-003 LHSI
~24 days5 2.8x10-7 3.2x10-7 Total =
1.4x10-6 An estimated CDP of 1.4x10-6 (including internal fires) from these windowed events is estimated to be added to CDP for the failure of PORV 656A.
4 LER 498-24-001 is not listed in this table because the concurrent SG PORV failures provides a negligible risk impact to the failure of pressurizer PORV 656A. An initiating event analysis was performed for LER 498-2024-004, which uses the CCDP risk metric. Therefore, this result was not added to the overall results of the degraded condition assessment of the failure of PORV 656A, which uses the CDP risk metric.
5 This exposure time is associated with LHSI pump A failure for all hazards. The additional 341-day exposure time for seismic hazards (only) is a negligible risk contributor.
LER 498-2024-006 A-1 Appendix A: Key Event Tree Figure A-1. Hurricane (Bin 2) Event Tree Figure A-1. Weather-Related LOOP Event Tree IE-HCN-BIN-2 HURRICANE WIND EVENT BIN-2 (130 MPH - 156 MPH)
HCN-WARN PLANT SHUTDOWN DUE TO HURRICANE WARNING STR-CD-HCN-B2 CAT 1 STRUCTURAL FAILURE DUE TO BIN 2 HURRICANE WINDS LOOP-HCN-B2 LOOP DUE TO BIN 2 HURRICANE End State (Phase - CD)
Disabled Branch - Future Use 1
OK 2
@SD-LOOPWR 3
TRANS 4
LOOPWR 5
CD IE-LOOPWR LOSS OF OFFSITE POWER INITIATOR (WEATHER-RELATED)
RPS REACTOR TRIP EPS FS = FTF-LOOP EMERGENCY POWER AFW AUXILIARY FEEDWATER PORV PORVS ARE CLOSED LOSC RCP SEAL COOLING MAINTAINED HPI HIGH PRESSURE INJECTION FAB FEED AND BLEED OPR-02H OFFSITE POWER RECOVERY IN 2 HRS OPR-06H OFFSITE POWER RECOVERY IN 6 HRS SSC SECONDARY SIDE COOLDOWN RHR RESIDUAL HEAT REMOVAL LPR LOW PRESSURE RECIRC FAILS CFC CONTAINMENT FAN COOLERS HPR HIGH PRESSURE RECIRC End State (Phase - CD)
AFW-L 1
OK LOSC-L 2
LOOP-1 PORV-L HPI-L 3
OK 4
OK 5
OK 6
CD 7
CD 8
OK 9
CD 10 CD CFC-L 11 OK HPR-L 12 CD CFC-L 13 CD HPI-L 14 CD AFW-L FAB-L 15 OK 16 CD 17 CD CFC-L 18 OK HPR-L 19 CD CFC-L 20 CD FAB-L 21 CD 22 SBO 23 ATWS 24 CD